ESTIMATION OF THE OUTCROSSING RATE IN THE CHESTNUT BLIGHT FUNGUS, CRYPHONECTRIA-PARASITICA

The outcrossing rate in the chestnut blight fungus, Cryphonectria (Endothia) parasitica, was estimated in a natural population using data from six unlinked restriction fragment length polymorphism (RFLP) loci and DNA fingerprinting. This fungus was shown to have a mixed mating system with both self-fertilization and outcrossing in the same population. In a sample of 22 perithecia, all progeny segregated in 1:0, 1:1 or 0:1 ratios at RFLP loci; there was no evidence for multiple male parents in ascospore families from single perithecia. The multilocus estimate of the outcrossing rate was 0.74 for the RFLP data; the proportion of perithecia with progeny segregating for fingerprint fragments was 0.73. Vegetative compatibility was not a sensitive marker for detecting outcrossing as only 11 perithecia (50 per cent) showed segregation with this marker, compared with 16 (73 per cent) for DNA fingerprinting. We are not aware of any other report of a mixed mating system of a fungus in a natural population with an outcrossing rate significantly different from 1.0 or 0.0.