STUDIES OF MULTIPLE ALLELISM IN THE MYXOMYCETE DIDYMIUM IRIDIS

Heterothallism in three physiological races of Didymium iridis collected from Honduras (Mexico), Hawaii, and Australia are reported. Pure clone matings of these races are found to be governed by multiple alleles of incompatibility factors, heterozygosity of which leads to zygote formation. Two of these races behave identically in their mating reactions although their plasmodia are non-fusable. Existence of a two-locus system with linkage of parental alleles is suggested to explain the mating behavior of the pure clones handled and the absence of tetrapolarity. Because of aging and artificial culture, intraracial sterility develops in the pure clones. These clones retain their interracial fertility. Sporadic Plasmodia occasionally form in some of the pure clones probably because of a localized mutation. Zygote formation with the mutated cell apparently leaves the remaining clone population uncontaminated.