Genetic variation among populations of Pythium irregulare in southern Australia

Isolates of Pythium irregulare were sampled from seven cereal crops through out South Australia to determine the extent of genetic diversity within thi s pathogen and the scale of genetic differentiation among populations. Data derived from 29 individual restriction fragment length polymorphism (RFLP) loci differentiated 54 DNA fingerprints among the 92 isolates analysed. So me isolates had two alleles at several RFLP loci and were scored as heteroz ygous. One such isolate was selfed in vitro and segregation ratios in the p rogeny were not significantly different from those expected for allelic var iation in a diploid. These data provided evidence that outcrossing occurs w ithin P. irregulare and may contribute to the high level of genetic variati on within the species (D-T = 0.502). Allelic frequencies were significantly different among all seven populations and G(ST) values showed significant genetic differentiation between populations. The average genetic identity a mong populations was low and hierarchical cluster analysis provided no clea r evidence that populations formed geographically related groups. These ana lyses indicate low levels of interpopulation gene flow within P. irregulare and imply that population differentiation results from genetic drift.