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.
Highlights
Dendroctonus bark beetles (Curculionidae: Scolytinae) are key ecological agents of coniferous forests, and some of their species are the most destructive pests in these communities in North and Central America [1,2]
Because frontalin and ipsdienol were not previously identified in the gut of D. rhizophagus [40], we evaluated the potential for the de novo synthesis of frontalin and ipsdienol in pre-emerged adults
The highest similarity percentages of all MVA pathway genes from D. rhizophagus were found with the sequences from D. ponderosae followed by other Dendroctonus species
Summary
Dendroctonus bark beetles (Curculionidae: Scolytinae) are key ecological agents of coniferous forests, and some of their species are the most destructive pests in these communities in North and Central America [1,2] These bark beetles complete their life cycles under the bark of host trees, except for a brief dispersal period during which adults find a new host and mate. Pheromone production in bark beetles, including the Dendroctonus species, is an adaptive strategy favored by natural selection because pheromones provide information about food availability and sources, conspecific attraction, reproductive partners, and regulation of population density [6]. Pheromone production can vary based on insect health, mating system, colonization strategy, species aggressiveness, and geographical region [16,17,18,19]
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