Abstract
Most animals use olfaction to obtain important information from the environment, including the presence of food or mates. Insects detect odorants through receptors that are expressed in the sensory neurons of the olfactory sensilla, which cover the surface of the antennae. The olfactory capacities of an insect thus depend largely on the repertoire of the odorant receptors. Here, we study the repertoire of olfactory proteins in the stick insect Timema cristinae. We first generate transcriptomes from the antennae of adult males and females and identify, via homology searches, putative olfactory proteins of three different families: odorant binding proteins, odorant receptors, and chemosensory proteins (CSPs). We then attempt to categorize olfactory proteins likely involved in sexual communication by comparing gene expression between adults and juveniles, as well as between males and females. Notably, the olfactory proteins involved in the perception of food or abiotic environmental components, should be expressed in both adults and juveniles. By contrast, the olfactory proteins involved in sexual communication, such as the detection of sex pheromones, should be expressed in adults and often comprise different repertoires in males and females. Finally, we also tested whether olfactory proteins in general and the subset, with putative roles in sexual communication in particular, are under relaxed selection in the asexual species T. monikensis, a close relative of T. cristinae. We found that olfactory proteins are typically differentially expressed between juveniles and adults, but there is little overlap of differential expression between developmental stages and the level of sex bias in adults. Furthermore, while we find evidence that olfactory proteins are indeed under relaxed selection in the asexual species, there is no evidence that this is necessarily the case for olfactory genes with a putative role in sexual communication. Nevertheless, the list of olfactory genes generated in our study provides a useful tool for future studies on olfaction in Timema and other stick insects.
Highlights
Olfaction, the sense of smell, is of critical importance for survival and reproduction in insects
We identified a total of 132 putative olfactory genes in T. cristinae, 122 of which were expressed at least at a low level in the antennae (Table 1, Supplementary Table 1)
Factors influencing antennal expression pattern of olfactory genes varied by olfactory gene type, with sex inferencing odorant-binding proteins (OBPs) expression more than the developmental stage, and vice versa for chemosensory proteins (CSPs) and odorant receptors (ORs) (Figure 1)
Summary
The sense of smell, is of critical importance for survival and reproduction in insects. The molecular mechanisms underlying the detection and processing of olfactory cues for host plant selection or mate discrimination remain unexplored This constrains studies of how olfaction contributes to adaptive divergence between stick insect populations and species. Odorant-binding and CSPs are involved in the first step of the recognition of chemical signals, by binding to hydrophobic molecules from the environment and delivering them to the receptors (Hallem et al, 2006; Touhara and Vosshall, 2009; Leal, 2013) Members of these protein families are involved in functions not linked to the detection of odors. Members of the chemosensory protein family in particular have more diverse functions in chemoreception, growth, and development (Pelosi et al, 2018)
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