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Referring to interspecific hybrids, I have the following two questions:-

Quoting from wikipedia:-

The offspring of an interspecific cross are very often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct. Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.

1) The sterility of hybrids prevents interspecific gene exchange and is necessary in case the hybrid has odd number of total chromosomes (the hybridized species has odd and even pairs of chromosomes individually), where equal meiotic division would not be possible, and is also necessary to avoid sex determination problems that may arise from such hybridizations genetically. What is the molecular (genetic) reason behind the sterility? In other words, why is the expression of primary sexual organs and its further proliferation to effect reproduction not possible, although the genes required for it are already present in the hybrids? Is this due to epigenetic mechanisms or is the reason inherent in the hybrid nature of the genome?

2) This might probably sound a bit naive, but why are these hybrids viable? They have 2 non-homologous haploid pairs of chromosomes which have a huge variety(compared to two homologous pairs) of different non-allelic genes. Why are all the essential functions not hindered by the presence of such considerably unrelated set of chromosomes?

Probably related to the question is the fact that certain allopolyploids are viable and become an entirely new species, but others are not. Is it because here are 2 sets of each combining species' chromosomes (and hence allowing proper meiotic pairing) or is there some other reason pertaining to the nature of the combined sets of chromosome?

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    $\begingroup$ mitosis is not affected because there is no pairing of homologous chromosomes. Meiosis is possible affected because pairing is compromised. However, non-meiotic recombinations are possible. Why are all the essential functions not hindered by the presence of such considerably unrelated set of chromosomes? Hybrids are not possible between totally unrelated species. The similarity of the genes and the genetic networks perhaps allows the cells to carry out normal functions. $\endgroup$ – WYSIWYG Oct 28 '13 at 8:10
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    $\begingroup$ Can you add a referece to support your first point? Hybrid introgression and hybrids with lowered fitness (i.e. not fully sterile) has been shown to be relatively common in natural populations. $\endgroup$ – fileunderwater Oct 28 '13 at 14:44
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    $\begingroup$ @fileunderwater Please see the edit in my question. $\endgroup$ – Satwik Pasani Oct 28 '13 at 15:11
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    $\begingroup$ i think it is not just about odd number of chromosomes. Pairing would be compromised if there are no homologous syntenic regions. $\endgroup$ – WYSIWYG Oct 28 '13 at 17:04
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    $\begingroup$ Mallett 2008 might be of interest to you. It doesn't answer the questions about genetic mechanisms, but is a good perspective on hybridization viability/sterility. $\endgroup$ – Oreotrephes Dec 5 '13 at 19:47
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There are tons of explanations for this, but here is a starter. I am not including any sources because this is just from the back of my head and and knowledge I gained during my graduate school. During my graduate school, I worked on a classic natural inter-specific hybrid - wheat (an allopolyploid).

  1. What is the molecular (genetic) reason behind the sterility?

As you have already mentioned in your post, chromosomes from horse and donkey are not completely homologous, therefore they won't pair very well to be equally distributed to their respective gametes. Evolutionarily, they do share some homology that results in their weak pairing though. But what is actually causing this problem, as you asked, when all the required genes are present? Consider this, in humans or any intra-specific cross, when the chromosomes doubles during first meiosis division, exact same copies of every chromosome is made, then they are divided equally to two different cells. Now these two new cells have the equal number of chromosomes but from the same species. Then happens the second meiotic division when the chromosomes again lineup based on homology and again gets divided into two cells. In both these cases, recombination happens which results in the gene exchange. Now this is not an issue if both parents are from the same species because their genes are just getting swapped, meaning they are just exchanging alleles, therefore gene content remains same. However, in case of inter-specific hybridization, when recombination happens and chromosomal segments are swapped, genes are actually gained and/or lost. Depending on how severe is this loss, this will impact how the cells are divided. This will ultimately result in death of gametes.

Let's say theoretically, if recombination do not happen, can they produce viable gametes? Unlikely. Because, imagine again when chromosomes line up before cell division, it is highly highly highly unlikely that all horse chromosomes will be on the left side and donkey on the right side. It can happen but with a teeny tiny (almost zero) probability. Realistically, some horse chromosomes will lineup on the left and some on right, and same will happen for the donkey chromosomes. So when chromosomes go in different cells, the cells will have random mix of chromosomes from horse and donkey. For example, if cell1 has chrom 1 from donkey, then cell2 must have it from horse, therefore cell1 lacks the horse genes and cell2 lacks donkey genes. Hence, gene loss. And the fact that mule or hinnies have odd number of chromosomes, that just adds another layer of complication to the division.

  1. why are these hybrids viable?

This is a very valid question that is probably not fully understood yet. Borrowing information from my above answer, these different species are evolutionarily related. And the viability of their hybrids depends on how related they are. For example, donkey and horse are very related to each other if you look at their lineage, so they do share quite a bit of genome. Maybe their chromosomes are not fully homologous but they are quite enough to pair with each other. This is why they are viable and they survive. However, take a more extreme example, I can say with almost 100% certainty that you cannot get a hybrid from a donkey and a dog. Because they are very unrelated. In case of closely related species, some dosage dependent genes express less in the hybrids because now they only a single copy, which is sometime compensated if the other species have the same gene. But some genes are completely lost, whereas some genes are compensated. Depending on when and where this compensation occurs, they are classified as gametophytic/sporophytic compensation. And because of the fact that once a hybrid is conceived, it is all mitosis from there on, hybrid survives. Mitosis is much tolerant to recombination than meiosis because the resulting daughter cells still have all the gene content as the parent cells.

I hope this helps a bit and provide you with some ideas to read further.

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