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Abstract: Directed mushroom breeding requires relevant information about those genetic systems which control genetic exchange and gene flow between individuals from different populations The cultivated mushroom Agancus bitorquis occurs worldwide in diverse habitats Studies of the inter-relationships in a sample of twelve wild and commercial strains of A bitorquis, showing wide genetic variation, have revealed a complex of three groupings 1) Temperate, 2) Bridging, and 3) Tropical Mating tests identified 13 different alleles at the incompatibility locus in these groups Heterokaryosis in hybrids was confirmed by cytological studies, fruiting trials, and the recovery of parental matmg-type specificities from single spore progenies The genetic relatedness of the stocks was studied by RAPD analysis Correlation of classical and molecular analyses indicates that heterokaryon formation in the complex is controlled by homogenic and heterogenic incompatibility systems The appearance of partial and complete reproductive barriers, through heterogenic incompatibility, suggests that the populations from the tropical group are evolving sympatncally within the complex Adaptation to higher fruiting temperatures, in some members of the tropical group, may be an ecological factor inducing reproductive isolation by disruptive selection Early stages of sympatnc speciation in the A bitorquis complex involving decreased expression of sexuality (mating reactions) and reduced viability of the hybrids, are also accompanied by a substantial degree of genetic differentiation on the basis of RAPD analysis
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