The Differential Expression of Mevalonate Pathway Genes in the Gut of the Bark Beetle Dendroctonus rhizophagus (Curculionidae: Scolytinae) Is Unrelated to the de Novo Synthesis of Terpenoid Pheromones

Abstract

Bark beetles commonly produce de novo terpenoid pheromones using precursors synthesized through the mevalonate pathway. This process is regulated by Juvenile Hormone III (JH III). In this work, the expression levels of mevalonate pathway genes were quantified after phloem feeding—to induce the endogenous synthesis of JH III—and after the topical application of a JH III solution. The mevalonate pathway genes from D. rhizophagus were cloned, molecularly characterized, and their expression levels were quantified. Also, the terpenoid compounds produced in the gut were identified and quantified by Gas Chromatography Mass Spectrometry (GC-MS). The feeding treatment produced an evident upregulation, mainly in acetoacetyl-CoA thiolase (AACT), 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS), 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), phosphomevalonate kinase (PMK), and isopentenyl diphosphate isomerase (IPPI) genes, and males reached higher expression levels compared to females. In contrast, the JH III treatment did not present a clear pattern of upregulation in any sex or time. Notably, the genes responsible for the synthesis of frontalin and ipsdienol precursors (geranyl diphosphate synthase/farnesyl diphosphate synthase (GPPS/FPPS) and geranylgeranyl diphosphate synthase (GGPPS)) were not clearly upregulated, nor were these compounds further identified. Furthermore, trans-verbenol and myrtenol were the most abundant compounds in the gut, which are derived from an α-pinene transformation rather than de novo synthesis. Hence, the expression of mevalonate pathway genes in D. rhizophagus gut is not directed to the production of terpenoid pheromones, regardless of their frequent occurrence in the genus Dendroctonus.

Function of mevalonate pathway genes in the synthesis of frontalin in Chinese white pine beetle,Dendroctonus armandi(curculionidae: Scolytinae)

“…Upregulation was observed mainly in the HMGS , HMGR , and PMK genes in fed male intestines of D. armandi . The expression of AACT was upregulated only in female intestines fed for 24 h, which was similar to that in D. rhizophagus ; but no significant change was observed in male intestines, which was different from that in D. rhizophagus (Sarabia et al, 2019). After being stimulated by JHIII for 24−36 h, the expression levels of HMGS , HMGS , and MPDC in male intestines significantly increased, while a little change was observed in female intestines.…”

“…Most genes reached higher expression levels in fed rather than JHIII-stimulated beetles, with solitary and paired males showing the highest levels of almost all genes. This was true for D. rhizophagus also (Sarabia et al, 2019). Nevertheless, although both IPP and DMAPP were essential for the de novo production of frontalin, the expression of GPPS/FPPS and GGPPS genes was different from that in D. ponderosae (Keeling et al, 2013).…”

“…These findings agreed with those of previous reports on the expression of these genes in bark beetles, indicating that MVA pathway genes had higher expression levels in males than in females in the majority of analyzed genes. Some studies reported that JHIII induced the expression of some of these genes in I. pini (Bearfield et al, 2006; Hall et al, 2002; Keeling et al, 2004), I. confuses (Bearfield et al, 2009), I. paraconfusus (Tittiger et al, 1999), D. ponderosae (Aw et al, 2010), D. jeffreyi (Tittiger et al, 2000) and D. rhizophagus (Sarabia et al, 2019), with high expression levels shown mainly in males, or at least higher expression levels compared with those in females (Tittiger et al, 2000; Tittiger et al, 2003).…”

“…Zhao et al (2017) collected frontalin from newly invaded virgin females. This suggests that the tactile stimulus of a gallery, rather than feeding itself, may stimulate pheromone production in females (Sarabia et al, 2019). At the same time of treatment, the amount of frontalin synthesized by fed males was significantly more than that by JHIII‐induced males, and the paired male produced more frontalin compared with the solitary ones.…”

“…Additionally, the shortest feeding duration in this study was 24 h, after which both solo and paired males produced relatively large quantities of frontalin. Interaction between male presence and feeding duration on frontalin production by males may occur such that the effect of female presence would only be dominant at feeding durations less than 24 h (Sarabia et al, 2019). RNAi was successfully used in the present study to reduce the transcript levels of MVA pathway genes significantly.…”