Why are hybrids infertile?

Question

Let's take a quote from Wikipedia about zebroids.

Donkeys are closely related to zebras and both animals belong to the horse family. These zebra donkey hybrids are very rare. In South Africa, they occur where zebras and donkeys are found in proximity to each other. Like mules, however, they are generally genetically unable to breed, due to an odd number of chromosomes disrupting meiosis.

First, if I understand meiosis, the resulting cells don't actually end up with half the number of chromosomes, but closer to a full set of halves of chromosomes. How is the meiotic process disrupted?

Then,

A donkey has 62 chromosomes; the zebra has between 32 and 46 chromosomes.

Apparently this difference doesn't obstruct producing (infertile) offspring. How comes the process of recombination of such vastly different number of chromosomes in gametes is viable? What happens to chromosomes that don't find their 'pair'?

And then,

Horses have 64 chromosomes, while most zebroids end up with 54 chromosomes.

54 is an even number. How comes zebroids can't just normally produce fertile offspring with other zebroids of the same number of chromosomes?

Answer 1

A critical step in meiosis is the formation of tetrads. In diploid organisms like donkeys, they have a paternal version and a maternal version of each chromosome. Two chromosomes 2, for instance. During prophase 1, these two "matching" or homologous chromosomes form a tetrad and cross over, swapping alelles. There is no recombination and no tetrad formation in mitosis, which is used for growth and daily living.

How is the meiotic process disrupted?

In a hybrid, there are no matching maternal and paternal chromosomes. In the zebroid testes or ovary, a lonely donkey chromosome 2 is wandering around, looking for another donkey chromosome 2, and there isn't a homologous chromosome. This mucks up Prophase 1, and the first division pulls crazy numbers of chromosomes to each daughter cell, and the gametes from this division are likely full of extra chromosomes and missing some critical ones.

Apparently this difference doesn't obstruct producing (infertile) offspring. How comes the process of recombination of such vastly different number of chromosomes in gametes is viable?

An embryo needs a solid set of genes to grow up, but chromosomes don't have to be homologous for mitosis, and organisms use mitosis for growth and living, not meiosis. As long as the crazy mis-matched chromosomes in a zebroid have everything needed for a healthy organism, the hybrid will be healthy. It isn't until the zebroid starts to make gametes via meiosis that a problem occurs, as the homologues ONLY form tetrads in making eggs and sperm. The inability to recombine only occurs in the making of gametes.

54 is an even number. How comes zebroids can't just normally produce fertile offspring with other zebroids of the same number of chromosomes?

Because even chromosome number doesn't help if they are not each homologous with another chromosome.

Answer 2

I am a little bit out of practice but i researched the genetics of the horse tribe during the 1990s at the start of the Internet. Male hybrids of the diverse equine species are all infertile. However, a fairly large number of fertile female mules have been observed for over a century and much genetic research has been done to them, both in the West as in China. A few fertile female hinnies have also been proven. With zebroids only one fertile mare, a zebra-horse mix, is known and sadly her remains as that of her foal have not been subjected to serious research. That donkeys are closer related to zebras as horses is not generally accepted. There exist three species of zebras -and possibly one recently extinct species, the Quagga- but all three species have interbred and at least a few of the female hybrids have produced foals, in the wild even!

Answer 3

If you think about the complexity of successful reproduction -- sperm recognizing egg, entering egg, the intricate choreography of cell division and differentiation -- it's staggering that any two individuals of the same species ever successfully interbreed, and it's obvious that enormous selective pressure is needed to maintain this perfect compatibility. It's much less surprising that genetic drift would randomly lose one or more of the thousands of adaptations that are needed to guarantee compatibility.

So random drift is probably the reason for most genetic incompatibility, but in cases where closely-related species come into contact (whether through sympatric speciation or through changes in the range of one or both), there's also positive selection that will actively prevent cross-species interfertility. Hybrids are almost certainly going to be less well adapted to either parental niche than are purebreds, and so investing energy into developing hybrids is likely to be much less evolutionarily successful than putting the same energy into purebreds.