The California population of Agaricus bisporus comprises at least two ancestral elements

Genotypes of a sample of field-collected heterokaryon isolates of Agaricus bisporus from California were compared with those of a comprehensive sample of cultivar isolates that are likely to be of European ancestry. Isolate genotypes were determined for 14 nuclear DNA restriction fragment length polymorphism markers and seven allozyme markers, and were compared in a pairwise fashion to produce a matrix of genotypic dissimilarities between all isolates. This matrix was evaluated with regard to the affinities of individual California isolates, both among themselves and with a reconstructed population ancestral to the cultivars. Most isolates from California fell into one of two apparent groups; these either: 1) had high affinity with the cultivars or 2) differed substantially from the cultivars and aggregated in a distinct, cypress-associated "Californian" group. A few isolates had an intermediate character. This result, supported by habitat correlations and the distribution of characteristic mitochondrial DNA types, indicates that the California population presently comprises introduced cultivars, indigenous cypress-associated strains, and a less distinct group that could include spontaneous hybrids between these two ancestral elements, or strains with other affinities. If hybridization is indeed occurring, the two major ancestral elements have introgressed only slightly