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Sexual Reproduction- Why only between male and female?


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Hi- I'm new to this group but I have an interesting question posed by a friend.

 

I know that some organisms reproduce asexually (like slime mold), and some organisms can flip between male and female (like fish), but among organisms that reproduce sexually, why did they evolve into just two distinctive groups--male and female--and not many different groups.

 

Thanks in advance!

 

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The most likely answer is that sexual reproduction is inherently inefficient. If you think about it, you will notice that it carries a two-fold cost over asexual reproduction. I.e. an asexually reproducing organism has a much higher potential to reproduce (i.e. double as fast as sexual reproduction with all other things equal). More groups further limit reproductive potential without providing any additional benefit of having two sexes.

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There is a suggestion that two sexes maximises the entropy to cost ratio, giving the best possible diversity and minimum cost [1]. I think the idea of looking at entropy is older than this.

Reference
[1] Bo Deng, The Origin of 2 Sexes Through Optimization of Recombination Entropy Against Time and Energy, Bull Math Biol. 2007 Aug;69(6):2105-14. arXiv:q-bio/0703021 [q-bio.PE]

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If males fertilize and females are fertilizied, how can there be anything other?

Although it's usually more mutable than sex, morphology can be more varied. Larvae or juveniles are clearly distinct. I recall that dominant male primates may exhibit differences.

Edited by MonDie
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If males fertilize and females are fertilizied, how can there be anything other?

 

You could have a single sex, where any two individuals can exchange gametes.

 

You could have three sexes where a contribution is needed from all three to form viable offspring.

 

You could have N sexes where a contribution is needed from all N to form viable offspring.

 

I'm not sure how you would make the N > 2 scenario work with the existing structure of DNA.

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If males fertilize and females are fertilizied, how can there be anything other?

The open question is why has nature chosen this model?

 

Interestingly, most terrestrial gastropods are hermaphrodites, which may blur this slightly, but still there is essentially two sexes.

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The open question is why has nature chosen this model?

 

Interestingly, most terrestrial gastropods are hermaphrodites, which may blur this slightly, but still there is essentially two sexes.

I think hermaphroditism is a failsafe strategy in case sexual reproduction is not available. The downside to it is that there is no genetic diversification. Some types of female plants will produce male parts to make pollen in the absence of male plants..

Edited by StringJunky
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Perhaps it's an arbitrary matter. In the event of a three-way system, for example an identical donor egg fusing with the egg prior to fertilization, the egg donor's sex would be unclear. I think then, and only then, would biologists be forced to abrtitrate and further define "female".


However, even if all systems are technically male-female, that doesn't mean there aren't variations on this system.

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ajb

The open question is why has nature chosen this model?

 

 

Good question ajb and I am no expert in this field so anything I propose would be speculation.

 

However I see a parallel with the mathematics of chemical reaction kinetics.

 

If two substances, A and B, react then most reaction rates are proportional to the concentrations of the two reagents.

 

This is because for two molecules to meet and react they must be in the same place at the same time.

Statistical kinetic considerations yield equations proportional to the concentrations. ie the higher the concentration the greater the chances of molecule A meeting molecule B and reacting.

 

If we extend that to more reacting molecules, say C, the chances of three molecules being in the right place at the right time is reduced compared for only two and so on.....

 

 

The other consideration that occurs is that there is les scope for error in single cell fission than two cell fusion and even more scope for error if a third...fourth.. are required to play an input.

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...The other consideration that occurs is that there is les scope for error in single cell fission than two cell fusion and even more scope for error if a third...fourth.. are required to play an input.

In genetics, two-cell fusion is less error-prone than single-cell division because a pathological mutation of an allele in one cell could be negated by a mon-mutated copy in the other; it's not guaranteed because you have recessive and dominant but the risk for error is reduced.. Necessitate 3 different sexes to mate and the probability of reproductive success goes down because more elements need to be at the same place at the same time which, probably, outweighs the added advantage of greater diversity than two..

Edited by StringJunky
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If males fertilize and females are fertilizied, how can there be anything other?

 

 

I recall an Iain Banks novel where there were an alien race with three sexes, a male would deposit his seed into an apex-gendered egg, who would then implant the embryo into a female for gestation with its 'reversible vagina'.

 

It seems like that's just an extra, unnecessary step, but what if there was an advantage to reproducing in this manner?

Edited by kisai
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In genetics, two-cell fusion is less error-prone than single-cell division because a pathological mutation of an allele in one cell could be negated by a mon-mutated copy in the other; it's not guaranteed because you have recessive and dominant but the risk for error is reduced.. Necessitate 3 different sexes to mate and the probability of reproductive success goes down because more elements need to be at the same place at the same time which, probably, outweighs the added advantage of greater diversity than two..

 

I printed the paper to read later.

Often the challenge isn't mating per se, but selecting the fittest mate or winning that mate. Even as three-way, if only one progenitor nurtures, then only that progenitor will be choosey.

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Actually, diversification has been put forward quite often as a the reason of having sexual reproduction. However, that is not enough. Many asexually reproducing organisms are able to have exchange of genetic material. Thus, theoretically they can claim both advantages, recombination as well as more efficient reproduction.

 

The question why even two sexes exist has not been conclusively solved yet. But if that is already an issue, it is clear while adding more is even more unlikely. Some advanced hypotheses anchor the stability around maintenance of certain cellular systems that co-evolved with the meiotic machinary, IIRC.

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Actually, diversification has been put forward quite often as a the reason of having sexual reproduction. However, that is not enough. Many asexually reproducing organisms are able to have exchange of genetic material. Thus, theoretically they can claim both advantages, recombination as well as more efficient reproduction.

 

The question why even two sexes exist has not been conclusively solved yet. But if that is already an issue, it is clear while adding more is even more unlikely. Some advanced hypotheses anchor the stability around maintenance of certain cellular systems that co-evolved with the meiotic machinary, IIRC.

 

I read this the other day:

 

Since in many species, sperm is males' only contribution to reproduction, biologists have long puzzled about why evolutionary selection, known for its ruthless efficiency, allows them to exist.

 

Now British scientists have an explanation: Males are required for a process known as "sexual selection" which helps species to ward off disease and avoid extinction.

A system where all offspring are produced without sex -- as in all-female asexual populations -- would be far more efficient at reproducing greater numbers of offspring, the scientists said.

 

But in research published in the journal Nature on Monday, they found that sexual selection, in which males compete to be chose by females for reproduction, improves the gene pool and boosts population health, helping explain why males are important.

 

An absence of selection -- when there is no sex, or no need to compete for it -- leaves populations weaker genetically, making them more vulnerable to dying out.

 

http://www.reuters.com/article/2015/05/18/us-health-sexual-males-idUSKBN0O31M220150518

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If males fertilize and females are fertilizied, how can there be anything other?

 

The highest number of "mating types" I recall encountering within a single species was iirc 8, 6 of them distinct. I can't remember of that was an alga or a fungus - probably a fungus.

 

At any rate, more than two.

 

Here's an informal description: https://blog.mycology.cornell.edu/2010/06/02/a-fungus-walks-into-a-singles-bar/

Edited by overtone
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I read this the other day:

 

Cheers, I will take a look at it. Quite fresh off the press, too. After skimming the abstract it seems that the next step to look into would be the molecular mechanisms affecting fitness.

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Luckily, I have access and if there is interest in details I can provide/discuss them (of course I cannot share the pdf online).

 

The overall finding was that fitness was maintained better after conditions exposing mutation load (i.e. inbreeding), which, afaik one of the few data that actually demonstrate the proposed benefit (and no doubt the reason it got into Nature at all).

 

That being said, it is of course just one study (others have been far less clear in their outcome) and it will require more work to see whether this is something special to the system tested or more universal (especially when looking into cases were sexual selection is not that dominant).

Edited by CharonY
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Luckily, I have access and if there is interest in details I can provide/discuss them (of course I cannot share the pdf online).

 

The overall finding was that fitness was maintained better after conditions exposing mutation load (i.e. inbreeding), which, afaik one of the few data that actually demonstrate the proposed benefit (and no doubt the reason it got into Nature at all).

 

That being said, it is of course just one study (others have been far less clear in their outcome) and it will require more work to see whether this is something special to the system tested or more universal (especially when looking into cases were sexual selection is not that dominant).

Was the study design, in terms of the number and variety of study arms, good in your opinion? What were the weak spots in the design with the beetles?

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A few quick comments on the paper:

The major strength and also limitation is that they tested one specific hypothesis: whether sexual selection can alleviate fitness constraints by selection of males. Thus the study is specifically geared to show this effect experimentally but thereby obviously only addresses only one part of the puzzle. The result is that empirically it can. Though it is not clear how that relates to species where sexual selection is not known to be a major factor. Or maybe there is more sexual selection going on than acknowledged universally as some of the competition may happen on the molecular side (but that is material for further study).

 

The study design itself is alright, strength being that they let it run for 6 years (long-term experiments are just so rare, as most are capped to the time limit when you shove the grad student or postdoc out). They did two lines, the first with 90 F to 10 M or 10 F and 90 M with only three replicates each (still must have been a logistic nightmare to maintain). Then they did a divergent line with competition of five males to one female (12 replicates) or monogamy (20 replicates). From these again 3 lines each were maintained.

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