Mixed mating in natural populations of the chestnut blight fungus, Cryphonectria parasitica

As in plants, fungi exhibit wide variation in reproductive strategies and mating systems. Although most sexually reproducing fungi are either predominantly outcrossing or predominantly selfing, there are some notable exceptions. The haploid, ascomycete chestnut blight pathogen, Cryphonectria parasitica, has previously been shown to have a mixed mating system in one population in USA. In this report, we show that both selfing and outcrossing occur in 10 additional populations of C. parasitica sampled from Japan, Italy, Switzerland and USA. Progeny arrays from each population were assayed for segregation at vegetative incompatibility (vic) and DNA fingerprinting loci. Outcrossing rates (t̂m) were estimated as the proportion of progeny arrays showing segregation at one or more loci, corrected by the probability of nondetection of outcrossing (α̂). Estimates of t̂m varied from 0.74 to 0.97, with the lowest rates consistently detected in USA populations (0.74–0.78). Five populations (four in USA and one in Italy) had t̂m significantly less than 1, supporting the conclusion that these populations exhibit mixed mating. The underlying causes of variation in outcrossing rates among populations of C. parasitica are not known, but we speculate that – as in plants – outcrossing is a function of ecological, demographic and genetic factors.