Influence of the large–Z effect during contact between butterfly sister species

Abstract

Closely related species of butterfly sampled from southern suture zones in North Amer-1 ica exhibit a continuous pattern of gene flow and population difference measures (index 2 values) for autosomes, but not for the Z chromosome; When populations are compared 3 through their Z chromosomes, index values obtained from samples of the same species are 4 separated from those of closely related species by a gap of "missing" values, suggesting a 5 discrete "on-off" criterion for species delimitation. Here, we explore the possibility that 6 some, or all of the index data for suture zones reflects secondary contact between species 7 formed in glacial refugia. We simulate fusion of butterfly populations limited by negative 8 fitness interactions between genes in hybrids, assuming that interactions between auto-9 somes and the Z chromosome are stronger than those among autosomes, and that hybrid 10 fitness effects conform to Haldane's rule. We find that weakly interbreeding populations 11 trace out a path toward equilibrium consistent with the data for butterfly suture zones, 12 in which index values for the Z chromosome lag behind those for autosomes, leading to 13 a similar gap of missing values when species become indistinguishable through their au-14 tosomes, but no evidence of a sudden change in index values for the Z chromosome on 15 longer timescales. As a result, we find that the gap can be explained by a process in 16 which the pattern of index data for the Z chromosome is, ultimately, continuous. 17