Abstract
BackgroundOne of the challenges in insect chemical ecology is to understand how insect pheromones are synthesised, detected and degraded. Genome wide survey by comparative sequencing and gene specific expression profiling provide rich resources for this challenge. A. ipsilon is a destructive pest of many crops and further characterization of the genes involved in pheromone biosynthesis and transport could offer potential targets for disruption of their chemical communication and for crop protection.ResultsHere we report 454 next-generation sequencing of the A. ipsilon pheromone gland transcriptome, identification and expression profiling of genes putatively involved in pheromone production, transport and degradation. A total of 23473 unigenes were obtained from the transcriptome analysis, 86% of which were A. ipsilon specific. 42 transcripts encoded enzymes putatively involved in pheromone biosynthesis, of which 15 were specifically, or mainly, expressed in the pheromone glands at 5 to 120-fold higher levels than in the body. Two transcripts encoding for a fatty acid synthase and a desaturase were highly abundant in the transcriptome and expressed more than 40-fold higher in the glands than in the body. The transcripts encoding for 2 acetyl-CoA carboxylases, 1 fatty acid synthase, 2 desaturases, 3 acyl-CoA reductases, 2 alcohol oxidases, 2 aldehyde reductases and 3 acetyltransferases were expressed at a significantly higher level in the pheromone glands than in the body. 17 esterase transcripts were not gland-specific and 7 of these were expressed highly in the antennae. Seven transcripts encoding odorant binding proteins (OBPs) and 8 encoding chemosensory proteins (CSPs) were identified. Two CSP transcripts (AipsCSP2, AipsCSP8) were highly abundant in the pheromone gland transcriptome and this was confirmed by qRT-PCR. One OBP (AipsOBP6) were pheromone gland-enriched and three OBPs (AipsOBP1, AipsOBP2 and AipsOBP4) were antennal-enriched. Based on these studies we proposed possible A. ipsilon biosynthesis pathways for major and minor sex pheromone components.ConclusionsOur study identified genes potentially involved in sex pheromone biosynthesis and transport in A. ipsilon. The identified genes are likely to play essential roles in sex pheromone production, transport and degradation and could serve as targets to interfere with pheromone release. The identification of highly expressed CSPs and OBPs in the pheromone gland suggests that they may play a role in the binding, transport and release of sex pheromones during sex pheromone production in A. ipsilon and other Lepidoptera insects.
Highlights
One of the challenges in insect chemical ecology is to understand how insect pheromones are synthesised, detected and degraded
We identified 17 transcripts predicted to encode esterases in the A. ipsilon pheromone gland, and the BLASTx results showed that all have very high amino acid identities with the antennal esterases of S. littoralis (Table 4), we named them as AipsCXE1-AipsCXE16 and AipsCXE20 following the nomenclature in S. littoralis
The black cutworm A. ipsilon is a destructive pest of many crops [66,67] and mainly controlled by chemical pesticides, which has led to the development of resistance to various compounds [68]
Summary
One of the challenges in insect chemical ecology is to understand how insect pheromones are synthesised, detected and degraded. Most sex pheromone blends of Lepidoptera insects are synthesised de novo via modified fatty acid biosynthesis pathways [2,6,7] and gland-specific enzymes are involved in desaturation, chain shortening, reduction and acetylation [1,2]. Once unsaturated pheromone precursor with a specific chain-length is produced, the carboxyl carbon is modified to form one of functional groups (aldehyde, alcohol or acetate ester) These modifications require the enzymes fatty acid reductase to produce the alcohols from the fatty acyl precursor [22], which in some species may be oxidized to aldehydes serving as pheromone components [23], and to acetate esters (OAc) by acetyltransferase [24]. Using RNA interference, Matsumoto and colleagues showed that two pheromone gland-specific enzymes (acyl-CoA desaturase and a fatty-acyl reductase) are responsible for pheromone production in the silk moth Bombyx mori [29,30,31]
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