Multiple CRISPR/Cas9 modifications of an esterase reveal its role in influencing acetate esters in the pheromone blend of a moth

Abstract

Sexual signalling by pheromones is essential for mate finding and mate choice in moths and plays an important role in reproductive isolation. Acetates (i.e., acetate esters) produced by females of Heliothis (Chloridea) subflexa Fabricius, 1777 (Lepidoptera: Noctuidae) attract conspecific males but repel Heliothis virescens Fabricius, 1777 (Lepidoptera: Noctuidae) males. A QTL (quantitative trait locus) harbouring carboxylesterases and lipases was previously shown to affect acetates, and CRISPR/Cas9-induced knockouts increased acetate amounts by blocking hydrolysis of the esters as expected. A second, unlinked QTL, containing a cluster of three different carboxylesterases (CXEs), unexpectedly yielded decreased acetate amounts. In one of these genes, esterase CXE24, we found a naturally occurring transposable element insertion in exon 8. A CRISPR/Cas9-induced frameshift at the same position yielded the same results. The paradox was resolved by a CRISPR/Cas9-induced frameshift in exon 2 of CXE24 which increased acetate amounts. The frameshift in exon 2 produced a truncated protein lacking the substrate binding site and the catalytic triad, while the frameshift in exon 8 removed only the third residue of the catalytic triad. In silico modelling showed that the exon-8-truncated protein could not hydrolyse the esters by itself, which likely explains the decreased acetate amounts. To place our findings in an evolutionary context, we explored variation in the esterase cluster in 16 species of Lepidoptera with completely sequenced genomes. Geographic and temporal variation in acetates has been observed in H. subflexa, and variation in the frequency of the transposable element could be a possible explanation.