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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Volvox meeting posters

I thought I had already done this, but if so I can’t find it. Here are the full-resolution versions of all four Volvox meeting posters:

  • First International Volvox Meeting (Volvox 2011, Tucson, Arizona) pdf jpg
  • Second International Volvox Meeting (Volvox 2013, Fredericton, New Brunswick) pdf jpg
  • Third International Volvox Meeting (Volvox 2015, Cambridge, U.K.) pdf jpg
  • Fourth International Volvox Meeting (Volvox 2017, St. Louis, Missouri) pdf jpg (there wasn’t really a poster for this one; this is the cover of the abstract booklet)

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The Two Jakes

The organizers of the Volvox 2017 meeting put together a Volvox trivia quiz, and one of the questions had to do with a 1990 movie in which Volvox had had a cameo appearance. I was stumped. I knew Europa Report was much more recent. I probably didn’t know what Volvox was in 1990, and I don’t think I had seen the movie in question, which turned out to be The Two Jakesa sequel to 1974’s Chinatown

The Two Jakes

Because that’s how dedicated I am, I rented and watched both videos and screen captured the clip in question. Sorry there’s no sound. [somewhat important spoiler below the fold]

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Exxon still loves Volvox

I hope Exxon’s scientists know more more algal taxonomy than their ad team. We’ve seen before that they don’t know the difference between Chlorophyte green algae and cyanobacteria (Exxon loves Volvox). Some of the things they’ve lumped together in that video are more distantly related than humans are to mushrooms.

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Mechanics of Volvox inversion

Variation is everywhere in biology. Structural variation is present at molecular, cellular, organismal, and population levels, and functional variation occurs in processes from metabolism to development to behavior. In spite of this, we often describe biology in typological terms, and this is often a source of confusion.

Some variation is crucial; for example, evolution is dependent on genetic variation, and behavioral variation within ant and bee colonies ensures that all the necessary jobs get done. Much variation, though, is simply biological noise, an unavoidable consequence of the mostly analogue nature of living systems. In extreme cases, variation of this sort can complicate and even derail development, but in general development is remarkably robust. A variety of regulatory mechanisms prevent small amounts of variation early in development from being amplified into large variations in adults.

Pierre Haas and colleagues have posted a preprint to arXiv describing variation in the developmental process of inversion in Volvox globator. Facultatively sexual organisms such as Volvox are great for studying non-genetic sources of variation, because it’s pretty simple to produce millions of genetically identical individuals. When they are raised in identical conditions, variation due to environmental differences is minimized, and most of the observed variation is stochastic.

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