SELECTION OF DNA PROBES USEFUL FOR ISOLATE IDENTIFICATION OF 2 PYTHIUM-SPP

DNA fragments from regions of the mitochondrial chromosome of Pythium oligandrum and P. sylvaticum showing intra- and interspecific variation provided probes with varying levels of isolate and species specificity. In this genus, the mitochondrial DNA (mtDNA) is most commonly a circular molecule composed primarily of an inverted repeat region separated by small and large single-copy unique regions. The small unique region was the most variable portion of the genome for P. oligandrum. A 0.1-kb clone containing sequences entirely from within the inverted repeat region adjacent to the small unique region hybridized strongly to DNA of 10 isolates of P. oligandrum from geographically diverse areas and three of 27 other Pythium spp. A clone that spanned the single-copy, small unique region and also contained some sequences from the inverted repeat region hybridized strongly to P. oligandrum and P. acanthicum. Several clones containing sequences entirely from within the single-copy small unique region were specific for those isolates of P. oligandrum that shared the same restriction map of this region. For P. sylvaticum, a probe with a significant level of isolate specificity was constructed from a portion of the mtDNA associated with an insertion-deletion event in the inverted repeat region.