RIBOSOMAL-RNA SEQUENCE DIVERGENCE WITHIN THE PYTHIACEAE

Partial sequences of 28S RNA genes from 49 strains belonging to 23 spe cies of the Pythiaceae were compared to assess their phylogenetic rela tionships. A high level of diversity was found within Pythium and the eight species studied were classified in four distinct subgroups. In c ontrast to Pythium, Phytophthora appeared to be very homogeneous, with small phylogenetic distances among all 15 species investigated, inclu ding representatives of the six commonly accepted taxonomic groups. Fi nally, we found within Pythium a strict correlation between groups of species defined by molecular taxonomy and groups defined on the basis of a morphological criterion which has been traditionally considered o f secondary importance for classification: the sporangial form.