THE HETEROTHALLIC LIFE-CYCLE OF AGARICUS-BISPORUS VAR BURNETTII AND THE INHERITANCE OF ITS TETRASPORIC TRAIT

The new taxon Agaricus bisporus var. burnettii was recently proposed f or a tetrasporic population of the species discovered in California. T his tetrasporic variety is interfertile with the familiar bisporic var iety of the species. In the present study we determined the heritabili ty of parental basidial morphologies in 91 first-generation intervarie tal hybrids descended from either of two genetically distinct strains of var. burnettii. Large samples of basidiospore offspring of one of t hese parental var. burnettii strains, and of one of the first-generati on intervarietal hybrids, were evaluated with respect to their ploidy (n vs n + n). The life cycle of var. burnettii was deduced and the mat ing type alleles of the two var. burnettii parents stocks were determi ned. We found that: (1) The life cycle of this variety, of which more than 90% of the basidia were four-spored, was predominantly heterothal lic. About 90% of the basidiospores were homokaryotic. (2) Its incompa tibility system was unifactorial. Four different, previously unknown m ating type alleles were found. (3) The tetrasporic trait was always tr ansmitted to first-generation hybrids between bisporic and tetrasporic varieties. Of 91 intervarietal hybrids, 88 had a majority of four-spo red basidia while 3 had predominantly three-spored basidia, and none h ad more than a small percentage of bisporic basidia. (4) The basidiosp ore offspring of these hybrids germinated normally, producing a high p ercentage of homokaryons. For one hybrid sporocarp this percentage was estimated at 77% by three methods (growth rate, mating competence, an d multilocus allozyme genotype) which generally agreed with one anothe r. These data shed new light on the genetics of A. bisporus and will b e useful in related breeding work. (C) 1994 Academic Press, Inc.