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

The outcrossing rate in the chestnut blight fungus, Cryphonectria (End othia) parasitica, was estimated in a natural population using data fr om six unlinked restriction fragment length polymorphism (RFLP) loci a nd DNA fingerprinting. This fungus was shown to have a mixed mating sy stem with both self-fertilization and outcrossing in the same populati on. In a sample of 22 perithecia, all progeny segregated in 1:0, 1:1 o r 0:1 ratios at RFLP loci; there was no evidence for multiple male par ents in ascospore families from single perithecia. The multilocus esti mate of the outcrossing rate was 0.74 for the RFLP data; the proportio n of perithecia with progeny segregating for fingerprint fragments was 0.73. Vegetative compatibility was not a sensitive marker for detecti ng outcrossing as only 11 perithecia (50 per cent) showed segregation with this marker, compared with 16 (73 per cent) for DNA fingerprintin g. 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.