Sex Determination in the Social AmoebaDictyostelium discoideum

Abstract

The genetics of sex determination remains mysterious in many organisms including some that are otherwise well-studied. Here we report the discovery and analysis of the mating-type locus of the model organism Dictyostelium discoideum. Three forms of a single genetic locus specifies this species' three mating-types: two versions of the locus are entirely different in sequence, and the third resembles a composite of the other two. Single, unrelated genes are sufficient to determine two of the mating-types, while homologues of both these genes are required in the composite type. The key genes encode polypeptides that possess no recognisable similarity to established protein families. Sex determination in the social amoebae thus appears to use regulators unrelated to any currently known.Most eukaryotes are sexual, but little is known in molecular detail about sex across most branches of the eukaryotic tree. One aspect, the genetic basis of sex determination, is well understood in several animal, fungal and plant lineages (1-5), but across the protozoan kingdoms we know little, and nothing in comparable detail. The social amoebae are members of the Amoebozoa, and have an unusual sexual cycle that leads to the formation of dormant, walled macrocysts (6) ( Fig. 1 A and B). To produce a macrocyst, a pair of haploid amoebae of different sexes fuse (7) to form a diploid zygote, which then attracts surrounding haploid cells (8). 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).The most-studied species of social amoeba, Dictyostelium discoideum, is notable for having three sexes (hereafter called mating-types I, II, and III; supporting online text, S1), as well as uncommon self-fertile homothallic strains (12)(13)(14). Each of the three sexes can pair with each of the other two, but not with itself, giving three possible classes of zygote: type-I/type-II, type-I/type-III, and type-II/type-III. 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 * 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. For this purpose we performed comparative genomic hybridizations using DNA microarrays composed of probes for around 8500 of the 10500 predicted genes in the sequenced type-I D. discoideum genome (17).We analysed ten strains derived from independent wild isolates (table S1) usi...

Origins of Eukaryotic Sexual Reproduction

“…In most modern eukaryotes, this handicap has been assuaged by the evolution of countless systems for identifying the opposite type via pheromones and mating behaviors, enhancing the efficiency of finding a mate, but options are still limited to half the population. In some lineages, the odds have been improved by the presence of multiple mating types-three or more in Dictyostelium (Bloomfield et al 2010(Bloomfield et al , 2011, seven in Tetrahymena (Phadke et al 2012;Cervantes et al 2013;Umen 2013), 13 in Physarum polycephalum (Collins and Tang 1977;Clark and Haskins 2010), and often thousands in the basidiomycetes (Raper 1966)-and in these cases more encounters are sexually fertile (.98% in some basidiomycetes). But more outcrossing is not always better, and in basidiomycete fungi there are multiple independent transitions from the tetrapolar outcrossing species with thousands of sexes back to bipolar species with just two mating types, where it has been suggested that different environments favor outcrossing versus inbreeding sexual cycles ).…”

Evolution of cooperation and control of cheating in a social microbe

“…In a major recent advance, the sex-determining locus was identified and the presence of three mating types was confirmed, clearly establishing the genetic basis of sex (95). The sexual cycle is somewhat of an enigma because it rarely leads to recombinant progeny under laboratory conditions (42), but estimates of recombination rates of natural clones indicate they are very high, with a population ρ of 37.75 and baseline linkage disequilibrium achieved between 10 and 25 kb (59).…”

A Surface Glycoprotein Indispensable for Gamete Fusion in the Social Amoeba Dictyostelium discoideum

“…Raw spectral data were processed using SEQUEST software to extract peak lists. The obtained peak lists were analyzed using the MASCOT program against the D. discoideum protein database extracted from dictyBase (http://dictybase.org) and supplemented with B and MatC (mat locus gene products in type II) and D, S, and MatT (mat locus gene products in type III) sequences (2).…”