On the Evolution of Heterothallism in Fungi

Three major types of life cycles in the fungi - homothallism, unbalanced heterothallism, and balanced heterothallism-and their possible evolutionary relationships to each other are discussed in the light of recent genetic discoveries in microorganisms and higher forms. It is concluded that the most likely explanation of the origin of one-locus two-allelic heterothallism in the fungi is to be found in the compound-locus hypothesis. This would involve the occurrence - independently or simultaneously - and association of pseudoallelic self-sterility mutations in a compound locus (of two or more subunits). It is proposed that the compound nature of the multipleallele compatibility loci of bipolar and tetrapolar higher basidiomycetes may have resulted from successive duplications during evolution.