Abstract
Introduction Olive (1958), in his essay on the evolution of heterothallism in the fungi, concluded that the most liltely explanation of the origin of one-locus, twoallelic heterothallism is still to be found in the compound locus concept. However, a direct implication of the acceptance of this view xvould be that recombination of the pseudoalleles could give rise to a sinall number of homothallic descendants. Olive states that St. Lawrence obtained several singleascospore isolates of N. crassrr which, although not self-fertile, could cross wit11 either the A or the a strain. Lindegren (1934) reported on an unusual ascus of N. crassrr containing four bisexual and four altaryotic spores. The n~ycelia from each of the four bisexual spores produced conidia asevually (as well as perithecia sexually). Cultures were made from single conidia of each of the four lines. Since conidia are usually multinucleate, most of the n~ycelia grown from the four lines would be expected to be bisexual. This was thc case, but unisexual conidia were also produced in each instance. Asci produced directly by four bisexual mycelia were analyzed and no bisexuals were encountcred. Several matings wcre also made between the unisexual strains obtained by somatic segregation from the sex-heteroltaryons. hTo bisexual ascosporcs were found among their offspring. Accorling to Lindegren thcsc experiments suggest that the third division in the ascus may, in some cases, function as a device to insure incorporation of two nuclei of opposite sex in cach ascospore. A similar case was observed by hloreau and Moruzi (1932) in N. sitophila. Four-spored asci were found with large ascospores, hon~othallic in their matingtype reaction. Recently, Ncwmeycr (1965) reported on bisexuality in N. crassrr. This phenotype is attributed to heterozygosity of the mating-type locus which resolves itself by the formation of homozygous A and rr nuclei, probably by mitotic crossing over.
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