Mating systems of haploid species such as fungi, algae, and bryophytes are either heterothallic (self-incompatible) with two sex phenotypes (male and female, or mating type minus and plus in isogamous species) or homothallic (self-compatible) with only a bisexual phenotype producing zygotes within a clone. The anisogamous volvocine green alga Pleodorina starrii is a haploid species previously reported to have a heterothallic mating system. Here, we found that two additional culture strains originating from the same water system of P. starrii were taxonomically identified as P. starrii and produced male and female gametes and zygotes within a clone (bisexual). Sequences of rapidly evolving plastid genome regions were identical between the bisexual and unisexual (male or female) P. starrii strains. Intercrossings between the bisexual and unisexual strains demonstrated normal thick-walled zygotes and high survivability of F1 strains. Thus, these strains belong to the same biological species. P. starrii has a new haploid mating system that is unique in having three sex phenotypes; namely, male, female, and bisexual. Genetic analyses suggested the existence of autosomal “bisexual factor” locus independent of volvocine male and female determining regions. The present findings increase our understanding of the initial evolutionary step of transition from heterothallism to homothallism.

The dataset was composed of the rbcL gene alignment used for the construction of the tree in Figure 1B and the results of three phylogenetic analyses.

Supplementary pdf files are DIC and DAPI images of Pleodorona starrii strain P85 (NIES-4479) showing a male nucleus within a female gametic cytoplasm.

rbcL_tree.nexus: the rbcL gene alignment

rbcL_Baysiantree.tre: results of Baysian inference

rbcL_MLtree.tre: results of ML analysis

rbcL_MPtree.tre: results of MP analysis

Supplementary Information (published on Zenodo)

DIC-P85.pdf: DIC image

DAPI-P85.pdf: DAPI image