Variations in the Phytophthora infestans population in Nepal as revealed by nuclear and mitochondrial DNA polymorphisms

Phytophthora infestans isolates collected from potato and tomato crops from various parts of Nepal during the 1999 and 2000 crop seasons were characterized for nuclear and mitochondrial DNA polymorphisms using restriction fragment length polymorphism markers. The nuclear DNA probe RG57 detected 11 multilocus genotypes among 280 isolates. Three genotypes were detected 21 times or more, constituting 94% of the total population, whereas frequencies of other genotypes ranged from 0.004 to 0.014. The overall genotypic diversity as estimated by the Gleason index was 1.78. Most of the overall diversity was present at the highest level (i.e., interregional, 46%), indicating limited gene flow among regions. Cluster analysis of multilocus genotypes derived from RG57 and mating type data for Nepalese isolates and representative isolates worldwide showed Nepalese isolates grouping into four clusters. Characterization of 67 isolates for mitochondrial DNA polymorphisms revealed the presence of two mt-haplotypes, la and Ib with the proportions of 0.88 and 0.12, respectively. Polymorphisms in nuclear and mitochondrial DNA revealed a moderate level of diversity in this population. Genotype NP3 had an identical RG57 fingerprint to US1 and had mt-haplotype Ib, confirming the presence of an old population in Nepal. Most of the genotypes had a different RG57 fingerprint than that of US1 and mt-haplotype Ia, the common characteristics of new populations. The presence of a new population at high proportions in Nepal was consistent with the global trend of mt-haplotype distribution, and suggests the displacement of old populations. This study indicates at least three possible introductions of P. infestans to Nepal.