Abstract
In insects, flight and sophisticated olfactory systems go hand in hand and are essential to survival and evolutionary success. Females of many Lepidopteran species have secondarily lost their flight ability, which may lead to changes in the olfactory capabilities of both larval and adult stages. The gypsy moth, Lymantria dispar, an important forest pest worldwide, is currently undergoing a diversification process with three recognized subspecies: the Asian gypsy moth (AGM) L. dispar asiatica, the Japanese gypsy moth L. dispar japonica (JGM) and the European gypsy moth (EGM) L. dispar dispar. Females of EGM populations from North America have lost their flight capacity whereas the JGM and AGM females are flight capable, making this an ideal system to investigate the relationship between flight and olfaction. We used next-generation sequencing to obtain female antennal and larval head capsule transcriptomes in order to (i) investigate the differences in expression of olfaction-related genes among populations; (ii) identify the most similar protein sequences reported for other organisms through a BLAST search, and (iii) establish the phylogenetic relationships of these sequences with respect to other insect species. Using this approach, we identified 115 putative chemosensory genes belonging to five families of olfaction-related genes. A principal component analysis revealed that the gene expression patterns of female antennal transcriptomes from different subspecies were more similar among them than to the larval head capsules of their respective subspecies supporting strong chemosensory differences between the two developmental stages. An analysis of the shared and exclusively expressed genes for three populations shows no evidence that loss of flight affects the number or type of genes being expressed. These results indicate either that (a) loss of flight does not impact the olfactory gene repertoire or that (b) the secondary loss of flight in the American EGM populations may be too recent to have caused major changes in the genes being expressed. However, we found higher expression values for IRs, OBPs GRs and ORs in EGM females, suggesting that differences in transcription rates could be an adaptation of flightless females to their chemical environment. Differences in olfactory genes and their expression in the
Highlights
Flight is a leading factor contributing to the evolutionary success of insect species, enabling them to locate food and shelter, avoid predation and competition, and search for optimal oviposition sites for their offspring (Barbosa et al, 1989; Sattler, 1991; Hunter, 1995)
To fulfill these aims we focused on five groups of chemosensory gene families: odorant receptors (ORs), odorant binding proteins (OBPs), chemosensory proteins (CSPs), gustatory receptors (GRs), and ionotropic receptors (IRs)
We chose B. mori because it has one of the best characterized genomes of the Lepidoptera. This analysis showed that an average L. dispar transcript encodes just over half the expected protein sequence based on the best blastx hit to B. mori, possibly due to a high proportion of partial sequences (Supplementary Figure 1)
Summary
Flight is a leading factor contributing to the evolutionary success of insect species, enabling them to locate food and shelter, avoid predation and competition, and search for optimal oviposition sites for their offspring (Barbosa et al, 1989; Sattler, 1991; Hunter, 1995). The females of many Lepidopteran species have secondarily lost their ability to fly, shifting the responsibility of host selection partly or entirely to the larvae (Barbosa et al, 1989; Sattler, 1991; Hunter, 1995) In this context, it is interesting to investigate whether the loss of flight has an impact on the olfaction of adults and larvae. The gypsy moth Lymantria dispar is one of the most important forest pest species worldwide, currently undergoing a diversification process involving the loss of flight by females of some populations (Schweitzer, 2004; Pogue and Schaeffer, 2007). These features make L. dispar an ideal model to explore changes in expression of olfaction-related genes that are associated with flight ability
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