Volvox 2015: development

Replica of Antonie van Leeuwenhoek's microscope.

Ray Goldstein‘s working (!) replica of Antonie van Leeuwenhoek’s microscope.

At the start of the Development session, I asked for a show of hands of people who self-identify as developmental biologists. About four went up. That’s not quite fair, since there’s some ambiguity in the question (primarily? exclusively?), but my point was that what all of us who are interested in the evolution of multicellularity study is the evolution of development. In fact, it might fairly be said that the origin of multicellularity is the origin of development.

Development is how multicellular organisms translate genotype to phenotype, and regardless of whether we are interested in life cycle evolution, cell differentiation, or the genetic bases of these traits, I think it’s fair to say that what most of us are really after is understanding how and why development has changed in the transition from unicellular to multicellular life.

Pierre Haas, from the University of Cambridge, presented research on the biomechanics of inversion. Inversion is crucial for the members of the Volvocaceae, because at the end of cell division, they end up with their flagella on the inside, where they’re no use for motility. Since inversion occurs in all known members of the Volvocaceae, and since the Volvocaceae are, by most indications, monophyletic, it seems probable that the progenitor of the Volvocaceae underwent inversion. It’s quite surprising, then, that at least two very different forms of inversion occur within this family, and working out the details of how this came to be will be fascinating. I wrote about some of this research in an earlier post (“The most important time of your life“), and more details about so-called “type-B” inversion, including a very cool movie, can be found here.

Given the orientation of the flagella at the end of cell division, it would be tempting to conclude that inversion is simply a result of a phylogenetic constraint — that it is the only way large, spheroidal colonies could have evolved in this group. It would be a tempting conclusion, that is, if it weren’t for the existence of Astrephomene. Astrephomene (the pronunciation of which was the topic of several discussions) is a member of the Goniaceae, the sister group to the Volvocaceae, and manages to form colonies of up to 64 cells without the need for inversion. This has always bothered me, and I’ve often wondered how they do it. Shota Yamashita, from the University of Tokyo, has studied Astrephomene development in detail and worked out just how it manages to develop relatively large, spheroidal colonies without inversion. I think a detailed study of Astrephomene development is long overdue, since Astrephomene probably represents an independent origin of spheroidal colonies from that in the Volvocaceae.

Finally, Alexey Desnitskiy, from Saint Petersburg State University, gave a pre-recorded talk on Volvox biogeography. Like most aspects of volvocine ecology, this is a topic about which we know surprisingly little. The nominal genus Volvox is almost certainly polyphyletic, with at least two and probably more than two independent origins within the Volvocaceae. And while these separate lineages have converged on a common body plan, they have not always achieved this in the same way. Professor Desnitskiy has written a great deal about the developmental differences among various species of Volvox, and his talk focused on possible connections between these differences and the biogeographic history of the genus. After his talk, he answered questions via Skype.

Edit: I originally misattributed the owner of the van Leeuwenhoek replica.

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