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 (tˆ m) were estimated as the proportion of progeny arrays
showing segregation at one or more loci, corrected by the
probability of nondetection of outcrossing (a# ). Estimates of tˆ m
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 tˆ m 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.