Signatures of selection in loci governing major colour patterns in Heliconius butterflies and related species

BACKGROUND: Protein-coding change is one possible genetic mechanism underlying the evolution of adaptive wing colour pattern variation in Heliconius butterflies. Here we determine whether 38 putative genes within two major Heliconius patterning loci, HmYb and HmB, show evidence of positive selection. Ratios of nonsynonymous to synonymous nucleotide changes (ω) were used to test for selection, as a means of identifying candidate genes within each locus that control wing pattern. RESULTS: Preliminary analyses using 454 transcriptome and Bacterial Artificial Chromosome (BAC) sequences from three Heliconius species highlighted a cluster of genes within each region showing relatively higher rates of sequence evolution. Other genes within the region appear to be highly constrained, and no ω estimates exceeded one. Three genes from each locus with the highest average pairwise ω values were amplified from additional Heliconius species and races. Two selected genes, fizzy-like (HmYb) and DALR (HmB), were too divergent for amplification across species and were excluded from further analysis. Amongst the remaining genes, HM00021 and Kinesin possessed the highest background ω values within the HmYb and HmB loci, respectively. After accounting for recombination, these two genes both showed evidence of having codons with a signature of selection, although statistical support for this signal was not strong in any case. CONCLUSIONS: Tests of selection reveal a cluster of candidate genes in each locus, suggesting that weak directional selection may be occurring within a small region of each locus, but coding changes alone are unlikely to explain the full range of wing pattern diversity. These analyses pinpoint many of the same genes believed to be involved in the control of colour patterning in Heliconius that have been identified through other studies implementing different research methods.