Algae in the family Volvocaceae are (with one exception) little spheroids that swim around in freshwater lakes, ponds, and puddles. Volvox is by far the most famous of these algae, but there are a number of smaller genera, including Eudorina, Pleodorina, and Pandorina:

Fig. 1 from Herron 2016. Examples of volvocine species; D-O are in the family Volvocaceae. (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. Figure Credit for A and B: Deborah Shelton.

All of the members of this family have a problem: at the end of cell division, they find themselves in an awkward configuration, with their flagella on the inside. Each cell has two flagella, and the algae need them on the outside to be able to swim. They achieve this through a developmental process called inversion, essentially turning themselves completely inside-out during embryogenesis. Even the one member of the family that is not spheroidal, Platydorina (F in the figure above), undergoes inversion before flattening into a horseshoe shape. The ways in which they do this are complex and diverse (see for example “Pleodorina inversion” and “The most important time of your life“), but not the topic of this post.

The sister group to the Volvocaceae, the Goniaceae, also includes a spheroidal genus, Astrephomene (C in the figure above). Although Astrephomene looks a lot like some of the Volvocaceae, say Eudorina (I) or Pleodorina (J), it doesn’t undergo inversion!

How, then, does Astrephomene end up with its flagella on the outside? Shota Yamashita and colleagues have investigated Astrephomene development to find the answer. In a new article in BMC Evolutionary Biology, researchers from the University of Tokyo and Hosei University report detailed observations of Astrephomene development:

The present study was undertaken to evaluate the cellular or subcellular mechanisms underlying the formation of spheroidal colonies of Astrephomene; to this end, we used light microscopy time-lapse imaging of an actively growing culture of a newly established strain and compared it with that of a volvocacean Eudorina, which has a similar cell number and colony size.

The lead author, Shota Yamashita, presented some of these results at the Third International Volvox Meeting (see “Volvox 2015: development“), but the new paper includes much more detail. Instead of the flashy drama of inversion, Astrephomene moves its flagella to the outside by gradual rotations of the protoplasts (basically the cell minus the cell wall):

Figure 7 from Yamashita et al. 2016. Schematic diagrams of the two mechanisms of spheroidal colony formation in the volvocine algae. In Astrephomene, rotation of daughter protoplasts occurs in conjunction with the movement of basal bodies during successive cell divisions. In Eudorina, protoplast rotation is lacking during successive divisions; a spheroidal colony is formed by means of inversion after successive divisions.

The paper, which is open access, includes time-lapse movies of several views of Astrephomene development. I’ve embedded just one here; head to the article to see the rest.