Alexey Desnitskiy from St. Petersburg State University has published a short review of the process of embryonic inversion in the genus Volvox. It is a translation, by the author, of his Russian-language paper in the journal Ontogenez (Desnitskiy, AG. 2018. Ontogenez 49:147-152). The article, in the Russian Journal of Developmental Biology, isn’t listed as open access, but it also doesn’t seem to be paywalled.
Inversion occurs during the development of all known species in the family Volvocaceae (Colemanosphaera, Eudorina, Pandorina, Platydorina, Pleodorina, Volvox, Volvulina, and Yamagishiella), where it serves to turn the embryo inside-out and get the flagella on the outer surface of the colony. The paper discusses the two distinct inversion processes found in different Volvox species:
…the inversion of “type A” and the inversion of “type B,” represented by the two species most thoroughly studied, respectively V. carteri f. nagariensis and V. globator (Hallmann, 2006; Höhn and Hallmann, 2011). The principal difference between these two types of inversion is that this process begins at the anterior pole of the embryo in the first case, while in its posterior hemisphere in the second case. Coordinated displacements of cells relative to the system of intercellular cytoplasmic bridges play, along with changes of the cell shape, an important role in the inversion process in embryos of both Volvox species. In V. globator, though, the spindle-shaped cells could be observed not in the entire embryo but only in the posterior hemisphere at the stage of its compression.
“Type A” inversion is characteristic of Volvox carteri, V. africanus, V. gigas, and V. tertius, while “type B” occurs in V. globator, V. capensis and V. rousseletii. There is some disagreement over the details of inversion in Volvox aureus, and Prof. Desnitskiy recommends further study. He also briefly discusses inversion other Volvocaceaean genera:
During the inversion of Pleodorina (Höhn and Hallmann, 2016), the embryonic cells undergo a change in shape not simultaneously (unlike Pandorina, Platydorina, and Eudorina), but it proceeds in a wave-like manner towards the posterior pole, resembling the inversion of type A in Volvox. On the other hand, only the cells in the posterior hemisphere of the Pleodorina embryo acquire a spindle-like shape, similar to the type B inversion. Thus, various changes in the process of the Pleodorina embryo inversion might have led, in the course of evolution, to an inversion of type A or type B in algae of the genus Volvox.
I wrote about Höhn and Hallmann’s Pleodorina work back in 2016:
A detailed picture of the evolution of inversion would be a valuable contribution, but it is probably a long way off. A full phylogenetic reconstruction of the steps involved would first require identifying those steps in a much larger sample of species. A thorough description of the process in just one species requires a great deal of work, but each such description brings us a bit closer to understanding the evolution of this crucial developmental process.
Desnitskiy, A.G. 2018. Comparative analysis of embryonic inversion in algae of the genus Volvox (Volvocales, Chlorophyta). Russ. J. Dev. Biol., 49: 129–133. DOI: 10.1134/S1062360418030025
Hallmann, A. 2006. Morphogenesis in the family Volvocaceae: different tactics for turning an embryo right-side out. Protist 157, 445–461. DOI: 10.1016/j.protis.2006.05.010
Höhn, S. and Hallmann, A. 2011, There is more than one way to turn a spherical cellular monolayer inside out: type B embryo inversion in Volvox globator. BMC Biol. 9, 89. DOI: 10.1186/1741-7007-9-89
Höhn S. and Hallmann A. 2016. Distinct shape-shifting regimes of bowl-shaped cell sheets – embryonic inversion in the multicellular green alga Pleodorina. BMC Dev. Biol. 16:35. DOI: 10.1186/s12861-016-0134-9