Molecular evolutionary analysis of a gender-limited MID ortholog from the homothallic species Volvox africanus with male and monoecious spheroids

PLoS One. 2017 Jun 30;12(6):e0180313. doi: 10.1371/journal.pone.0180313. eCollection 2017.


Volvox is a very interesting oogamous organism that exhibits various types of sexuality and/or sexual spheroids depending upon species or strains. However, molecular bases of such sexual reproduction characteristics have not been studied in this genus. In the model species V. carteri, an ortholog of the minus mating type-determining or minus dominance gene (MID) of isogamous Chlamydomonas reinhardtii is male-specific and determines the sperm formation. Male and female genders are genetically determined (heterothallism) in V. carteri, whereas in several other species of Volvox both male and female gametes (sperm and eggs) are formed within the same clonal culture (homothallism). To resolve the molecular basis of the evolution of Volvox species with monoecious spheroids, we here describe a MID ortholog in the homothallic species V. africanus that produces both monoecious and male spheroids within a single clonal culture. Comparison of synonymous and nonsynonymous nucleotide substitutions in MID genes between V. africanus and heterothallic volvocacean species suggests that the MID gene of V. africanus evolved under the same degree of functional constraint as those of the heterothallic species. Based on semi quantitative reverse transcription polymerase chain reaction analyses using the asexual, male and monoecious spheroids isolated from a sexually induced V. africanus culture, the MID mRNA level was significantly upregulated in the male spheroids, but suppressed in the monoecious spheroids. These results suggest that the monoecious spheroid-specific down regulation of gene expression of the MID homolog correlates with the formation of both eggs and sperm in the same spheroid in V. africanus.

MeSH terms

  • Blotting, Southern
  • Evolution, Molecular*
  • Genes, Plant*
  • Ovule
  • Phylogeny
  • Pollen*
  • Polymerase Chain Reaction
  • Reproduction
  • Reverse Transcriptase Polymerase Chain Reaction
  • Species Specificity
  • Spheroids, Cellular*
  • Volvox / classification
  • Volvox / genetics*
  • Volvox / physiology

Grant support

TH and HN were supported by Grants-in-Aid for Scientific Research (grant numbers 16H06734, 15K14590 and 16H02518) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT)/Japan Society for the Promotion of Science (JSPS) KAKENHI ( PJF was supported by the National Aeronautics and Space Administration (; grant number NNX13AH41G) and the National Science Foundation (; grant number MCB-1412395). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.