Figure 1 from Sekimoto 2017. The life cycle of Chlamydomonas reinhardtii. Vegetative cells (V) differentiate into mt+ and mt− gametes (G) during nitrogen starvation (−N). Mating types are restricted by mating-type loci (+ and −). When gametes are mixed, the plus and minus agglutinin molecules on their flagellar surfaces adhere to each other, and this adhesion results in increased intracellular cAMP levels. The signal triggers gamete cell wall release and mating-structure activation. Cells then fuse to form binucleate quadriflagellated cells. Zygotes with thick cell walls germinate in response to light and nitrogen supplementation, and undergo meiosis to release four haploid vegetative cells.

A brief comparative section reviews the genetics of sex and sex determination in the volvocine algae for which this is known, mostly Volvox carteri and Gonium pectorale. Much of this information was covered by Takashi Hamaji and colleagues, and some was reviewed by Desnitskiy, but the Sekimoto paper has a good, concise  summary:

As indicated previously, sexual differentiation in Ch. reinhardtii is controlled largely by the MID gene. Nozaki et al. (2006) isolated an orthologous MID gene from the oogamous volvocine alga, Pleodorina starrii (Nozaki et al. 2006). The gene, named PlestMID, is present only in the male genome and the protein is abundantly present in sperm nuclei. This finding strongly suggests that maleness was probably established from the minus mating type of its isogamous unicellular ancestor during the evolution of oogamy. MID orthologues have also been identified in the isogamous volvocine genus Gonium (Hamaji et al. 2008, 2013). Only one of the two complementary mating types in each of the six heterothallic species had a MID orthologue. In addition, a MID orthologue has been found in a homothallic strain of G. multicoccum, suggesting the involvement of MID in the sexual reproduction of homothallic species.

From the comparison of sex-determining regions (SDRs) of G. pectorale, two MT− limited genes (MID and MTD1) and one MT+ limited gene (FUS1) of Ch. reinhardtii were found to be conserved in the MT− and MT+ loci, respectively (Hamaji et al. 2016b). The product of G. pectorale FUS1 (GpFUS1) was specifically localized at the mating structure of the mt+ gamete, indicating a conserved function in the conjugation of two isogamous volvocine algal species.

In contrast, 5 female-limited and 10 male-limited genes were identified by a comparison of the SDRs of both alleles of Volvox carteri (Ferris et al. 2010). Among them, only two male-limited genes, Volvox MID (VcMID) and Volvox MTD1 (VcMTD1), showed similarities to sex-limited genes of Ch. reinhardtii; however, no FUS1-like gene was detected. Because both VcMID and VcMTD1 are expressed constitutively, their transcription might be uncoupled from sexual differentiation. Following treatment with a sex-inducing pheromone, transgenic female V. carteri expressing a male VcMID produced sexual germ-cell precursors in a pattern similar to that of female eggs, but then the precursors divided similar to androgonidia to produce functional sperm packets. In contrast, sexually induced transgenic males with VcMID knockdown produced functional eggs, or self-fertile hermaphrodites, although the early embryonic patterning was similar to that of the wild-type male (Geng et al. 2014). From these results, it was suggested that a master regulator of mating-type determination in an ancestral unicellular species was reprogrammed to control sexually dimorphic gamete development in a multicellular descendant.

Stable links:

Desnitskiy AG (2017) Differentiation of reproductive structures and experimental sex change in Volvox (Chlorophyta, Volvocaceae). International Journal of Plant Reproductive Biology, 9, 65–70.

Hamaji, T., Y. Mogi, P. J. Ferris, T. Mori, S. Miyagishima, Y. Kabeya, Y. Nishimura, et al. 2016. Sequence of the Gonium pectorale mating locus reveals a complex and dynamic history of changes in volvocine algal mating haplotypes. G3: Genes|Genomes|Genetics 62386:1–42.

Sekimoto, H. 2017. Sexual reproduction and sex determination in green algae. J. Plant Res.