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“):
Figure 1A from Nozaki et al. 2015: Surface view of asexual Volvox reticuliferus spheroids.
By my count, Nozaki’s lab has described 10 new volvocine species in the 21st century:
Pleodorina starrii and P. thompsonii
Volvox kirkiorum and V. ferrisii
Colemanosphaera charkowiensis and C. angeleri
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:
Fig. 2 from Nozaki et al. 2015: Phylogenetic analysis of the Eudorina group, as inferred from 6021 base pairs of five chloroplast genes.
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):
Fig. 1K and L from Nozaki et al. 2015: Two views of mature Volvox reticuliferus zygotes with a reticulate wall.
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]
In 1971, Richard Starr published a paper entitled “Sexual reproduction in Volvox africanus”, in concluding which he wrote (p. 66): “I do not propose at this time to establish new species to include the several sexual types. Further study is needed…”
Now such a study has been fulfilled! A new species of Volvox, V. reticuliferus Nozaki has been described (Nozaki et al., 2015. PLOS One), which is a sister species of Volvox africanus.
According to this work, Volvox reticuliferus is represented by several new Japanese strains from Lake Biwa and three old strains from the Texas Collection of Algae: UTEX 1890, UTEX 1891, and UTEX 2907, which were previously considered to be Volvox africanus strains.
It may be remarked that the strain UTEX 2907 has been recently investigated (along with several other representatives of the genus Volvox) with respect to somatic and gonidial cell numbers (Shelton et al., 2012. Evol. Ecol. Res. 14: 707–727) and some aspects of asexual development (Herron et al., 2010. J. Phycol. 46: 316–324). Now it is evident that these authors dealt with Volvox reticuliferus rather than Volvox africanus.
Therefore, the information from these two papers may be helpful for description of Volvox reticuliferus. In UTEX 2907 strain, asexual colonies consist of 1–12 gonidia and 1366–4522 somatic cells (Shelton et al., 2012, p. 715, Table 2) versus 2–8 gonidia and 800–3000 somatic cells in V. reticuliferus Japanese strains (Nozaki et al., 2015). The asymmetric division was demonstrated during embryo cleavage in UTEX 2907 strain. In some embryos, gonidial initials are already formed at the 16-celled stage, whereas in many other embryos the asymmetric division occurs at a later stage (Herron et al., 2010, p. 319, Fig 1, A–E).
Good point, Alexey; I hadn’t thought of that!