Abstract
The phytophagous mirid bugs of Apolygus lucorum, Lygus pratensis as well as three Adelphocoris spp., including Adelphocoris lineolatus, A. suturalis, and A. fasciaticollis are major pests of multiple agricultural crops in China, which have distinct geographical distribution and occurrence ranges. Like many insect species, these bugs heavily rely on olfactory cues to search preferred host plants, thereby investigation on functional co-evolution and divergence of olfactory genes seems to be necessary and is of great interest. In the odorant detection pathway, olfactory receptor co-receptor (Orco) plays critical role in the perception of odors. In this study, we identified the full-length cDNA sequences encoding three putative Orcos (AsutOrco, AfasOrco, and LpraOrco) in bug species of A. suturalis, A. fasciaticollis, and L. pratensis based on homology cloning method. Next, sequence alignment, membrane topology and gene structure analysis showed that these three Orco orthologs together with previously reported AlinOrco and AlucOrco shared high amino acid identities and similar topology structure, but had different gene structure especially at the length and insertion sites of introns. Furthermore, the evolutional estimation on the ratios of non-synonymous to synonymous (Ka/Ks) revealed that Orco genes were under strong purifying selection, but the degrees of variation were significant different between genera. The results of quantitative real-time PCR experiments showed that these five Orco genes had a similar antennae-biased tissue expression pattern. Taking these data together, it is thought that Orco genes in the mirid species could share conserved olfaction roles but had different evolution rates. These findings would lay a foundation to further investigate the molecular mechanisms of evolutionary interactions between mirid bugs and their host plants, which might in turn contribute to the development of pest management strategy for mirid bugs.
Highlights
Due to long-term adoption of transgenic Bt (Bacillus thuringiensis) cotton and the associated reduction in broadspectrum insecticide used for controlling Helicoverpa spp. (Wu et al, 2008), several species of the mirid bugs (Hemiptera: Miridae) including Apolygus lucorum, Lygus pratensis as well as three Adelphocoris spp., including Adelphocoris lineolatus, A. suturalis and A. fasciaticollis have become most important pest species in cotton fields of northern China (Lu et al, 2010)
Among the five plant bug species, two olfactory receptor co-receptor (Orco), AlinOrco from A. lineolatus and AlucOrco from A. lucorum were identified in our previous work (Zhou et al, 2014; Xiao et al, 2016)
A 400 bp fragment encoding putative Orco was amplified from A. fasciaticollis, A. suturalis, FIGURE 2 | Gene structure and intron insertion loci of five Orcos. (A) Location of extrons and introns in different Orco genes. (B) Insertion loci labeled using black triangle of different introns in Orco sequence
Summary
Due to long-term adoption of transgenic Bt (Bacillus thuringiensis) cotton and the associated reduction in broadspectrum insecticide used for controlling Helicoverpa spp. (Wu et al, 2008), several species of the mirid bugs (Hemiptera: Miridae) including Apolygus lucorum, Lygus pratensis as well as three Adelphocoris spp., including Adelphocoris lineolatus, A. suturalis and A. fasciaticollis have become most important pest species in cotton fields of northern China (Lu et al, 2010). Conventional ORs demonstrate low sequence identity, whereas Orco is strikingly well conserved across insect species. Substitution rates and tissuesbiased expression of Orco orthologs from five bug species were investigated to further figure out the evolutionary divergence in different mirid bugs.
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