A major locus controls a biologically active pheromone component in Heliconius melpomene

Abstract

Understanding the production, response, and genetics of signals used in mate choice can inform our understanding of the evolution of both intraspecific mate choice and reproductive isolation. Sex pheromones are important for courtship and mate choice in many insects, but we know relatively little of their role in butterflies. The butterfly Heliconius melpomene uses a complex blend of wing androconial compounds during courtship. Electroantennography in H. melpomene and its close relative Heliconius cydno showed that responses to androconial extracts were not species specific. Females of both species responded equally strongly to extracts of both species, suggesting conservation of peripheral nervous system elements across the two species. Individual blend components provoked little to no response, with the exception of octadecanal, a major component of the H. melpomene blend. Supplementing octadecanal on the wings of octadecanal‐rich H. melpomene males led to an increase in the time until mating, demonstrating the bioactivity of octadecanal in Heliconius. Using quantitative trait locus (QTL) mapping, we identified a single locus on chromosome 20 responsible for 41% of the parental species’ difference in octadecanal production. This QTL does not overlap with any of the major wing color or mate choice loci, nor does it overlap with known regions of elevated or reduced FST. A set of 16 candidate fatty acid biosynthesis genes lies underneath the QTL. Pheromones in Heliconius carry information relevant for mate choice and are under simple genetic control, suggesting they could be important during speciation.

Clustering of loci controlling species differences in male chemical bouquets of sympatric Heliconius butterflies

“…Most compounds mapped did not produce significant QTL at either the p = 0.05 or the Bonferroni-adjusted cutoff levels (the latter taking into account the number of compounds mapped) (Figures S1 and S2). A total of 21 genital compounds and 14 wing compounds produced significant QTL, including three peaks already published in (Byers et al, 2020, Darragh et al, bioRxiv) for octadecanal, 1-octadecanol, and ( E )-β-ocimene. Of these QTL, nine genital and ten wing compounds were significant at the more conservative Bonferroni threshold (Tables 2 and 3; Table 4).…”

“…Pheromones were collected from male individuals (both parental species and offspring of mapping crosses) following (Darragh et al 2017). Wing and genital data from H. melpomene and wing data from H. cydno have previously been published in (Darragh et al, 2017, Darragh et al, 2019, Byers et al, 2020). Briefly, males of 7-14 days post eclosion had their wings and genitals removed and soaked in dichloromethane plus 1 ng/μL 2-tetradecyl acetate (DCM+IS) for one hour, after which the DCM+IS was removed to a fresh vial and stored at −20°C before gas chromatography-mass spectrometry analysis.…”

“…Ten families were constructed as backcrosses to H. melpomene (representing 89 individuals for the wing phenotype, 81 individuals for the genital phenotype, and 78 individuals for the hindwing relative androconial area phenotype) and fifteen families were constructed as backcrosses to H. cydno (127 individuals for the wing, 114 individuals for the genital phenotype, and 124 individuals for the hindwing relative androconial area phenotype) for a total of 216 and 195 hybrid individuals for wing and genital studies respectively. DNA extraction, library preparation, and QTL map construction are detailed in (Byers et al, 2020, Darragh et al, bioRxiv). Briefly, Qiagen DNeasy kits (Qiagen) were used for DNA extraction, and individuals then genotyped either by RAD-seq or low-coverage whole genome sequencing using nextera-based libraries (Picelli et al, 2014, Davey et al, 2017, Merrill et al, 2019).…”

“…One challenge of studying the entire chemical profile is determining which (combination of) compounds are behaviorally active. The chemical bouquets of wings and genitals are complex, often consisting of 30-70 compounds (Mann et al, 2017, Darragh et al, 2017, 2020, Byers et al, 2020, this study), but it remains unknown how many of these are important for signaling. A single wing compound in H. melpomene , octadecanal, is known to be biologically active in H. melpomene and H. cydno (Byers et al, 2020), while the genital compound ( E )-β-ocimene acts as the main anti-aphrodisiac in H. melpomene (Schulz et al, 2007).…”

“…In combination with their importance in inter- and intraspecific mate choice, such simple genetic control and relatively lower risk of pleiotropy makes them ripe material for adaptation and speciation. We here conduct quantitative trait locus (QTL) analyses for wing androconial area and wing and genital compounds that differ between Heliconius melpomene L. and H. cydno Doubleday (Figure 1; see also Byers et al, 2020, Darragh et al, 2020). We then investigated the patterns of QTL distribution across the genome to test for clustering of loci across chromosomes.…”