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 Immetta respectively appear to rely primarily upon inbreeding or outbreeding reproductive strategies. These two divergent syndromes depend upon the 'ploidy level' (n vs n+n) of offspring. This in turn is determined by the number of spores produced upon the basidia, which are respectively predominantly bisporic or tetrasporic. This study investigated the genetic basis of control over the reproductive syndrome by analyzing transmission of basidial spore number traits in two intervarietal hybrid pedigrees. For two different pedigrees, 103 or 71 homokaryotic offspring of a first generation intervarietal hybrid were all crossed with a single homokaryon, from a bisporic parent, to produce a second generation of sibling heterokaryons. In each pedigree, the average basidial spore number, or ASN, had a bimodal frequency distribution and was the most useful discriminant variable for resolving the two classes of offspring. Our results indicate that basidial spore number is primarily determined by a single genetic locus (BSN). Statistical analyses of joint segregante indicate that the locus is linked to the mating type locus (MAT) and other loci on chromosome I. Using fruiting tests, mating tests, and genotype analysis, it was shown that the offspring of preponderantly bisporic or tetrasporic second generation hybrids were respectively preponderantly heterokaryotic (n+n) or homokaryotic (n). Homokaryons were capable of normal mating behavior, unlike most of their heterokaryotic siblings. This is consistent with earlier observations on this and other species. We propose that BSN is the primary locus regulating the two alternative reproductive modes.