Adaptive divergence and the evolution of hybrid trait mismatch in threespine stickleback

Selection against mismatched traits in hybrids is the phenotypic analogue
of intrinsic hybrid incompatibilities. Mismatch occurs when hybrids
resemble one parent population for some phenotypic traits and the other
parent population for other traits, and is caused by dominance in opposing
directions or from segregation of alleles in recombinant hybrids. In this
study, we used threespine stickleback fish (Gasterosteus aculeatus L.) to
test the theoretical prediction that trait mismatch in hybrids should
increase with the magnitude of phenotypic divergence between parent
populations. We measured morphological traits in parents and hybrids in
crosses between a marine population representing the ancestral form and
twelve freshwater populations that have diverged from this ancestral state
to varying degrees according to their environments. We found that trait
mismatch was greater in more divergent crosses for both F1 and F2 hybrids.
In the F1 the divergence--mismatch relationship was caused by traits
having dominance in different directions, whereas it was caused by
increasing segregating phenotypic variation in the F2. Our results imply
that extrinsic hybrid incompatibilities accumulate as phenotypic
divergence proceeds.