The effect of nuclear dominance in monokaryotic oidium formation from dikaryotic mycelia in Pholiota nameko was examined. Over 90% of oidium isolates from dikaryotic mycelia were monokaryotic. Although only one parental nuclear type was recovered from an average of about 80~/o in these isolates, the nuclear selection process in oidium formation seems essentially to produce split nuclear type composition in oidium products. The hierarchy of relative dominance among the nuclear types of the parental dikaryons in monokaryotic oidium formation was determined. The two hierarchies in nuclear selection between monokaryotic oidium formation and monokaryotic mycelium formation coincided at a level of at least 75~. Key Wordsbasidiomycete; monokaryotization; nuclear selection; oidium formation; Pholiota nameko.Many basidiomycetous mushrooms produce oidia or other asexual spores from both monokaryotic and dikaryotic mycelia. Oidium formation occurs in such mushrooms as Coprinus cinereus (Schaeff.: Fr.) S.F. Gray (Rao and Niederpruem, 1969), Flammulina velutipes (Curt.: Fr.) Singer (Brodie, 1936;Takemaru, 1954), Favolus arcularius (Fr.) Ames (Kitamoto, unpublished data), Hypsizygus marmoreus (Peck) Bigelow (Yamanaka, 1995), and Pholiota nameko (T. Ito) S, Ito & Imai in Imai (Arita, 1979). Most oidia from dikaryotic mycelium of F. velutipes (Brodie, 1936;Takemaru, 1954;Masuda, 1996) and P. nameko (Cao et al., 1999) are in a monokaryotic state. Although both parental nuclear types of monokaryotic oidia were isolated from the majority of hybrid dikaryons, one type was recovered in a much higher rate (85~ in the oidia from most hybrids (Masuda, 1996). Therefore, the process of spontaneous monokaryotization and the occurrence of dominant nucleus selection might be involved in oidium formation from dikaryotic mycelia in various mushrooms. A similar monokaryotization process in monokaryotic mycelia from dikaryotic mycelia in P. nameko has been reported (Masuda et al., 1995).We have previously demonstrated that P. nameko produced abundant oidia on aerial hyphae from monokaryotic and dikaryotic test stocks, but rarely did so on submerged hyphae (Cao et al., 1999). Observation of various test stocks on slide cultures revealed thatCorresponding author. about 80~ of oidia were produced from a secondary branched hypha and about 20~ from the terminal hyphal cell of the main hypha. About 82~ and 70~ of the oidia from monokaryons and dikaryons, respectively, had only one haploid nucleus, while the remainders were multinucleate. Among the stocks tested, most oidia had a DNA content corresponding to a haploid at the G1 phase of the cell cycle, while a few contained twice as much, corresponding to the G2 phase.In the present study, we have sought an empirical rule for dominant nucleus selection in monokaryotic oidium formation from dikaryotic mycelia in P. nameko. We have also compared the hierarchies of relative dominance in the selection of one of the two nuclei of dikaryotic cells in monokaryotization between mycelium monokaryotization and ...

Pholiota nameko is a wood-rotting edible mushroom that carries a bipolar A incompatibility factor gene. The linkage analysis of the multiple allelomorphic A factor gene demonstrated that sexual reproduction produced a monospore isolate carrying a new A factor gene in addition to two parental mating types of isolates. However, 10%-30% of the modified monospore isolates could not produce a dikaryon with both of the parental monokaryons by crossing. It is concluded that the bipolar A incompatibility factor gene of P. nameko is constituted of two functional subunits, Aα and A , which might be successively located beside each other with an apparent genetic distance of 0.3 centi-Morgan between them on the same chromosome. Further, some monospore isolates that did not conjugate with both parental monokaryons could produce dikaryons with different monokaryotic stocks with either one of the parental mating types. This result suggests that the crossing capability of these isolates were essentially those for one of the mating types of the parental monokaryons, but that their function for mating activity was made partially by unequal crossingover in the process of sexual recombination.

Pholiota nameko produced abundant oidia on aerial hyphae from monokaryotic and dikaryotic test stocks, but oidia were rare on submerged hyphae. The oidia from the former stocks had a layer of hydrophobic protein between the cell wall and the inner cell membrane, which was absent in the oidia from the latter. The only remarkable differences in the morphological features of the oidia from monokaryotic and dikaryotic mycelia was the slightly larger size of the latter. Observation of various test stocks on slide cultures revealed that about 80% of oidia were produced from the secondary branched hypha, and about 20% from the terminal hyphal cell of the main hypha. In the former, the secondary hyphae were segmented to form several oidium cells; in the latter, a single or several oidia were formed at the terminal end of the main hypha. Most oidia from monokaryons and dikaryons had only one haploid nucleus, while the remainders were multinucleate. Among the stocks tested, most oidia had a DNA content with a haploid amount at the G 1 phase of the cell cycle, but a few contained twice that amount corresponding to the G2 phase.