Analysis of the Agrotis segetum pheromone gland transcriptome in the light of sex pheromone biosynthesis

Background: Moths rely heavily on pheromone communication for mate finding. The pheromone components of most moths are modified from the products of normal fatty acid metabolism by a set of tissue-specific enzymes. The turnip moth, Agrotis segetum uses a series of homologous fatty-alcohol acetate esters ((Z)-5-decenyl, (Z)-7-dodecenyl, and (Z)-9 tetradecenyl acetate) as its sex pheromone components. The ratio of the components differs between populations, making this species an interesting subject for studies of the enzymes involved in the biosynthetic pathway and their influence on sex pheromone variation.
Results: Illumina sequencing and comparative analysis of the transcriptomes of the pheromone gland and
abdominal epidermal tissue, enabled us to identify genes coding for putative key enzymes involved in the
pheromone biosynthetic pathway, such as fatty acid synthase, β-oxidation enzymes, fatty-acyl desaturases (FAD), fatty-acyl reductases (FAR), and acetyltransferases. We functionally assayed the previously identified Δ11-desaturase
[GenBank: ES583599, JX679209] and FAR [GenBank: JX679210] and candidate acetyltransferases (34 genes) by
heterologous expression in yeast. The functional assay confirmed that the Δ11-desaturase interacts with palmitate and produces (Z)-11-hexadecenoate, which is the common unsaturated precursor of three homologous pheromone component acetates produced by subsequent chain-shortening, reduction and acetylation. Much lower, but still visible, activity on 14C and 12C saturated acids may account for minor pheromone compounds previously observed in
the pheromone gland. The FAR characterized can operate on various unsaturated fatty acids that are the immediate acyl precursors of the different A. segetum pheromone components. None of the putative acetyltransferases that we expressed heterologously did acetylate any of the fatty alcohols tested as substrates.
Conclusions: The massive sequencing technology generates enormous amounts of candidate genes potentially
involved in pheromone biosynthesis but testing their function by heterologous expression or gene silencing is a
bottleneck. We confirmed the function of a previously identified desaturase gene and a fatty-acyl reductase gene
by heterologous expression, but the acetyltransferase postulated to be involved in pheromone biosynthesis remains
illusive, in spite of 34 candidates being assayed. We also generated lists of gene candidates that may be useful for
characterizing the acetyl-CoA carboxylase, fatty acid synthetase and β-oxidation enzymes.