Abstract
Hybrid zones, where distinct populations meet and interbreed, give insight into how differences between populations are maintained despite gene flow. Studying clines in genetic loci and adaptive traits across hybrid zones is a powerful method for understanding how selection drives differentiation within a single species, but can also be used to compare parallel divergence in different species responding to a common selective pressure. Here, we study parallel divergence of wing colouration in the butterflies Heliconius erato and H.melpomene, which are distantly related Müllerian mimics which show parallel geographic variation in both discrete variation in pigmentation, and quantitative variation in structural colour. Using geographic cline analysis, we show that clines in these traits are positioned in roughly the same geographic region for both species, which is consistent with direct selection for mimicry. However, the width of the clines varies markedly between species. This difference is explained in part by variation in the strength of selection acting on colour traits within each species, but may also be influenced by differences in the dispersal rate and total strength of selection against hybrids between the species. Genotyping-by-sequencing also revealed weaker population structure in H.melpomene, suggesting the hybrid zones may have evolved differently in each species, which may also contribute to the patterns of phenotypic divergence in this system. Overall, we conclude that multiple factors are needed to explain patterns of clinal variation within and between these species, although mimicry has probably played a central role.
Highlights
Studying clines in genetic loci and adaptive traits across hybrid zones is a powerful method for understanding how selection drives differentiation within a single species, but can be used to compare parallel divergence in different species responding to a common selective pressure
We show that clines in these traits are positioned in roughly the same geographic region for both species, which is consistent with direct selection for mimicry
Races were collapsed into a single “noniridescent” group, north of the sequenced and WGS, to their corresponding reference genomes, eihybrid zone, since our results show there is no genetic structure bether Heliconius melpomene version or Heliconius tween them based on race
Summary
Number FP differences between populations are maintained despite gene flow. Studying clines in genetic loci and adaptive traits across hybrid zones is a powerful method for understanding how selection drives differentiation within a single species, but can be used to compare parallel divergence in different species responding to a common selective pressure. This demonin the programs NGSadmix, to estimate ancestry proportions from strates the mean strength of effective selection on loci underlying a a varying number of genetic clusters (k), and PCAngsd to confirm trait required to maintain a cline of width (w), given the dispersal dispopulation clustering by principal components (PCA) Yelpopulation structure, or the sparse sampling of genomic data within ow bar the ML estimates of the c ine width were rough y four times wider in H. melpomene than in H. erato Tab e For both traits, the peaks of the likelihood profiles did not overlap (Figure 5), by pairwise regression analysis, as the linear model was always with the difference being significant for iridescence (p = .01). Given that the widths of the ye ow bar and sion analysis indicated that the admixture clines were concordant iridescence clines were not different within each species, similar es-
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