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Abstract: A. bisporus is one of several Basidiomycetes which produce, largely, 2-spored basidia. Fertility (i.e., sporophore production) of monosporous isolates is characteristic of several bisporic species including the cultivated mushroom. Three different bisporic species were reported by Sass (10) to produce heterokaryotic spores which were self-fertile as the result of two nuclei of different mating types migrating into each spore following meiosis. Cytological (2, 3, 4, 8, 9, 10) studies on the meiotic division in the basidium of A. bisporus, have shown that two nuclei migrate into each of the two basidiospores, but the genetic basis for fertility of monosporous isolates has remained obscure until recently (6). Proof of the existence of sexuality in A. bisporus required: (1) a technique for isolating and identifying homokaryotic, haploid mushroom strains, (2) the combination of appropriate homokaryotic isolates to form heterokaryons and (3) progeny analyses to demonstrate a normal inheritance pattern. Several investigators (1, 3, 5) have observed that individual basidia of A. bisporus occasionally produce four instead of the normal two basidiospores per basidium. Miller (6) demonstrated that monosporous isolates from 4-spored basidia, obtained from two morphologically distinct strains, were sterile (i.e., produced no sporophores) and that certain matings between monosporous isolates were fertile (i.e., produced sporophores) while other matings were sterile. Fertility of paired isolates occurred in a pattern suggesting a bipolar mating system. Analysis of monosporous isolates from 4-spored basidia obtained from a cross between strains that differed in sporophore morphology provided confirming evidence. Fruit-bodies produced by the Fi resembled one parent. Fertile monosporous progeny from an Fi sporophore segregated for the sporophore morphology characteristic of the parents. Recovery of both parental types suggested that fusion of parental nuclei occurred in the basidia and that this was followed by random segregation and recombination of two haploid nuclei in each fertile spore. These preliminary results indicated that sexuality of the cultivated mushroom should be classified as secondary homothallism similar to other bisporic Basidiomycetes. Matings of sterile isolates produced a sufficiently consistent reaction with regard to sporophore production to indicate the existence of a simple bipolar mating system. Although a bipolar sexual pattern was postulated, certain inconsistencies suggested that the possibility of other factors affecting sexuality should not be excluded. Matings between certain monosporous isolates from 4-spored basidia resulted in ambiguous fruiting responses (e.g., matings with W6 and W7 in Table 1). There were several possible explanations for these ambiguities. Raper, et al. (1), have shown that some monosporous isolates associated with the ambiguous fruiting responses were heterokaryotic for prototrophic and auxotrophic components. It was possible that isolates such as W6 and W7 were also heterozygous for mating type. The mushroom strain from which most of the monosporous isolates from 4-spored basidia were obtained, was the W strain. The W strain had an unusual morphology and produced a relatively high percentage of 4-spored basidia, but produced few sporophores on commercial compost. The characteristic of poor sporophore production of the W strain and of matings between monosporous W strain isolates from 4-spored basidia may reflect unusual nutritional or environmental requirements rather than incompatible mating types. The procedure used to measure fertility of paired monosporous isolates was to use rye grain spawn to inoculate conventional mushroom compost. During the test period, sterile isolates were exposed to contamination by basidiospores from other mushrooms which could have rendered a normally sterile isolate or a combination of sterile isolates fertile. Experiments described in this paper involved a mushroom growing technique that minimized the possibility of accidental matings. The mushroom strain used in this study was consistently fertile under the growing procedure employed and was obtained by mating two sterile monosporous isolates (W3 and W8) obtained from 4-spored basidia. Monosporous isolates from 4-spored basidia of the Fi sporophore were tested for fertility singly and in all possible matings.
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