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I imagined a tetraploid species of teleost. This species has both XY and ZW sex-determination systems. I call this strange thing the VYW sex-determination system.

Genetic females have three V chromosomes and a W chromosome, two of the three V chromosomes are inherited from their father, and the other V chromosome and the W chromosome are inherited from their mother.

Genetic males have three V chromosomes and a Y chromosome, two of the three V chromosomes are inherited from their mother, and the other V chromosome and the Y chromosome are inherited from their father.

Sometimes, there are genetic intersexes: some have two V chromosomes, a Y chromosome, and a W chromosome; at the opposite, some have four V chromosomes; however, in both cases, they are sub-fertile, which means they CAN reproduce, but they do produce MUCH less gametes than dyadic individuals. Genetic intersexes with both a Y chromosome and a W chromosome can self-fertilize, but genetic intersexes with four V chromosomes cannot.

The V chromosome, the Y chromosome, and the W chromosome all have two pseudo-autosomal regions. However, this species needs at least one V chromosome to survive.

So, I wonder if the laws of genetics of my teleost species are realistic.

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    $\begingroup$ I don't think this system would be stable. The offspring will have VVVV, VVVY, VVVW or VVWY chromosomes, each with 25% probability. If two of these combinations have reduced ability to reproduce and are selected against like you state, the system will quickly evolve towards something simpler that produces a fertile combination every time. $\endgroup$
    – Cloudberry
    Nov 13, 2022 at 11:59
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    $\begingroup$ (1) A "teleost actinopterygian gnathostome" is a funny doubly pleonastic way of saying "teleost". All teleosts are actynopterigians by definition; all actynopterigians are gnathostomes by definition. (2) Why does this complicated mechanism exist? What advantages does it bring to the population? This is the key question. Natural selection is ruthless and unforgiving. If you can come up for a compelling reason for this mechanism to exist, then it can exist; otherwise it cannot. $\endgroup$
    – AlexP
    Nov 13, 2022 at 13:13
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    $\begingroup$ @Daron: In the real world, females are the sex which can (potentially, at least) lay eggs or give birth to live young. In mammals, females have XX genotype; in birds, they have ZW genotype -- in birds, females are the heterozygotic sex. Other animals use different sex determination mechanisms; for example, in some reptiles sex is not determined genetically, but developmentally. (And a tetraploid teleost is, quite obviously a tetraploid teleost. For example, trout and salmons are partially tetraploid teleosts.) $\endgroup$
    – AlexP
    Nov 13, 2022 at 21:56
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    $\begingroup$ @Cloudberry You presume each is an equal probability. If the species had mechanisms to encourage the fertile options and the other less fertile options were thus rare 'mistakes' then the occasional subfertile individual may not be a sufficiently harmful to reproductive success, and may be a worthwhile tradeoff if this system provided other forms of utility, such as increased genetic diversity. I could point to plenty of 'sub fertile' phenotypes that show up in humans on occasion, though I think it's politically safer to not give actual examples ;) $\endgroup$
    – dsollen
    Nov 15, 2022 at 22:27
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    $\begingroup$ @Cloudberry if you presume the lower fertility types are rare enough already, or nearly as fertile, it could be that negative selection has already occurred decreasing their frequency. They may stick around if some other positive trait was provided by the sex determination system. In essence the negative effects of rare less-fertile children was offset by some other positive aspect, like say greater fecundity or higher genetic variance. Just like allergies are an accidental, potentially fatal, side effect of our immune system, but worth accepting due to advantages our immune system provides $\endgroup$
    – dsollen
    Nov 17, 2022 at 14:39

2 Answers 2

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Zygote-gamete Cycling

First some terms that everybody probably already knows. The usual scheme for organisms with sex is for each parent to contribute half of their genetic code. The parent has two sets of DNA and is called diploid. The parent produces specialized cells called gametes, each carrying one set of DNA, and this is called haploid. The gametes join to produce a new full set of DNA, and the newly formed organism is a zygote. This is all very much highschool biology, and is much more complicated than I have described here.

Suppose the species had two phases, one where zygotes grew to adulthood and reproduced under one scheme. And another where gametes grew to an alternative adulthood and reproduced under a different scheme. The gametes are haploid, the zygotes diploid. There would be one pair of sexes for zygotes. And a different set for gametes.

So instead of XX and XY as mammals arrange, it might be XX and XY in zygotes, and X and Y in gametes. The gametes might reproduce through some different scheme such as self-cloning. The X and Y gametes might even be sufficiently different as to not obviously be the same species.

There could be distinct sorts of X and Y. For example, X1, X2, X3, etc. When any of them is joined to a Y, the result might be an ordinary looking zygote. But the gametes witth X1 might look quite different to those with X2 or X3. So, in principle, there could be many very different looking creatures, each capable of mating with any Y type gamete.

There would be some environmental factor that would switch from one phase to the other. Perhaps if the food supply is unusually good then the gametes join up to produce some zygotes. Or perhaps there is a fixed time between the two phases, say a prime number of years. Or perhaps something in the climate is cyclic, such as drought for 10 years then wet weather for 10 years. Maybe the immature zygote requires pools of water to develop. There are many possible things that could switch the phase.

Metamorphosis

There are some organisms that, at some point in their life, go through a metamorphosis. For example, frogs have a tadpole stage, then the tadpole grows legs and lungs and becomes an air breather.

Some species of salamander have a stage where they have gills and live under water. They must metamorphose into land creatures, gain lungs and lose their gills, in order to reach adulthood.

Some creatures that do this can get "stuck" at the pre-metamorphosis stage. Axolotls are neotonous. This means, they do not go through metamorphosis, and breed without reaching the "adult" stage. At least a few varieties can be induced, through injections of iodine, to metamorphose into the form typical of salamander adults. (It is not recommended since it often results in the death of the creature.)

Imagine a creature with the ability to breed either before or after such a metamorphosis. That is, there might be a gills-live-in-water adult form. And a second lungs-live-on-land form. And either form could breed. The pre-met form could have male and female. And the post-met form could also have male and female. But a pre-met female might become a post-met male. That is, there might be pre-met male and female that were distinct from post-met male and female. Possibly resulting from the expression of different genes. Or possibly turning on/off a different collection of genes on the same chromozome.

So there are four combos depending on sex before and after met. m-m, m-f, f-m, f-f. Each of these could be quite different. The specific pattern could be determined by environmental factors.

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    $\begingroup$ While I know all those terms having a passing interest in evolution I think your giving way too much credit saying 'everyone' knows them. In fact other then zygote I'm pretty sure the rest weren't even taught to me in high school, I had to learn them on my own while playing around on wikipedia. $\endgroup$
    – dsollen
    Nov 15, 2022 at 22:30
  • $\begingroup$ @dsollen xkcd.com/2501 $\endgroup$
    – Nosajimiki
    Dec 2, 2022 at 14:22
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    $\begingroup$ I dunno. I took all of these terms in high school biology. Indeed, they made us hunt through a local forest and find examples, make drawings, etc. $\endgroup$
    – Boba Fit
    Dec 2, 2022 at 15:14
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Yes, this is possible. But it won't work as you think.

For starters, you have 12 possible combinations. Your genetic male and female are just two of them. "Absolute" males and females would be a minority.

This looks like c. elegans, except you'd have females. In the elegans worm population you find males and hermaphrodites. When hermaphrodites mate or self inseminate, you get 300 eggs per individual. But when a male inseminates an hermaphrodite, the latter can lay up to 1,000 eggs.

Elegans sex is determined by an X0 system, which is simpler, but still the ratio of males to hermaphrodites is 1/200. This is because since hermaphrodites can and will self inseminate if they can't find a partner, and since they can also inseminate each other, you will always have more XX to XX pairings.

In your species, something similar would happen. You may still need a partner to mate. Intersex may be less fertile than absolute males or females, but since their pairings would be more frequent (barring some social custom being a factor), population would be more skewed towards intersex variations, just like with c. elegans.

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    $\begingroup$ There are not 81 possibilities--chromosomes aren't ordered, which brings the possible combinations down to only 12: VVWY, VWWY, VWYY, VVVW, VVVY, VWWW, WWWY, VYYY, WYYY, VVWW, VVYY, WWYY $\endgroup$ Jan 11, 2023 at 1:56
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    $\begingroup$ @LoganR.Kearsley good point. Thanks for that, I fixed that in my answer. $\endgroup$ Jan 11, 2023 at 3:42

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