PHYTOPHTHORA-SOJAE RACES HAVE ARISEN BY CLONAL EVOLUTION AND BY RARE OUTCROSSES

An extensive set of nuclear and mitochondrial restriction fragment len gth polymorphisms (RFLPs) was used to examine the genetic relationship s among 48 isolates of Phytophthora sojae, an oomycete pathogen of soy bean, This organism is diploid and homothallic. The isolates examined encompassed 25 physiological races of the pathogen, including races 36 and 37 which we describe here for the first time. The results reveal a moderate degree of diversity within the species; about 18% of all de tected nuclear restriction fragments were polymorphic in at least one isolate. One group of isolates, representing seven physiological races , had RFLPs nearly identical to isolates of the first described race, race 1, and probably arose from race 1 isolates clonally by mutation. There was much more genetic variation among the remaining isolates. Th e distribution of RFLP alleles among these isolates suggests most of t he genetic variation in the species is found in four genotypes (progen itor lines) represented by isolates P1658 (race 1), P7064 (race 7), P7 074 (race 17), and P7076 (race 19), respectively. All the other isolat es appear to have been produced by rare outcrosses between representat ives of these four genotypes. The distribution of avirulence phenotype s against Rps genes (soybean resistance genes against P. sojae) is con sistent with the reassortment of single avirulence genes as a result o f the same outcrosses. Therefore it is proposed that occasional outcro sses have been a major contributor to the origin of new physiological races of P. sojae, in addition to clonal evolution. In concordance wit h these mechanisms, no correlation was observed between particular RFL Ps and race types.Thus it will be very difficult to use RFLP or RAPD m arkers to directly identify race types of new field isolates unless th e markers are derived directly from avirulence genes.