Population genomics of parallel hybrid zones in the mimetic butterflies,H. melpomeneandH. erato

Abstract

Hybrid zones can be valuable tools for studying evolution and identifying genomic regions responsible for adaptive divergence and underlying phenotypic variation. Hybrid zones between subspecies of Heliconius butterflies can be very narrow and are maintained by strong selection acting on color pattern. The comimetic species, H. erato and H. melpomene, have parallel hybrid zones in which both species undergo a change from one color pattern form to another. We use restriction-associated DNA sequencing to obtain several thousand genome-wide sequence markers and use these to analyze patterns of population divergence across two pairs of parallel hybrid zones in Peru and Ecuador. We compare two approaches for analysis of this type of data-alignment to a reference genome and de novo assembly-and find that alignment gives the best results for species both closely (H. melpomene) and distantly (H. erato,~15% divergent) related to the reference sequence. Our results confirm that the color pattern controlling loci account for the majority of divergent regions across the genome, but we also detect other divergent regions apparently unlinked to color pattern differences. We also use association mapping to identify previously unmapped color pattern loci, in particular the Ro locus. Finally, we identify a new cryptic population of H. timareta in Ecuador, which occurs at relatively low altitude and is mimetic with H. melpomene malleti.

No evidence for maintenance of a sympatricHeliconiusspecies barrier by chromosomal inversions

“…Notably, none of these candidate inversions were located near loci known or suspected to determine species differences in wing pattern or any other trait with known locations (Nadeau et al. 2014, Davey et al. 2016; we have transferred the H. melpomene loci to positions in the ordered Hmel2 genome in Table S8 and also included a table of all candidate inversion positions for comparison in Table S7; the BD region has been narrowed based on the results of Wallbank et al.…”

What Can We Learn About Adaptation from the Wing Pattern Genetics of Heliconius Butterflies?

“…14 Baxter et al (2009). 15 The Ro locus was mapped to linkage group 13 by means of a hybrid zone association study (Nadeau et al 2014). 16 L and Wo are linked loci that control forewing white elements in H. cydno and may be homologous to Ac (Linares 1996).…”

“…'Postman' races typically have a bright red forewing band, while Amazonian forms have orange dennis and ray patterns. Another locus that has been better characterised is Ro, which generates a rounded forewing band phenotype such as that seen in H. e. notabilis (Salazar 2012;Papa et al 2013;Nadeau et al 2014). Some of the most beautiful but poorly characterised are the iridescent blue and green colours that result from structural variation in the wing scales.…”

Transitions between phases of genomic differentiation during stick-insect speciation

“…We conducted 24 no-choice 23 copulation trials (eight trials each with 'conspecific native population perfume', 'heterospecific 24 perfume', or 'no perfume') between one male and one female T. cristinae from FHA (males 25 choose mates in Timema) 49 . Each individual perfume consisted of CHCs that we extracted 26 from six cold-euthanized females, and that we gently transferred to the live female in each 27 trial.…”

“…No-choice copulation trials were based on previously published protocols 69 . For each 28 trial, we kept one male and one female T. cristinae in a 10-cm Petri dish for 4 h, and we scored 29 the latency to copulate (i.e., minutes until copulation) 49 These analyses focused on seven T. cristinae populations previously studied for sexual 36 isolation, for which we also obtained data on colour-pattern, CHCs, and FST (n = 21 pairwise 37 comparisons). We estimated the strength of sexual isolation between populations by 38 calculating the IPSI index (theoretical range -1 to +1, where -1 = complete disassortative 39 mating, 0 = random mating, +1 = complete sexual isolation; all our empirical values were 40 positive) 70 .…”