Abstract
BackgroundGenes underlying signal production and reception are expected to evolve to maximize signal detection in specific environments. Fireflies vary in their light signal color both within and between species, and thus provide an excellent system in which to study signal production and reception in the context of signaling environments. Differences in signal color have been hypothesized to be due to variation in the sequence of luciferase, the enzyme that catalyzes the light reaction. Similarly, differences in visual sensitivity, which are expected to match signal color, have been hypothesized to be due to variation in the sequence of opsins, the protein component of visual pigments. Here we investigated (1) whether sequence variation in luciferase correlates with variation in signal color and (2) whether sequence variation in opsins correlates with inferred matching visual sensitivity across populations of a widespread North American firefly species, Photinus pyralis. We further tested (3) whether selection has acted on these loci by examining their population-level differentiation relative to the distribution of differentiation derived from a genome-wide sample of loci generated by double-digest RADseq.ResultsWe found virtually no coding variation in luciferase or opsins. However, there was extreme divergence in non-coding variation in luciferase across populations relative to a panel of random genomic loci.ConclusionsThe absence of protein variation at both loci challenges the paradigm that variation in signal color and visual sensitivity in fireflies is exclusively due to coding variation in luciferase and opsin genes. Instead, flash color variation within species must involve other mechanisms, such as abdominal pigmentation or regulation of light organ physiology. Evidence for selection at non-coding variation in luciferase suggests that selection is targeting luciferase regulation and may favor differ expression levels across populations.
Highlights
Genes underlying signal production and reception are expected to evolve to maximize signal detection in specific environments
We examined population structure in P. pyralis using a set of neutrally-evolving ddRADseq single nucleotide polymorphism (SNP) and tested for correlations between genotype at luciferase or opsins and emission color or habitat while controlling for this population structure
Variation at signaling loci To examine variation at signaling loci in the context of signaling environment, we Sanger sequenced the three signaling loci: adult luciferase, LW opsin, and UV opsin, of 191 individual males collected from 12 populations (15– 16 individuals/population) across the P. pyralis North American geographical range
Summary
Genes underlying signal production and reception are expected to evolve to maximize signal detection in specific environments. Fireflies vary in their light signal color both within and between species, and provide an excellent system in which to study signal production and reception in the context of signaling environments. The effects of natural and sexual selection driving the evolution of sensory systems in particular directions should be detectable in the genes underlying signal production and detection. Such evidence for selection should be observable both across species and across populations within a species that inhabit different environments
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