More evidence for co-option in the evolution of soma

One of the reasons Volvox was developed as a model organism was that it has the minimum number of cell types something with cellular differentiation can have: two. This property focuses investigations of cellular differentiation in a way that an organism with many cell types could not. In describing their move from studying avian and mammalian models to studying Volvox, Marilyn and David Kirk said,

The thing that appealed to us most about V. carteri – in addition to the genetic accessibility that Starr (1970) had already demonstrated – was the fact that it presented the germ-soma dichotomy in such a clear and simple form. Each asexual adult (or “spheroid”) of V. carteri contains only two cell types: small, biflagellate somatic cells, and large asexual reproductive cells, called gonidia (figure 1). The somatic cells are mortal; once they have provided the organism with motility for a few days they die. The gonidia, in contrast, are potentially immortal; each mature gonidium acts as a stem cell, dividing to produce a juvenile organism containing a new cohort of gonidia and somatic cells. No one has ever found a way to make wild-type somatic cells divide, but the only way to prevent gonidia from dividing is by withholding energy or poisoning them. Who could ask for a clearer presentation of one of the central issues of developmental biology: how are cells with extremely different phenotypes produced from the progeny of a single cell?

Kirk & Kirk 2004 Fig. 1

Figure 1 from Kirk & Kirk 2004. A young adult spheroid of V. carteri consists of thousands of small, biflagellate somatic cells that are embedded at the surface of a transparent sphere of extracellular matrix, and about 16 large asexual reproductive cells, called gonidia, that are located just internal to the somatic cells.

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Two new gene expression studies in Volvox

One of the most remarkable things about multicellular organisms is the differentiation of genetically identical cells into functionally specialized cell types. It’s difficult to say exactly how many cell types a given species has, since we would first have to say how different two cells need to be to count as different types. Nevertheless, it’s clear that there’s a wide range among different multicellular groups. Within animals, for example, placozoa have around five cell types, mammals over a hundred.

Amazingly, all of these very different cell types share a genome: your liver cells are pretty much genetically identical to your brain cells (and your skin cells, your kidney cells, your muscle cells…). The dramatic differences in form and function among all these cell types are mainly a result of differences in gene expression.

Volvox has just two cell types: a dozen or so big cells that are responsible for reproduction and one or two thousand smaller cells that bear the flagella that colonies use to swim:

Matt & Umen Fig 1A

Figure 1A from Matt & Umen 2017. Micrographs of an intact adult Volvox carteri spheroid with fully mature somatic and gonidial cells (left), isolated somatic cell (top right), and isolated gonidial cell (bottom right).

This was one of the main attractions for the researchers who developed Volvox as a model organism. With only two cell types, Volvox retains something close to its original form of cellular differentiation, making questions about how such differentiation evolved much more tractable.

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Cells, colonies, and clones: individuality in the volvocine algae

Biological Individuality

As I mentioned previously, I have a chapter in the newly published book Biological Individuality, Integrating Scientific, Philosophical, and Historical Perspectives. The chapter was actually written nearly five years ago, but things move more slowly in the philosophy world than that of biology. Finally, though, both the print and electronic versions are now available; here is the electronic version of my chapter. The book currently has no reviews on Amazon, so if you want to give it a read, yours could be the first. If you’re interested in current and historical views on individuality, there is a lot of good stuff in here, including contributions by Scott Lidgard & Lynn Nyhart, Beckett Sterner, Andrew Reynolds, Snait Gissis, Olivier Rieppel, Michael Osborne, Hannah Landecker, Ingo Brigandt, James Elwick, Scott Gilbert, and Alan Love & Ingo Brigandt.

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J. S. Huxley part 2: Volvox

Last time, I wrote about Julian Huxley’s 1912 book, The Individual in the Animal Kingdom, and his use of the volvocine algae as an example. I liked most of what he had to say, though I took issue with his assertion that

…all the other members of the family except Volvox…are colonies and nothing more—their members have united together because of certain benefits resulting from mere aggregation, but are not in any way interdependent, so that the wholes are scarcely more than the sum of their parts.

This is, of course, a matter of how we define a multicellular organism, but I think any definition that excludes, for example, Eudorina, is not a very useful one.

This time, I’ll look at the rest of what Huxley had to say about the volvocine algae, most of which is about Volvox:

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J. S. Huxley part 1: Gonium

Julian Huxley was one of the biologists responsible for the merging of Mendelian genetics and Darwinian evolution in the early 20th century, the modern synthesis. His most influential work was Evolution: The Modern Synthesis, published in 1942. Thirty years earlier, though, he published a book on biological individuality, The Individual in the Animal Kingdom. Thankfully, the copyright on this book has expired, so it is now part of the public domain, and a scanned version is available for free in pdf and epub versions from Google.

Huxley Cover

Any book with Volvox on the cover can’t be all bad!

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Sex change (in Volvox)

Alexey Desnitskiy from Saint Petersburg State University has published a new review of sexual development in the genus Volvox in the International Journal of Plant Reproductive Biology. 

The article includes an up-to-date review of Professor Desnitskiy’s own work describing four developmental “programs” in the various species of Volvox:

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Pleodorina inversion

Stephanie Höhn and Armin Hallmann have published a detailed study of the developmental process of inversion in Pleodorina californica. Pleodorina is one of the two genera we usually refer to as ‘partially differentiated’ (the other is Astrephomene), meaning that some of their cells are specialized for motility and never reproduce (soma) and some perform both motility and reproductive functions. P. californica is pretty big, up to about 1/3 of a millimeter, easily visible to the naked eye (though you’d need better vision than mine to make out any details).

Stephanie Höhn sampling a pond near Cambridge University during the Volvox 2015 meeting.

Stephanie Höhn sampling a pond near Cambridge University during the Volvox 2015 meeting.

Like all members of the family Volvocaceae, P. californica undergoes complete inversion during development:

After the completion of the cell division phase and before inversion, the embryos of Gonium, Pandorina, Eudorina and Pleodorina consist of a bowl-shaped cell sheet, whereas the embryonic cells of Volvox form a spherical cell sheet. With exception of the genus Astrephomene, all multicellular volvocine embryos face the same “problem”: the flagellar ends of all the cells point toward the interior of the bowl-shaped or spherical cell sheet rather than to the exterior, where they need to be later to function during locomotion. [References removed]

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Multicellularity rundown

Too many papers, not enough time: each of these deserves a deep dive, but my list just keeps getting longer, so I’m going to have to settle for a quick survey instead. To give you an idea of what I’m up against, these papers were all published (or posted to bioRxiv) in July and August, 2016. By the time I could possibly write full-length posts about them all, there would probably be ten more!

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Graduate student position in the Nedelcu lab

If you’re a fan of Volvox and the volvocine algae and have recently received an undergraduate degree in biology or a related field, now’s your chance to get serious about studying them. Aurora Nedelcu is looking for a graduate student to join her lab at the University of New Brunswick. Professor Nedelcu is a major player in the Volvox community, having published foundational papers on diverse aspects of volvocine biology and organized the first two international Volvox meetings. This is a great opportunity to join a vibrant and growing research community:

A graduate student position is available in the laboratory of Aurora Nedelcu, in the Department of Biology at the University of New Brunswick, Fredericton, CANADA. Research in our laboratory is directed towards understanding general, fundamental issues in evolution – such as the evolution of multicellularity, development, cell differentiation, sex, programmed cell death, altruism.  Our research is rooted in the framework of transitions in individuality and evolution of complexity (at a conceptual level), and of cellular responses to stress (at a more mechanistic level).  The experimental model-system we are currently using is the green algal group, Volvocales (see our Volvocales Information Project; http://www.unbf.ca/vip). Highly motivated students with interests in either theoretical/genomics or experimental/molecular approaches, and previous research experience are encouraged to apply. Interested applicants should e-mail a CV, summary of research experience and interests, unofficial transcripts, and contact information for three referees to [email protected].

Applicants should meet the minimum requirements for acceptance in the Biology Department Graduate Program (see http://www2.unb.ca/biology/Degree_Info/Graduate.html).