Reproduction SystemThe system that give birth to the colorful world

Research Group: 张嘉农杨心怡兰青柳斯琦

Abstract

1. The Evolutionary Tree2. Detailed Features 3. General description about the evolution process

of reproduction system4. Comparisons and analysis5. Summary

The evolutionary Tree

Aschelminthes

Annelida

Arthropoda

Roundworm

Earthworm

crayfish

locust

Necator Americanus(Round worm)

• Gonad: Long, coiled and uncomplicated

• Main Structure: Male: testis—sperm duct—genital pore …single

Female: ovary—oviduct—genital pore …pair

Very few adjunctive organs are formed

Anatomic image of female roundworm Anatomic image of male roundworm

Uterus

Ovary

• Reproductive capacity:

Usually about 100~1000(Astonishing large quantity) eggs are produced, that is, N.Americanus has very strong reproductive capacity

• Dioecious

• Internal fetilization

Transversal image of roundworm

Lumbricus terrestris(Earthworm)

• Gonad: Reduced in length, but more complicated both in structure and function

• Systerm structure: Resembles the N.Americanus, with more adjunctive organs added, like the prostates and the sperm funnels

Anatomic image of an earthworm

Complicated reproduction organs

• Reproductive capacity: one sexually matured L.terrestris can only produce 0.

05 juveniles every day on average. And the average survival rate of the larva is 90%.

• Monoecious

• Internal fertilization

Transversal image of earthworm

Reproduction system is not seen as clear as the roundworm

Clitellum of an earthworm

Clitellum

The clitellum is a cocoon for the deposition of eggs and sperm. Position: 14-16 segments

The deriving of clitellum:

when an earthworm is sexually matured, certain glands in the body wall inflate into a ring that circles the body, to form the clitellum

Chemical formation of clitellum

Mucoid, chitinous materials…

Motion of the clitellum

We see an unique reproduction method in the species: 1.After two earthworms finished copulation, the sperms are temporally stored in a sac called seminal receptacles.

2.Then the worm begins to back out of it, meanwhile, the eggs are released into the clitellum through oviduct.

3.Fertilizing occurs when it move pass the seminal receptacles.

4.At last, the clitellum is separated from the earthworm and the development of the young takes place in the clitellum.

Homarus Ameicanus(Crayfish)

• Gonad: main structure is the same with the phylum talked before. The length is relatively shorter and no significant changes had taken place.

• External fertilization

• Direct development: the adult is directly developed from the embryos.

The most distinct feature of H.americanus is its gonoducts.

Female genital pore

Male genital pore

Female crayfish

Male crayfish

In fact, the 1st pair pleopods of the male crayfish is specialized to be a sperm transferring tube. During copulation, the sperms are shed through the genital pore, then they are transferred through this peculiar appendage and are conducted to the space near the ventral part of a female, where the fertilization occurs.

Locusta migratoria (locust)

• Gonad: main structures remain the same, while adjunctive organs are evolved specifically for terrestrial living and hatching.

adjunctive organs:

female: collateral gland (helps to secret a sheath over the eggs for

protection)

male: a paired gland ( help to activate the sperms)

• Internal fertilization• Reproductive

capacity: The number of eggs a locust produce is about 110 in average.

It is a large number and may lead to disaster if not properly controlled.

The reproduction process of locust

After summarizing our results and findings of the experiments, we can draw a rough draft about the evolutionary direction of the reproduction system. The results are gathered in the table given below.

At the same time, many problems presented as we are trying to give an general principle of evolution. And those are the questions we are to explain.

Comparisons and analysis

The evolution of reproduction systemFemale male

Nematodes(dioecious&dimorphic)

A pair of ovary

A pair of oviducts

A pair of uterus that unite to form a vagina

The vagina opens to the outside through a genital pore

A pair of testes

Three pairs of seminal vesicle

single sperm duct

Oligochaete

(Monoecious)

A pair of ovaries A pair of short oviduct

The oviduct open to the outside through a genital pore

The genital pore open at the base of the 3rd periopod

A pair of testes

Four prostate (a kind of adjunctive gonad)

Crayfish (dioecious

&dimorphic)

A pair of ovaries

A pair of oviduct that assemble to a total oviduct

A pair of testes

A pair of sperm ducts

A pairs of seminal vesicle

Locust

(dioecious

&dimorphic)

A pair of ovaries

A pair of oviduct that assemble to a total oviduct

A seminal receptacle

A tubular gland

A pair of testes

A pair of sperm ducts

A pair of seminal vesicle

A tubular adjunctive gonad

They both have complicated adjunctive gonads, which have been described in detail

Through the table, we can figure out such a evolutionary process:

Original structure of the reproduction system: gonad—gonoduct—genital pore

Adjunctive organs are gradually added

External reproduction

Internal reproduction

Asexual reproduction

Sexual reproduction

Low survival rate

High survival rate

Why is lobster’s reproduction system so undeveloped?

Seeing from the table, it’s clear to learn that the reproduction system of a lobster is not so complicated or even below the stage of the earthworm, however, it’s taxonomic position is higher than the later one, Why?

Crayfish (dioecious&dimorphic)

A pair of ovaries

A pair of oviduct that assemble to a total oviduct

A pair of testes

A pair of sperm ducts

A pairs of seminal vesicle

It doesn’t have special adjunctive organs!

After discussion, we come to the conclusion:

The evolutionary state of every system is not always at the same pace during revolution. For example, once a tremendous progress happens to the respiration system, its nervous system may stayed in the previous stage, and that can be applied to other systems which we may regard as degeneration.

This concept can be presented in a coincide-model.

diploblastic Non-coelom pseudocoelomata coelomata

The time table

Developing stage of reproduction system

Coincided part

Stage 1Stage 2

Stage 3Stage 4

In fact, now our taxonomic standard is coelom. When deciding the evolutionary stage of a species, evidences are given due to the development of coelom.

But as the coinciding effect, higher level of a particular system can be found in a lower level of taxonomy, while on the other hand, lower level of a certain system can be equally found in a higher level of taxonomy.

According to this theory, we can give an explanation to the “strange degeneration” phenomenon of lobsters.

External vs Internal

It is concluded from the comparisons that the general trends of fertilization method is evolving from external fertilization to internal.

This change is obvious if we compare a worm with a mammal. But we should not take it for granted. It’s significant to raise the question Why.

When we were discussing this question, we encountered some disputes. We formed two views towards this question.

1.The survival efficiency of internal fertilization is far higher than external method.

a specie has two ways to maintain the quantity of a population: a. large numbers of eggs but low survival rate

b. small numbers of eggs but high survival rate

Adopting strategy a requires more energy and mass because every egg needs nutrition and the nutrition all come from the female. In order to save the energy, the evolution select strategy b. And this strategy is accomplished by internal fertilization.

2. The result mainly lies on the living environment

If the environment is fierce, they need to develop a secure system to protect their fertilized eggs.

Taking roundworm and locust for example, roundworm parasites in other animal’s gut, and faces the threat of the host’s immune system, if the eggs are exposed to the outside, the chance of being killed increases vertically. For insect—the primary immigrant to terrestrial life, water is so precious and it’s probable to die without water supply from its mother.

In the ecosystem, the parent generation pays no attention to the cost of reproduction, no matter it will consume a lot of energy. It’s just a matter of environmental pressure that results in the change.

Dioecious or monoecious?

This question is not so difficult to think.

We all know that the higher the differentiation, the more powerful a certain tissue will be. It is analogous to the reproduction system. If the system is specialized for merely one sex, its function will be stronger. That’s why dioecious species seem to be more developed.

This world is filled up with wonders, among which is sex.

We know female and male are basic elements of many species. We also learn that some species can reproduce without mating.

But we are curious about which system is more adaptive, asexual or sexual.

SexualAsexual

John Maynard Smith’s modelSexual reproduction is complicated, costly and dangerous, but why is this method so wildly used?

John Maynard Smith(1978) approached this question by developing a null model, containing 2 assumptions.

a. the reproduction mode doesn’t affect the number of offspring

b. The reproduction mode doesn’t affect the survival chance

Under that assumptions, we have the table below:Sexual

F×M

F×M F×M

F×M F×MF×M F×M

Asexual

F

F F F F

F F F F F F F F

F F F F F F F F

We can see that in every generation, the population of asexual will be 2 times larger than sexual. So before long, the world is overwhelmed by asexuals.

But does that happen?

Dunbrack’s experiment:

Living Pressure(malathion)

Replace

Sexual reproductionevolved Not

evolved

Mix

1 generation

After n generations

Predicted result: the asexual takes over, the sexual be eliminated

Experimental result: the sexual takes over, the asexual disappears

After all, the sexual can overcome the pressure and survive. And also the stronger the Malathion, the shorter it takes for sexual to take over, thus reflecting the advantage of sex.

So the null model is incorrect, we need an advanced theory to explain this question.

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time arrow

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Sexual reproduction means genetic combination.

Under this direction, scientists had developed 2 explanations

1. Genetic drift, in combination with mutation, can make sex beneficial

In asexual reproduction, a deleterious genetic mutation will pass on and on, while sexual reproduction process can produce mutation-free offspring immediately, by mating mutation-free mate.

H.J.Muller conclude that deleterious mutation will accumulate in asexuals. With deleterious genes accumulated, the asexual will end in extinction

But sexual reproduction breaks that ratchet, as they can create zero-mutation genotypes even if it is extinct in parent generation

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Accumulation in asexual reproduction

2. Selection by a changing environment can make sex benificial

In constant environment, it will be safer to choose asexual reproduction.

However, the core problem is our environment is changing every moment.

Imagine a situation: Environment A favors genotype BBYY

Environment B favors genotype BByy

(Certain population have gene B, b, Y, y)

Environment A most BBYY few BByy

Environment B BBYY decline to extinct BByy remains and propagate

Environment B few BBYY most BByy

Environment A BBYY remains and propagate BByy decline to extinct

change

After a period

change

Cycling…

This theory is proved by the Red Queen Hypothesis Experiment

Summary During the process of evolution, the structure and

function of the reproduction system gradually develops towards the destination of a more effective, higher quality, and more adaptive system.

The success owes to 2 factors, the inner one and the outer one. The inner factor is associated with the evolution of coelom and other systems that give rise to a boom to the creature. The outer one is the living pressure from the environment—the natural selection, and also the living habit of certain specie.

This concept is very important in researching the evolution and solving questions.

References

• Zoology

• Evolution

• Explaining density-dependent regulation in earthworm populations using life-history analysis Jan E. Kammenga, David J. Spurgeon, Claus Svendsen, Jason M. Weeks (2003)

• Morphological and behavioural characteristics of a gynandromorph of the desert locust, Schistocerca gregaria KOUTARO MAENO, SEIJI TANAKA (2007)

• 昆虫生理学 王荫长 2004

• 城市绿地虫害及防治 迟德富 严善春 2001

Experiment display

Thank youThank you

By group 7 All rights reserved 2007.11.24