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Vitztlampaehecatl t1_jbhtm7v wrote

Reply to comment by actuallyserious650 in Is there a fertile creature with an odd number of chromosomes? by TheBloxyBloxGuy

> the Y continuing to dwindle down to nothing and then we’re just like stick bugs.

I don't think that would be likely given that human sex development is decided by the presence or absence of the SRY gene. Without that gene activating, you develop female. So without a Y chromosome, the SRY would have to find somewhere else to live.

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shagieIsMe t1_jbhx2su wrote

Not soon... but eventually.

https://www.sciencealert.com/the-y-chromosome-is-slowly-vanishing-a-new-sex-gene-could-be-the-future-of-men

> The sex of human and other mammal babies is decided by a male-determining gene on the Y chromosome. But the human Y chromosome is degenerating and may disappear in a few million years, leading to our extinction unless we evolve a new sex gene.

> The good news is two branches of rodents have already lost their Y chromosome and have lived to tell the tale.

> A new paper in Proceedings of the National Academy of Science shows how the spiny rat has evolved a new male-determining gene.

(the paper is Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9 - https://www.pnas.org/doi/10.1073/pnas.2211574119 )

Chasing links and searches:

NYT A Gene Mystery: How Are Rats With No Y Chromosome Born Male?

> ...

> Both female and male Amami spiny rats have only one X chromosome, an arrangement only known to occur in a handful of rodents among mammals. Arata Honda, associate professor at the University of Miyazaki and the lead author of the paper, said in an email that he was partly motivated to study Amami spiny rats in the hope that learning about them might reduce their risk of extinction.

> No one knows how or why, but at some point the rats lost their Y chromosome and, along with it, an important gene called SRY that’s considered the “master switch” of male anatomical development in most mammals.

And this also lead's to the OP's question: https://en.wikipedia.org/wiki/Ryukyu_spiny_rat

> The Ryukyu spiny rat (Tokudaia osimensis) is a species of rodent in the family Muridae. Endemic to Amami Ōshima island in the Amami Islands of the Ryukyu archipelago of Japan, its natural habitat is subtropical moist broadleaf forest. The karyotype has an odd diploid number, 2n = 25. Like its relative T. tokunoshimensis, it has lost its Y chromosome and SRY gene.

http://www.departments.bucknell.edu/biology/resources/msw3/browse.asp?id=13001850

> The unique chromosomal complement of this species (2n = 25, with no X in the female or visible Y in the male) first documented by Honda et al. (1977) and corroborated by Kimiyuki et al. (1989)

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CMxFuZioNz t1_jbiwuuq wrote

"leading to our extinction" is a bit dramatic.

The y chromosome is decreasing because there's either pressure to do so, or no pressure to stop it.

If the y chromosome disappearing meant that our species started to decline, then there would be pressure to either find a new sex determination method as has happened in the rodents or simply continue on with the y chromosome. There's no existential threat.

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Average_Cat_Lover t1_jbj4q18 wrote

> The y chromosome is decreasing because there's either pressure to do so, or no pressure to stop it.

It seems that sex chromosomes (as well as "parasitic" beta chromosomes) are always under a pressure to degrade over time. But, IIRC there is also a counter-pressure for new chromosomes to arise before this.

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Jkarofwild t1_jbj7zph wrote

Without going to read all those sources, where is the new male-determining genre? And is it wholly new, it is it SRY in a weird place? I've heard of a condition in humans where SRY can wind up on the X chromosome in some sperm cells, with the sister Y chromosome not carrying it, leading to (exceedingly rarely) XX male humans or XY female humans.

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SpecterGT260 t1_jbj4oay wrote

This seems like they are missing something important. Evolution is driven by those genes that get passed on. If absence of the gene produces females and if females do not ever carry the sex gene it's basically impossible for the gene to be lost. They are over extending the prediction based on the chromosome getting smaller but to suggest the key gene will just disappear is just silly. For a genotype to become dominant in a species it needs to convey some sort of advantage. Usually it's a survival advantage as this correlates with reproductive success. But here we are strictly talking about a reproductive advantage. It's just impossible for that to become the dominant trait as it is a direct disadvantage. The gene (or lack there of) can't actually get passed on and therefore it can't become the dominant genotype. This is strictly regarding the whole "extinction" argument btw. Evolution just doesn't work that way

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