Variation in the ribosomal DNA repeat unit within single-oospore isolates of the genus Pythium

Abstract

Restriction analyses of the DNA repeat unit coding for the production of 5.8S, 17S, and 26S ribosomal RNA (rDNA) isolated from several different Pythium species revealed polymorphic forms within a single-oospore isolate. Restriction mapping of cloned rDNA from Pythium paroecandrum indicated that two major forms were present in approximately equal ratios (A and B, 11.6 and 11.9 kb in size, respectively), which differed in the nontranscribed spacer region for numbers and locations of restriction sites for HaeIII, SalI, and KpnI and an insertion–deletion adjacent to the 3′ end of the 26S RNA coding region. The insertion–deletion makes form B 0.27 kb larger than form A. An additional insertion–deletion in both forms of approximately 60 bp in the same region makes a total of four polymorphic types in the isolate investigated. Similar results also were obtained for Pythium spinosum following restriction digestion of purified rDNA. A survey of other Pythium species indicates that the presence of polymorphic forms in the same isolate, variation in the number or polymorphic forms in different isolates of the same species, and insertions–deletions in multiples of approximately 60 bp adjacent to the 3′ end of the 26S coding region are common in the genus.Key words: Pythium spp., rDNA.