A comparison of the olfactory gene repertoires of adults and larvae in the noctuid moth Spodoptera littoralis

Abstract

To better understand the olfactory mechanisms in a lepidopteran pest model species, the cotton leafworm Spodoptera littoralis, we have recently established a partial transcriptome from adult antennae. Here, we completed this transcriptome using next generation sequencing technologies, namely 454 and Illumina, on both adult antennae and larval tissues, including caterpillar antennae and maxillary palps. All sequences were assembled in 77,643 contigs. Their analysis greatly enriched the repertoire of chemosensory genes in this species, with a total of 57 candidate odorant-binding and chemosensory proteins, 47 olfactory receptors, 6 gustatory receptors and 17 ionotropic receptors. Using RT-PCR, we conducted the first exhaustive comparison of olfactory gene expression between larvae and adults in a lepidopteran species. All the 127 candidate olfactory genes were profiled for expression in male and female adult antennae and in caterpillar antennae and maxillary palps. We found that caterpillars expressed a smaller set of olfactory genes than adults, with a large overlap between these two developmental stages. Two binding proteins appeared to be larvae-specific and two others were adult-specific. Interestingly, comparison between caterpillar antennae and maxillary palps revealed numerous organ-specific transcripts, suggesting the complementary involvement of these two organs in larval chemosensory detection. Adult males and females shared the same set of olfactory transcripts, except two male-specific candidate pheromone receptors, two male-specific and two female-specific odorant-binding proteins. This study identified transcripts that may be important for sex-specific or developmental stage-specific chemosensory behaviors.

Figure 1. Distribution of S. littoralis contigs annotated at GO level 2.

Figure 2. Maximum likelihood tree of candidate odorant-binding proteins (OBPs) from S. littoralis and other Lepidoptera.

Sequences used were from B. mori , H. melpomene , H. virescens , , , M. sexta , and from three other species of the genus Spodoptera (S. exigua, S. frugiperda and S. litura). Signal peptide sequences were removed from the data set. Branch support was estimated by approximate likelihood-ratio test (aLRT) (circles: >0.95) . Images were created using the iTOL web server . SlitOBPs are in bold and the new SlitOBPs identified in this study are in red.

Figure 3. Maximum likelihood tree of candidate ORs from S. littoralis and other Lepidoptera.

Sequences used were from B. mori , C. pomonella , H. virescens , , M. sexta , D. plexippus and H. melpomene . Branch support was estimated by approximate likelihood-ratio test (aLRT) (circles: >0.95) . Images were created using the iTOL web server . SlitORs are in bold and the new SlitORs identified in this study are in red.

Figure 4. Maximum likelihood tree of candidate ionotropic receptors (IRs) from S. littoralis and other insects.

Sequences used were from B. mori , C. pomonella , D. plexippus , D. melanogaster, Apis mellifera and Tribolium castaneum . Branch support was estimated by approximate likelihood-ratio test (aLRT) (circles: >0.95) . Images were created using the iTOL web server . SlitIRs are in bold and the new SlitIRs identified in this study are in red.

Figure 5. Distribution of chemosensory genes in S. littoralis adults and larvae.

RT-PCRs were performed on male and female adult antennae and caterpillars antennae and maxillary palps. OBP: odorant-binding protein, PBP: pheromone-binding protein, GOBP: general odorant-binding protein, CSP: chemosensory protein, OR: olfactory receptor, IR: ionotropic receptor, GR: gustatory receptor.