THE INDIGENOUS COASTAL CALIFORNIAN POPULATION OF THE MUSHROOM AGARICUS-BISPORUS, A CULTIVATED SPECIES, MAY BE AT RISK OF EXTINCTION

This study employed nuclear and mitochondrial markers to assess the pr esent-day composition of the population of Agaricus bisporus in coasta l California. Favourable weather in the fall, winter and spring of 199 0-91 furnished an uncommon opportunity to collect and study field mate rial of the 'button mushroom' A. bisporus, a cultivated species, from the region. The previous such season occurred 13 years earlier. Ninety -five nonredundant cultures from field material were prepared and geno typically characterized. These data were combined with data from earli er studies. Multilocus nuclear and mitochondrial genotypes were determ ined for 123 individuals. Genotypes were compared in pairwise fashion both within the sample and between this sample and others of diverse g eographical origin or commercial provenance. Using parametric analysis and cluster analysis of nuclear similarities, and also mitochondrial data, two elements-indigenous and European-were apparent within the sa mple. This was consistent with our earlier results on a much smaller s ample. At least 10 mitochondrial haplotypes (MTs) were present; based on genotypic similarities of associated nuclei, five (or six) MTs were Californian, four were European, and one was ambiguous. Based on MT o rigins, 54% of the 121 classifiable individuals in California were of European ancestry; natives constituted a minority at 46%. Even in the indigenous Monterey cypress habitat, where 84% of all individuals from California were sampled, nonnative A. bisporus appeared to have achie ved parity (at 48-49%) with the native population. In all other habita ts, which are far more extensive, European individuals outnumbered Cal ifornian natives by 4:1. Some evidence of hybridization between the tw o ancestral groups was found. European strains appear to have been res ident in California for approximately one century. The extensive occup ancy of the native habitat by non-native germ plasm, the apparent inab ility of native strains to occupy or compete in non-native habitats, a nd the disproportionately large inoculum reservoirs represented by non -native habitat and agronomic activities all suggest that the native p opulation is under considerable competitive pressure in what may be a very dynamic situation. If this surmise is correct, the native populat ion may be at serious risk of further contraction, irreversible diluti on through interbreeding, and possibly even extinction.