Ok Ok this is the last one!!
 
In captivity it is not uncommon to find different and very distinct species
interbreeding and the hybrids producing Fx generations. As expected sometimes
through inbreeding loss of genetic diversity occurs and some lethal phenotype
appears. I have a hunch (just not the money, time or space,) that if one was
to do enough initial interspecies crossing (between let say 200 males of
species X and 200 females from species Y) and take the F1 offspring and use
them as an initial breeding stock one can produce Fx number of generations
that would be very healthy and fertile.
I have personally seen crossings in cichlids from Lake Malawi and crossing
between genera. The offspring have then produced fertile F2 individuals. In
short, I have seen healthy hybrids reproduce to the F3 generation.
 
Last one I promise,
Emilio