Hisayoshi Nozaki and colleagues have done it again: in a new PLoS One article, they have described yet another new species of Volvox, V. reticuliferus (see also “Volvox 2015: taxonomy, phylogeny, & ecology“):
By my count, Nozaki’s lab has described 10 new volvocine species in the 21st century:
and Volvox reticuliferus.
The new species is similar to Volvox africanus, and indeed it phylogenetic analyses based on chloroplast and nuclear genes recover V. reticuliferus as sister to V. africanus:
The main differences between V. africanus and V. reticuliferus are in sexual characters. Volvox africanus is homothallic, that is, there are not separate male and female strains. Rather, a single V. africanus strain produces both male and dioecious (producing both sperm and eggs) spheroids. Volvox reticuliferus is heterothallic, with separate male and female strains (and thus genetic sex determination). Zygotes of the two species also exhibit differences, with those of V. africanus having smooth walls and those of V. reticuliferus having reticulate walls (thus the name):
Attempts to cross V. africanus males with V. reticuliferus females failed, suggesting that the two are sexually isolated.
Finally, the authors address the problems with volvocine classification (see also “Volvox 2015: taxonomy, phylogeny, & ecology“) and suggest a reorganization of the ‘sections’ of the nominal genus Volvox:
As in previous multigene phylogenies, our phylogenetic analyses robustly resolved that three volvocacean genera Volvox, Pleodorina, and Eudorina are not monophyletic, and the lectotype species V. globator is robustly separated from Volvox sect. Merrillosphaera sensu Smith (Fig 2). Therefore, V. africanus and V. reticuliferus should not be classified to the genus Volvox when the generic classification is strictly based on the monophyletic concept. As discussed previously, however, division of the genus Volvox into four monophyletic genera (Fig 2) would not resolve problems for nonmonophyly of the genera Eudorina and Pleodorina. New phenotypic characters are still needed to clearly delineate monophyletic genera proposed in future within the Volvocaceae…here we propose a new classification system (four monophyletic sections) of Volvox at section level. Thus, V. powersii and V. gigas should be removed from section Merrillosphaera and assigned to section Besseyosphaera (type species: V. powersii), and section Copelandosphaera (type species: V. dissipatrix) should be synonymized under section Merrillosphaera based on the phylogenetic results. [references removed; see original for citations]