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iayork t1_jbhkcye wrote

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).


Modifien t1_jbi7jbp wrote

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.


mini-rubber-duck t1_jbi7xqr wrote

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.


Modifien t1_jbice55 wrote

Lol, I need to read more to make sure I understand, but I will get back to you later!


Maximum-Mixture6158 t1_jblwk1z wrote

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."


madmaxgoat t1_jbo0z1q wrote

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.


lunas2525 t1_jbj0krf wrote

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


mothmvn t1_jbj3qzr wrote

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.


Tyrosine_Lannister t1_jbjcvyo wrote

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.


almightySapling t1_jbkhsfx wrote

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.


Rather_Dashing t1_jcym0rf wrote

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.


ukezi t1_jbknqm4 wrote

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.


ScipioAfricanisDirus t1_jblw9du wrote

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.


peteroh9 t1_jbj8j0b wrote

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.


Techiedad91 t1_jbjcc69 wrote

Are you referring to Speciation?


Nick-Uuu t1_jbjhj27 wrote

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


Rather_Dashing t1_jcymabj wrote

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.


Nick-Uuu t1_jbjhc1h wrote

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.


DaSaw t1_jbk1n11 wrote

Red wolves, for example, may be a stable hybrid of grey wolves and coyotes.


lunas2525 t1_jbky25n wrote

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


lollerkeet t1_jbj861j wrote

Hybridization brings genetic diversity. Lots of small differences to choose from.


encoder123 t1_jbjhtn7 wrote

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.


almightySapling t1_jbkfwpm wrote

>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.


TheBloxyBloxGuy OP t1_jbhl65t wrote

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.


EmilyU1F984 t1_jbie16i wrote

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.

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.


Dr_Vesuvius t1_jbihbun wrote

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.


viridiformica t1_jbkeika wrote

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?


Navvana t1_jbism7z wrote

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.


lostmyselfinyourlies t1_jbj4gqk wrote

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 :)


Navvana t1_jbj7blc wrote

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.


WendysForDinner t1_jbjj4ug wrote

I see why mycologists claim there are thousands of unidentified species of fungi. It makes sense that many variants would occur.


bookmonkey786 t1_jbi7mqm wrote

Why do they actively want to prevent mules from breeding? (besides wanting them available foe work).


Dr_Vesuvius t1_jbihxj4 wrote

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.


Duke_Shambles t1_jbiikpz wrote

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.


stabliu t1_jbijdp2 wrote

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?


TastiSqueeze t1_jbira3v wrote

The cases I've read about, an abnormal pachytene phase during mitosis resulted in an extra chromosome that completed the pair.


KarlDeutscheMarx t1_jbjypeq wrote

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.


Welpe t1_jbldmmr wrote

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.


Rather_Dashing t1_jcymma4 wrote

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.


Tjuo t1_jbl7mzd wrote

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.


orick t1_jbjd05x wrote

Why are farmers trying to prevent mules and hinnies from breeding? Did a quick google and didn't find out.


DisapprovingCrow t1_jblkpve wrote

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.