Figure 4 from Haas et al. 2018. Average shapes of Volvox globator embryos for 10 stages of inversion (red lines), obtained from N = 22 overlaid and scaled embryo halves (lines in shades of blue on the left) and corresponding standard deviation shapes (shaded areas on the right).

One of my search alerts turned up a blog post about Volvox inversion, “Upside Down and Inside Out: Inversion in Volvox.” The author wasn’t identified at the top, but by the third paragraph it was clear that the post was written by someone with a deep familiarity with the subject:

In order to be able to swim, the colony must therefore turn itself inside out through a hole at the top of the cell sheet. This process is called inversion, and proceeds in different ways (type-A and type-B inversion) in different species. (It is not clear why Volvox evolved to have its flagella on the inside after cell division: the closely related alga Astrephomene divides into spherical colonies without the need for inversion.

Huh? Astrephomene, type-A and B inversion…that’s serious Volvox geekery. I thought if this were written by a science journalist, they were uncommonly well informed. I confess I skipped ahead at this point and scrolled to the bottom to find the author’s name: Pierre Haas. Now-Dr. Haas did his Ph.D. in Ray Goldstein’s lab at Cambridge University, where he studied, among other things, the physics of Volvox inversion. I have written about his work here, here, and here.

The post, then, is a very readable description of Volvox inversion and its relevance from one of the world’s leading experts:

Volvox inversion is driven by a programme of cell shape changes: different cells undergo different shape changes at different times, but the key cell shape change is the formation of wedge-shaped cells. These cells are thicker at one end than at the other; this splays the cells and thereby bends the cell sheet.

Check it out: https://www.magd.cam.ac.uk/news/inversion-in-volvox