Volvox: a colony of cells

In 1950, a young assistant professor at Princeton University published an essay about Volvox in Scientific American, “Volvox: a colony of cells.” The essay touches on several themes that will be familiar to regular readers of Fierce Roller, including cellular differentiation, inversion, and what it means to be an individual.

The author was John Tyler Bonner, whose (much) more recent work I’ve written about previously (“Chance favors the minute animalcule: John Tyler Bonner on randomness“).

John Tyler Bonner

John Tyler Bonner, ca. 1957. Image from the Guggenheim Foundation.

Among many other contributions, Bonner was a pioneer in the development of the social amoeba (or cellular slime mold) Dictyostelium discoideum as a model system for multicellular development and cell-cell signaling. A member of the National Academy of Sciences and a fellow of the American Association for the Advancement of Science, he has published over twenty books and mountains of peer-reviewed papers.

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Volvox newsletter

Volvox newsletter cover

As David Kirk pointed out, what we normally call the First through Fourth International Volvox Meetings are really about the fifth through eighth, as they were preceded by several meetings in the ’70s. The very first meeting was hosted by David and Marilyn Kirk at Washington University in St. Louis. Richard Starr, then at Indiana University, reported on the meeting in the first Volvox Newsletter (Dr. Starr would later move to the University of Texas, and his strains would form the beginning of the UTEX Culture Collection, which is still in operation).

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Mary Agard Pocock

Alexey Desnitskiy, Stuart Sym, and Pierre Durand have published a new paper in Transactions of the Royal Society of South Africa recounting the contributions of South African phycologist Mary Agard Pocock to Volvox research [full disclosure: Pierre Durand and I were labmates in Rick Michod’s lab at the University of Arizona for a time, and Alexey Desnitskiy is a friend and collaborator].

Pocock, who defended her Ph.D. in 1932, made careful observations of both sexual and asexual development in several species of Volvox that she collected in southern Africa: V. africanus, V. capensis,V. rousseletii, and V. gigas (which she originally described). For some of these species, hers are still the only detailed descriptions of their ontogeny:

Pocock studied almost all aspects of asexual and sexual development in several African Volvox species, with the exception of sexual differentiation control…Pocock’s data on embryonic inversion in V. africanus, V. capensis, V. gigas and V. rousseletii retain their importance today. Her description of inversion during asexual development in V. africanus and V. capensis remains the only detailed study of this process in these two species and her observations of embryonic inversion in V. gigas and V. rousseletii were corroborated almost 40 years later. [references omitted]

Pocock 1933 Fig. 2L-O

Figure 2L-O from Pocock 1933. Inversion in Volvox gigas.

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CRISPR/Cas9 mutagenesis in Volvox

Researchers in Stephen Miller’s lab at the University of Maryland, Baltimore County have successfully used CRISPR/Cas9 to knock out several developmentally important genes in Volvox carteri. CRISPR/Cas9 is a relatively new technology that allows heritable mutations to be introduced into living cells at specific locations within the genome.

This advance was announced in a new paper in The Plant Journal by José A. Ortega-Escalante, Robyn Jasper, and Stephen M. Miller (Jasper and Ortega-Escalante are listed as equal contributors). They were able to transform wild-type V. carteri with inversion-deficient and somatic-regenerator mutations, and they transformed somatic regenerator mutants with a gonidialess (no specialized reproductive cells) mutation.

I have never used CRISPR/Cas9, and I don’t know as much about it as I should, so I’m sure any explanation I gave would be riddled with errors. Here’s someone who seems to know what she’s talking about:

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Volvox email

I got an email from a science teacher in India. This is the internet being what we thought it would be in the 1990s. I did my best to answer, but feel free to weigh in in the comments.

Dear Matthew,

This is Subhashini, I am a science teacher and a content writer for higher secondary school in India. I have gone through your research papers about Volvox. I still have few questions about Volvox. As I do not want children to get confused need some clarification. I would appreciate if you can help me in answering few questions regarding the same.

Q.1 Is Volvox unicellular, multicellular or colonial organism? Why? (I understand the evolutionary process and the relation of the same but need the explanation about specific cellularity.)

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This awkward condition

I’ve written several times about the process of inversion that occurs during the development of algae in the family Volvocaceae. Today I was going through a paper I’d read (and even written about) before, and I came across a turn of phrase I appreciated regarding inversion:

The fully cleaved embryo contains all of the cells of both types that will be present in an adult but it is inside out with respect to the adult configuration. This awkward condition is quickly corrected by a gastrulation-like inversion process.

The quote is from Benjamin Klein, Daniel Wibberg, and Armin Hallmann’s 2017 paper, “Whole transcriptome RNA-Seq analysis reveals extensive cell type-specific compartmentalization in Volvox carteri.” Setting aside that animal gastrulation and Volvox inversion may not be as similar as they are often portrayed, I love the description of inside-out colonies as “awkward”. As if they just realized they left the house with their shirt half tucked in and inversion is their way of saying “Excuse me while I get myself sorted out here.”

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Are the multicellular volvocine algae monophyletic?

One of the strengths of the volvocine algae as a model system is that they span a range of sizes and degrees of complexity. Sizes range from tens of microns to a couple of millimeters, cell numbers range from one to 50,000 or so, some species do and some don’t have cellular differentiation, and some do and some don’t undergo inversion during development. This variation makes the volvocine algae ripe for comparative analyses, which I and many others have done. It also allows many of the intermediate steps between unicellular and complex multicellular life to be identified, as David Kirk did in his “twelve-step” paper.

The volvocine algae have clearly taken some of those steps more than once. Cellular differentiation, for example, has evolved at least three times, in the genus Astrephomene, in the so-called Volvox section Volvox (a.k.a. Euvolvox), and in the lineage that includes Pleodorina and the other Volvox species. One thing they seem to have only done once, though, is to evolve multicellularity itself.

There have been dozens of studies addressing the evolutionary relationships among various species of volvocine algae. Most have been from Hisayoshi Nozaki’s lab, though I and many others have weighed in as well. Nearly all of them, at least those that address the topic, agree that the three families that make up the multicellular volvocine algae–the Tetrabaenaceae, Goniaceae, and Volvocaceae–uniquely descend from a common ancestor. In other words, the multicellular volvocine algae are monophyletic.

Three important cladistic terms are used to summarize the evolutionary relationships among a group of species. If all of the members of the group descend from a common ancestor, and nothing else descends from that ancestor, the group is called monophyletic. Mammals, for example, are monophyletic. A monophyletic group is also called a clade. If all group members are descended from a common ancestor, but so are some non-group members, the group is called paraphyletic. Reptiles, for example, are paraphyletic, because there is no clade that includes all reptiles that doesn’t also include birds. The word ‘paraphyletic’ should nearly always be followed by ‘with respect to’: reptiles are paraphyletic with respect to birds.

The bottom of the barrel, in terms of evolutionary relationships, is polyphyly. A group is considered polyphyletic if its members don’t share a recent common ancestor at all, in other words, if they have multiple evolutionary origins. Flying animals are polyphyletic. Algae are polyphyletic. The genus Volvox is polyphyletic. Polyphyletic taxa are the scum of the phylogenetic Earth. Telling a taxonomist that a group she has named is polyphyletic is a deadly insult.

The prevailing view of volvocine evolutionary relationships is that the family Volvocaceae is sister to the Goniaceae (that is, each is the other’s closest relative), and the Tetrabaenaceae are sister to the Volvocaceae + Goniaceae. Two new papers infer relationships among volvocine algae and their unicellular relatives, and one of them challenges the view of multicellular monophyly.

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Everything Flows

Everything Flows cover

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

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Extreme variation in male Volvox carteri from Taiwan

Nozaki et al. 2018 Fig. 1 A-D

Figure 1 a-d from Nozaki et al. 2018. Light microscopy of asexual spheroids in Taiwanese strains of Volvox carteri f. nagariensis. a Surface view of a spheroid showing undivided gonidia (G). 2016‐tw‐nuk‐6‐1. b Optical section of a spheroid in (a) with gonidia (G). c Surface view of spheroid. Note no cytoplasmic bridges between somatic cells. 2016‐tw‐nuk‐6‐1. d Surface view of spheroid showing individual sheaths of the gelatinous matrix. Stained with methylene blue. 2016‐tw‐nuk‐8‐1. e Optical section of gonidium. 2016‐tw‐nuk‐6‐1. f, g Pre‐inversion plakea or embryo (E) showing gonidia (G) of the next generation outside. 2016‐tw‐nuk‐8‐1.

Most of what we know about the developmental genetics of Volvox comes from the Eve strain of Volvox carteri forma nagariensis, which was collected by Richard Starr from Kobe, Japan in 1967. Eve is the strain that David Kirk and colleagues used for most of their experiments and from which most of the important developmental mutants are derived.

It’s natural, then, to think that Eve is representative of V. carteri f. nagariensis and that what’s true for Eve is generally true for this forma. Recent work from Hisayoshi Nozaki and colleagues shows that, at least in one respect, this is a bad approximation.

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