Figure 1 from Nozaki et al. 2017. Pleodorina sphaerica.

There really aren’t enough people looking for volvocine algae. There’s a suspicious tendency for the geographical centers of volvocine diversity — southern Africa, central North America, southeast Asia — to include the home institutions of phycologists studying volvocine diversity — Mary Pocock, Richard Starr, Hisayoshi Nozaki, respectively. I find it much more likely that this is an artifact of sampling effort than that, for example, central Africa and Central and South America are depauperate of volvocine algae.

Dr. Nozaki’s lab is really the only one studying volvocine diversity in any kind of systematic way (although lots of us dabble), and they have described at least ten new species in the 21st century. So things are getting missed. For example, I find it unlikely that Volvox capensis lives in southern Africa, Montana, and nowhere in between (see “African Volvox in Montana“).

A really, really disjunct distribution?

Even less likely is that Pleodorina sphaerica, which Dr. Nozaki’s team recently found in Thailand, has been lying low since it was last seen in India in 1951. A detailed description of the rediscovered alga has just been published in Phycologia (full disclosure: I’m a junior author). Pleodorina sphaerica is a bit of an odd duck, because it has somatic cells all the way from the front end to the back (see Figures 1, 6, and 7 above). Although Volvox has somatic cells from front to back, all of the other (known) species of Pleodorina have somatic cells only in the front, as in these P. starrii colonies:

32- and 64-celled colonies of Pleodorina starrii.

This distribution of somatic cells is a big deal, because the division of labor between reproductive and somatic cells is a simple form of cellular differentiation. This is a big part of the reason that Volvox was developed as a model system for understanding the evolution of multicellularity. Because P. sphaerica has somatic cells all over, like Volvox, it has been suggested that it might represent some kind of intermediate between Pleodorina and Volvox, or even that it should be classified as a species of Volvox.

Because modern molecular phylogenetic methods didn’t exist in 1951, the evolutionary relationship of P. sphaerica to other volvocine algae was uncertain. By comparing chloroplast DNA sequences from P. sphaerica to those of other volvocine algae, Nozaki’s team has resolved their relationships:

Figure 22 from Nozaki et al 2017. Phylogenetic position of Pleodorina sphaerica Iyengar as inferred from 6021 base pairs of five chloroplast genes. The tree was constructed by maximum likelihood (ML) method. Branch lengths are proportional to the genetic distances, which are indicated by the scale bar above the tree. Numbers on the left or right side at the branches represent bootstrap values (50% or more, based on 1000 replicates) obtained with the ML and maximum parsimony analyses, respectively. Asterisks at the branches indicate 100% bootstrap values by the two methods.

It turns out that P. sphaerica is a close relative to two other species of Pleodorina, P. californica and P. japonica (guess where those are from).  This doesn’t support the idea of P. sphaerica having a particularly close relationship to any Volvox species; in fact, P. sphaerica, P. californica, and P. japonica are all equally distantly related to V. aureus.

Until we know more about the genetic mechanisms underlying somatic cell differentiation across volvocine species, it will be hard to say how this trait evolved. We know a great deal about the genetics of soma in Volvox carteri, but very little about any other volvocine species (see “Volvox 2015: cell differentiation“). An important piece of this puzzle has just come to light, though. I’ll be blogging about it soon, but if you want a sneak preview, check out “Genetic basis for soma is present in undifferentiated volvocine green algae” by Zach Grochau-Wright and colleagues.

Stable links:

Grochau-Wright, Z. I., E. R. Hanschen, P. J. Ferris, T. Hamaji, H. Nozaki, B. J. S. C. Olson, and R. E. Michod. 2017. Genetic basis for soma is present in undifferentiated volvocine green algae. Journal of Evolutionary Biology 38:42–49.

Nozaki, H., W. Mahakham, S. Athibai, K. Yamamoto, M. Takusagawa, O. Misumi, M. D. Herron, F. Rosenzweig, M. Kawachi. 2017. Rediscovery of the species of “ancestral Volvox”: morphology and phylogenetic position of Pleodorina sphaerica (Volvocales, Chlorophyceae) from Thailand. Phycologia 56:469–475.