Genetic Exchanges in the Macrocysts of Dictyostelium discoideum

Abstract

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 Al) 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 mouldsDictyostelium mucoroides, Dictyostelium giganteum and Polysphondylium pallidum -and are reminiscent of the results reported for germinated zygospores of Phycomyces blakesleeanus.

Sex Determination in the Social AmoebaDictyostelium discoideum

“…These help to lay down external layers of cellulose around the developing mass of cells before being cannibalized by the zygote (9). After a period of dormancy the cyst germinates, releasing haploid progeny that arise most likely after meiosis and multiple mitoses (10). Population genetics of wild isolates indicate that mating and recombination are probably frequent in the wild (11).…”

“…Genetic analysis suggested that mating-type is stable, and determined by a single locus with two or * To whom correspondence should be addressed. garethb@mrc-lmb.cam.ac.uk Phone: +44 (0) (10,14,16). We argued that it might be possible to identify this postulated locus by searching for genes which are present in any member of one mating-type but absent (or highly diverged) in any member of another.…”

“…Although several genes are known to be involved during the sexual cycle (15), the determinant of mating-type has proved elusive. Genetic analysis suggested that mating-type is stable, and determined by a single locus with two or more alleles (10,14,16). We argued that it might be possible to identify this postulated locus by searching for genes which are present in any member of one mating-type but absent (or highly diverged) in any member of another.…”

A Comparison of the Macrocyst and Fruiting Body Developmental Pathways in Dictyostelium discoideum

“…It is presumed in some instances that the giant cell is a zygote and that the final cells emerging from the macrocyst are the haploid progeny. There is evidence in some species to support this idea (MacInnes & Francis, 1974;Francis, 1975;Wallace & Raper, 1979) but in other cases the possibility of asexual development or selfing remains strong (Bozzone & Bonner, 1982).…”

Induction of Macrocyst Germination in the Cellular Slime Mould Dictyostelium mucoroides

“…Macrocysts are large, thickwalled, resting structures formed after the fusion of starving amoebae, and are probably sexual zygotes (Erdos et al, 1973a, 6 ;MacInnes & Francis, 1974;Francis, 1975;Wallace & Raper, 1979). Some species, like Dictyostelium mucoroides, are homothallic, and the first sign of macrocyst formation is the formation of large cell aggregates in a process showing some resemblance to asexual aggregation.…”