On the Evolution of Heterothallism in Fungi

Abstract

Three major types of life cycles in the fungi are discussed; namely, (1) homothallism, (2) unbalanced heterothallism, and (3) balanced (true) heterothallism. Their possible evolutionary relationships to each other are considered in the light of recent genetic discoveries in microorganisms and higher forms. The synthesis in the laboratory of unbalanced heterothallism from homothallism in Sordaria fimicola is described. This type of life cycle involves the crossing of non-allelic sterility mutants, the progeny of which include not only the parental genotypes, but also self-fertile and double mutant, sterile recombinants. It is concluded that the most likely explanation of the origin of 1-locus 2-allelic heterothallism in fungi is to be found in the compound-locus hypothesis. This entails the occurrence[long dash]independently or simultaneously[long dash]and association of pseudoallelic self-sterility mutations in a compound locus (of 2 or more subunits). Instances of true morphological heterothallism appear to be rare in the fungi, only 1 species, the ascomycete Ascosphaera apis, have been experimentally proven to belong to this type of life cycle. It is proposed that the compound nature of the multiple-allele compatibility loci of bipolar and tetrapolar higher basidiomycetes may have resulted from successive duplications during evolution. The most primitive form of heterothallism in basidiomycetes is believed to be simple 1-locus 2-allelic heterothallism as found in the great majority of investigated rust fungi, probably indicating evolution of the group from a heterothallic ascomycetous ancestor.