Candidate chemosensory genes in the Stemborer Sesamia nonagrioides

Abstract

The stemborer Sesamia nonagrioides is an important pest of maize in the Mediterranean Basin. Like other moths, this noctuid uses its chemosensory system to efficiently interact with its environment. However, very little is known on the molecular mechanisms that underlie chemosensation in this species. Here, we used next-generation sequencing (454 and Illumina) on different tissues from adult and larvae, including chemosensory organs and female ovipositors, to describe the chemosensory transcriptome of S. nonagrioides and identify key molecular components of the pheromone production and detection systems. We identified a total of 68 candidate chemosensory genes in this species, including 31 candidate binding-proteins and 23 chemosensory receptors. In particular, we retrieved the three co-receptors Orco, IR25a and IR8a necessary for chemosensory receptor functioning. Focusing on the pheromonal communication system, we identified a new pheromone-binding protein in this species, four candidate pheromone receptors and 12 carboxylesterases as candidate acetate degrading enzymes. In addition, we identified enzymes putatively involved in S. nonagrioides pheromone biosynthesis, including a ∆11-desaturase and different acetyltransferases and reductases. RNAseq analyses and RT-PCR were combined to profile gene expression in different tissues. This study constitutes the first large scale description of chemosensory genes in S. nonagrioides.

Keywords: Sesamia nonagrioides.; chemosensory receptors; next-generation sequencing; pheromone biosynthesis; transcriptome.

Figure 1

Distribution of S. nonagrioides contigs annotated at GO level 2.

Figure 2

SnonOBP and CSP sequence logos. Degree of amino acid sequence conservation along the primary sequence axis of odorant-binding proteins (OBPs) and the chemosensory proteins (CSPs) of S. nonagrioides. Depicted amino acid character size correlates to relative conservation across aligned sequences. Green asterisks indicate the conserved six and four cysteine motifs of OBPs and CSPs, respectively.

Figure 3

Maximum likelihood tree of candidate odorant-binding proteins (OBPs) from S. nonagrioides and other Lepidoptera. Sequences used were from B. mori , S. littoralis , , , H. melpomene , H. virescens , , and M. sexta , . 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 . The SnonOBPs identified in this study are in red.

Figure 4

Maximum likelihood tree of candidate ORs from S. nonagrioides and other Lepidoptera. Sequences used were from B. mori , S. littoralis , , , 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 . The SnonORs identified in this study are in red.

Figure 5

RT-PCRs of S. nonagrioides OR transcripts (SnonORs) in male and female antennae.

Figure 6

Maximum likelihood tree of candidate ionotropic receptors (IRs) from S. nonagrioides and other insects. Sequences used were from B. mori , S. littoralis , , , 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.