Female Behaviour Drives Expression and Evolution of Gustatory Receptors in Butterflies

Abstract

Secondary plant compounds are strong deterrents of insect oviposition and feeding, but may also be attractants for specialist herbivores. These insect-plant interactions are mediated by insect gustatory receptors (Grs) and olfactory receptors (Ors). An analysis of the reference genome of the butterfly Heliconius melpomene, which feeds on passion-flower vines (Passiflora spp.), together with whole-genome sequencing within the species and across the Heliconius phylogeny has permitted an unprecedented opportunity to study the patterns of gene duplication and copy-number variation (CNV) among these key sensory genes. We report in silico gene predictions of 73 Gr genes in the H. melpomene reference genome, including putative CO2, sugar, sugar alcohol, fructose, and bitter receptors. The majority of these Grs are the result of gene duplications since Heliconius shared a common ancestor with the monarch butterfly or the silkmoth. Among Grs but not Ors, CNVs are more common within species in those gene lineages that have also duplicated over this evolutionary time-scale, suggesting ongoing rapid gene family evolution. Deep sequencing (∼1 billion reads) of transcriptomes from proboscis and labial palps, antennae, and legs of adult H. melpomene males and females indicates that 67 of the predicted 73 Gr genes and 67 of the 70 predicted Or genes are expressed in these three tissues. Intriguingly, we find that one-third of all Grs show female-biased gene expression (n = 26) and nearly all of these (n = 21) are Heliconius-specific Grs. In fact, a significant excess of Grs that are expressed in female legs but not male legs are the result of recent gene duplication. This difference in Gr gene expression diversity between the sexes is accompanied by a striking sexual dimorphism in the abundance of gustatory sensilla on the forelegs of H. melpomene, suggesting that female oviposition behaviour drives the evolution of new gustatory receptors in butterfly genomes.

Genomic adaptation to polyphagy and insecticides in a major East Asian noctuid pest

“…1d). These observations are similar to GR expression patterns reported in adult tissues of H. melpomene 14 and in diverse developmental stages and tissues in H. armigera 21 . Intriguingly, four bitter GR genes on Chr 25 and 14 bitter GR genes on Chr 14 were mainly expressed in moth proboscis (Fig.…”

“…GRs, especially, are highly variable among species, which could be a major factor for host plant adaptation. GRs are categorized into three classes-CO 2 receptors, sugar receptors and bitter receptors-among which bitter receptors are most variable, while CO 2 and sugar receptors are conserved [14][15][16][17][18] . Manual annotation identified 237 GR genes in the S. litura genome ( Table 2, Fig.…”

“…Manual annotation identified 237 GR genes in the S. litura genome ( Table 2, Fig. 1a and Supplementary Table 13), whereas in the other lepidopteran species investigated to date, most of which are mono-and oligophagous, only about GRs are reported 8,11,14,16,19,20 . Since large expansions of GR genes were also reported recently in S. frugiperda 5 and in another polyphagous noctuid, Helicoverpa armigera 21 , the expansion of GRs may be a unique adaptation mechanism for polyphagous Noctuidae to feed on a wide variety of host plants (Table 2).…”

Carbon dioxide receptor genes in cotton bollworm Helicoverpa armigera

“…The orthologous genes of these threeCO2 receptors were also identified in other mosquito species such as Aedes aegypti (Erdelyan et al 2012) and Culex quinquefasciatus (Robertson and Kent 2009). Recently published genome and transcriptome sequences from various insect species revealed that putative CO2 gustatory receptors are conserved in many insect species including mosquitoes, moths, butterflies, beetles and flies (Anderson et al 2009;Briscoe et al 2013;Dahanukar et al 2007;Erdelyan et al 2012;Obiero et al 2014). However, CO2 GRs are absent in other species like honeybees, human louses, wasps, ants and blacklegged ticks (Robertson and Kent 2009), suggesting that these latter species may use different mechanisms for CO2 sensation.…”

“…However, CO2 GRs are absent in other species like honeybees, human louses, wasps, ants and blacklegged ticks (Robertson and Kent 2009), suggesting that these latter species may use different mechanisms for CO2 sensation. Among the insect species that have CO2 GRs, the numbers of genes present in the genome vary (Anderson et al 2009;Briscoe et al 2013;Dahanukar et al 2007;Erdelyan et al 2012;Obiero et al 2014;Robertson and Kent 2009). Beetles, moths, butterflies and mosquitoes possess three CO2 GR genes in their genomes while Drosophila only has two (Briscoe et al 2013;Robertson and Kent 2009), and a recent study on tsetse fly reported that there are four GR genes orthologous to the Drosophila CO2 receptors (Obiero et al 2014).…”