The longicorn beetle, Monochamus alternatus, is a major vector for the transmission of pine wilt disease, which is caused by a nematode pathogen, Bursaphelenchus xylophilus (or also possibly by B. mucronatus) that is spread by the beetle as it feeds on pine trees. In this study, the mitochondrial genome sequences of four longicorn species (Coleoptera: Cerambycidae: Lamiinae) were determined to further elaborate the phylogenetic relationships of Lamiinae. RT-qPCR was also used to assess the expression of eight mitochondrial protein-coding genes in M. alternatus when carrying B. mucronatus or not, so as to explore the relationship between these two species. The results showed that expression of mitochondria-encoded genes was elevated in M. alternatus beetles that were infected with B. mucronatus, suggesting that B. mucronatus putatively activates an immune response, which significantly affects the metabolic processes of M. alternatus. These results are of significance for further understanding the phylogenetic relationships of longicorn beetles and controlling the spread of pine wilt disease.
We determined the mitochondrial gene sequence of Monochamus alternatus and three other mitogenomes of Lamiinae (Insect: Coleoptera: Cerambycidae) belonging to three genera ( Aulaconotus, Apriona and Paraglenea) to enrich the mitochondrial genome database of Lamiinae and further explore the phylogenetic relationships within the subfamily. Phylogenetic trees of the Lamiinae were built using the Bayesian inference (BI) and maximum likelihood (ML) methods and the monophyly of Monochamus, Anoplophora, and Batocera genera was supported. Anoplophora chinensis, An. glabripennis and Aristobia reticulator were closely related, suggesting they may also be potential vectors for the transmission of the pine wood pathogenic nematode ( Bursaphelenchus xylophilus) in addition to M. alternatus, a well-known vector of pine wilt disease. There is a special symbiotic relationship between M. alternatus and Bursaphelenchus xylophilus. As the native sympatric sibling species of B. xylophilus, B. mucronatus also has a specific relationship that is often overlooked. The analysis of mitochondrial gene expression aimed to explore the effect of B. mucronatus on the energy metabolism of the respiratory chain of M. alternatus adults. Using RT-qPCR, we determined and analyzed the expression of eight mitochondrial protein-coding genes ( COI, COII, COIII, ND1, ND4, ND5, ATP6, and Cty b) between M. alternatus infected by B. mucronatus and M. alternatus without the nematode. Expression of all the eight mitochondrial genes were up-regulated, particularly the ND4 and ND5 gene, which were up-regulated by 4–5-fold ( p < 0.01). Since longicorn beetles have immune responses to nematodes, we believe that their relationship should not be viewed as symbiotic, but classed as parasitic.