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Abstract: The prospect for breeding the edible mushroom to combine desired characteristics of different strains depends upon an understanding of the life cycle that has not been available. The life cycle can be understood only with specific answers to a number of questions based upon knowledge of related forms (Raper, 1966). (1) What is the nature of the vegetative nucleus, whether haploid, diploid, or polyploid? (2) Is there heterokaryosis, the intimate association of two or more genetically distinct nuclei in a vegetative hyphal system or mycelium ? (3) Is there nuclear fusion and meiosis to recombine inherited characteristics ? (4) What is the relationship of the nuclei of the basidiospores? (5) Is there a sexual cycle? If so, what is the pattern and nature of its control? Virtually nothing is known about the first two of these aspects. Hyphal anastamosis, observed by Evans (1959) and others, is a prerequisite to the occurrence of heterokaryosis but does not prove its existence. No morphological signs of heterokaryosis, such as those seen in other basidiomycetes — the development of septal clamp connexions, paired nuclei, and conjugate division — are visible in A. bisporus. There is cytological evidence for nuclear fusion and meiosis in the basidium, but reconstruction of dynamic events through the examination of fixed material is difficult and unreliable in the absence of genetic corollaries. The clues available for an understanding of the development of the basidiospore are, again, principally cytological: A. bisporus bears mostly 2-spored basidia, and its spores have 2-4 nuclei (Sass, 1928; Colson, 1935; Sarazin, 1938 ; Evans, 1959), but it is not known whether the nuclei of the spores are homo- or heterogenic, i.e., genetically identical or dissimilar. Most monosporous isolates of this species are self-fertile, i.e., produce fruit-bodies or sporocarps, but the underlying basis for self-fertility is unknown. Dr R. E. Miller (1971), a contributor to this Congress, has described a pattern of fertility among self-sterile monosporous isolates of rare 4-spored basidia that suggests a bipolar pattern of sexuality and secondary homothallism as characteristic of this species. His work indicates that self-sterility of isolates from 4-spored basidia is due primarily to inclusion of a single mating-type factor and that interaction of opposite mating types in matings of compatible strains is required for fruiting. Fertility, however, may be prevented by other factors (Raper and Raper, 1966). This suggests that most monosporous isolates of 2-spored basidia are fertile because of the inclusion in the spores of factors for two mating types.
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