Homothallic, monoicous species of the green alga genus Volvox produce both male and female gametes within a single sexual spheroid. Thus, these taxa are considered strictly inbred. However, the evolution and diversification of such monoicous species have not been studied using molecular data. Volvox sp. Sagami is a homothallic, monoicous alga distinguished from other Volvox species by the morphological characteristics of its zygotes. Nevertheless, this alga has not been formally classified because the sequence of its internal transcribed spacer (ITS) region of nuclear ribosomal DNA is highly similar to that of its sister species, V. ferrisii. To investigate the species distinctness and diversity within Volvox sp. Sagami and V. ferrisii, we examined five single-copy nuclear genes (psbO, atp2, actin1, efl and MID) and the ITS region. These six nuclear DNA fragments were examined in ten culture strains of V. sp. Sagami and ten culture strains of V. ferrisii, originating from various freshwater localities in Japan. The analysis of the five gene sequences revealed a clear distinction between V. sp. Sagami and V. ferrisii. Concatenated sequences of the six nuclear DNA fragments revealed minimal genetic divergence within each species and no evidence of hybridization or gene flow between the two species. This finding potentially represents the typical molecular evolution of homothallic, monoicous species in natural populations. Consequently, Volvox sp. Sagami exhibits a state of reproductive isolation from its sister species, V. ferrisii. Here, Volvox sagami sp. nov. is described.
