It’s embarrassing, really. I’ve been studying Volvox and its relatives for 15 years now, and until today I couldn’t have told you who the most famous member of the group, Volvox carteri, was named for. Sure, I know Colemanosphaera is named for Annette Coleman, Volvox ferrisii for Patrick Ferris, and Volvox kirkiorum (“of the Kirks”) for David and Marilyn Kirk, but that’s because they were all named after I started studying Volvox.
But do you recall…the most famous algae of all?
Figures 13-20 from Nozaki et al. 2019*. Light microscopy of female strain of Volvox zeikusii Nozaki. Abbreviations: c, cytoplasmic bridges; d, daughter spheroid or developing embryo; e, egg; i, individual sheath; p, pyrenoid; s, stigma.
Figs 13–19. Asexual spheroids. Fig. 13. Optical section of spheroid. Scale bar = 50 μm. Fig. 14. Optical section of spheroid stained with methylene blue. Scale bar = 50 μm. Fig. 15. Front view of somatic cells showing cytoplasmic bridges. Scale bar = 20 μm. Fig. 16. Front view of somatic cells showing individual sheaths of the gelatinous matrix stained with methylene blue. Scale bar = 20 μm. Fig. 17. Lateral optical section of somatic cells positioned in anterior region of spheroid. Scale bar = 20 μm. Fig. 18. Surface view of somatic cells positioned in anterior region of spheroid. Scale bar = 20 μm. Fig. 19. Surface view of newly formed daughter spheroid. Scale bar = 50 μm. Fig. 20. Sexual female spheroid. Scale bar = 200 μm.
Hisayoshi Nozaki and colleagues have discovered a new species of Volvox, Volvox zeikusii. Or more accurately, they have discovered new strains of an old species and decided that some of the old strains with that name are something else.
One of the strengths of the volvocine algae as a model system is that they span a range of sizes and degrees of complexity. Sizes range from tens of microns to a couple of millimeters, cell numbers range from one to 50,000 or so, some species do and some don’t have cellular differentiation, and some do and some don’t undergo inversion during development. This variation makes the volvocine algae ripe for comparative analyses, which I and many others have done. It also allows many of the intermediate steps between unicellular and complex multicellular life to be identified, as David Kirk did in his “twelve-step” paper.
The volvocine algae have clearly taken some of those steps more than once. Cellular differentiation, for example, has evolved at least three times, in the genus Astrephomene, in the so-called Volvox section Volvox (a.k.a. Euvolvox), and in the lineage that includes Pleodorina and the other Volvox species. One thing they seem to have only done once, though, is to evolve multicellularity itself.
There have been dozens of studies addressing the evolutionary relationships among various species of volvocine algae. Most have been from Hisayoshi Nozaki’s lab, though I and many others have weighed in as well. Nearly all of them, at least those that address the topic, agree that the three families that make up the multicellular volvocine algae–the Tetrabaenaceae, Goniaceae, and Volvocaceae–uniquely descend from a common ancestor. In other words, the multicellular volvocine algae are monophyletic.
Three important cladistic terms are used to summarize the evolutionary relationships among a group of species. If all of the members of the group descend from a common ancestor, and nothing else descends from that ancestor, the group is called monophyletic. Mammals, for example, are monophyletic. A monophyletic group is also called a clade. If all group members are descended from a common ancestor, but so are some non-group members, the group is called paraphyletic. Reptiles, for example, are paraphyletic, because there is no clade that includes all reptiles that doesn’t also include birds. The word ‘paraphyletic’ should nearly always be followed by ‘with respect to’: reptiles are paraphyletic with respect to birds.
The bottom of the barrel, in terms of evolutionary relationships, is polyphyly. A group is considered polyphyletic if its members don’t share a recent common ancestor at all, in other words, if they have multiple evolutionary origins. Flying animals are polyphyletic. Algae are polyphyletic. The genus Volvox is polyphyletic. Polyphyletic taxa are the scum of the phylogenetic Earth. Telling a taxonomist that a group she has named is polyphyletic is a deadly insult.
The prevailing view of volvocine evolutionary relationships is that the family Volvocaceae is sister to the Goniaceae (that is, each is the other’s closest relative), and the Tetrabaenaceae are sister to the Volvocaceae + Goniaceae. Two new papers infer relationships among volvocine algae and their unicellular relatives, and one of them challenges the view of multicellular monophyly.
Figure 1 a-d from Nozaki et al. 2018. Light microscopy of asexual spheroids in Taiwanese strains of Volvox carteri f. nagariensis. a Surface view of a spheroid showing undivided gonidia (G). 2016‐tw‐nuk‐6‐1. b Optical section of a spheroid in (a) with gonidia (G). c Surface view of spheroid. Note no cytoplasmic bridges between somatic cells. 2016‐tw‐nuk‐6‐1. d Surface view of spheroid showing individual sheaths of the gelatinous matrix. Stained with methylene blue. 2016‐tw‐nuk‐8‐1. e Optical section of gonidium. 2016‐tw‐nuk‐6‐1. f, g Pre‐inversion plakea or embryo (E) showing gonidia (G) of the next generation outside. 2016‐tw‐nuk‐8‐1.
Most of what we know about the developmental genetics of Volvox comes from the Eve strain of Volvox carteri forma nagariensis, which was collected by Richard Starr from Kobe, Japan in 1967. Eve is the strain that David Kirk and colleagues used for most of their experiments and from which most of the important developmental mutants are derived.
It’s natural, then, to think that Eve is representative of V. carteri f. nagariensis and that what’s true for Eve is generally true for this forma. Recent work from Hisayoshi Nozaki and colleagues shows that, at least in one respect, this is a bad approximation.
If I didn’t study Volvox, I would probably study placozoa. Placozoa are animals, but you wouldn’t know it to look at them. They look and behave very much like giant amoebae, big enough to be visible to the naked eye.
Trichoplax adhaerens. By Bernd Schierwater – Eitel M, Osigus H-J, DeSalle R, Schierwater B (2013) Global Diversity of the Placozoa. PLoS ONE 8(4): e57131. doi:10.1371/journal.pone.0057131, CC BY 4.0, Link
ICZN logo by Martyn E. Y. Low. Licensed under CCBY 3.0.
While writing the previous post, I wanted to know if the long-standing tradition of not naming species after oneself is just a tradition or an actual rule. Rules and guidelines for this sort of thing are set by the International Commission on Zoological Nomenclature, so I checked their website. In the section about Latinizing proper names, I found this gem:
32- and 64-celled colonies of Pleodorina starrii. Not to scale. 
Pleodorina starrii by Matthew Herron is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
I spent a year in graduate school trying to cross male and female strains of the volvocine green alga Pleodorina californica. A year. I did some other stuff in that time, but I spent an awful lot of it trying to convince these algae to get busy. I threw everything I could think of at them: four different mating media, different temperatures, different lighting conditions…nothing worked. I never recovered a single viable zygote. I needed to cross them to generate some genetic variation for an ambitious artificial selection study, my ‘official’ dissertation project. Eventually, my advisor suggested I ask Hisayoshi Nozaki for advice.
There is little doubt that Dr. Nozaki is the world’s leading expert on volvocine biodiversity, having described about half of the known species (see for example New Volvox Species, Volvox ovalis, and African Volvox in Montana). He responded that the strains of Pleodorina californica I had been failing to breed had been collected many years ago and had probably lost the ability to reproduce sexually (a problem I mentioned in Why don’t we revise volvocine taxonomy?). I had been spinning my wheels, never realizing that I had no hope of success. I should have contacted Dr. Nozaki about eleven months earlier.
Fig. 1 from Herron 2016. Examples of volvocine species. A: Chlamydomonas reinhardtii, B: Gonium pectorale, C: Astrephomene gubernaculiferum, D: Pandorina morum, E: Volvulina compacta, F: Platydorina caudata, G: Yamagishiella unicocca, H: Colemanosphaera charkowiensis, I: Eudorina elegans, J: Pleodorina starrii, K: Volvox barberi, L: Volvox ovalis, M: Volvox gigas, N: Volvox aureus, O: Volvox carteri. A and B by Deborah Shelton.
The meeting review for the Third International Volvox Conference is now available online at Molecular Ecology (doi: 10.1111/mec.13551). The editors warned me ahead of time that the challenge for this paper would be to make it of broad interest to the readership of Molecular Ecology, so there is a lot of background information that will be old news to members of the Volvox community.
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.
Volvox ovalis, strain NIES-2569.
Volvox ovalis by Matthew Herron is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
Volvox ovalis was described by Hisayoshi Nozaki and Annette Coleman in 2011 from a strain collected near College Station, Texas. Colonies are often distinctly egg-shaped, up to 450 µm long, with 1000-2000 somatic cells and 8-12 gonidia. A member of the section Merrillosphaera, it is closely related to V. tertius and V. spermatosphaera: