ISOZYME VARIATION WITHIN AND AMONG POPULATIONS OF RHYNCHOSPORIUM-SECALIS IN EUROPE, AUSTRALIA AND THE UNITED-STATES

Isolates of Rhynchosporium secalis from diverse geographical origins in Australia, Europe, Japan and the United States were scored for genetic variability at eight isozyme loci. There were 21 putative alleles at the eight loci, and seven loci were polymorphic (frequency of the most common allele less than 0.99) in at least one of the 12 locations sampled. Allelic diversities within populations were generally high compared to most fungi, indicating that founder effects and genetic drift have not caused an extreme loss of genetic variation in most R. secalis populations. Population subdivision analysis revealed a moderate level of genetic differentiation among regions (G(ST) = 0.16), although genetic differentiation among locations within regions was low (G(ST) from 0.0 to 0.10). There was no genetic differentiation between isolates from Hordeum vulgare and H. leporinum in Australia (G(ST) = 0.0), and isolates from other wild grass hosts had the same alleles as R. secalis from cultivated barley. Thus there was no genetic evidence for distinct R. secalis populations on different host species. There were 34 multilocus electrophoretic genotypes among the 150 isolates tested, and from 1 to 15 genotypes in each location sampled. Although the sample sizes were limited, these results are consistent with migration of R. secalis from a common centre of origin, and either recent specialization or continuing gene flow among R. secalis populations on different hosts.