PINEWOOD NEMATODE SPECIES COMPLEX - INTERBREEDING POTENTIAL AND CHROMOSOME-NUMBER

Interbreeding potential, chromosome number, and host range were compar ed among several isolates and species of Bursaphelenchus from diverse geographic areas. Some isolates from North America, Japan, and France had a wide-ranging interbreeding potential, whereas others were restri cted in their potential to hybridize with other isolates. Although int erbreeding occurred in the laboratory between some ''M'' and ''R'' for ms of B. xylophilus, interbreeding of B. xylophilus and B. mucronatus was rare. The hybrids had the pathogenicity of the parent with the bro ader host range. This fact suggests that virulence may be inherited as a dominant character or that increased virulence may have resulted fr om differences in hybrid vigor. The haploid chromosome number of the d ifferent isolates separated the isolates into three groups and disting uished B. xylophilus from B. mucronatus. The findings suggest that the pinewood nematode species complex consists of sibling species that ha ve evolved by reproductive isolation, that the French isolate is a new species, and that B. xylophilus and B. mucronatus have evolved from a common ancestor.