Genetic exchanges in the macrocysts of Dictyostelium discoideum

Crosses were made between strains of Dictyostelium discoideum involving two drug resistance markers and the mating-type locus. Over 6000 progeny from 263 individual germinated macrocysts from four single-factor crosses, five two-factor crosses and one three-factor cross were characterized. In most cases the progeny from a single macrocyst were of one genotype, although in the population of macrocysts from any two-factor cross all possible parental and recombinant genotypes were recovered. There was no evidence of linkage between any of the markers examined. No selection against progeny carrying the methanol or the cycloheximide resistance markers was found in two-factor crosses, but selection against progeny carrying both resistance markers was found in the three-factor cross. Germination of macrocysts in all crosses was poor, only once exceeding 2.5% of the total macrocyst population. A variety of crosses and back-crosses with different parental strains indicated that germination might be influenced by both extrinsic (environmental) and multiple genetic factors. About 10% of the macrocysts yielded progeny spores that were ambivalent in their mating reactions. After extensive recloning these populations could be resolved to the normal matA (formerly A1) and mata (formerly A2) mating-types and might therefore have represented aneuploids. The results obtained with D. discoideum macrocysts differ from those obtained with other cellular slime moulds--Dictyostelium mucoroides, Dictyostelium giganteum and Polysphondylium pallidum--and are reminiscent of the results reported for germinated zygospores of Phycomyces blakesleeanus.