Temperature effects on interspecific hybridisation and growth of hybrid-derived populations between Bursaphelenchus xylophilus and B. mucronatus

Summary Bursaphelenchus xylophilus is the causative agent of pine wilt disease and is indigenous to North America. It has been introduced into Japan where the non-pathogenic close relative, B. mucronatus , is widely distributed. Both nematode species have similar life cycles with vectors of cerambycid beetles of the genus Monochamus . When interspecific hybrids are backcrossed with B. xylophilus nematodes repeatedly, a small proportion of B. mucronatus genes is infused into the B. xylophilus genome (introgression) and may lead to adaptation to locally harsh environments. Though cool temperatures had inhibited the development of pine wilt disease, B. xylophilus started expanding the range to cool-summer areas after suspending its range expansion for 10 years in Japan. To consider if the range expansion is related to the introgression hybridisation between the two Bursaphelenchus species, some traits of newly and already formed hybrid-derived populations were compared between three temperature conditions of 16, 20 and 25°C. Formation of F1 hybrids was not affected by temperature. There was no difference in the time required for hybrid formation and for population establishment at 16°C between an interspecific crossing and two intraspecific crossings of B. xylophilus . Using three hybrid-derived populations reared over multiple generations at 25°C, there was no difference in the early rate of population growth between each of two hybrid-derived populations and its parental-lineage population of B. xylophilus at 16°C. Thus, the results did not reject a hypothesis that introgression hybridisation may have enhanced the fitness components such as pathogenicity of B. xylophilus populations at cool temperatures.