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North-Pea-4926 t1_jbhf4h0 wrote

If you have 13 important chromosomes, one of them is getting left behind when your cells divide to produce gametes.

There are insects that use haplo-diplody (spl?) where the males can have an odd number of chromosomes (n) and females have the regular diploid number (2n)

If you reproduce asexually, I can’t think of a reason why having an odd number of chromosomes would be a problem.

There are some animals where different sexes have a different number of chromosomes, and one of them is an odd number.


TheBloxyBloxGuy OP t1_jbhki69 wrote

I looked it up and, stick bugs have 21 chromosomes in males and 22 in females.

Moreover, they can reproduce asexually but the offspring will be female. If they reproduce sexually, the offspring will have a 50% chance of being male.

So that answers the question. Male stick bugs have an odd number of chromosomes yet they can still have an offspring.


actuallyserious650 t1_jbhs1rt wrote

That’s interesting because in humans the Y chromosome is substantially smaller than the X chromosome (all our other chromosome pairs are equally matched).

It wouldn’t be hard to imagine the Y continuing to dwindle down to nothing and then we’re just like stick bugs.


Vitztlampaehecatl t1_jbhtm7v wrote

> 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.


shagieIsMe t1_jbhx2su wrote

Not soon... but eventually.

> 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 - )

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:

> 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.

> 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)


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.


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.


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.


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


CyberneticPanda t1_jbi6tye wrote

That is happening right now. The Y chromosome used to be bigger and is shrinking and will be gone in a few million years. Most mammals have a similar situation to humans, who have an X chromosome with 900 genes and a Y chromosome with 55 genes. One of those 55 genes is the one that causes male sex characteristics to start to develop - don't tell your right wing friends, but everyone is female at first and the males TRANSform at about 9 weeks.

The platypus has 2 equal sized sex chromosomes, and we diverged from them evolutionarily 166 million years ago. We have lost 845 Y chromosome genes in that time, which means we will lose the rest in about 11 million years. All is not lost, though.

We know of a couple of other mammals that lost their Y chromosome and still produce males. In most mammals, a gene called SRY on the Y chromosome (sex region y) triggers another gene on another chromosome called SOX9 to start the process of developing into a male. In a couple of spiny rat species from Japan, the y chromosome has disappeared. The males have a duplication mutation near the SOX9 gene that turns it on and the females don't have that mutation. Humans could evolve (or splice in) a similar mutation to keep producing males once we have lost SRY.


PuddyVanHird t1_jbj7tin wrote

> That is happening right now. The Y chromosome used to be bigger and is shrinking and will be gone in a few million years.

Can you really extrapolate that it will disappear altogether from the fact that it's shrinking? There's no evolutionary reason that's obvious to me why the Y chromosome needs to be particularly large, but there is an evolutionary reason to have one at all. It's certainly true that there are alternative possibilities, but the probability of losing SRY at a population level is still significantly lower than the probability of losing some other random gene that isn't expressed. Unless there's an established mechanism that means the Y chromosome has to keep shrinking?


CyberneticPanda t1_jbjp2bu wrote

The Y Chromosome is mostly non-coding DNA. We know that it has been losing genes for millions of years. The reason it shrinks while others don't is that it has no duplicate partner to repair itself from, like every other chromosome has including the X chromosome, though X only has a partner in women. We also know that it has been lost in other mammals. Some of them found alternative ways to keep producing males. We don't know for sure, but it is a reasonable hypothesis that others did not and went extinct.


PuddyVanHird t1_jbjp9ku wrote

Interesting, thanks.


Lurker_IV t1_jbkt0a3 wrote

Producing and carrying offspring is far more costly and risky than just producing sperm. One excellent example of this is flatworm penis-fencing where they battle to impregnate their opponent while avoiding it themselves.

Some point in our evolutionary history as mammals some mutation made it impossible for one side to get pregnant at all and only able to impregnate others thus freeing up resources for males to focus on getting as many females pregnant as they could. This strategy also carries the danger of relying entirely on others to reproduce. If females develop the ability to select only female offspring and not males then this can eliminate y-chromosomes entirely, something that has been theorized to have happened more than once already in our evolutionary past until a y-chromosome able to overcome this selectivity happened.

There are entire books on the topic of male-female reproductive strategies and cost-benefit analysis at the genome level which I won't go into as I don't have a teaching degree.


[deleted] t1_jbi7q92 wrote



CyberneticPanda t1_jbi8ed9 wrote

It's SRY that gets lost, and it would probably not get lost all at once, but instead get a mutation that makes it work less well and shifts the chances of being born male lower. That will create selective pressure for the mutation near SOX9 (or another that substitutes for SRY) to spread through the population.


JerGigs t1_jbi21fp wrote

I read about that, apparently it'll just get replaced. I don't remember much of that article, I read it a long while ago. Now I might go fall down a wiki hole just thinking about it


WantsToBeUnmade t1_jbl2fjj wrote

In the creeping vole (Microtus oregoni) females are XO and males are XX. The offspring always get an X from their father, but their mother gives either an X or nothing.

This only works because their X chromosome carries some of the information that the Y carries in other species.

There is also a clever bit of coding that in most mammals causes less expression of the genes on the X chromosome in females (so that females don't have twice as many proteins running around their body.) And in creeping voles it only does so in males.

Creeping Vole Sex Determination


pineappledan t1_jbm2txa wrote

This is called an XO sex determination system and has been independently evolved in mammals, so it can happen.


B4byJ3susM4n t1_jbi2o82 wrote

Some creatures like stick bugs or grasshoppers have an X0 sex determination system. So when eggs are fertilized, some sperm may have an X chromosome and some don’t. If the result of fertilization is two X chromosomes (even number of total chromosomes), then the offspring is female. If the result is only one X chromosome (an odd number of chromosomes), then the offspring is male.


EmilyU1F984 t1_jbiep5a wrote

This is true, but it is too simplified.

You can easily have differing 2n in even mammals. All it requires is fusion of only the X fragment but not the Y chromosome.

See the Indian muntjac: 2n=6/7

Which funnily enough can actually breed with the Chinese 2n=46

Additionally plenty of chromosomes do weird fusion and duplication stuff, meaning you can actually have specific chromosomes that can be dropped with no I’ll effect.

There‘s also insects with males having a single chromosome less than the females.

But that should only possible with ZW/ZZ insects, and not our XX/XY relatives.

And it’s always the case for insects completely lacking Y chromosomes. Take for example stick insects. The females have XX chromosomes the males just have X0 chromosomes.

Like they lack the second sex chromosome in regular existence.

But really as I said: if you have chromosomes that can be dropped wirh no Ill effect, uneven is possible.

And since males are the genetically inferior variant of the ancestral asexual progenitors, usually it‘s the males that will have uneven chromosomes, because their Y chromosome can in many cases do whatever and still leave a fully viable animal.