BSN, THE PRIMARY DETERMINANT OF BASIDIAL SPORE NUMBER AND REPRODUCTIVE MODE IN AGARICUS-BISPORUS, MAPS TO CHROMOSOME-I

In the mushroom species Agaricus bisporus, heterokaryotic individuals of the geographically isolated varieties bisporus and burnettii respec tively appear to rely primarily upon inbreeding or outbreeding reprodu ctive strategies. These two divergent syndromes depend upon the 'ploid y level' (n vs n + n) of offspring. This in turn is determined by the number of spores produced upon the basidia, which are respectively pre dominantly bisporic or tetrasporic. This study investigated the geneti c basis of control over the reproductive syndrome by analyzing transmi ssion of basidial spore number traits in two inter-varietal hybrid ped igrees. For two different pedigrees, 103 or 71 homokaryotic offspring of a first-generation intervarietal hybrid were all crossed with a sin gle homokaryon, from a bisporic parent, to produce a second generation of sibling heterokaryons. In each pedigree, the average basidial spor e number, or ASN, had a bimodal frequency distribution and was the mos t useful discriminant variable for resolving the two classes of offspr ing. Our results indicate that basidial spore number is primarily dete rmined by a single genetic locus (BSN). Statistical analyses of joint segregants indicate that the locus is linked to the mating type locus (MAT) and other loci on chromosome I. Using fruiting tests, mating tes ts, and genotype analysis, it was shown that the offspring of preponde rantly bisporic or tetrasporic second generation hybrids were respecti vely preponderantly heterokaryotic (n + n) or homokaryotic (n). Homoka ryons were capable of normal mating behavior, unlike most of their het erokaryotic siblings. This is consistent with earlier observations on this and other species. We propose that BSN is the primary locus regul ating the two alternative reproductive modes.