> Actually I was thinking of the case of horses and donkeys. I agree that
>this isn't the only cause, but neither is Drosophila necessarily
>representative of other species. As I recall some of that is based on the
>presence of an intracellular bacterium. In general, karyotypic changes that
>lead to non-functional meiosis will necessarily lead to reproductive
>isolation. The point is that karyotype changes are common; I don't know if
>some of the other factors occur as widely. Karyotype differences (in
>particular translocations that lead to acentric or dicentric chromosomes on
>recombination) are effective blocks to interbreeding. Whether they are the
>first cause of isolation is open to study for any species.
The genetics of hybrid sterility in Drosophila that Tony Long mentions
is observed when species of the melanogaster subgroup, D. simulans,
D. seychellia, and D. mauritiana, are crossed. The reproductive isolation
caused by an intracellular bacterium was observed when D. simulans was
crossed to D. simulans from separate geographic populations (Hoffman,
Turelli, and Simmons, 1986 Evolution 40:692-701). Perhaps intracellular
bacteria are important in some cases of reproductive isolation, but the
work of Jerry Coyne, Allen Orr, and Chung-I Wu have mapped genes on
all major chromosomes of Drosophila that contribute to hybrid sterility.
Karytypic changes are not always associated with reproductive isolation.
Populations of D. pseudoobscura and D. subobscura have chromosomal
inversions segregating on one or more of their chromosomes. These
inversions suppress recombination in inversion heterozygotes but there
is no evidence that these have caused reproductive isolation. The limited
comparisons of genetic maps among species of Drosophila do show that
chromosomes have been translocated in Drosophila evolution. These data,
however, cannot be used to suggest that the translocations caused the many
speciation events in Drosophila.
Stephen W. Schaeffer
Institute of Molecular Evolutionary Genetics
The Pennsylvania State University