Tetrapolar Sexuality

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Abstract
The basic life cycle of hymenomy-cetous fungi is defined by the cyclic progression: spore to homokaryotic mycelium, to dikaryotic mycelium, to sporocarp, to spore. Three patterns of sexuality occur in hymenomycetes: homothallism, in which the dikaryotic phase is initiated by an intramycelial association of nuclei; and 2 types of heterothallism, bipolarity and tetrapolarity, in which the dikaryotic phase is initiated by obligatory intermycelial association of compatible nuclei. Self-sterility and cross-fertility are detd. in bipolar species by incompatibility factors at 1 locus, and in tetrapolar species by incompatibility factors at 2 independent loci. Tetrapolarity is distinguished by the production of progeny of 4 distinct mating types by each sporocarp: A1A2B1B2 yield A1B1; A1B2; A2B1; A2B2. Fertile matings occur between the different matings which restore the double heterozygote. Occurrence of "new" mating types among sibling monosporous mycelia has long been interpreted as the result of mutations of incompatibility factors, whereas recent work indicates that the A incompatibility factor of Schizophyllum commune is composed of a no. of distinct genes between which crossing-over yields new incompatibility factors by genetic recombinations. Mating of homokaryotic mycelia in all intertype combinations yields a series of reactions which exactly reflect specific intermycelial combinations of incompatibility factors.
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