Enzyme Polymorphism and Genetic Differentiation Among Geographic Isolates of the Rice Blast Fungus

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Abstract
Polymorphisms in 12 enzymes were examined among 335 rice isolates of Magnaporthe grisea from 12 rice-growing regions of the world. Six electrophoretic types (ETs) were found. Ninety-five percent of the 335 isolates belonged to two common ETs. All isolates were monomorphic at 16 of the 18 putative enzyme loci. When isolates were pooled as a single population, the proportion of polymorphic loci (P) was 0.11 with an average gene diversity (.hivin.H) of 0.03. In contrast, a sample of 34 nonrice isolates of M. grisea from several gramineous hosts showed more allelic variation (P = 0.5, .hivin.H = 0.21). Among the rice isolate populations, geographic differentiation was observed at the lactate dehydrogenase-1 (Ldh 1) locus, where the distribution frequencies of a functional allele (Ldh1100) and a null allele (Ldh1n) appeared to follow a north-south geographic trend. However, because there was also a strong association between Ldh1100 with japonica rice and Ldh1n with indica rice, it was not certain whether the distribution of the Ldh1 alleles was influenced by climatic factors or by rice geographic races.