1 1Laboratory of Forest Zoology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
2 2Laboratory of Forest Zoology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan; Environmental Restoration and Conservation Agency of Japan, Kawasaki, Kanagawa 212-8554, Japa
3 3Laboratory of Forest Zoology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
4 4Kyushu Regional Breeding Office, Forest Tree Breeding Centre, Forestry and Forest Products Research Institute, Koshi, Kumamoto 861-1102, Japan
To determine the likelihood of introgressive hybridisation between Bursaphelenchus xylophilus and B. mucronatus, interbreeding experiments were performed. Of 240 trials involving 24 reciprocal crosses (ten trials per cross, ten virgin females and ten virgin males per trial) between three B. xylophilus isolates and four B. mucronatus isolates under conditions that allowed F1 hybrids to backcross with parents, 174 produced hybrid juveniles. The others failed to generate juveniles. Twenty trials in four crosses successfully established populations which persisted for 65.8-179.5 days, corresponding to 10-40 generations. Analyses of ribosomal DNA (rDNA) fragments for 20 hybrid-derived populations and analysis of inheritance mode of mitochondrial DNA indicated that three crosses produced populations whose members had the B. mucronatus nuclear genome and B. mucronatus or B. xylophilus cytoplasm, whereas the remaining interbreeding produced populations in which 79% of members examined had the B. xylophilus rDNA and cytoplasm and 17% had rDNA fragments of the two species. Populations composed of heterospecific cytonuclear individuals took longer to eat up Botrytis cinerea mycelial mat in a rearing flask than others. Formation of a tail tip mucro in adult females, a characteristic of B. mucronatus, was suggested to be controlled by nuclear and cytoplasmic genes. The results suggested that the introgression of B. xylophilus genes into the B. mucronatus genome was easier than introgression in the opposite direction.