Submitted by TheBloxyBloxGuy t3_11mdtz4 in askscience
So I know it needs an even number to be divided. And I know that there are creatures like mules that are infertile hybrids with odd numbers. As far as I know there are zero. But for some reason when I try to research it, it always says "usually". So what is the reason?
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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.
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Even though “everyone knows” that mules are infertile, there are actually quite a few well documented cases of fertile mules. This page lists some older examples going back to the 19th century; some more recent (peer-reviewed) cases are listed in
With dozens of instances being documented in spite of farmers actively trying to prevent mules and hinnies from breeding, it's likely that a fairly significant percentage (though of course a minority) of them are fertile.
There are several cases of plants with odd numbers of chromosomes, such as Homeria flavescens (2n = 9). These plants can generally (always?) reproduce through self-compatibility and autogamy, which reduces the issues of odd chromosome numbers.
There are a number of species in which the males have odd chromosome counts, such as the Indian muntjac (6 chromosomes in the female, 7 in the male).
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.
What's really interesting is that I have found multiple examples of organisms with odd-numbered males and even-numbered females, but I can't seem to find a single odd-numbered female.
Genetics are delightfully complicated. However, even with the too simple Mendel square, you can see it happen. You put in the extra chromosome, and watch as it drops into one gamete and another ends up with the normal count.
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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.
A man can dream
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> 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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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 - 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)
I’m a forensic dna analyst and I unfortunately can’t answer your direct question but I can offer some insight on some things you’ve mentioned or other commenters have mentioned.
Haploid and diploid: our fully developed human cells are diploid—we reproduce sexually. We have 23 pairs of chromosomes for a total of 46. This is our diploid number. One “partner” of each pair originated from our bio “mom” (egg donor) and the other from our bio “dad” (sperm donor). Our sex cells or gametes (sperm or eggs) are haploid. They contain just ONE partner from each pair. Our diploid number is 23. Each egg or sperm is completely random whether it has the partner chromosome we inherited from our mom or dad, for EACH pair (if I am the egg source, my egg could have all the partners that originated from my mom or all the partners from my dad or any combination of partners from mom vs dad—but just one partner of each pair, for a haploid total of 23 chromosomes). My haploid egg is then fertilized by a haploid sperm also containing one partner of each chromosome pair inherited from its bio parents, randomly). This is why we share half our dna with each of our parents but may share no dna or all of our dna with our full siblings—random sorting of diploid chromosomes into haploid sex cells randomly combining during fertilization to make a new combo of diploid cells :)
There are some human genetic conditions where we sometimes end up with an extra partner or missing a partner in a chromosome pair, giving an odd number of total chromosomes. This usually occurs when the pairs are being copied and divided to produce eggs or sperm by our bio parents. Trisomy 21 (downe syndrome) is an example most people have heard about—an extra copy of chromosome 21 is inherited from one of bio parents. This can also happen with the X and Y chromosome (one copy of X is Turner syndrome, two copies of X with Y is klinefelter syndrome, whereas typically we inherent either two x or one x and one y).
Sorry for the book. Hope it helps. Always glad to help :)
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Nope. Nope nope nope.
Several species of insect have odd chromosomal numbers. Both male and female Japanese Oak Silkmoths have 31 chromosomes (not pairs).
The male Indian muntjac has 7; the female has 6.
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So plants don't have centrioles (I think that's right? I'm a plant ecologist not a geneticist) and as a result, the chromosomal number doesn't matter as much. Because of this, many plants are polyploid - they can be diploid, triploid, or even tetraploid, with additional chromosomal replicates believed to confer some advantages in specific conditions, depending on the species. You could, then, have a triploid plant with an odd number of chromosomes.
EDIT: Apparently fertility in triploid plants is greatly reduced, which I was unaware of, though reproduction is possible: https://journals.ashs.org/hortsci/view/journals/hortsci/51/8/article-p968.xml#:~:text=Triploids%20are%20typically%20highly%20infertile,et%20al.%2C%202011).
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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
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.
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There are some unisexual salamanders that can be triploid or even pentaploid, and it doesn't really affect their fertility because they are all female and reproduce by mating with males of closely-related species.
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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.
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Thank you for this rabbit hole! I am stunned that creatures with such a huge difference in chromosome number can produce viable offspring! Reading about chromosomal fusion, genome shock, and stunning, is freaking incredible, and I'm going to be obsessing about this for a while. I pity they people in my life who are about to get the most random, intense, infodump soon.
Why do they actively want to prevent mules from breeding? (besides wanting them available foe work).
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I’m all ears. I’ve been inundated with researching other, more pressing and unpleasant things, and would love a reprieve by someone enthusiastic about the topic.
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.
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Lol, I need to read more to make sure I understand, but I will get back to you later!
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I just wanted to say thank you for this read!
It‘s the Y chromosome in Indian muntjacs that‘s added on.
Or rather the Y chromosome doubled at some point, with one of them becoming a non sex determining variant.
Since a ‚functioning‘ Y chromosome that determines sex only actually needs a single gene to switch from the normal female type to the male type; it can get pretty wonky without much trouble.
I‘d assume in other species it‘s the same.
Also the X chromosomes in Indian muntjacs is stuck into a different chromosome.
So the female muntjac has two ‚regular‘ chromosome pair, one pair with a tiny X portion stuck to the two tops. While the male muntjac has the same two regular pairs; and then the next pair has the X portion only stuck to a single one in the pair, and the Y being a tiny additional chromosome.
So really, the males have a higher number of chromosomes because their Y chromosome is free floating and doesn‘t ‚attach‘ to the spot where the second X would go in females
As single X chromosome individuals are perfectly viable in virtually all species, it seems the Y chromosome can really do whatever it wants and things will still work.
Btw the Chinese muntjac has 46 chromosomes and can interbreed with the Indian muntjac with 6/7 chromosomes.
And the change from the 46 variant to the 6/7 one is pretty recent.
Like they have the same number of genes. They just Stuck all those 46 chromosomes together in a very chaotic way for some reason.
While staying perfectly healthy throughout.
https://www.jstage.jst.go.jp/article/cytologia/70/1/70_1_71/_pdf
For the chromosomes.
Since the Y chromosome is the ‚inferior‘ variant of the X chromosome, I.e. it carries virtually zero essential genetic information, because only XX individuals are ‚whole‘; it is much more free to just ‚be‘ and still work.
It really only needs the SRY gene and it‘ll work.
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: https://www.jstage.jst.go.jp/article/cytologia/70/1/70_1_71/_pdf 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.
Do you want to see something interesting? Check out the Indian Muntjac:
The X fragment is always fused to the autosome, while the Y chromosome stays unfused. So 2n is 6 in females and 7 in males.
Their close relative the Chinese muntjac has 2n=46
And they can interbreed with sterile offspring.
Like an n=3 gamete can fuse with an n=23 gametes and it ‚works‘
They both come from n=70 ancestral deer.
The Indian one just went let’s fuse all the chromosomes.
https://www.jstage.jst.go.jp/article/cytologia/70/1/70_1_71/_pdf
Not sure about odd or even numbers but potatoes technically reproduce via triploid seeds, triplets instead of pairs of chromosomes. It is why you almost never see actual potato seeds for sale because it is the work of a lifetime to actually breed stable genetics in something like that. I think there may be one or two named seed varieties, usually they are grown from "seed potatoes" which are just potatoes or root tubers, that can sprout. They aren't actual seeds. When they sprout the leaves produce enough energy to produce more potatoes thus repeating the cycle.
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From this logic it would follow that in species which don’t use the Y chromosome to determine sex, like birds or crocodilians, we would expect to see different patterns.
Well, assume mules and hinnies are functionally infertile. Breeding is zero reward, but not zero risk. As well as the risk of injury, there is the risk that a male mule mating with a fertile female will result in a miscarriage of an unviable foetus rather than the young you actually want. There is also a smaller risk than a female mule mating with a fertile male will cause him to be unable to successfully stud for the fertile female you want to breed him with, although this is a lesser concern.
Generally, individuals with an odd number of chromosomes are not fertile. Such individuals cannot make proper pairs of homologous chromosomes during meiosis. Thus such organisms cannot make viable gametes, and hence they are infertile.
But in the real world, nothing is static. There are undoubtedly some exceptions, so there are several species of animals that have odd numbers of chromosomes and are fertile, such as certain types of fish, amphibians, and reptiles. For example, the Mexican tetra fish and the Chinese soft-shelled turtle.
Also if you aren't careful she might just kick the stud in the head and kill him. There are a lot of risks for a statistically improbable event where you could just have a gelded male that is useful for work and easier to control, which is why someone would own a mule besides wanting a violent and obstinate pet.
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For the mules and hinnies that are fertile did they end up with even number of chromosomes or are they fertile despite having odd number of chromosomes?
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The cases I've read about, an abnormal pachytene phase during mitosis resulted in an extra chromosome that completed the pair.
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Do a general search for sex-determination systems. There are all sorts of patterns that can arise from other systems that aren’t XY.
In particular ZO/ZZ sex determination where the female only has one sex chromosome and the male has two.
It only occurs in some species of moth though.
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"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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I have always wondered if hybridization wasn't actually more commonly possible. I mean if the theory of evolutionary origin for life on this planet is true. Viable hybrids would need to exist like a lungfish and something to make a land dwelling amphibian
Yes, there are fertile creatures with an odd number of chromosomes!
A whole lot of them, actually. But it's not related to hybrids.
Namely some insects, due to something known as haplodiploidity, where males of some species will be haploid, having only one set of chromosomes, instead of e.g. two like the females of their species, or all humans.
Meaning that in those species that have an uneven number of chomosome pairs, the males will have an uneven number of chromosomes overall.
One good example woule be the Jack Jumper Ant, where males are haploid, possessing just a single Chromosome!
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This is how I now imagine Amazon women because I don't have to worry about them killing off male babies
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You have it a bit backwards: evolution doesn't advance through hybrids, it advances through the best-adapted individuals surviving. Like a lungfish with a mutation that gives it slightly more developed limbs, or slightly better chances at surviving outside of water. This lungfish mates with another, overall average lungfish, and their children may have that one cool parent's mutations with better access to a world most lungfish don't visit.
Repeat to the power of N (even bigger limbs, even better lungs, proto-claws, proto-fur, etc). If it gives the creature an advantage over the other members of its species, the creature has a better chance of making babies before dying, and the trait is passed on more often. There is no objective measure of what's a better or worse trait, of course — whales evolved back into water because that, too, was advantageous in a way.
Hybrids don't really have a role in the typical evolution pipeline, is the point. Sorry if this is old news, of course, just figured there's no harm in writing it out.
I read it as leading to men’s extinction since they go on to talk about the rats.
May I suggest a Bearded Dragon. They're only violent and obstinate on bath day.
Not relevant to the original topic but it blew my mind when I found out that there are mushrooms with thousands of sexes. Super weird little fungi :)
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
> 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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Yea Fungi mating types aren’t so much sexes as we understand them and more decoder rings.
It’d be like if we had 4 sex chromosomes instead of two, and multiple letters instead of binary.
XAYZ sex vs LBXK sex, and you can only mate with someone whose every chromosome is different.
That, in very broad strokes, is the method behind some fungi reproduction. Others are pretty analogous to human sexes. It’s a very wide range of strategies with fungi.
> 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?
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.
Hybridization brings genetic diversity. Lots of small differences to choose from.
Is that what causes that? I knew that many plant species had a ridiculous number of redundant chromosomes, but I didn't know it was because of a leak of centrioles. How do the cells separate the DNA during cell division, then?
But if the number of chromosomes changes as species evolve, there would most likely be a point where an organism with one number of chromosomes is mating with one that has a different number of chromosomes.
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The most crazy examples of animals are usually parthenogenetic insects. And in the distant future, even human males may lose their Y chromosome (although another chromosome would need to take over). Here is a case study featuring the spiny rat: https://greekreporter.com/2022/11/30/rat-without-y-chromosome-human-genetic-future/
But do any have an odd number of chromosomes?
Are you referring to Speciation?
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I feel like this ignores the common ancestry of all life, though.
Like, sapiens-neanderthalensis "interbreeding" is a great example.
We diverged for a while, likely just due to geographic isolation, then re-crossed, and now a significant fraction of people are "hybrids", even if neanderthal proper aren't around anymore.
Why are farmers trying to prevent mules and hinnies from breeding? Did a quick google and didn't find out.
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Evolution is a complicated thing and hybridisation shouldn't be brushed off. It's quite common in more closely related animals, which leads to different results than random mutation. It's likely what you said was taught to you at one time but evolution is one of those things that's always overly simplified and it annoys most biologists I know.
Speciation isn't one and done, it's a shift in the average of a whole population, so this would be part of it but definitely not the norm
Today, hybridization is recognized as pretty common, and many hybrids are fertile. It also is considered an important evolutionary process, which shaped the evolution of many organisms, including humans. The previous belief that hybridization is rare and is an "evolutionary dead-end" is long gone.
I see why mycologists claim there are thousands of unidentified species of fungi. It makes sense that many variants would occur.
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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.
Interesting, thanks.
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I didn't know a mammal could have so few chromosomes, figured only worms and such would have less than 10, but I'm just a layman so guess I could be missing a few chromosomes as well.
Red wolves, for example, may be a stable hybrid of grey wolves and coyotes.
>Generally, individuals with an odd number of chromosomes are not fertile.
An exception would be a man with XYY syndrome. They have normal fertility rates.
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Not only that but plants can have different numbers of chromosomes in different parts of the plants due to inaccurate cell wall growth after cell division. It’s how we got seedless grapes, naval oranges and pink grapefruit.
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I searched the comments and surprised that no one has yet mentioned humans with XXY (Klinefelter Syndrome or KS). It is possible for them to reproduce, although usually with the help of fertility services and fertility rates are extremely low. It is possible for them to reproduce naturally as well though it's pretty rare.
A couple related articles:
https://pubmed.ncbi.nlm.nih.gov/21207006/
https://www.nichd.nih.gov/health/topics/klinefelter/conditioninfo/faqs
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Seems like it's still contentious, but since avian sex regulation appears likely to be dependent on having two copies of the Z chromosome to induce maleness, the W chromosome that females have is likely to be analogously expendable to the Y chromosomes - so you would see the same patterns just with the sexes flipped?
>I have always wondered if hybridization wasn't actually more commonly possible.
It's incredibly possible. It happens all the time. The only reason you think it doesn't is because the definitions of words.
The entire concept of the taxonomic tree is human made arbitrary decisions. By definition, when hybrids are "common", we group them together as one species.
But like, pretend you are an archaeologist going through bones. Would you call a Chihuahua the same thing as a Rottweiler? That's totally a hybrid. There's so many, we call them all "dogs" and just use a different word: breed.
If that doesn't convince you, look up Ring Species, which are incredibly cool and totally make you rethink how you think about species.
Was there a period where sapiens and neanderthals couldn't interbreed? I guess what I'm trying to understand is what formally makes them different species in the first place.
Seems to me that "hybrids," as a concept, have less to do with biology and more to do with our arbitrary classification of it.
Our current definition of different species required then to not be compatible and apparently sapiens sapiens and sapiens neandathalensis were compatible at least to a certain point.
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.
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Exactly just like guppies and Betta are very different from natural and in the case of guppies there are like 4 or 5 species that they can cross with endlers, swordtails, mollies, platty, guppies can all interbreed with some complications some hybrids are too big for the mother to birth. Eg these are not viable Platy male and guppies female... Mollies male and female guppies, endlers female to anything except endlers. Where as swap the gender and you can hybrid.
And like some one else said hybridization is not something to separate from evolution as it can give leaps towards bigger changes if they are not viable they die if they end up beneficial to survival hybrid lives to mate and join the gene pool for either a whole new species or or in the case of what we believe happened to neanderthal proto humans out bred and some hybridization occured so basically their genes got poured into the pool and the hybrids diluted down
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.
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Is their fertility due to improper division as a gamete? Like, one gamete ended up with 10 chromosomes and the other ended up with 8 instead of both ending up with 9? Oversimplifying, of course.
You’re associating amount of chromosomes with overall species complexity or advancement or something, which isn’t exactly how it works. More chromosomes isn’t necessarily “better” and fewer isn’t worse, and some of the species with the largest amounts of chromosomes are butterflies and various plants, with hundreds and even over a thousand chromosomes, while some mammals are…well, the Indian Muntjac. There are also some ~10 and into the low teens.
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Because of the risks associated with mating and pregnancy Farmers generally try to prevent ‘unnecessary’ mating. While I’m sure the mule wouldn’t consider it unnecessary, farmers generally want to restrict breeding opportunities to their chosen ‘studs’ (the males with the best stats essentially).
There is always a slight risk of injury during mating, and a pretty high risk during pregnancy. These are very valuable animals and even if you didn’t care about their well-being you would want to restrict mating and pregnancy to only be happening under optimal conditions.
Letting a mule try to get another animal pregnant to see if they are fertile or not just isn’t really worth it when raising livestock.
Hello fellow plant ecologist! Also not a geneticist but aren’t triploid plants sterile? They can’t evenly split their chromosomes during meiosis.
Well, it looks like you are right - I apologize, I was not aware that triploids are generally sterile. Apparently they can reproduce but it's not as common: https://journals.ashs.org/hortsci/view/journals/hortsci/51/8/article-p968.xml#:~:text=Triploids%20are%20typically%20highly%20infertile,et%20al.%2C%202011).
It now makes sense why I've heard of triploidy in Aspen in particular, as they are clonal species :)
There isn't really one authoritative "current definition" of a species the way we're taught in lower-level science courses, at least not one so clear-cut and universal. If you ask a molecular biologist, a botanist, a zoologist, an ecologist, a geneticist, and a paleontologist to define a species you'll get six different, and sometimes contradictory, answers. Hell, if you ask two biologists in the same field you'll occasionally get competing answers.
These different definitions are called species concepts, of which probably the most common is the biological species concept. This is the one that you're referring to, which defines species based upon reproductive isolation. But it's not a perfect nor universal definition; it's entirely useless for asexual organisms, isn't informative in cases of horizontal gene tranfer, can't be directly tested in certain circumstances like when dealing with fossil species, and even breaks down with extant sexual populations in situations like ring species or many cases of hybridization (which we're learning is a lot more common and complex than previously thought). Other species concepts work better when dealing with asexual populations, or extinct groups, or when working specifically at the genomic level.
Most biologists work within the framework of whatever species concept best fits their field day-to-day as a shorthand but recognize there's a lot of nuance to the biological reality. That is to say, it's not as simple as can interbreed or can't.
The rule of animal husbandry is "never put anything beyond horney creatures" or "anything that can go wrong will go wrong, including your best mule can't plow because she's got one 1⃣ the way with no daddy within 20 miles."
This is called an XO sex determination system and has been independently evolved in mammals, so it can happen.
Yeah! A lot of grasses and other grass like plants are triploid as well.
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At present there's at least a species of ladybug that often has an odd number of chromosomes - some members of that species has a couple of chromosomes fused. This doesn't significantly impact fertility: if a ladybug only has one fused chromosome then during meiosis the unfused counterparts just line up end to end next to it.
The ancestors of humans also had a similar deal but afaik there are no modern humans with unfused chromosome 2.
I cannot recommend the podcast episode on hybrids on 'in our time' enough if you find the subject interesting. To me it was one of the best episodes they've ever done.
I won't spoil it too much, but I can say that the very idea of speciation is being re-evaluated because of new insights related to hybrids.
Or virgin births. There exist all-female lizard species whose children are clones of their mothers. Like these ones: https://en.m.wikipedia.org/wiki/New_Mexico_whiptail
Species is just an arbitrary classification. Interbreeding is only one factor used to determine what is a species. Its thought that only female neanderthal human hybrids were fertile and not males, so that one justification for considering us seperate species. Just how likely an offspring is to be fertile could also be taken into account. If two species have to breed a million times to produce 1 fertile offspring, it doesnt mean the two are the same species, there is never going to be considerable gene flow between those two groups.
Yes, in fact there are plenty of species which have differing numbers of chromosomes within that species. All different chromosome numbers mean (if everything else is the same) is that there is slightly higher chance of genetic abnormalities and the offspring is slightly higher likelihood of being infertile.
Fewer chromosome doesnt mean less genes or genetic material, it just means the chromosome are all joined up together. Species with more chromosomes have smaller chromosomes.
[deleted] t1_jbhc71k wrote
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