Distribution of the sex-determining gene MID and molecular correspondence of mating types within the isogamous genus Gonium (Volvocales, Chlorophyta)

PLoS One. 2013 May 16;8(5):e64385. doi: 10.1371/journal.pone.0064385. Print 2013.


Background: Isogamous organisms lack obvious cytological differences in the gametes of the two complementary mating types. Consequently, it is difficult to ascertain which of the two mating types are homologous when comparing related but sexual isolated strains or species. The colonial volvocalean algal genus Gonium consists of such isogamous organisms with heterothallic mating types designated arbitrarily as plus or minus in addition to homothallic strains. Homologous molecular markers among lineages may provide an "objective" framework to assign heterothallic mating types.

Methodology/principal findings: Using degenerate primers designed based on previously reported MID orthologs, the "master regulator" of mating types/sexes in the colonial Volvocales, MID homologs were identified and their presence/absence was examined in nine strains of four species of Gonium. Only one of the two complementary mating types in each of the four heterothallic species has a MID homolog. In addition to heterothallic strains, a homothallic strain of G. multicoccum has MID. Molecular evolutionary analysis suggests that MID of this homothallic strain retains functional constraint comparable to that of the heterothallic strains.

Conclusion/significance: We coordinated mating genotypes based on presence or absence of a MID homolog, respectively, in heterothallic species. This scheme should be applicable to heterothallic species of other isogamous colonial Volvocales including Pandorina and Yamagishiella. Homothallism emerged polyphyletically in the colonial Volvocales, although its mechanism remains unknown. Our identification of a MID homolog for a homothallic strain of G. multicoccum suggests a MID-dependent mechanism is involved in the sexual developmental program of this homothallic species.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Chlorophyta / classification
  • Chlorophyta / genetics
  • Chlorophyta / metabolism*
  • Female
  • Genotype
  • Male
  • Phylogeny

Grants and funding

This research was supported by the Japan Society for the Promotion of Science (Research fellowship numbers 19-7661 and 23-5499, for TH; Grants-in-Aid for Scientific Research numbers 24112707 and 24247042, for HN). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.