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Hyperparasites Influence Population Structure of the Chestnut Blight Pathogen, Cryphonectria parasitica

First, fourth, and fifth authors: Department of Plant Biology, Michigan State University, East Lansing 48824; first and fifth authors: Program in Ecology, Evolutionary Biology, and Behavior, Michigan State University, East Lansing 48824; second author: Biology Department, University of Wisconsin-La Crosse, La Crosse 54601; and third and fifth authors: Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing 48824.

Vegetative compatibility (VC) is commonly used to characterize structure and diversity in fungal populations. In the chestnut blight fungus, Cryphonectria parasitica, high VC diversity is hypothesized to be responsible for the failure of hyperparasitic mycoviruses to spread through pathogen populations in North America. To test this hypothesis, we assessed VC diversity at three recovering sites in Michigan where mycoviruses had invaded and compared them with four epidemic population sites where mycoviruses were absent. VC diversity was assessed for samples collected in 1996 and 2009, which allowed us to determine how C. parasitica populations changed with time. Twelve VC types were found in 1996 while 29 were found in 2009; 75% of types overlapped between the sample dates. Sites where mycoviruses were present had unique VC structures with the exception of the recovering population site at County Line where the main VC group was also detected at two epidemic sites. With one exception, epidemic sites contained more VC groups and displayed higher population level diversity than recovering sites. Mating-type analyses of blight populations revealed that two of three recovering populations were significantly skewed for MAT2 suggesting asexual reproduction, while epidemic sites with a long history of blight infection had ratios near 50:50 suggesting sexual reproduction. We propose that selection in the largely asexual C. parasitica populations at two recovering sites favors the most-fit fungal genotype by mycovirus combination and results in reduced diversity relative to the sexually reproducing pathogen populations at epidemic sites.

Additional keywords: balancing selection, frequency-dependent selection, mating-type alleles, vegetative incompatibility.