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.

Luckily, Dr. Nozaki had recently described two new species of Pleodorina, P. starrii and P. thompsonii, and he had deposited several male and female strains of P. starrii in the Japanese culture collection. A few weeks later (I remember some difficulties setting up a wire transfer in Yen), I had them in the lab. I tried all of the methods I had previously tried with P. californica, and everything worked! The artificial selection project was up and running soon after.

Pleodorina is special for those of us who study the evolution of multicellularity. Most of its cells are undifferentiated, like those of Chlamydomonas and Eudorina, that is, they both contribute to swimming by beating their flagella and divide to produce offspring. The rest, however, are somatic cells: they beat their flagella to help the colony swim, but they never divide to produce offspring. This condition is called ‘partial differentiation’ to distinguish it from the ‘complete differentiation’ of some Volvox species, in which the reproductive cells are also specialized. Partial differentiation is often thought of as a step on the path to complete differentiation, so understanding how and why it evolves is relevant to understanding the first steps toward ‘complex’ multicellular organisms with lots of different cell types.

Pleodorina starrii was originally isolated from Lake Sagami and Lake Tsukui in Kanagawa Prefecture, Japan. The specific epithet is in honor of Richard Starr, a phycologist at Indiana University and later the University of Texas, Austin who studied Volvox and established what is now the UTEX Culture Collection. Although P. starrii is morphologically similar to P. californica, phylogenetic analysis of five chloroplast genes showed that it is actually a close relative of P. indica:

Figure 3 from Nozaki et al. 2006. Maximum likelihood tree based on five chloroplast gene sequences. Pleodorina starrii is sister to P. indica and only a distant relative of P. californica.

Pleodorina starrii also played a pivotal role in unearthing the origin of males and females from isogamous ancestors (see Origins of the sexes: Takashi Hamaji on mating type determination and Volvox 2015: all about sex). Nozaki and colleagues found a P. starrii gene that is only present in males, and this gene turned out to be similar to one found only in the ‘minus’ mating type in Chlamydomonas reinhardtii. The gene was later named “OTOKOGI,” meaning “Japanese male chivalry.” OTOKOGI appears to only be expressed in males that have have entered the sexual cycle and begun producing sperm.

Pleodorina starrii typically has 32 or 64 cells, 25-38% of which are somatic, and can reach 175 μm in diameter. The P. starrii genome is being sequenced in Bradley Olson’s lab at Kansas State University.