EVIDENCE OF GENETIC-DIVERGENCE IN 2 POPULATIONS OF AGARICUS-BISPORUS

Allele frequencies in a sample of a wild Californian population of Agaricus bisporus were compared with those in a reconstructed sample of a European population from which the commercial cultivar strains of the species are believed to have been drawn. Reconstruction employed genealogical analysis using a model of trait inheritance based on known genetic behaviour of the species and historical accounts of cultural practices, to distinguish probable derived genotypes from putative ancestral, wild genotypes. Frequencies of the more common alleles were similar in both samples; if selectively neutral they are most likely ancestral. The uncommon alleles differ significantly in frequency between the two population samples (pooled chi-square test; P < 0·001); most of these occur in only one sample, usually the one from California. Pairwise comparison of genetic similarity, using a corrected, symmetrical measure, between all putative original genotypes, indicated that the divergence of the two populations is not extensive; rather it appears that a large number of uncommon, potentially recent alleles may be spreading within their population of origin. Heterozygosity at three of six loci in the Californian sample differed significantly from expectations under a null hypothesis of Hardy-Weinberg conditions. Two of these loci were deficient in heterozygotes while the third was deficient in homozygotes. Wright's F consequently varied enormously between loci. The observed heterozygote deficiencies and high interlocus variation in F values provide indirect evidence for‘intramixis’ (automictic reproduction with intrameiotic restitution of diploidy or heterokaryosis) in A. bisporus in nature. The simplest cytogenetic model of intramixis predicts that heterozygote deficiency at single loci will be negatively correlated with linkage to elements of the genome which either interfere with the recombinational loss of heterozygosity, e.g. centromeres, or which, when physically recombined, lead to homozygosity which renders progeny infertile (mating-type loci). This model therefore suggests a novel means of ordering markers on genetic maps of advanced intramictic species in which meiotic crossing-over may be largely suppressed. Population heterozygosity data, which sample intramictic meiotic processes over evolutionary time, should, given certain assumptions, generally reflect the relative linkage of marker loci to centromeres. In exception to the predicted relationship, anomalous locus-specific heterozygote surplus, as is found at the PEP2 locus, may indicate linkage to deleterious recessive mutations which render some hemizygous and homozygous offspring unfit or even inviable. Cladistic analysis of genotypes containing rare alleles indicates that, unless an improbably high degree of evolutionary convergence upon a limited number of electromorphs has occurred, sexual outcrossing has also been an important factor in producing the extant wild genotypes of A. bisporus. © 1990, British Mycological Society. All rights reserved.