Nicole Haloupek on germ-soma differentiation

Volvox carteri

Volvox carteri by Gavriel Matt & James Umen.

Back in December, I wrote about two studies that compared global patterns of gene expression between germ cells and somatic cells in Volvox carteri, one by Benjamin Klein, Daniel Wibberg and Armin Hallmann from the University of Bielefeld in Germany and one by Gavriel Matt and Jim Umen from Washington University in St. Louis and the Donald Danforth Plant Science Center, respectively. The Matt & Umen paper has also been highlighted on the Genetics Society of America blog, Genes to Genomes.

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Origins of the sexes: more on MID

Last week, I wrote about Takashi Hamaji’s new paper characterizing the mating-type/sex-determining loci in Eudorina and Yamagishiella. That paper showed that the sex-determining region of anisogamous Eudorina is, surprisingly, considerably smaller than the mating-type loci of isogamous Chlamydomonas, Gonium, or Yamagishiella. Because only one gene, MID, is present in the male version of the sex-determining region in Eudorina, Hamaji and colleagues concluded that

…the evolution of males in volvocine algae might have resulted from altered function of the sex-determining protein MID or its target genes.

I commented that

…we’re still left with two (non-mutually exclusive) possibilities: changes to the MID gene itself may have changed which genes it interacts with (or how it interacts), or there may have been changes in the genes whose expression is controlled by MID.

Now Sa Geng and colleagues have provided at least a partial answer. In a new paper in Development, they swapped versions of MID among different volvocine species* (unfortunately, no unpaywalled version of the paper is currently available; I will add a link when I find one). We already knew that MID is necessary and sufficient for male development: genetically male Volvox carteri colonies that have MID expression turned off produce eggs, and genetically female colonies transformed with MID produce sperm packets (“Sex change (in Volvox)”). But that’s MID from the same species. It’s somewhat surprising that a single gene can cause Volvox to switch sexes, but at least Volvox MID evolved side-by-side with the genes whose expression it controls.

What would be really surprising is if MID from other species, species that diverged from the Volvox lineage ~200 million years ago, worked in Volvox. It would be extraordinarily surprising if MID from a species that doesn’t even have males  could control their development in Volvox. It won’t work. Waste of time; don’t bother trying.

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That beautiful animalcule: Andrew Pritchard on Volvox

Pritchard figure

Plate from Pritchard 1834. Image from Google Books.

Andrew Pritchard’s 1834 book The Natural History of Animalcules includes several species he classifies as Volvox. Most of them were probably not Volvox, but his Volvox globator certainly was. His description of Volvox begins on page 39. A scanned version is available online at The Biodiversity Heritage Library, but I have used the slightly higher quality scan in Google Books for the plate above.

The animalcules belonging to this genus are of a globular form, and revolve in the water. Some of the species are so large as to be discerned by unassisted vision, while others are very diminutive. Ehrenberg has not demonstrated their digestive organization; but in a note to his table, conceives they ought to follow the monads. In this genus is included that beautiful animalcule, called the Volvox globator, which forms so interesting a spectacle in the Solar and Gas Microscopes.

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Zombie Volvox on PhysOrg

Ueki & Wakabayashi Fig. 4A-C

Figure 4A-C from Ueki and Wakabayashi 2018. Ca2+-dependent changes in the direction of axonemal beating. (A) Experimental setups for observation of live or demembranated spheroids in a chamber. (B) Frames from high-speed recordings of regions near the anterior (Top) and posterior (Bottom) poles of a live spheroid. The observation using setup A was under stationary conditions in continuous light (Left) and after photostimulation (Right). (Scale bar: 100 μm.) (C) Typical sequential flagellar waveforms in a single beating cycle under each condition. Waveforms recorded as in B were traced (time interval of 1/500 s). (Scale bar: 10 μm.).

Last month, I reported on mad scientists Noriko Ueki and Ken-ichi Wakabayashi’s reanimation of dead (demembranated) Volvox rousseletii spheroids. PhysOrg is also carrying the zombie Volvox story:

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How did I miss this?

Volvox Group Limited

Volvox Group Limited:

Volvox Group Limited manufactures and distributes automotive lighting and electrical products in the United Kingdom. It distributes its products to transport industries, workshops, leisure, and the industrial consumables market. Volvox Group Limited also involves in delivering mains power in vehicles from the 12v battery to developing eco-friendly wind-up torches, as well as offers automotive bulbs and industrial tools. Volvox Group Limited was incorporated in 2005 and is based in Leeds, United Kingdom. The company is a former subsidiary of Ring Lamp Company Ltd.

Incorporated in 2005 means there’s no chance I ever had a Volvox light bulb in my ’77 MGB.

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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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The Essential Tension

The Essential Tension

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.

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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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