Polymorphisms in the nuclear DNA of Achlya species: some taxonomic implications

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Abstract
Within the genus Achyla, which belongs to the class of fungi known as the Oomycetes, taxonomic judgments have traditionally been made using a variety of sexual criteria. We have used restriction fragment length polymorphisms as a new taxonomic character to examine intra- and inter-specific variation within this genus. Using a cDNA clone coding for the Achlya 18S rRNA gene as a hybridization probe, a 10-kb fragment of ribosomal DNA (rDNA) from Achlya ambisexualis strain E87 was cloned and then mapped for selected restriction enzyme sites. In Southern blot hybridizations, both this rDNA fragment and cloned 18S cDNAs revealed differences in the rDNA organization of A. ambisexualis E87 (male) and a female isolate of A. ambisexualis strain 734. No differences in the rDNAs were detected between the two heterothallic isolates A. ambisexualis E87 and A. bisexualis 65-1. Southern blot hybridizations suggested that two different rDNA organizations may exist within the genome of the homothallic strain A. heterosexualis B14. cDNA clones coding for two different hormonally regulated genes revealed the same relationships between the four isolates studied as those determined with rDNA probes. Two homothallic Achlya strains recently isolated from nature were found to have additional DNA polymorphisms not detected in the laboratory strains. Phenetic analysis distinguished the same similarities that were evident upon inspection of the hybridization data. Taken together, these data suggest different relationships between the isolates examined than do the previous taxonomic criteria by which species have been delimited within this genus.Key words: Achlya; restriction fragment length polymorphisms; ribosomal DNA; taxonomy.