GENETIC STABILITY IN A POPULATION OF A PLANT-PATHOGENIC FUNGUS OVER TIME

Collections of the plant pathogenic fungus Mycosphaerella graminicola were made from the same field of wheat over a 3-year period. The field was planted with small plots containing four varieties of wheat grown in pure stand and in all possible two-, three- and four-way mixtures. In each year, the wheat field was recolonized by a local source of in oculum of unknown origin. Allele frequencies at 10 RFLP loci were comp ared at two different times within a growing season and over the 3-yea r period. No significant differences in allele frequencies were found for any of the RFLP loci over any of the time periods. DNA fingerprint s were used to identify clones produced by asexual reproduction. Genot ypic diversity based on the frequency of each clone was compared for e ach collection. No significant changes in genotypic diversity were fou nd within a year or between years. Identical genotypes were found in t he field at different times within a season, but no clones were conser ved between years. No clone existed in a high frequency in any year, s uggesting that selection for particular asexual lineages was weak. The founding population each year probably originated from wind-borne asc ospores of the teleomorph, which may exist as an indigenous population on alternative hosts, such as Poa annua (annual bluegrass).