Expert: Dana Krempels, Ph.D. - 10/6/2009

Question
QUESTION: Dear Dana

Even after reading some resources on the Net (eg Wikipedia), I still struggle to understand any theory of origin of sexual reproduction among eukaryotes.

Of course, I would understand that the meisosis is a crucial process here. I know that the meiosis is a process of formation of haploid cells out of diploid cells through the division of the latter; after the meiosis couples of haploid cells can form  zygotes, which can give rise to new individuals.

What I do not understand is, by what mechanism the two specialised types od haploid cells, i.e. sperms and eggs, evolved, and how the division of haploid cells into sperms and eggs was sustained.

As a complete layperson I can also imagine the world of eukaryotes, in which individuals produce only one type of haploid cells, and couples of haploid cells of the same type produced by different (but not sexually differentiated) individuals merge and create new zygotes. On the other hand, I can also think about advantages of division of individuals of the same species into males and females (and the corresponding division of haploid cells produced by those individuals into sperms and eggs).

However, what I cannot comprehend at all is any particular evolutionary mechanism which might have lead to sexual division of individuals and their haploid cells coming into existence and being sustained. And if I read about any theories trying to explain such evolutionary events and processes, I always get stuck due to the lack of my biological education.

Please, could you help me by some easy, layperson-friendly explanation of the nitty-gritty of one or two currently most respected theories of the evolutionary mechanism, which lead to the sexual division among eukaryotes? And could you maybe also offer some reasonable relevant internet resources, suitable to the lay public?

Thank you very much in advance.



ANSWER: Dear Dan,

You are treading in oft-traveled waters when you try to understand the origin and maintenance of sexual reproduction.  Numerous scholarly journal articles and books have been written on this topic, and there is still no consensus.  It's especially challenging because it's impossible to go back and see how it happened and also very difficult to subject to rigorous experimental manipulation.

The best we are able to do is study the respective advantages and disadvantages in sexual vs. asexual reproduction by studying species that do both.  And, as you suspect, there *are* species that do not have morphologically distinct gametes (sperm and eggs), but which are isogametic (i.e., the two complementary gametes are not physically distinguishable), but they almost invariably come in two different mating "types", usually referred to as "=" and "+" (analogous to male and female).

The conventional wisdom is that if this pattern has recurred so often in different lineages, there must be an adaptive advantage to having different types of gametes.

I'm not sure if you've already found this resource or not, but Matt Ridley is a good author for making scientific information accessible and fun.  Please see:

http://www.pbs.org/wgbh/evolution/sex/advantage/index.html

If there are specific points that are not clear to you because of a limited background in biology, then please write back and tell me exactly what those "sticking points" are, and I'll do my best to clarify.

I hope this helps, for starters.

Dana

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QUESTION: Dear Dana

Thank you very much for your great and extremely knowledgeable answer.

Please, could you answer also these further questions:

1) As regards the isogametic species: can "=" and "+" be also seen as a sort of markers, whose function is to stop gametes from the same individual to merge with each other?

2) As regards the sex of anyone, is it strictly defined by their DNA? In other words: are all pairs of identical twins the same sex pairs? Or if you have only small samples of diploid human cells, is it possible to tell the sex of their (former) owners only on the basis of the DNA testing?

3) As regards the particular inherited features of our sex organs and gametes, are they inherited (fully or predominantly) from our parents of the same sex as that one of which we are? Do men inherit sizes and shapes of their penises and testicles, or particular features of their sperms, (fully or predominantly) from their fathers and other male ancestors in the paternal lineage, rather than from their male ancestors in the maternal lineage? And is the analogical true for females and their female ancestors in the maternal lineage in case of the female sex organs and eggs?

Thank you very much in advance also for your help in resolution those my questions.

Yours sincerely

Dan

Answer
Dear Dan,

In answer to your questions...

1) As regards the isogametic species: can "=" and "+" be also seen as a sort of markers, whose function is to stop gametes from the same individual to merge with each other?

I suppose one could look at it either way.  But since there are species that do self-fertilize, this might not be the full story.  In fact, in some types of organisms, a single individual can produce both + and - gametes.  In species that self-fertilze, two opposite type gametes from the same individual can fuse.  But in other species, there are separate mechanisms (possibly at the level of plasma membrane receptors, but I'm not sure) that prevent successful fertilization between gametes from the same individual.

2) As regards the sex of anyone, is it strictly defined by their DNA? In other words: are all pairs of identical twins the same sex pairs? Or if you have only small samples of diploid human cells, is it possible to tell the sex of their (former) owners only on the basis of the DNA testing?

All pairs of identical twins are the same sex, unless there has been a very unusual somatic (i.e., in one of the cells of the developing embryo at a *very* early stage, not in a gamete) mutation.  For example, if there was a human zygote with XY sex chromosomes, and at the first division the sex chromosomes did not migrate properly such that one daughter cell received the normal XY and the other received only an X (the Y might have "lagged", been excluded from the nucleus as the membrane re-formed, and then degraded in the cytoplasm), you could have otherwise identical twins with one being male (XY) and the other being a female with Turner Syndrome (XO).  But that would be really, really rare. I don't know of any accounts of that happening.  But it's hypothetically possible.

At this time, sexing of mammal cells is done via examination of the sex chromosomes, since that's what mostly determines femaleness or maleness in mammals.  So the rest of the DNA doesn't tell us much.  At least at this point.

3) As regards the particular inherited features of our sex organs and gametes, are they inherited (fully or predominantly) from our parents of the same sex as that one of which we are? Do men inherit sizes and shapes of their penises and testicles, or particular features of their sperms, (fully or predominantly) from their fathers and other male ancestors in the paternal lineage, rather than from their male ancestors in the maternal lineage? And is the analogical true for females and their female ancestors in the maternal lineage in case of the female sex organs and eggs?

The formation of the genitalia in both males and females is very complex, and involves the activation and inactivation of many genes that direct specific cell divisions, programmed cell death, and other functions necessary for the formation of a complete structure.  This is true of many other complex structures, from your nose to your toes.  And the genes that control the shapes of those structures are inherited from *both* parents.

The male and female genitalia are homologous, with the penis of the male developing from the same embryonic precursor cells as the clitoris of the female.  The labia of the female are homologous to the scrotum of the male.  Etc.  Hence, the genes that control the ultimate shape of any of these can be inherited from both parents.  The development of the structures is also guided by sex hormones, and the proper timing and effect of these is often (but now always) directed by genes on the sex chromosomes (with a son always inheriting his Y chromosome from his father).  For example, the early formation of the external male genitalia is directed by testosterone, and later by its derivative, dihydrotestosterone (DHT). (We know this from the study of mutant individuals who cannot maniufacture DHT, and have only partly formed "ambiguous" male genitalia.)  So one could say that, indirectly at least, the genes of the father do have a strong effect on the formation of the son's genitalia simply by being the parent whose Y chromosome strongly affects the action of the male hormones..  But the mother has a genetic contribution, as well.

Hope this helps!

Dana