A natural intra-specific hybridization between populations of B. mucronatus with European and East Asian genotypes, in pine forests

Bursaphelenchus mucronatus Mamiya & Enda, 1979 is a wood nematode widely distributed over Palearctic coniferous forests. It has two subspecies, that is, B. mucronatus mucronatus and B. mucronatus kolymensis, which present molecularly different East Asian and European genotypes, respectively. The European subspecies is found mainly in Europe and Siberia, while the East Asian subspecies occupies mostly Eastern regions of Asia. However, local isolates of both subspecies have been occasionally reported from various localities in Europe and Asia. Our field isolation and molecular (DNA ITS-RFLP) identification of 21 recently collected populations of B. mucronatus revealed only 4 isolates representing clearly East Asian or European genotypes, while the remaining 17 isolates showed intermediate genotypes with electrophoretic band characters of both the above types. Further individual crossbreeding, in vitro, of nematodes representing European, East Asian, and intermediate genotypes provided experimental evidence for the process of intraspecific hybridization between both subspecies, spontaneously taking place in the forest. Such a widening of the B. mucronatus genetic variation may have its effect on a range of reported in the literature direct interactions between this native, nonpathogenic nematode species and the genetically similar, causative agent of the pine wilt disease B. xylophilus which can colonize the same host trees, use the same insect vectors, compete, and crossbreed inter-specifically.