Ted Shoemaker  wrote or quoted:

> Can someone explain the mechanisms involved in sterility of many
> hybrids?  Probably the best-known example is the mule: You can get
> children from a horse and an ass, but you can't get grandchildren from
> them.  This has been a well-known fact for thousands of years.  But
> can someone point me to the specifics of HOW mules (or other hybrids)
> are sterile?

Sterility is mostly due to malfunction - due to conflicting sets of 
genetic instructions.

Too many malfunctions and the organisms is dead.  Slightly fewer and
it lives - but fails to work properly - i.e. it is sterile.  A few
less again, and it merely suffers from reduced fertility.

So: there are probably as many ways of being sterile as there are
for the reproductive system to malfunction - and there are many,
*many* ways for such complex systems to go wrong.

Some examples:

Genetic complexity underlying hybrid male sterility in Drosophila.

 - http://calorierestriction.org/pmid/?n=15020468

Little qualitative RNA misexpression in sterile male F1 hybrids of 
Drosophila pseudoobscura and D. persimilis

 - http://www.doaj.org/abstract?id=81386&toc=y

Association of misexpression with sterility in hybrids of Drosophila 
simulansand D. mauritiana.

 - http://calorierestriction.org/pmid/?n=15486701

Genetic analysis of X-linked hybrid sterility in the house mouse

 - http://calorierestriction.org/pmid/?n=15366371

The genetics of inviability and male sterility in hybrids between 
Anopheles gambiae and An. arabiensis.

 - http://calorierestriction.org/pmid/?n=15166154
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