Abstract
Comparisons of genomes from recently diverged butterfly populations
along a suture zone in central Texas have revealed high levels of
divergence on the Z chromosome relative to autosomes, as measured by
fixation index, $F_{st}$. The pattern of divergence appears to
result from accumulation of incompatible alleles, obstructing
introgression on the Z chromosome in hybrids. However, it is unknown
whether this mechanism is sufficient to explain the data. Here, we
simulate the effects of hybrid incompatibility on interbreeding
butterfly populations using a model in which populations accumulate
cross–incompatible alleles in allopatry prior to contact. We compute
statistics for introgression and population divergence during contact
between model butterfly populations and compare them to statistics
obtained for 15 pairs of butterfly species interbreeding along the Texas
suture zone. For populations that have evolved sufficiently in
allopatry, the model exhibits high levels of divergence on the Z
chromosome relative to autosomes in populations interbreeding on time
scales comparable to periods of interglacial contact between butterfly
populations in central Texas.Levels of divergence on the Z chromosome
increase when interacting groups of genes are closely linked, consistent
with interacting clusters of functionally related genes in butterfly
genomes. Results for various periods in allopatry are in qualitative
agreement with the pattern of data for butterflies, supporting a picture
of speciation in which populations are subjected to cycles of divergence
in glacial isolation, and partial fusion during interglacial contact.