Sommerhalder, Rubik J.*; McDonald, Bruce A.; Zhan, Jiasui. Analysis of the mating type distribution and of the mitochondrial and nuclear genetic structure of the wheat pathogen Stagonospora nodorum indicate the importance of sexual reproduction, gene flow, and absence of selection in shaping the populations genetic structure. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-05.

Molecular analysis of nuclear and mitochondrial DNA can provide deeper and novel insights into the evolutionary forces acting on natural populations of fungal plant pathogens. To test the hypothesis that Stagonospora nodorum undergoes regular cycles of sexual recombination, we analyzed mating type frequency and distribution using PCR amplification of the mating type locus for a total of 1207 isolates sampled from 18 fields in 12 geographical regions in six countries. To investigate the effects of selection, gene flow, and sexual reproduction on the population genetic structure and evolutionary history of the wheat pathogen S. nodorum we compared patterns of mitochondrial restriction fragment length polymorphism (RFLP) diversity with patterns of nuclear RFLP diversity of a total of 315 fungal isolates from Texas, Oregon, and Switzerland. Both mating types were often found on the smallest spatial scales tested, including within the same lesion, the same leaf, and the same 1 m2 plot. In only one case out of the 18 fields tested was there a significant departure from the expected 1:1 ratio.  The joint analysis of nuclear and mitochondrial DNA revealed forty-two different mitochondrial haplotypes and 298 different nuclear haplotypes. Two most frequent mtDNA haplotypes were present in every population and represented 32% of all isolates and high levels of gene flow, low levels of population subdivision, and no evidence of natural selection for local adaptation or host specificity were inferred from the analysis of both genomes. The combination of the results on mating type frequency and distribution with the high genetic variation found in the nuclear and mitochondrial genome and the concordance in estimates of population genetic parameters from both genomes suggest that this pathogen undergoes regular cycles of sexual recombination, has large effective population sizes, and that the two genomes underwent similar evolutionary processes.

Keywords: Stagonospora nodorum, mitochondrial diversity, recombination, gene flow

*Institution: ETH-Zurich

Email: rubiks@agrl.ethz.ch