Annotation and evolutionary analysis of chemosensory gene sequence data in the Colorado potato beetle, Leptinotarsa decemlineata

Plants and plant-feeding insects comprise the majority of global species
diversity, and their coevolutionary dynamics provide an important window
into the mechanisms that mediate niche evolution. In particular, there is
considerable interest in understanding the nature of genetic changes that
allow host-plant shifts to occur and to determine whether functional
genomic diversity varies predictably in relation to host-plant breadth.
Insect chemosensory proteins play a central role in mediating insect-plant
interactions, as they directly influence plant detection and sensory
stimuli during feeding. This large group of gene families is known to
evolve rapidly, yet it remains unclear how these genes evolve in response
to host-shifts and host specialization. Here we investigate whether
selection at chemosensory genes is linked to host-plant expansion in the
Colorado potato beetle (CPB), Leptinotarsa decemlineata (Coleoptera:
Chrysomelidae), and whether rates of selection vary among ten closely
related Leptinotarsa species. To develop functional hypotheses of
chemosensory genes involved in the detection of potato host-plants, we
combine gene expression analysis of the antennae and maxillary-labial
palps using RNA sequencing with genomic evidence of natural selection. We
show that expression of chemosensory genes differs among pest populations
of Leptinotarsa decemlineata and numerous genes are under positive
selection. We also find that rates of positive selection on olfactory
receptors are higher in host-plant generalists, whereas rates are higher
for gustatory receptors and olfactory binding proteins in host-plant
specialists.