Mule Gives Birth

!splut writes "Thumbing it's nose as science, a Moroccan mule has given birth. Mules, hybrids between horse and donkey, are normally infertile, due to differences between the number and structure of horse and donkey chromosomes. Nevertheless, for reasons not well understood, fertile mules do occur, infrequently, with some 60 documented live births since 1527."

Comments

The reason for sterility...

[DLWormwood]

...given in the article has to do with if the hybrid creature has an even number of chromosomes. I was under the impression that sterility of offspring was one of the determinations of speciation used by biologists. (And one of the frequent "disproofs" of evolution by creationists.) If a hybrid offspring had an even number of chromosomes, could it be fertile?

Are there known hybrids/half-breeds can have normal fertility, as opposed to requiring a "miracle" to occur? This is a sincere question, even though I know full well the trolls that are going to be attracted to this post.

Offspring fertile?

[crow]

So what is really interesting is whether the offspring are fertile. If so, then we can start breeding mules from mules, and we have a new species.

And where does the infertility in mules normally lie? Is it a male thing or a female thing? Or both?

Re:Offspring fertile?

[geoswan]

So what is really interesting is whether the offspring are fertile. If so, then we can start breeding mules from mules, and we have a new species.

And where does the infertility in mules normally lie? Is it a male thing or a female thing? Or both?

I looked into this when we discussed cloning Mammoths, or harvesting frozen Siberian Mammoth sperm a few months ago.

It was my impression that the very rare offspring of a mule mare and a horse sire, or a mule mare and a donkey sire, are the same species as the father. The mule has a mixture of donkey and horse chromosomes. Sperm and ovum are haploid cells -- they have one chromosome, not a pair. That is how sexual reproduction works. It was my impression that most ovum will have a mixture of horse and donkey chromosomes. But very occasionally, by chance, an ovum will have entirely horse chromosomes or donkey chromosomes.

Male mules are almost always gelded, to curb their agressiveness.

Hobbyists cross donkeys with zebras. They call the offspring "golden zebras". Hobbyists cross lions and tigers. These crosses are, apparently, a bit nuts. Lions are, of course, social. And tigers are, of course, solitary. The hybrid is drawn both ways.

The Moroccan foal looks a bit like a baby donkey and a bit like a baby mule - but not exactly like either.

The site I found about crossing cats talked about the differences between lion tiger crosses where the lion was the mother and when the tiger was the mother. When the tiger was the mother the hybrid is larger than a tiger. The maternal influence on the foal's embryonic environment has an influence on how the genotype is expressed.

Another anecdote. You can tell whether a mule's mother was a horse or a donkey by putting it in a corral that contained both donkeys and horses. The mule will go hang out with the kind of animals it was raised with.

Liger/Tigon

[crow]

A quick google search on Liger and Fertile tells me that male hybrids are rarely fertile. Another page states, "No fertile male ligers have yet been found and it is assuemd all are sterile." Though reading a bit more, there may be some fertile males, or males that are fertile for a brief period.

Fertility

[ktulus cry]

One possible explantion for this rare occurance, one that I can see, anyway - Horses have 64 chromosomes in their normal diploid state, with haploid gametes having 32. Donkeys have 62 chromosomes in normal diploid state, with haploid gametes having 31. This gives a mule 63 chromosomes. If one gamete had experienced non-disjunction during meiosis (that is, one pair of homologues did not split correctly, giving one daughter cell 2 copies of a chromosome, and one none), a mule could have 62 or 64 chromosomes (depending on which gamete it received). This is not a very rare occurance, Down Syndrome or Kleinfelter's Syndromes in humans (trisomy 21 and XXY respectively). While most cases of non-disjunction produce abnormal offspring, in humans, for example, an XXX female is completely normal, except for a usually smaller stature. A case like this, where the offspring IS normal, is relatively rare, and a similar situation could be occuring in these fertile mules.

Speciation

[f97tosc]

The truth is that speciation is not very well understood. In many ways 'species' is a convenient abstraction - we humans like to put horses and donkeys into two discrete buckets and not think much about anything in between. In reality there is no law of nature that says that all living entities must belong to exactly one of our convenient buckets.

The 'infertile children' definition works pretty well, but it is not perfect. Another problematic example is that of a species of birds that live in different territories around the globe. The birds can mate with other birds in their own or in adjacent territories. In other words, the birds in the first territory can mate with those in the second territory, and those in the second with those in the third, and so on all the way around the globe - and finally the birds of the last territory can again mate with those in the first. However, it turns out that the birds cannot mate with birds several territories away. Our convenient species definition breaks apart.

Tor

Re:The reason for sterility == odd chromosomes?

[geoswan]

The reason for sterility given in the article has to do with if the hybrid creature has an even number of chromosomes.

The beeb and the British Mule org may have said mules are infertile because they have an odd number of chromosomes. But I am skeptical.

Here is an excerpt from a page about the Przewalski Horse

Some authorities feel strongly that the Przewalski horse is the ancestor of all modern breeds. Others point out that it is a different species from the domesticated horse, having 66 chromosomes as compared to the 64 of the domestic horse. They further point out that while crosses between the Przewalski and domestic horses result in a fertile hybrid, the offspring has 65 chromosomes. Subsequent crosses result in 64 chromosomes and bear little resemblance to the Przewalski. The Foundation for the preservation and protection of the Przewalski's Horse, in Rotterdam, the Netherlands, report that only a few Przewalski horses are tamable, in proportions similar to a Zebra.

So, even if this site is mistaken to say that the 65 chromosome hybrid is fertile, what if you crossed a 62 chromosome Ass with a 66 chromosome Przewalski's Horse? That hybrid would have 64 chromosomes. Would that make it fertile?