Pathogenicity and boarding ability of hybrid-derived populations between Bursaphelenchus xylophilus and B. mucronatus (Nematoda: Aphelenchoididae)

Summary The pine wood nematode, Bursaphelenchus xylophilus (Steiner et Buhrer), is the causative agent of pine wilt disease. It is native to North America and has been devastating pine forests in East Asia and West Europe since its introduction. The pathogenic nematode is transmitted by cerambycid beetle adults of Monochamus alternatus Hope in East Asia and M. galloprovincialis (Olivier) in West Europe. Bursaphelenchus mucronatus Mamiya et Enda is widely distributed in Eurasia and has no or little pathogenicity to pine species. If hybrids between the two species are backcrossed with B. xylophilus nematodes repeatedly, a small proportion of B. mucronatus genes will be infused into B. xylophilus genome (introgression) and may lead to adaptation to locally harsh environments. Inoculation tests in this study revealed that two of eight hybrid-derived populations from crossing B. xylophilus and B. mucronatus caused pine seedling mortalities of 36-70%. The pathogenic hybrid-derived populations carried nuclear ribosomal DNA (rDNA) of two species and B. xylophilus extra-nuclear genes, whereas non-pathogenic populations carried B. mucronatus rDNA and extranuclear genes of B. xylophilus or B. mucronatus. Thus, it is inferred that a gene or genes associated with pathogenicity are present in the nuclear genome of B. xylophilus. A laboratory experiment revealed that the pathogenic hybrid-derived population had as great an ability to board M. alternatus adults as the parental-lineage B. xylophilus isolate. Thus, the introgressive hybridisation is considered to occur in pine forests if two species cohabit in host trees.