The Heterothallic Life Cycle of Agaricus bisporus var. burnettii and the Inheritance of Its Tetrasporic Trait

Kerrigan, R. W., Imbernon, M., Callac, P., Billette, C., And Olivier, J.-M. 1994. The heterothallic life cycle of Agaricus bisporus var. burnettii and the inheritance of its tetrasporic trait. Experimental Mycology 18, 193-210. The new taxon Agaricus bisporus var. burnettii was recently proposed for a tetrasporic population of the species discovered in California. This tetrasporic variety is interfertile with the familiar bisporic variety of the species. In the present study we determined the heritability of parental basidial morphologies in 91 first-generation intervarietal hybrids descended from either of two genetically distinct strains of var. burnettii. Large samples of basidiospore offspring of one of these parental var. burnettii strains, and of one of the first-generation intervarietal hybrids, were evaluated with respect to their ploidy (n vs n + n). The life cycle of var. burnettii was deduced and the mating type alleles of the two var. burnettii parents stocks were determined. We found that: (1) The life cycle of this variety, of which more than 90% of the basidia were four-spored, was predominantly heterothallic. About 90% of the basidiospores were homokaryotic. (2) Its incompatibility system was unifactorial. Four different, previously unknown mating type alleles were found. (3) The tetrasporic trait was always transmitted to first-generation hybrids between bisporic and tetrasporic varieties. Of 91 intervarietal hybrids, 88 had a majority of four-spored basidia while 3 had predominantly three-spored basidia, and none had more than a small percentage of bisporic basidia. (4) The basidiospore offspring of these hybrids germinated normally, producing a high percentage of homokaryons. For one hybrid sporocarp this percentage was estimated at 77% by three methods (growth rate, mating competence, and multilocus allozyme genotype) which generally agreed with one another. These data shed new light on the genetics of A. bisporus and will be useful in related breeding work.