I haven’t been blogging much lately, and here’s one of the reasons: Peter Conlin, Will Ratcliff, and I have been editing a book on the evolution of multicellularity, which the publisher says will come out in late March, 2022. It’s available for preorder now, at a 20% discount.
The Evolution of Multicellularity, cover art by Pedro Márquez-Zacarías
Fig 2A & B from Nozaki et al. 2020. Light microscopy of Eudorina compacta Nozaki sp. nov. originating from Lake Victoria. (A) Surface view of 32-celled vegetative colony showing compactly arranged cells. (B) Optical section of 32-celled vegetative colony showing a hollow structure.
Hisayoshi Nozaki and colleagues have described another new species of volvocine algae, a member of the genus Eudorina from Lake Victoria in Tanzania. Unlike most species in this genus, the cells of Eudorina compacta are tightly packed around the surface of the colony, which is ellipsoidal. They coexist in Lake Victoria with Colemanosphaera charkowiensis, another species that Dr. Nozaki and colleagues described in 2014.
That life is hierarchically organized, with species composed of populations, populations of individuals, individuals of cells, cells of organelles, organelles of genomes, genomes of chromosomes, and chromosomes of genes, is so obvious an observation that it is quite remarkable that we have no general explanation of why this is so. –Leo Buss, The Evolution of Individuality, p. 183
Back when I was a cocky grad student, I wrote a paper that was, in some ways, critical of the work of one of the biggest names in my field. David Kirk, who passed away last year, was among the most important figures in establishing Volvox as a model system for development, genetics, and evolution, among other things. He had published a paper that I thought was unnecessarily progressivist, and I said so in terms that, in retrospect, could have been more diplomatic. In response, Dr. Kirk, whom I had never met, sent me a very thoughtful email thanking me for pointing out some of the problems and politely disagreeing on some other points. Its tone was kind and respectful when annoyed and argumentative would have probably been justified.
In that email, he offered a bet, the stakes of which were to be a beer, that one of the things I had suggested would turn out to be wrong. The issue had to do with inversion, a process that the (mostly) spheroidal algae in the family Volvocaceae undergo during development. I have written about inversion many times on Fierce Roller; in a nutshell, these algae start their lives inside-out, with their flagella on the inside, and invert to get the flagella on the outside, where they can be used for swimming. Their relatives in the genus Gonium also undergo a process of partial inversion, changing from cup-shaped (with the flagella on the concave side) to flat or slightly cup-shaped in the other direction. Dr. Kirk had interpreted Gonium‘s partial inversion as a probable intermediate step that led to the complete inversion characteristic of the Volvocaceae. My reconstructions suggested that incomplete inversion in Gonium had evolved separately from complete inversion in the Volvocaceae, and Dr. Kirk bet me that this would turn out to be wrong.
Image from Goodreads.
One of my pet topics is the concept of biological individuality, which I’ve written about quite a lot here. One question that comes up often, in fact what I initially asked Dr. Pepper when he used to carry on about it, is why does it matter?
So much ink has been spilled trying to define what an individual is, in the peer-reviewed literature of philosophy and of biology, as well as several books dedicated to the topic. What is the point of all this, to justify so much intellectual effort and so many dead trees?
Process philosophy has really just recently come on my radar, and I’m not sure what to make of it. I have written before that I don’t have a particularly strong background in philosophy, and so I’m hesitant to judge what I may not understand. At least some of the descriptions I’ve seen strike me as quasi-mystical word salads:
In short, a becoming actual entity prehends, or “feels,” not only other, past actual entities (which may be seen as the metaphysical basis for causality wherein one entity becomes part of another entity’s formation process), but also eternal objects (i.e., “pure possibilities”), which introduces novelty into the process. –Lukasz Lamza in Nature Alive – Essays on the Emergence and Evolution of Living Agents
One reason to study green algae is because they can teach us something about the evolution of multicellularity. A number of related species in the Volvocalean family form a gradation of complexity between single-celled and simple multicellular organisms. The members of this family of algae differ in size, the number of cells they produce, and whether or not there is a split between germline and somatic cells. This split is thought to be central to understanding how a new level of individuality has evolved. — Calcott 2011, p. 39.
I have an interest with the philosophy of biology, but I’m a dilettante. My background is in evolutionary biology; I haven’t had a philosophy class since I was an undergrad at UCF. Nevertheless, if you study the so-called Major Transitions, you’re inevitably going to end up reading some philosophy. Topics such as multilevel selection, emergence, and the nature of biological individuality come up over and over again in this field, and philosophers of biology have made important contributions in all of them.
Among these, I find discussions of the nature of biological individuality fascinating, and I’ve written about it often here. Volvox and its relatives often come up in these discussions, and they have for a long time. A new edited volume, Nature Alive, continues this trend in a chapter by Lukasz Lamza (“Cells, organisms, colonies, communities–the fuzziness of individuality in modern biology”).
When I ran across The Essential Tension by Sonya Bahar, my first thought was that it sounded very much like something my PhD advisor could have written:
‘The Essential Tension’ explores how agents that naturally compete come to act together as a group. The author argues that the controversial concept of multilevel selection is essential to biological evolution, a proposition set to stimulate new debate.
The subtitle is Competition, Cooperation and Multilevel Selection in Evolution, which is more than vaguely reminiscent of the ‘cooperation and conflict’ framework Rick Michod has built over the last twenty years.
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:
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.