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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 differing 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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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. –  WYSIWYG Oct 28 '13 at 8:10
    
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. –  fileunderwater Oct 28 '13 at 14:44
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@fileunderwater Please see the edit in my question. –  Satwik Pasani Oct 28 '13 at 15:11
    
@SatwikPasani Great! The relative prevalence of sterility probably differs dramatically between taxa (depending on e.g. how species concepts are applied, genomic differences within taxa and the strength of regulation). –  fileunderwater Oct 28 '13 at 15:24
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i think it is not just about odd number of chromosomes. Pairing would be compromised if there are no homologous syntenic regions. –  WYSIWYG Oct 28 '13 at 17:04

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