HKAL Biology – Genetics Evolution and Ecology 9789622790513

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Genetics Evolution and Ecology for 'N. Level YK.To

GREENWOOD PRESS 47 Pokfulam Road, Basement, Hong Kong. Telephone: 2546 8212

© Greenwood Press 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means - electronic, mechanical, photocopying, or otherwise - without the prior permission of the copyright owner.

First published 1983 Reprinted 1984, 1987, 1988, 1991, 1992, 1994, 1997.

PRINTED IN HONG KONG

Preface

Many students taking the Advanced Level Examination have difficulty in finding relevant information in the topics of Genetics, Evolution and Ecology. This book is written in note form with numerous illustrations that enable students to have a rapid and concise revision of these topics. Great emphasis has been put in Ecology which includes the Basic Principles in Ecology, Micro-organisms and Man, Pollution, Conservation and the study of a Local Habitat This book can also be regarded as the Part Four of my series of Biology for Higher Level Examination since it covers all the related topics of Genetics, Evolution and Ecology in the Core Syllabus as well as Option 2 (Man and the Environment) and Option 3 (Man and Microorganisms) of the Biology syllabus in the Hong Kong Higher Level Examination. Y.K.TO, B.Sc. (Hons), Dip.Ed.

Contents

Preface

Part 1 Genetics and Evolution Chapter 1 Genetics 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1 .10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19

Introduction, 3 The natural occurrence of variation, 3 Mendel and his breeding experiments, 5 Modification of the 3 : 1 phenotypic ratio, 14 Dihybrid inheritance (dihybrid cross), 18 Dihybrid test cross, 23 Reasons accounting for the success of Mendel, 24 Genetic interaction, 25 Complex genetic interaction-polygenic inheritance (Multiple-genes inheritance; multiple-factors inheritance), 29 Multiple alleles, 32 Linkage, 34 Crossing-over,38 Sex determination, 46 Sex-linkage, 48 Inheritance of human characters, 5 3 Worked examples, 53 Genetic material - DNA, 76 Geneconcept,92 Mutations, 93

Chapter 2 Evolution 2.1 2.2 2.3

Introduction, 99 Evidences of organic evolution, 99 Mechanism of evolution, 110

Part 2 Inter-relationship

of Organisms with Each Other and

with their Environment Chapter 3 Ecology

3.1 3.2 3 .3 3.4

Some ecological terms, 121 Factors making up the physical environment, 123 Soil, 130 Structure of ecosystem - interdependence of plants and animals, 15 3 3.5 Food chains, food webs and ecological pyramids, 155 3.6 Functional aspect of ecosystem, 160 3 .7 Interactions in ecosystem, 172 3 .8 Parasites, 1 84 3 .9 Adaptations of organisms to the environment, 199 3.10 Population dynamics, 214 Chapter 4 Man and Micro-Organisms 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14

Viruses (L. virus, a poisonous or slimy liquid), 224 Bacteria, 231 Fungi, 241 Rhizopus (black bread mould), 243 Albugo (Cystopus), 248 Yeast (Saccharomyces), 253 Penicillium, 257 Aspergillus, 260 Athlete's foot, 261 Plasmodium (malaria parasite), 261 Body's defences against disease, 266 Antibiotics, 270 Prevention of infection, 2 72 Useful application of micro-organisms, 2 75

Chapter 5 Man's Effect on His Environment - Pollution 5 .1 5 .2 5 .3 5 .4 5 .5 5.6 5 .7 5 .8

Pollution, 282 Air pollution, 284 Water pollution, 290 Organic pollution, 291 Inorganic nutrients pollution, 296 Chemical pollution, 297 Thermal pollution, 300 Oil pollution, 300

5 .9 5 .10 5 .11 5 .12 5 .13

Silt pollution, 302 Control of water pollution, 303 Land pollution, 308 Noise pollution, 312 Radiation pollution, 315

Chapter6 Conservation 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10

Meaning of conservation, 317 Natural resources, 317 Forestry, 319 Agriculture, 321 Fishery, 322 Wildlife, 323 Minerals, 324 Fossil fuels, 324 Land, 325 Human population and birth control, 328

Chapter7 Study of Local Habitats- Sea Shore 7.1 7 .2 7 .3 7.4 7 .5

Sea shore habitats, 334 Zona ti on of the exposed rocky shore, 340 Adaptation - solutions to problems on rocky shore, 348 Biological inter-relationship within the ecosystem, 354 Method of study of a rocky shore, 359

Index, 373

Partl Genetics and Evolution

ChapterI

Genetics

1.1 INTRODUCTION Reproductionis the process of producing a new generation of offspring which resemble their parents. Such resemblances between the offspring and parents are due to heredity (inheritance). But no two offspring are exactly identical with each other e.g. children of the same family differ in respect to their height, weight and etc. In fact, these variation (differences)are characteristics of living organisms.

)

GENETICSis the branch of biology concerned with the study of the laws governing similarities (heredity) and differences (variation) between the offspring and their parents.

1.2 THE NATURAL OCCURRENCE OF VARIATION Offspring of the same species resemble each other in all their major characteristics, but they also bear many differences (variation) among them.

A.

Continuous variation (quantitative inheritance)

Many human characteristics such as height, weight, hand span, rate of heart beat, skin colour, body form and intelligence are not quite clear cut and cannot be divided into distinct contrasting groups. In fact there is a continuous range of intermediate forms between two extremes. Variation of this type is known as continuous variation. If the number of individuals are plotted against height, a series of histograms will be formed. Joining these histograms together will produce a normal distributioncurve.(Fig. 1.1)

Other examples of continuous variation are milk production in cows, egg production in hens, kernel colour of wheat (a continuous gradation is observed from red to white) and coat colour in mice (agouti, black, brown, cinnamon and albino). 3

180 160 140

..,

.c

-~ .,

.c .c

120

u

.,ro

.... 0

100

C: a,

E

80

0

.,....

..c

E :::,

60

z

40 20

148

150 152

154 156

158 160

162 164

166 168 170 172 174

176 178 180

182 184

Height in cm

Fig. 1.1 A normal distribution

B.

curve

Discontinuous variation (qualitative inheritance)

Some human characteristics are very definite and clear cut, they can be divided into distinct groups with few or no intermediates between them. For example, some people can roll their tongues into a U-shape but some cannot; and some people can taste the bitter nature of the PTC paper (paper that has been dipped into the phenyl-thio-carbamide solution) while some cannot taste it. These variations are known as discontinuous variation (qualitative inheritance).

It is impossible to obtain a normal distribution curve for such variation because the histograms of the distinct groups are widely separated apart. Other examples of discontinuous variation in man: 1.

4

Ear lobes (a) Some people have a small lump of flesh (lobe) hanging down from the ear. They are regarded as having ear lobes (or free lobes).

(b)

2.

Others have the lower edge of the ear running smoothly downwards to fuse with the side of the head. They are regarded as having no ear lobes (or attached lobes). (Fig. 1.2)

Albinism (a) Albino is an individual whose pigments fail to develop resulting in having a light skin, white hair and pink eyes. (b) Most people have their pigments properly developed are said to be normally pigmented.

3.

Haemophilia (a) Some people may have the disease called haemophilia. They cannot clot their blood properly and may face a fatal continuous bleeding (haemorrhage) once they are hurted. (b) Most people have normal blood that can clot properly whenever they are injured.

4.

Blood groups Each person only belongs to one of the following blood groups: A, B, AB and 0. They will not have any blood group at intermediate forms among the above four groups.

5.

Eye colour The colour of the iris of man belongs to one of the following colours: brown and blue; no intermediate form is present.

(A)

I

I

Ear lobe-

I 1 I

(Bl

No ear lobe

Fig. 1.2 (A) With ear lobe (8) No ear lobe

1.3 MENDEL AND HIS BREEDING EXPERIMENTS The basis of our present theories of inheritance was founded by the Austrian monk, Gregor Mendel. From 1858 to 1866 Mendel carried out many breeding experiments on the garden pea, Pisum sativum, in his monastery garden at Brunn (now known as Brno). He published his results in 1866. But his findings were not appreciated nor understood by his contemporaries until 1900. The same laws of inheritance were independently rediscovered by Hugo de Vries in Holland, Karl Correns in Germany and Erich van Tschermak in Austria.

A.

Monohybrid inheritance (Monohybrid cross)

Mendel studied the inheritance of single pairs of contrasting characters, which is known as monohybrid inheritance. It is done by a cross between two individuals which differ in one character, thus such cross is also called monohybrid cross. An example of contrasting characters is that one variety of pea plant is tall, about 1.8 m high, while the other is dwarf, about 0.3 m. 5

1.

He prepared pure breeding tall pea plants by selfpollinating the plants for several generations. Self-pollination was secured by enclosing the flower with paper bag since the bud stage. All offspring produced had tall stems i.e. a pure line was established. He also prepared pure breeding dwarf pea plants by the same method of self-pollination. The flower of the pea plant, as in many legumes, has petals completely enclosing the sex organs, the stamens (male organs) and the pistils (female organs). Thus self pollination is the usual practice of the pea plants. Selfpollination is ensured by enclosing the whole flower inside a paper bag.

2.

He cross-pollinated the pure breeding tall plants with the pure breeding dwarf plants. It was done by opening the flower bud, removing the immature stamens and dusting the stigma with pollens (male gametes) from the other variety of the pea plant. (Fig. 1.3) Then the flower was enclosed in a paper bag to prevent other cross-pollination with other plants occur. ·

(Bl Open the bud and cut the immature stamens away (A) Pea flower bud with petals completely enclosing the sex organs

Petal

Stigma r-::;:::;tc=-Stamens (to be cut away)

(C) Introduce pollens from the flowers of the other variety

A!illlll'"'"-----

stamens

Fig. 1.3 Cross-pollination

6

of the pea plants

Hair brush with pollen grains

3.

The seeds produced by the cross-pollination of the parent pea plants were sown until they had developed into mature plants. All these new plants form the first filial generation, (F 1 ). All of the F 1 plants were tall, none of them were dwarf. They were called the hybrid plants because they were produced by dissimilar parents.

4.

These F 1 hybrid tall plants were then self-pollinated. The seeds produced were again sown until they had developed into mature plants. The offspring formed the second filial generation, (F 2 ). They consisted of 787 tall plants and 277 dwarf plants i.e. giving a ratio of 787 : 277 or 2.84 : 1, approximately 3 : 1. (Fig. 1.4)

Parental generation (P) Tall stem (pure breeding)

:r

~

X

~

Dwarf stem

~ (pure breeding)

+ Cross-pollination

ff~'.

Tall stem (hybrid)

Self~oll;aafoa

1\ l'"'\

Tall stem

:} Tall plants (all)

~

Tall plant

Dwarf plant

3

Fig. 1.4 The result of a monohybrid

7"""\

:r ,t Tall plant

Dwarf plant

DwTs,em

,t, : Dwarf plant (all)

3

inheritance of the pea plants

7

5.

Characters studied Shape of seed

The similar pattern of inheritance was found in other breeding experiments: (Table 1.1) (a) The F 1 always resembled ONE of the two parents. (b) In F 2 both parental types reappeared but in a ratio of 3 to 1.

Parent group

~

€) round seed

Colour of seed

F2 Ratio

F 1 (hybrid)

.

wrinkled seed

round seed only

5474

1850

2.96 : 1

0

0 yellow seed

green seed

yellow seed only

6022

2001

3.01 : 1

smooth pod

constricted pod

smooth pod only

882

299

2.95 : 1

axial flower

terminal flower

axial flower only

651

207

3.14:1

Shape of pod

Position of flower

Table 1.1 The pattern

8

of monohybrid

inheritance

of some characteristics

of the pea plants

B.

Mendel's conclusions of the monohybrid inheritance

1.

Gene concept Mendel found that neither the F 1 nor F 2 generations were there any plants of intermediate height. From this fact he concluded that inheritance was not a process in which the characters of the two parents were blended (mixed) together to produce an intermediate effect. Instead inheritance was a process in which definite structures, called the factors by Mendel (now we call them GENES), were transmitted from the parents to the offspring.

2.

Dominance and recessiveness Mendel found that no dwarf plants appeared in F 1 generation though one of the parent plants was dwarf However some dwarf plants reappeared in F 2 generation. From these facts Mendel drew two conclusions: (a) Although the F 1 plants were tall, they had received a factor (gene) for dwarfness from their dwarf parents. The factor (gene) for dwarfness remained hidden in the outward appearance of the F 1 plants but expressed out in the F 2 plants. (b) As the factor (gene) for dwarfness was unable to show itself in the F 1 generation, it had been in some way masked by the factor (gene) for tallness. Only in the absence of this tall factor (gene) would the dwarf factor be able to express itself in the outward appearance of the plant. Thus the factor (gene) for tallness is dominant to the factor (gene) for dwarfness and the dwarf factor (gene) is recessive. The capital letter is used to represent the dominant factor (gene) and the small letter represents the recessive factor (gene). e.g. T for the tall factor (gene) and t for the dwarf factor (gene).

C.

Mendel's First Law (the Law of Segregation)

According to the experimental results Mendel proposed his First Law which is also known as the law of Segregation. In modern language this law states that when two pure bred individuals showing a pair of contrasting characters are crossed, the characters will segregate (separate) in def°mite proportions in the second filial generation (F 2 ).

D.

Interpretation of monohybrid inheritance (Fig. 1.5)

1.

Mendel suggested that a character such as the height of pea plants was controlled by a pair of genes. The gene for tallness (a domin9

ant character) was represented by T and the gene for dwarfness (a recessive character) was represented by t. Thus the genotype (genetic make-up) of the pure breeding tall parent plant was TT and that of the pure breeding dwarf parent plant was tt.

2.

When a pure breeding tall plant and a pure breeding dwarf plant were crossed, TT X tt, each parent would contribute only one gene which passed to the offspring in the gametes. Thus all the F 1 offspring were hybrid, Tt. Since the tall gene (T) dominates the dwarf gene (t), the phenotype (the external visible appearance of an organism) of all the F 1 plants would be tall plants.

Homozygous Tall

Parents (Pl

Homozygous Dwarf

TT

tt

!

!

Meiosis

Gametes formed by Meiosis

Meiosis

0

0 ~tion Tall

F,

Tt

Growth Pollen-producing cells in anther

Tt

Tt

I

I

M . . e1os1s

~

0

~

0 -,,._

0

/

F,

Tt Tt ~ 2 Heterozygous tall

1 homozygous tall

I

Fig. 1.5 Diagram to show the monohybrid

10

---.1

I

tt 1 homozygous dwarf

1 Dwarf (1 /4) (25%)

3 Tall (3/4) (75%)

inheritance

Eggs

I

-

TT

Egg-producing cells in ovary

of pea plants

3.

4.

When the F 1 plants (Tt) were self-pollinated Tt X Tt, each gene such as T separates (segregates) from its member such as t and passes into a different gamete so that each gamete has either a T or a t gene. The random fusion of gametes in fertilization produced three different genetic combinations in the F 2 plants: TT, Tt and tt. Again, since tallness is dominant, three-fourths of the F 2 generation would be tall and one-fourth dwarf, producing the definite 3 : 1 ratio of tall to dwarf plants. No plants were intermediate in height. The genotypes resulted from each cross can be represented in a Punnett square. The different gametes supplied by each parent are listed on the top and left sides of the checkerboard. The possible genetic combinations resulting from this cross-fertilization are represented in the cells of the Punnett square. (Fig. 1.6)

E.

Common terms used in genetics

l.

Gene Gene is a hereditary unit which forms part of a chromosome. A gene either by itself or interacting with other genes deter• mine the development of certain characteristics. It is chemically made of DNA (deoxyribose nucleic acid) and is capable of self replicating and undergoing changes (i.e. mutation).

2.

Locus It is a site on the chromosome where the gene exists.

3.

Mutation It is a sudden change in the structure of gene and is inherited to the next generation. The offspring so produced is called the mutant.

4.

Alleles (Allelomorphs) (Gk. allelon, of another) Chromosomes exist in pairs. Each member of the homologous pair carries, at identical loci, a gene controlling the same character but may have an expression different from the original form. Such alternative forms of the gene are known as alleles.

~ Gametes

., ... .,"'

T

t

T

TT

Tt

t

Tt

tt

.. E

Cl

-0

Fig. 1.6 A Punnett square to show the Tt X Tt

e.g. The gene for height of the pea plants exists in two different allelic forms, an allele T for tallness and an allele t for dwarfness. Alleles are usually formed by mutation. 5.

Genotype (Gk.genes, race+ type) It is the genetic make-up (constitution) of an organism. e.g. TT, It, tt.

11

6.

Phenotype (Gk. phaneros, showing+ type) The external visible appearance of an organism resulting from certain inherited trait (characters). e.g. tallness and dwarfism.

7.

Homozygote It is an organism containing two identical alleles (such as TT or tt) at the corresponding loci of the homologous chromosomes. (ad1:homozygous)

8.

Heterozygote It is an organism containing two different alleles at the corresponding loci of the homologous chromosomes. (adj. heterozygous).

9.

Dominant gene It is the gene which produces its effect no matter it is in homozygous or heterozygous condition. e.g. T causes the plant to be tall in either TT or tt condition.

10.

Rece§sive gene

It can only produce its effect in homozygous condition. e.g. t causes the plant to be dwarf in tt condition.

F.

Back cross

Due to complete dominance the tall pea plants may be homozygous (TT) or heterozygous (Tt ). The genotype of the tall plant can be found by self-pollination. 1.

If all the offspring are tall, the parent plant is homozygous (TT). (Fig. 1.7)

Parents( Homozygous)

Gametes

X

TT

+

TT

(Self-pollination)

+

0~0 TT (Tall)

Fig. 1.1 Self-pollination

12

of the homozygous

tall pea plant

2.

If a mixture of tall and dwarf pea plants are obtained, the parent plant is heterozygous (Tt). (Fig. 1.8)

A

Parents

Gametes

X

Tt

0

Tt

l

ISelf-polHoafoo)

ef

0

I

~ I

I I

I

--

F,

TT

Tt

G.

I tt

'-------v-----Dwarf

Tall Fig. 1.8 Self-pollination

--..1

Tt

of the heterozygous tall pea plants

Test cross

Most animals cannot carry out self fertilization. The genotype of an organism showing dominant character is found by crossing it to an individual with homozygous recessive (double recessive) of the character concerned. For exa.mple the normal wings of the fruit fly, Drosophila melanogaster, are longer than the length of its abdomen. These flies are regarded as the wild type. However some have vestigial wings which are short and functionless. The normal long-wing is dominant to the vestigial wing. The gene for wild-type is represented by + and the gene for vestigial wing is presented by v. 1.

If all the offspring are long-winged, the long-winged parent fly is homozygous(++). (Fig. 1.9) Parents

(Homozygous)

long-winged fly

X

Vestigial winged fly

Gametes

F,

+v (all are long-winged) Fig. 1.9 Result of the test cross of a homozygous organism

13

2.

If half of the offspring have long-wings and half have vestigial wings, the long-winged parent fly is heterozygous (+v). (Fig. 1.10) Parents

{Heterozygous)

long-winged fly

X

Vestigial winged fly

Gametes

+v

vv

Long-winged flies

Vestigial winged flies

Fig. 1.10 Result of the test cross of a heterozygous organism

1.4 MODIFICATION

OF THE 3 : 1 PHENOTYPIC RATIO

The breeding of many other organisms produce results different from that of Mendel's experiments on monohybrid inheritance. It is due to the interaction of genes.

A.

Incomplete dominance

Neither genes of a given character is dominant. The offspring produced by two individuals resemble neither of the parents but have character intermediate between the two.

1.

Flower colour of snapdragon (a) A snapdragon with red flowers crossed with white flower (b)

(c)

14

plants produce plants all of PINK flowers in the F 1 . Crossing the pink flowers in the F 1 produces offspring in the F 2 at the ratio of 1 red : 2 pink : 1 white. i.e. the red and white phenotypes reappear in the F 2 • (Fig. 1.11) The gene controlling red flower is represented by R and the gene controlling white flower is represented by r. (The incomplete dominant genes should properly be represented by lower case letters with numeral subscripts. e.g. r 1 for red flower gene and r2 for white flower gene). In incomplete dominance the F 2 phenotypic ratio of 1 red : 2 pink : 1 white is the same as the genotypic ratio of 1 RR : 2 Rr : 1 rr.

2.

Hair colour of short-horn cattle Crossing the short-horn cattle of red coat (RR) with those of white coat (rr) produces offspring with coat composed of a mixture of red and white hairs, the roan colour (Rr). (Fig. 1.11) Parents

Red flower plant X White flower plant

n

RR

t

t

0

0

Gametes

-----i------(Pink)

X

Rr

Rr (Pink)

A 0

Gametes

0 I

I I

--..J

RR

Rr

(Red)

Rr

rr

(Pink)

(White)

2

1

Fig. 1.11 The result of the inheritance of the incomplete dominant genes in Snapdragon

3.

M-N blood group of man If the wife has blood group M (her genotype is MM) and the

husband has blood group N (his genotype is NN), all their children must have blood group MN. (Fig. 1.12)

Parents

Wife

X

MM

NN

t

Gametes

Husband

t

®------r® MN

Fig. 1.12 The result of the cross between the wife of blood group Mand the husband of blood group N

15

B.

Lethalgenes

Some genes may directly cause the death of an organism. They are known as the lethal genes. Most lethal genes are recessive. 1.

16

Most lethal genes prevent the embryo of the bearer to develop or permit it to develop but eventually it aborts. e.g. the haemophiliac genes of the human female and the genes controllLng the fur colour in mice. (a) The gene controlling the yellow fur (Y) of mice is dominant to the gene controllin.g the grey fur (y). (b) If a yellow mouse is mated with another yellow mouse, the result is always the same: (i) Two thirds (2/3) of the offspring are yellow and one third (1/3) are grey i.e. yellow : grey are in 2 : 1 ratio instead of the usual 3 : 1 ratio. (ii) The litter size of such cross is always one quarter (1/4) smaller than those of other crosses e.g. yellow X grey or grey X grey. {c) Explanation: (i) The production of both types of fur colour in the F 1 offspring suggests that both parents are heterozygous (Yy) yellow. (ii) The impossibility of obtaining the 3 : l ratio in the F 1 offspring is explained by postulating that the homozygous condition of the yellow genes (YY) is a lethal combination that kills the mice in the embryonic stage. (Fig. 1.13) (iii) This hypothesis is supported by two other findings: ( 1) The number of dead embryos found in the uteri of the yellow mothers who have been crossed with yellow males is about one quarter of the total offspring. No such dead embryos are found in the uteri of the black mothers or in that of the yellow mothers crossed with black males because no homozygous condition of the yellow genes (YY) can be produced by such cross. (Fig. 1.14) (2) Repeated crossing yellow mice with yellow mice never produce any generation exclusively consisting of yellow mice i.e. impossible for yellow mice to breed true. This can only be explained by suggesting that all living yellow

Yellow

Parents

X

Yellow

V Yy

A 0 0

Gametes

Vy

p Die in embryonic stage

Yellow

Fig. 1.13 The lethal genes combination

Parents

Grey

Yellow

in mice

Grey (cl)

Yellow (9)

~ Vy

Gametes

A 0 0 Yy

Yellow

VY

A 0 0 yy

Grey

Fig. 1.14 Back cross of the yellow mouse shows that it is heterozygous (Yy.J

17

mice must be heterozygous (Yy) and no living homozygous yellow mice e:,cists.This can be proved by back cross. (Fig. 1.14) 2.

Some lethal genes produce their effects post-embryonically. (a) Albinism (absence of chlorophyll) in tobacco or some cereal plants produce white and spindly seedlings which will die after they have used up the food reserve in their seeds. (b) Similarly a gene for kidney malfunction in human expresses its lethal effect after birth when the baby no longer excretes the nitrogenous wastes through the placenta.

3.

In animals the lethal combination of genes is often shown by the death of the embryos. Therefore the existence of such lethal combination of genes is revealed by the conspicous deviation of the ratio of the offspring from that of the normal crosses. e.g. 2 : 1 ratio instead of the normal 3 : 1 ratio.

1.5 DI HYBRID INHERITANCE (DI HYBRID CROSS)

A.

Mendel's breeding experiment on dihybrid inheritance

Mendel also studied the inheritance of two pairs of contrasting characters at the same time in each cross i.e. dihybrid inheritance.

18

1.

He crossed pure breeding tall pea plants having round seeds with pure breeding dwarf pea plants having wrinkled seeds. All the F 1 offspring are tall plants with round seeds.

2.

He self-pollinated the F 1 offspring, collected their seeds and sowed. He got four groups of offspring in F 2 at the ratio of 9 : 3 : 3 : 1. (Fig. 1.15)

B.

Conclusion of the dihybrid inheritance

1.

Since both parent plants were pure breeding (i.e. homozygous), all the F 1 had to be heterozygous (i.e. hybrid) with the dominant characters showing out. Therefore tall plant is dominant to dwarf plant and round seed is dominant to wrinkled seed.

2.

Suppose T represents the gene for tallness, t for dwarfness, R for round seed and r for wrinkled seed. Therefore the genotypes of the parent plants are TTRR for tall plant with round seeds and ttrr for dwarf plant with wrinkled seeds. All the F 1 offspring will be TtRr. i.e. tall with round seeds.

Parents

Tall with round seeds X Dwarf with wrinkled seeds

Tall with round seeds

l

Self•poti;"";'"

~~

Tall with round seeds

9

Tall with wrinkled seeds 3

Fig. 1.15 The result of the dihybrid

3.

Dwarf with round seeds

Dwarf with wrinkled seeds

3

inheritance of pea plants

Since there were four possible combinations of characters in the F 2 generation at the ratio of 9 : 3 : 3 : 1, Mendel concluded that during gamete formation the members of one pair of genes (T, t) separate (segregate) from each other independently of the separation of the members of the other pair of genes (R, r) and they come to be assorted freely in the resulting gametes. It is known as Mendel's Second Law or the Law of Independent Segregation or the Law of IndependentAssortment. Therefore half of the gametes of F 1 contained T and the other half gametes contained t. Similarly half of the F 1 gametes contained R and the other half contained r. But the gametes containing T might be assorted with either R or r, and similarly the gametes containing t might also be assorted with either R or r. As a result four different kinds of gametes were produced: TR, Tr, tR and tr in equal proportion.

4.

As each F 1 plant produced four types of gametes and these gametes fertilized at random, there were sixteen possible combinations of the gametes and their results were shown by a Punnett square (Fig. 1.16). 19

Parents

D

Tall with round seeds

0

~

X

Dwarf with wrinkled seeds

l

TTRR

ttrr

j

Segceg,Ser UCA

UUG

C

Amino Acid

u

UGU Tyr

UAC

Leu

CUC

G

A

Phe

l

The amino acids

GluN

J Ser

AspN AGC

AAC

C

::r

a

11. 12. 13. 14. 15. 16. 17.

18. 19. 20.

Alanine Arginine Asparagine Aspartic Acid Cysteine Glutamine Glutamic Acid Glycine Histidine lsoleucine Leucine Lysine Methionine Phenylalanine Praline Serine Threonine Tryptophan Tyrosine Valine

Abbreviation ala arg asn asp cys gin glu gly his ilu leu lys met phe pro ser thr try tyr val

Thr

A AUA AUG

ACA Met (initiator)

GUA GUG

GAU}

GCC

GUC Val (initiator) (initiatorl

Lys

AAG

ACG

GCU

GUU

G

AAA}

AGA} AGG

G

GGU

u

Asp

GCG

C

GGC

GAC

Gly

Ala GCA

A Arg

GAA} GAG

GGA

A

Glu GGG

Fig. 1.53 The genetic code (messenger RNA codons)

4.

Non-overlappingcode Once the bases on a DNA molecule have been assigned to form a codon, they will not be shared with other bases to form another codon i.e. non-overlapping.For example if the piece of messenger RNA (GAA GCU GAC) is overlapping, it may code for eight amino acids.

87

aa6 aa2 GAA

--,,.-, aal

aa5 aa8 GCU

GAC

~

aa4

~ ~

aa3

aa7

If it is non-overlapping, it only codes for three amino acids.

G.

GAA

GCU

GAC

aal

aa2

aa3

The synthesis of protein - for controlling cell activities

DNA controls cell activities by directing the synthesis of proteins in the cell. Since all enzymes are made of proteins, thus all metabolic activities are indirectly controlled by the DNA.

DNA (genotype) ---+

1.

transcription

bio-chemical reaction

m-RNA

---+

translation

----➔

protein synthesis (enzyme production)

metabolic activities of the cell (phenotype)

Transfer of genetic information from the nucleus to the cytoplasm The DNA is confined to the chromosomes inside the nucleus while the proteins are made in the cytoplasm. Thus the genetic code (base sequence) in DNA is transcribed (copied) into another molecule called messenger RNA (m-RNA) which leaves the nucleus through the nuclear pore to the ribosomes (structurally built with ribosomal RNA, r-RNA) in the cytoplasm to direct the synthesis of protein there.

2.

Formation of messenger RNA (m-RNA) (a)

88

The two nucleotide chains of the DNA helix separate from each other in the relevant region of the molecule.

{b)

Double{ helix of DNA

One of the chains acts as the template (mould) upon which the messenger RNA is made, with uracil pairs to adenine and cytosine pairs to guanine. The m-RNA grows in the 5' end to 3' end direction. Such synthesis requires the enzyme RNA polymerase. (Fig. 1.54)

GAG ..-l..LL__Jl...LL_LL.J._LLI_.LLJL___..L..L'---L.L.L_....L..L-'------'-'--'--_,_,__,_____,__,L....L-__._L..L-__._,

cue

Messenger RN A

.._,,.__,

1

CTC

Fig. 1.54 Transcription of a messenger RNA from an unzipped portion of DNA

3.

(c)

The genetic code transcribed from DNA into messenger RNA is a triplet code, that is, the bases in messenger RNA are read in groups of three called the codon. Each codon specifies a certain amino acid.

{d)

The messenger RNA then leaves the DNA and passes through the nuclear pore into the cytoplasm where the messenger RNA attaches itself on the surface of a ribosome.

Transfer RNA (t-RNA) (a)

Transfer RNA is transcribed from certain DNA and consists of a single chain folded upon itself to form a helix.

{b)

Each transfer RNA is specific for a single amino acid and is responsible for picking up its particular amino acid from the cytoplasmic amino acid pool and transfers it to the m-RNA which is attached on a ribosome. There are 20 t-RNA if there are 20 amino acids.

(c)

The three unpaired bases at one end of the transfer RNA form an anti-codon which is complementary to the codon of them-RNA. 89

4.

Transfer of amino acids to the growing polypeptide linked on a ribosome (Fig. 1.55; 1.56)

(a)

Each amino acid is activated with ATP by a specific amino acid activating enzyme (amino acyl synthetase) before it can attach to its transfer RNA (t-RNA) to form a complex. (Since there are 20 amino acids, there must be 20 amino acyl synthetases to catalyse their attachment with 20 transfer RNA).

(b)

The t-RNA charged with activated amino acid diffuses towards a ribosome.

(c)

The t-RNA binds temporarily with the m-RNA when its anticodon matches with the codon of the m-RNA by complementary base pairing.

(d)

Then the t-RNA binds strongly on the ribosome surface and is just adjacent to the preceding t-RNA which bears a growing polypeptide chain. When the amino acid of the newly arrived t-RNA forms a peptide bond with the polypeptide chain and becomes attached with it, the latter frees itself from the preceding t-RNA which also detaches from the m-RNA-ribosome surface as a free structure. Now the polypeptide chain has increased its length by one amino acid and is attached at the newly arrived t-RNA. The free t-RNA can be used again. (Fig. 1.55) .·..:

•-/ •-t._.., ,J,f,o/Hii:f l 1 f>• - •

Newly formed peptide chain

G

c

~

G C

cc\AAAC UUA Ill

CGC Ill

GGU ACA Ill

Ill

r.:-: \ ,,._. A

t/.

C

':'.'1

\ ,,,,.

4 C

A : ,.,., G C ·:J C

I

:: :

{:

:\:,G4u/AG

\ \{_~AU GGU UUG ::~UUA Ill 111:

15

"O Q)

~ 'I-

0

,._

10

Q)

..a

5

z

0

E ::,

(ii) 50

100

150

200

Number of plants grown per pot

Fig. 3.30 Number of seeds by each plant of shepherd's purse sown at different densities

(b)

When water flea (Daphnia pulex) were cultured at different densities: 1 per cc, 4 per cc, 8 per cc and 32 per cc, the maximum birth rate was obtained when the population density was lowest. (1) The fecundity (reproductive capacity) decreases with increasing density. (2) In all different cultures, birth rates show a consistent drop with increasing crowding. On plants (Fig. 3 .30) The number of seeds produced per plant of shepherd's purse was found to be very high when the density of the plants grown was very low. Slightly increase in the density of plant grown greatly decreased the number of seeds produced per plant. It was due to the competition for the limited resources, light in this case.

Territoriality Territory is an area occupied by an organism which will defend against other individuals of the same species entering that area. This social behavioural activity is called territoriality which is thus a kind of intraspeczfic competition.

This behavioural activity is observed in birds, mammals, fishes, social insects and lizards. Usually only individuals possessing the territory can get a mate. Surplus individuals are forced into poorer areas where qvailability of food and mates are greatly reduced. Territoriality serves to reduce conflict between members of the population, forces them to exploit all possible area in the habitat. It limits the population size in accordance with the ability of the area to supply food and living space.

2.

Interspecific competition

It is the competition between the individuals of different species for the same resources. The intensity of competition depends on the degree to which the two species share on that common resources.

According to Cause's study on competition, he proposed the competitive exclusion principle. This principle states that no

174

two species ever occupy exactly the same niche in the same area. If they did, one species would be more efficient than the other at exploiting the niche and would eventually replace the less welladapted species completely. There were two experiments used to support this principle. (a)

Competition bet}veen two (P. caudatum and P. aurelia)

species

of

Paramecium

Both species of Paramecium feed on bacteria. (i) When each species was cultured separately on a fixed amount of food (bacteria), each reproduced and its population growth achieved a typical sigmoid curve. (Fig. 3.31 A, B). P. aurelia separately

200 150 100 50 (A)

P. caudatum separately

200 150 100 50 (B)

In mixed population

200 OJ

E

P. aurelia

150

"' 100 0

>

50

P. caudatum

(C)

Days

0

Fig. 3.31

2

4

6

8

10 12

14 16 18 20 2 2 24

Growth of P. caudatum and P. aurelia, when grown separately

and in mixed culture medium

175

(ii)

(1)

(2)

(3) (A) Tribolium castaneum

(b)

When both species were put in the same culture with a limited amount of food, both species grew at the same rate during the first 6 to 8 days. But later there was a gradual decrease in the number of P. caudatum and a continuous increase of P. aurelia. On day 16 the P. aurelia increased up to a constant level which was still lower than that had been achieved when cultured separately. By this time the P caudatum almost vanished. (Fig. 3 .31 C) The P. aurelia did not kill the P. caudatum by predation nor secreting any harmful substances. P. aurelia won the competition simply by having a slightly higher growth rate and thus more successful in getting the limited food supply.

Competition between two species of flour beetles Tribolium (T. castaneum and T. confusum) (Fig. 3 .3 2)

The two species of flour beetles, Tribolium, were allowed to live, feed and breed in a jar of flour. Usually one species would eventually die out and the other was left as the successful competitor. Which $pecies won the competition was affected by the 'climatic' conditions such as temperature and humidity of the flour habitat. It was found that (i) (8) Tribolium confusum

Fig. 3.32

(ii)

The flour beetles, Tribolium (A) T. castaneum (8) T. confusum

C.

T. confusum survived in the cool-dry flour environment while T. castaneum survived in the hot-wet flour environment. Since there was plentiful supply of food, the flour, these two species were not competiting for food, but for space. The overcrowding condition coupling with the effect of humidity and temperature affects the birth rate and death rate and thus the survival value of each species.

Predation

Predation is a type of biological interaction in which one species (the predator) attacks and kills another species (the prey). Usually the predator population is much smaller than the prey population otherwise they may not have sufficient food to support them.

176

Experimentto illustratethe prey and predatorinteraction

1.

(a)

The experiment (Fig. 3 .33) Paramecium caudatum that feeds on yeast and bacteria are consumed by another ciliate, the Didinium nasutum i.e. P. caudatum is the prey and D. nasutum is the predator. (i) Five Paramecia (the prey) were put into a test tube which contains bacterial populations growing in an oat medium. The Paramecium population increased up to 120 in the second day because of the abundant nutrient supply, plentiful space and no predation. (ii) On day two of the experiment three Didinia (the predator)were also put into the same test tube. The Paramecium population decreased rapidly while the Didinium population increased to around 20 because the former as a prey was consumed by the latter, the predator. (iii) The Paramecium population continued to decrease until finally it became extinct. The Didinium population later also decreased and eventually became extinct too.

~ 120 ::::, "C

·;;;

=-c

·= 80 ..... 0

...

Q)

E 40 ::::,

z

Days 0

2

3

4

5

6

Fig. 3.33 Prey-predator interaction between Paramecium and Didinium

(b)

Interpretation (i) The predator population (Didinium) rose and fell at the same phase as that of the prey (Paramerium) but slightly lagged behind. (ii) In this simple situation the predator could find all the prey and ate them all, and then it died out due to starvation. Both species died.

177

2.

Prey and predator interaction in nature

In natural ecosystem the prey would survive because they could escape faster than the predators could find them and could hide in some shelters from the enemies. Thus the preys could reproduce and increase elsewhere. Therefore both the predators and preys could exist together, but followed some periodic oscillations. It is well shown by the population trends of the Canadian lynx (Lynx canadensis) and the snowshoe hare (Lepus amen·canus) in the Hudson Bay area of Canada. (Fig. 3 .34)

(i)

(a)

(ii)

The rise in the predator population consumes more prey resulting in the progressive decrease in prey population. With the decrease in the numbers of preys, the predator is left with less and less food, many predators die due to starvation resulting in the decline of the predator population. With less predation it allows the prey population to increase. LYNX (predator)

~-~1

~

160 140 "'

HARE (prey)

120

v )\' ~II

,1

"O C

~

100

i\

0

...,

.c

C

11

i

80

11 1 1 1

'-

~ E

60

Z

40·

I

i

l

I

1

I

__--Hare

-

:,

--~--

Lynx

20

1850

60

70

80

90

1900

10

20

30

Time (in years)

Fig. 3.34 The population trends of the Canadian lynx and snowshoe hares in the Hudson Bay area_of Canada

178

(iii)

3.

The rise in the prey population provides more food to support the increase in predator population.

(b)

The hares as the preys were greater in number than the lynx, the predators.Since the lynx depended on the hares for food supply, the rise and fall of the lynx population followed that of the hare population but lagged behind by one to two years. This cyclic population change occurred at a length of 9 .6 years.

(c)

Sometimes the hare population declined rapidly even in the presence of a small number of predators. (i) It might due to the intense intraspecific competition for food that led to starvation. (ii) The high density of population might encourage the occurrence of epidemics that caused the death.

Interferenceof the prey-predatorinteractionby man The prey-predator interaction well balances the populations of animals in nature. Man's interference may greatly disrupt the balance in nature. (a)

Prior to 1907 there were about 4,000 deer living on the Kaibab plateau just north of the Great Canyon in Arizona. Their number was maintained at this constant level by the predators (such as wolves and pumas).

(b)

From 1907 onwards these natural predators were deliberately killed by man in order to protect these deer.

(c)

In the absence of predation the deerincreased upto 100,000 in 1924. It was far beyond the supply of vegetation on the plateau. In the absence of sufficient food, mass starvation and death occurred, resulting in having 60% of the herd died in the two successive winters. The result was diastrous. (Fig. 3.35)

(d)

The excessive grazing had seriously damaged the vegetation which did not grow back to the 1906 state. The theoretical maximum carrying capacity of the plateau (i.e. the maximum number of herbivores that can be supported by the vegetation available) dropped from 30,000 deer in 1906 ta 10,000 deer in 1940.

179

Number of deer

100,000

/

50,000

60% of herd starved in 2 winters

40,000

Maximum carrying capacity in 1906

30,000

/

-0-----------10,000 / ~---__,... ......

Maximum carrying capacity in 1940

4000

Time 1905

1910

1915

1920

1925

1930

1935

Fig. 3.35 The effect of the removal of natural predators on the deer population

D.

1940

on the Kaibab Plateau

Symbiosis (Gk. symbiosis, to live together)

Symbiosis is a mode of life in which two organisms of different species live in intimate association with each other. Depending on the nature of the association , the relationship is designated as commensalism, mutualism and parasitism. 1.

Commensalism (L. cum, together; mensa, table)

When two types of organisms live together, only one organism, called the commensal, gains benefit, but the other, called the host, neither harmed nor benefited, such association is called commensalism.

(a)

Usually such association is not permanent and no physical connection is involved.

(b)

The commensals usually gain the benefits of feeding arrangement, protection (shelter) and support (anchorage).

Examples of commensalism

(a)

180

Epizoites (Gk. epi, upon; zoo, animal). Animals living on the surf ace of other animals are called epizoites. (i) The sea anemones live on the shells of the hermitcrabs (Fig. 3 .36)

The sea anemones as sessile animals gain the benefits of moving to a new environment with better food supply and obtaining the food remains dropped by the crabs. (The crab also gains some protection from the anemone's stinging cells and some form of camouflage.Since these benefits are not essential and their association is not permanent, it is regarded as commensalism).

(ii)

(iii)

Some barnaclesattach on the shells of crabs or even on the jaws of whale. These sessile commensals gain the benefits of locomotion and feed on the food remains of the host. A kind of fish (Remora) has its dorsal fin modified as a muscular sucker which attaches it on the body surface of the shark.(Fig. 3 .37) It gains benefit by feeding on scraps of food left uneaten by the shark and savingenergy in locomotion.

Sea anemone

Shell in which the crab in living

Fig. 3.36 Sea anemone on the hermit crab shows commensalism

Mouth

Fig. 3.37 The dorsal muscular sucker of Remora

Dorsal muscular sucker

181

(iv)

(v)

(b)

Peacrab (Pinnotheres) lives within the mantle cavity of some bivalve shell fish such as oyster. The peacrab, as a commensal, feeds on the sticky strings of food trapped by the filter-feeding mechanism of the bivalve.

(c)

Epiphytes (Gk. epi, upon;phuton, plant) Plants growing on surfaces of other plants are called epiphytes. Algae, mosses and other creeping plants grow on the tree trunks to gain support in order to receive more light. They do not draw nourishments from the host plants. Since the dense leafy canopy of the trees cut out most of the sunlight so that the humidity in a forest is rather high. The epiphytes can easily absorb sufficient amount of water from the atmosphere.

Fig. 3.38 A rhinoceros and an ox-pecker bird

2.

The bird Egret perches on the back of the Water Buffalo feeds on the insects, particular grass-hoppers, that are disturbed by the cow when the latter is moving around in the grass. The bird called ox-pecker standing on the back of the rhinoceros is a commensal which feeds on the insects disturbed by the rhinoceros and on the exoparasites such as ticks on the skin of the mammal. (Fig. 3 .38)

Mutualism When two types of organisms live together and both gain benefits from one another, such association is called mutualism.(Symbiosis, in a more restricted sense, can be used to describe this close association. The organisms involved are called symbionts). This association is usually a permanent one. In the absence of the other one, they do not live as successful as before or may even die. Examples of mutualisms

182

(a)

Lichens - an association of a green alga and a fungus. (Fig. 3.39) (i) The fungal hyphae entirely enclose the algal cells to protect them against desiccation, to anchor the plant body on the substratum and to absorb water and mineral salts. (ii) The algal cells carry out photosynthesis to supply food to the fungus.

(b)

Leguminous plants and nitrogen-fixing bacteria (i) The nitrogen-fixingbacteria(Rhizobium) living in the

(ii)

root nodules of leguminous plants (peas, beans, etc) change the atmospheric nitrogen gas into nitrogenous compounds. These compounds will be used by the leguminous plants to produce proteins for their growth. The leguminous plants in turn protect the bacteria and supply carbohydrates to support their energy requirement.

Fungus (for protection) Algal cells (for food production)

{--__ _ {

Fungal hyphae

Fungal rhizoid (for absorption and anchorage)

Fig. 3.39 Structure of a lichen

(c)

Green alga. (Zoochlorella) and freshwater green hydra (Chlorohydra) (i) The green alga living inside the endodermal cells of the Hydra gains protection, locomotion, carbon dioxide and nitrogenous compounds (from excretion) from the cells of Hydra. (ii) Hydragains oxygen and carbohydrates manufactured by the green alga.

183

(d)

Cellulase-producing bacteria and herbivorous mammals (i) The cellulase-producing bacteria living inside the alimentary canal of the herbivorous mammals (e.g. cows, rabbit or sheep) gain shelter, protection and food supply. (ii) The herbivorous mammals obtain cellulase from these bacteria to digest the cellulose of the plant tissues into soluble products.

(Termites obtain cellulase from its gut protozoa, the Trichonympha.) 3.

Parasitism (Gk. parasitos, one who eats at the table of another; ismos, condition). (a)

Definition Parasitism is a type of close association between two organisms in which one, the parasite, depending upon the other, the host, for supply of nutrients. Usually the parasite gains benefits (e.g. food and protection) from the host and imposes harm to it.

(b)

Types (i) Endoparasites They are parasites living inside the host's body, usually in their guts or tissue fluid e.g. Tapeworm is an animal parasite living inside the intestine of man. (ii) Ectoparasites They are parasites living on the body surface of their hosts and obtaining their food by piercing through the outer tissues of the hosts to -suck up their body fluids e.g. Dodder is a plant parasite living on clover plants.

3.8

PARASITES

A.

Tapeworm (Taenia solium)

Tapeworm is a flatworm which belongs to the Phylum Platyhelminthes. 1.

Habitat (a)

184

The adult stage lives in the small intestine of its primary host (e.g. man) i.e. it is an endoparasite.

(b)

1.

The immature stage lives inside the body of the secondary host (e.g. pig and ox).

External features of the adult Tapeworm (a)

Shape - It is flattened dorso-ventrally and is in the form of a white ribbon.

(b)

Size - It is about 6-10 metres long.

(c)

Body - The body consists of three parts, the scolex, neck and strobila.(Fig. 3.40) (i) Scolex (the head or holdfast) The scolex is a tiny muscular knob which is topped by a rostellum and is bordered by four suckers. (Fig;. 3 .41) (1)

(2)

Rostellum - This elevated top bears 22 to 23 downwardly directed chitinous hooks arranged in two rows. These hooks enable the scolex bury inside the intestinal wall. Suckers - The four muscular suckers anchor themselves firmly on the intestinal wall.

By means of the hooks and suckers the tapeworm is prevented to be swept away from the intestine.

Neck

Strobila Rostellum

Scolex

i

Neck[~~-

Strobil•{

Fig. 3.40 External features of Tapeworm

~--Hooks

Sucker

,,_,__ __

Excretory canal Young proglottids

Fig. 3.41 Anterior end of the Taenia solium

185

(ii)

Neck

(1) (2)

(iii)

2. ...,

Anterior

part of gut

Posterior

part of gut

Internal structure of the adult Tapeworm

(a)

Digestive system (i) The tapeworm has neither a mouth for taking in food nor an alimenta,y canal for digesting and absorbing food . (ii) It obtains its nutrients by diffusion and active transport througl1 the general body surface because it is bathed in a nutritive medium of digested food.

(b)

Respiratory system Since there is not much oxygen in the intestine, the tapeworm shifts to anaerobic respiration (respiration without oxygen) for its energy supply.

(c)

Locomotory system

:,

en C:

40

...,

.c

en 'iii

20

5

E

0

5 ro

o, I I

§

,._ 0 Q)

en

Immediately behind the scolex is the neck. This is the narrow region of actively dividing cells which give rise to a chain of segments called proglottids (sing. proglottis). Tapeworm continues its growth by producing new segments from this region. Strobila (1) It consists of 800-1000 proglottids which form the main portion of the body. (2) The proglottids near the neck are young, small and sexually immature while those further away are older, larger and sexually mature. The last few proglottids are continuously shed away and released from the body of the host. (3) Each proglottis is marked from the next one by the presence of a transverse groove. (4) The whole tapeworm is covered by a resistant cuticle (more properly called a tegument because it is found to respire).

40

~

C: Q)

~ Q)

20

a..

0

i---,----,--r---,----,--,-

l

4 p.m. 1200 Midnight Noon

1200 Noon

Fig. 3.42 Movement of tapeworm in the gut of a rat

186

The tapeworm has no locomoto,y system because there is no need for it to move about in finding food nor to escape from its enemies. (i) But new scientific findings suggest that during the day time (12:00 noon to 4:00 p.m.) the tapeworm moves from the anterior to the posterior part of the gut where the digestive actions are less active. (Fig. 3.42)

(ii)

During the night the tapeworm is found to move towards the anterior part of the gut where it is less active now and more digested nutrients are available.

(d)

Excretory system This system is well developed for eliminating metabolic wastes and water out of the body. (i) It consists of two parallel tubes, the excretory canals, with run along the entire length of the body and eventually open to the outside at the last proglottis. (ii) These two excretory canals are connected together by a transverse canal in the posterior region of each pro glottis. (iii) Branching from these two excretory canals are numerous fine ducts which end blindly in the excretory units called the flame cells. Flame cells are responsible for the excretion of wastes.

(e)

Sense organsand nervous system (i) Since the environment in the intestine of the host is rather constant, there is relatively no change in the external stimuli of the tapeworm, hence sense organs are absent. (ii) Due to the absence of sense organs and the sluggish way of life, co-ordination between the various systems of the tapeworm is not very important. Thus the nervous system is poorly developed. It consists of a nerve ring in the head and two lateral nerve cords running along the whole length of the body.

(f)

Reproductive system The tapeworm is a hermaphrodite(i.e. an organism possesses both male and female reproductive organs.) As a parasite in order to ensure the continuation of the species a large number of eggs has to be produced to increase its chance of survival. Every proglottis is a reproductive unit to produce eggs. (i) Since the male reproductive organs mature earlier than the female reproductive organs, the strobila is divided into three parts according to maturity. (1) The anterior young region - These newly formed proglottids either possess no reproductive organs or just bear the male reproductive organs.

187

(2)

The middle mature region - Each proglottis possesses both male and female reproductive organs. (3) The posterior old region - Each proglottis is almost completely filled with the enlarged uterus which is packed with numerous fertilized eggs while the male reproductive organs have degenerated. Male reproductive system (Fig. 3 .43) (1) It consists of many small rounded spermsproducing bodies, the testes which are present throughout the anterior and middle parts of a pro glottis. (2) The testes are connected by many fine ducts, the vasa efferentia, which unite to form the single, narrow, coiled sperm duct called the vas deferens. (3) This single sperm duct leads to a muscular penis which opens through the lateral genital pore.

(ii)

U/+f===--:---::--;?-)m

Transverse groove ___

Uterus

Nerve cord----r-t1 Excretory duct --

~::::C.3,---k;'---t-Hri- Fertilized egg

Sperm-duct ---+--~"'i'

Sperm sac

Penis Genital pore

Ovary

-----t-i-

1 ==c::--. "'Y'l.:-n,-

Vagina

Fig. 3.43

188

The life history of the tapeworm

Taenia solium

(iii)

3.

Female reproductive system (Fig. 3.43) (1) A pair of posteriorly situated ovaries is joined together by a common oviduct which branches into two tubes. (2) One tube called the vagina opens to the exterior at the genital pore. Sperms entered the vagina are stored at the sperm sac which is a dilated region of the vagina. (3) The other tube ends blindly into a bag-like structure, the uterus, which is used to store the fertilized eggs. (4) Before the oviduct enters the uterus, it is joined by ducts from the yolk (vitelline) gland and shell gland.

Life history (Fig. 3 .44) The life history of the tapeworm consists of three stages, the egg stage, the secondary host (for larval development) and the primary host stage (for adult development) (a) The egg stage - a free stage living outside any host. (i) Fertilization - Eggs passed out of the ovaries are fertilized by sperms from the sperm sac. (ii) Egg formation - Each fertilized egg receives a yolk cell from the yolk gland and is then enclosed in a protective layer, the egg shell, which is secreted by the shell gland. (iii) Six-hooked (Hexacanth) embryo - Inside the uterus the embryo of the egg develops up to a stage with six hooks and is called the six-hooked embryo. (iv) Release of eggs - The last few proglottids are filled with the enlarged uteri which contain numerous developed eggs. Eventually these old proglottids break of/from the worm and pass with the faeces out of the host. The decay of these proglottids releases the eggs on the ground. (v) Free stage - The eggs scattered on the ground are well protected from the surroundings by their egg shells and support their own life by the yolk cell. The embryos remain in the six-hooks stage until they are eaten by another animal, the secondary host. (Pig is the secondary host of T. solium ).

189

Bladder worm embedded in muscles of pig with head inverted

Raw or partly cooked infected pork eaten by man

_________....

Head pushed out in the gut of man

With head attached to the intestinal wall and growth of the proglottids begins Free embryo in gut of pig

n 'tt.;_,\i'

Eaten by pig \

Shell of the eggs

MAN (primary host)

~1/

:=9· '_:_:(~_ ··:~~---

·

~

Passes out with faece

Fig. 3.44

The life history of the tapeworm

Taenia solium

190

t

,': Mature tapeworm attaching

' ,

Six hooked embryo

\

PIG (secondary host)

(b)

~ast

to the gut wall of man

few proglottids

broken off from the strobila of the worm Ripe proglottids full of eggs

The secondary host stage (The development of bladder worms in the pig) (i) Release of the hexacanth embryo After the eggs have been eaten by the pig, the protective egg shells will be dissolved by the digestive juices of the pig's alimentary canal and the hexacanth embryos are set free. (ii) Passage to the muscle By means of the hooks, the embryos bore through the intestinal wall into the blood vessels and are then carried by blood to the muscles and embed there. (iii) Bladder-worm The embryos lose their hooks and encyst. Each encysted embryo then expands to form a fluid-filled bladder and has an invagination which eventually develops into an inverted head, the proscolex. This larval stage is called the bladder worm or cysticercus. (Pork infected with bladder worms has a brown spotted appearance and is called 'measly pork')

(c)

4.

The primary host stage (The development of the adult worm in man) ( i) Infected by the bladder worm - If the 'measely pork' is eaten raw or not properly cooked, the bladder worms will be brought into the intestine of man. (ii) Attachment to the intestine - The bladder is then dissolved by the digestive juices of the alimentary canal and the invaginated proscolex everts to form the scolex which by means of its hooks and suckers fastens itself firmly to the intestinal wall of the primary host. (iii) Growth of the adult - Porglottids are budded rapidly behind the scolex to form a new adult tapeworm.

Problems and adaptations of Tapeworm to the parasitic mode of life

There is a lot of problems in the parasitic mode of life. Parasites usually overcome them by developing special adaptive features. Tapeworm is taken to illustrate this principle. (a)

The problem of attachment to the host The hooks and suckers on the scolex enable the tapeworm attach itself firmly to the intestinal wall of the host and thus prevents it to be removed away during peristalsis of the host's gut.

(b)

The problem of an endoparasite to accommodate itself inside the host The elongated, dorso-ventrally flattened body of the tapeworm fits the shape of the intestine.

(c)

The problem of obtaining nutrients efficiently (i) As an endoparasite is bathing in the nutritive medium of digested food of the host, nutrients are absorbed by diffusion and active transport through the general body surface. (ii) Moreover the surface is increased by the elongated, flattened shape of the tapeworm. (iii) Thus the digestive system is degenerated. The sense organs are also degenerated because there is no need for it to detect its food.

(d)

The problem of the shortage of oxygen supply inside the host

191

The tapeworm shifts to anaerobic respiration to obtain energy.

(e)

(f)

The problem of the host reaction The endoparasite has the danger of being killed by digestive juices of the hosts. The cuticle (tegument) of adult tapeworm (i) is thick enough to protect the tapeworm against digestive juices of the primary host, and (ii) may even secrete 'anti-enzymes' to neutralize digestive juices.

the the the the

The problem of the continuation of generation The continuous production of new proglottids behind the neck ensures the presence of sufficient number of reproductive units. (ii} The adult tapeworm is a hermaphrodite. Each mature proglottis contains both the male and female reproductive organs so that there is no problem of fertilization though there is usually one tapeworm present in the primary host. (iii) Each proglottis produces a large number of eggs to increase the chance of infecting the secondary host for the continuation of the life cycle. (i}

(g)

5.

192

The problem of getting into the host (i} The hexacanth embryo is well protected by the egg shell against the tough conditions of the free stage. Its presence in the human faeces facilitates it to come into contact with the food of the pig, thus infecting the secondary host. (ii} The hexacanth embryo equipped with hooks enables it to penetrate through the pig's intestinal wall and gets into the pork. As pork is a favourite food of man that increases the chance of infecting the primary host in the form of bladder worms.

Effect on man (a)

The adult tapeworm draws away some of the man's nutrients so that the man may become very thin and unhealthy.

(b)

Sometimes some severe harmful effects may occur.

6.

Control of the Tapeworm Usually the control of tapewonn infection is done by interrupting its life cycle.

7.

(a)

The proper sewage-disposal prevents the human faeces embedded with hexacanth embryos to come into contact with the food of the secondary hosts such as pigs and cattle. In this case human faece is not a sanita1y fertilizer.

{b}

Before sale the meat has to be inspected carefully to discover the 'measely' meat and discard them.

(c)

The meat (especially pork) has to be cooked thoroughly to kill all the bladder worms before it is eaten by man.

{d)

TI1e tapeworm inside an infected person is removed either by applying vennifuges or imposing surgical operation. i.e. medical treatment.

A summary of the adaptation of Tapeworm to its parasitic mode of life Special features (a)

Elongated, body

Adaptation

flattened

(1)

(2)

(b)

Scolex with and suckers

hooks

(c)

TI1ick cuticle (tegument) covering body

To increase the surface area for absorbing the digested food of the host. To fit the course of the small intestine of the primary host.

To attach firmly on the intestinal wall of the host.

(1)

(2)

To resist the action of the digestive juice of the host. To secrete anti-enzymes to neutralize the action of the digestive juices of the host.

(d)

Anaerobic respiration

To provide a method of producing energy in the small intestine where is lack of oxygen.

(e)

Continuous proliferation of new proglottids behind the neck

To maintain a sufficient number of reproductive units-ensure continuation of generation.

193

(f)

Hermaphrodite

(g)

A large number eggs

(h)

Hexacanth embryo enclosed by egg shell

To withstand the adverse conditions of the free stage so that it can survive longer in soil.

(i)

The hooks of the hexacanth embryo

To facilitate the penetration of the embryo through the gut wall of the pig into the pork.

(j)

Bladder worm in pork

To enhance the infection of the primary host (man) because pork is a favourite food of this host.

(k)

Degeneration of (i) digestive system ( ii} locomotory system (iii) sense organs

Stem of Dodder

/Scale ~ leaf

To permit fertilization to occur though there is usually one tapeworm in the host. of

To increase the chance successful dispersal.

of

Bathed in a medium of digestive food. No need to find food which can easily be absorbed from its surroundings. Live in a relatively stable environment (small intestine) where there is no great change in stimuli.

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