VIEW ARTICLE

Genetics

Evidence for Mutation and Migration as Sources of Genetic Variation in Populations of Rhynchosporium secalis. Stephen B. Goodwin, Department of Genetics, University of California, Davis 95616, Present address: Department of Plant Pathology, 334 Plant Science, Cornell University, Ithaca, NY 14853; Robert K. Webster(2), and R. W. Allard(3). (2)Department of Plant Pathology, University of California, Davis 95616; (3)Department of Genetics, University of California, Davis 95616; Present address: P.O. Box 185, Bodega Bay, CA 94923. Phytopathology 84:1047-1053. Accepted for publication 25 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1047.

Four isolates of the barley scald disease pathogen, Rhynchosporium secalis, that differed both for pathogenicity to 14 barley differential cultivars and for isozyme genotype were coinoculated onto barley plants in the field and the greenhouse to test for mutation, migration, and parasexual recombination. Ten isozyme genotypes and 37 pathotypes were identified among 96 isolates recovered from 10 field plots. Most (62%) of the isolates recovered in the field experiment had an isozyme genotype like that of one of the four inoculated isolates. Among the remaining isolates were six different isozyme genotypes, two of which were probably introduced by migration; the other four could have arisen by mutation, migration, or parasexual recombination. Most of the new pathotypes probably arose by mutation or migration. Fifteen single-spore derivatives of the original isolates had changes for pathogenicity, which were almost certainly the result of mutation, to one or more of the differential cultivars, but there were no changes in isozyme genotype among single-spore derivatives. The four original isolates were also inoculated pairwise onto a susceptible barley cultivar in the greenhouse. No isozyme recombinants were found among 240 isolates recovered after two asexual generations; three greenhouse isolates were definitely migrants; one was a mixed culture of parental isozyme genotypes; and the remaining 236 were parental types. Among more than 1,000 isolates analyzed in this and two other studies, no three-banded (heterodimeric) isozyme phenotypes (indicating heterozygotes at intermediate stages in the parasexual cycle) were found. Taken together, these results indicate that parasexual recombination, if it occurs at all, probably plays at most a small role in generating genetic variability in R. secalis populations in nature. However, there is clear evidence that mutation and migration are very important, and the mutation rate for pathogenicity appears to be very much higher than that for isozymes.