The genetic underlayment of pheromone biosynthesis in pest spruce bark beetles
The potential of spruce bark beetles Ips typographus and Ips duplicatus to colonize living host trees depends on massive aggregation behaviour mediated by male-produced aggregation pheromone produced in hindguts and consists of terpenoid compounds. Disrupting pheromone biosynthesis by targeting genes is the sound scientific basis for gene drive-based approaches, responsible for the biosynthesis of key aggregation pheromones (Chiu et al, 2018) and could be an encouraging appeal for I. typographus control in beetle management (Ramakrishnan et al, 2022).
Both species produced 2-methyl-3-buten-2-ol (MEBU) and ipsdienol. Ips duplicatus has another active compound E-myrcenol and Ips typographus possesses cis-verbenol (cV) (Lanne et al, 1989). While MEBU, ipsdienol and presumably E-myrcenol are synthesized in male hindguts de novo via the so-called mevalonate pathway, cV is produced by hydroxylation of α-pinene sequestrated from the host during feeding (Schlyter et al, 1997). The genetic underlayment of the pheromone biosynthesis in Ips typographus was lately reported (Ramakrishnan et al, 2022). Based on the differential gene expression (DGE) in hindgut tissues from different developmental stages of beetles revealed candidate genes putatively involved in the MEBU and cV biosynthesis. Thus, were selected mevalonate pathway genes catalyzing the production of MEBU, including candidates of novel hemiterpene-synthesizing isoprenyl-di-phosphate synthase (IPDS) Ityp0927. As well as candidates for terminal cytochrome P450 genes which are supposed to be involved in cis/trans-verbenol synthesis. The biosynthesis of ipsdienol — another pheromonal component of both targeted Ips species, was studied in sibling bark beetle Ips pini (Say) (Coleoptera: Scolytidae) (Sandstorm et al, 2006). The intermediate produced by beetles de novo in the mevalonate pathway is myrcene (Song et al, 2013). Further steps involve hydroxylation of myrcene to ipsdienol by myrcene hydroxylase encoded by gene CytochromeP450 9T2. The Cytochrome P450 (P450s) genes play various roles in the metabolism of Ips species. They are known for example for the detoxification of tree-formulated defense terpenes by beetles or they oxidized odorants at antennae (Blomquist et al, 2021). Therefore, selecting and studying the specific members of the CYP450 family involved in the pheromone biosynthesis is challenging.
Pheromone biosynthesis is not yet studied in Ips duplicatus at the genetic level. The previous study has only indicated the synthesis of ipsdienol and E-myrcenol de novo with the utilization of enzyme inhibitor compactin which obstructs the activity of the enzyme hydroxy methylglutaryl-coenzyme A reductase (HMG-CoA). This enzyme catalyzes the synthesis of mevalonate, which is a pheromone precursor, and when inhibited by compactin, the pheromone production decreases (Ivarsson et al, 1993). Since the ratio of I. duplicatus pheromone components Ipsdienol: E-myrcenol varies in different populations, there is not exactly known its composition in the local population (Byers, et al 1990). For example, the I. duplicatus population from the Inner Mongolian showed a twice higher ratio of these compounds than those reported from Scandinavia, relating well with the dissimilarities in the optimal response ratios, analyzed by the electrophysiological and behavioral responses of the beetles (Zhang et al, 2007).
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