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“).
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.
All of that was still in the future, though, when Dr. Bonner penned his essay for Scientific American. Although Dictyostelium was his primary study organism, he returned to the subject of Volvox many times over the course of his long career, and some of his writings influenced my decision to study Volvox.
Of course there is no question that a tree or an elephant is one individual, and we have a very clear mental picture of what this means, for we ourselves are individuals. But there are lower forms in the borderland between one-celled organisms and multicellular organisms that are more bothersome in this respect.
A very good example of such an organism is Volvox, a curious inhabitant of freshwater ponds which is a kind of cross between a plant and an animal…
Now if you cut out any single cell from the colony and isolate it from its neighbors, it will round off slightly into the shape of a teardrop and swim about actively by means of its two flagella, apparently quite undismayed by its solitude. In this state it closely resembles Chlamydomonas, a one-celled form that is considered to be the ancestral type of Volvox.The only deficiency of this isolated cell is that it cannot reproduce and perpetuate itself; after a time it dies.
If the cells can live either independently or in a definitely organized community, which is the individual, the cell or the colony? I suppose one could argue that both are-or that neither is. It depends entirely upon how one defines “individual.”
Coincidentally, the article mentions Mary Agard Pocock, who I wrote about just last month:
The most beautiful and detailed account of Volvox, including its process of inversion, has been given by Mary A. Pocock of Rhodes University College in Grahamstown, South Africa; much that I write here has its origin in Dr. Pocock’s papers. Of the discovery of inversion she says: “It is hardly creditable that in so well known an organism as Volvox, which has been investigated again and again during the last two centuries, a phenomenon so striking as that of inversion of daughter colonies should have been completely overlooked. Yet such is apparently the case. No mention of it can be found in the extensive Volvox literature until Powers (1908) recorded its occurrence in his description of species of Volvox from Nebraska, and photographed some of the stages.”
Bonner, J. T. 1950. Volvox: a colony of cells. Sci. Am. 182:52–55. Available at https://www.jstor.org/stable/pdf/24967459.pdf