Gut transcriptome of two bark beetle species stimulated with the same kairomones reveals molecular differences in detoxification pathways

Abstract

Dendroctonus bark beetles are the most destructive agents in coniferous forests. These beetles come into contact with the toxic compounds of their host's chemical defenses throughout their life cycle, some of which are also used by the insects as kairomones to select their host trees during the colonization process. However, little is known about the molecular mechanisms by which the insects counteract the toxicity of these compounds. Here, two sibling species of bark beetles, D. valens and D. rhizophagus, were stimulated with vapors of a blend of their main kairomones (α-pinene, β-pinene and 3-carene), in order to compare the transcriptional response of their gut. A total of 48 180 unigenes were identified in D. valens and 43 704 in D. rhizophagus, in response to kairomones blend. The analysis of differential gene expression showed a transcriptional response in D. valens (739 unigenes, 0.58-10.36 Log2FC) related to digestive process and in D. rhizophagus (322 unigenes 0.87-13.08 Log2FC) related to xenobiotics metabolism. The expression profiles of detoxification genes mainly evidenced the up-regulation of COEs and GSTs in D. valens, and the up-regulation of P450s in D. rhizophagus. Results suggest that terpenes metabolism comes accompanied by an integral hormetic response, result of compensatory mechanisms, including the activation of other metabolic pathways, to ensure the supply of energy and the survival of organisms which is specific for each species, according to its life history and ecological strategy.

Keywords: Carboxylesterase; Cytochrome P450 monooxygenases; Dendroctonus rhizophagus; Dendroctonus valens; Detoxification process; Glutathione S-transferase.

Graphical abstract

Fig. 1

D. valens (dark colors) and D. rhizophagus (light colors) unigenes functional annotation. A) GO clasification. The GO terms were classified into of cellular component, molecular function, and biological process. B) KOG classification in four Macrogroups and 25 categories. Cellular processes and signaling: D, cell cycle control cell division, chromosome partitioning; M, cell wall/membrane/envelope biogenesis; N, cell motility; O, posttranslational modification, protein turnover, chaperones; T, signal transduction mechanisms; U, intracellular trafficking, secretion, and vesicular transport; V, defense mechanisms; W, extracelular structures; Y, nuclear structure; Z, cytoskeleton. Poorly characterized: R, general function prediction only; S, function unknown. Infotmation Storage and Processing: A, RNA processing and modification; B, chromatin structure and dynamics; J, translation, ribosomal structure and biogenesis; K, transcription; L, replication, recombination and repair. Metabolisms: C, energy production and conversion; E, amino acid transport and metabolism; F, nucleotide transport and metabolism; G, carbohydrate transport and metabolism; H, coenzyme transport and metabolism; I, lipid transport and metabolism; P, inorganic ion transport and metabolism; Q, secondary metabolites biosynthesis, transport and catabolism. C) KEGG classification in five major metabolic categories: M, metabolism; G, genetic information processing; E, environmental information processing; C, cellular processes; O, organismal systems.

Fig. 2

Main GO terms and KEGG pathway enriched in response to the blend kairomone. (a) Ration of GO terms in D. valens. (b) Ration of GO terms in D. rhizophagus. (c) Ration of KEGG pathways in D. valens. (d) Ration of KEGG pathways in D. rhizophagus. The color scale represent enrichment significance p < 0.05.

Fig. 3

Maximum likelihood phylogenetic tree based of P450s from Dendroctonus valens (Dval) and Dendroctonus rhizophagus (Drhi). The analysis included P450s from bark beetles D. frontalis (Dfro), D. armandi (Darm) and D. ponderosae (Dpon), as well as those from Anoplophra glabripennis (Agla), and Leptinotarsa decemlineata (Ldec). The amino acid evolution model was LG + G (- lnL = 229583.22, G = 1.646, AIC = 463465.501). Branch support was calculated with the approximate likelihood ratio test (aLRT). Support values ≥ 80% are indicated on the branches with black point.

Fig. 4

Maximum likelihood phylogenetic tree based of COEs from Dendroctonus valens (Dval) and Dendroctonus rhizophagus (Drhi). The analysis included COEs from bark beetles D. frontalis (Dfro), D. armandi (Darm) and D. ponderosae (Dpon), as well as those from Anoplophra glabripennis (Agla), and Leptinotarsa decemlineata (Ldec). The amino acid evolution model was VT + G (- lnL = 220061.043, G = 1.192, AIC = 443945.261). Branch support was calculated with the approximate likelihood ratio test (aLRT). Support values ≥ 80% are indicated on the branches with black point.

Fig. 5

Maximum likelihood phylogenetic tree based of GSTs from Dendroctonus valens (Dval) and Dendroctonus rhizophagus (Drhi). The analysis included GSTs from bark beetles D. frontalis (Dfro), D. armandi (Darm) and D. ponderosae (Dpon), as well as those from Anoplophra glabripennis (Agla), and Leptinotarsa decemlineata (Ldec). The amino acid evolution model was VT + G + I (- lnL = 220061.043, G = 1.800, I = 0.003, AIC = 41038.861). Branch support was calculated with the approximate likelihood ratio test (aLRT). Support values ≥ 80% are indicated on the branches with black point.

Fig. 6

Expression profile of detoxification genes from D. valens and D. rhizophagus based on TPM values. Genes expression levels were indicated with scale of color using a mean TPM value of two replicates.