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

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 H. 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.

Methods
This dataset consists of 4 major data subsets: pheromone analysis of parent species Heliconius cydno and H. melpomene (raw GC-MS output files and data tables); electroantennogram responses of both species (raw electroantennogram output and data tables); behavioral results (data table); QTL mapping information (linkage maps and QTL analysis scripts). GC-MS data were analyzed with AMDIS to produce data tables with compound amounts. Electroantennogram response strengths were manually marked in GcEad and peak heights exported to text files which were then put into data tables and analyzed with included scripts. Behavioral results were manually entered into data tables. QTL linkage maps were produced from whole genome data (separately archived on ENA under project PRJEB34160) and scripts for analysis of QTL using the R/qtl2 package are included.