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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Ulvophyte multicellularity: the sea lettuce genome

Ulva

Sea lettuce (Ulva sp.), Jericho Beach, Vancouver, BC, February 28, 2011.

David Kirk called the Chlorophyte green algae “master colony-formers” because multicellularity has evolved so many times within this class:

Although members of most chlorophycean genera and species are unicellular flagellates, multicellular forms are present in 9 of the 11 chlorophycean orders (Melkonian 1990). Multicellularity is believed to have arisen independently in each of these orders, and in some orders more than once.

In contrast, multicellularity has probably only evolved once or twice in the probable sister group of the Chlorophyceae, the Ulvophyceae. So when numbers like 25 get thrown around for the number of times multicellularity has evolved, something like half of those times were in the green algae.

We know a lot less about how multicellularity evolved in the Ulvophyceae than we do in the volvocine algae within the Chlorophyceae. A big step forward in understanding ulvophyte multicellularity happened last week, though, with the publication of the Ulva mutabilis genome.

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Uncommon Descent on Elizabeth Pennisi’s Science article

Two-headed quarter

Image from www.twoheadedquarter.net.

Yesterday, I ran a bit long about Elizabeth Pennisi’s new article in Science, “The momentous transition to multicellular life may not have been so hard after all.” I’m not the only one who noticed it, though; Uncommon Descent also commented (“At Science: Maybe the transition from single cells to multicellular life wasn’t that hard?“). There’s not much to it, just a longish quote from the article followed by this:

So at the basic level, there is a program that adapts single cells to multicellularity? Yes, that certainly makes multicellularity easier and even swifter but it also make traditional Darwinian explanations sound ever more stretched.

So if the evolution of multicellularity is easy, that’s evidence against “traditional Darwinian explanations.” Remember “Heads I win, tails you lose“?

…if multicellularity is really complicated, that’s evidence for intelligent design. But if multicellularity is really simple, that’s evidence for intelligent design.

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Multicellularity in Science

I spent the last week of June backpacking in Baxter State Park, Maine. When I finally emerged from the woods, my first stop was Shin Pond Village for a pay shower, a non-rehydrated breakfast, and free internet access. Among the week’s worth of unread emails were a nice surprise and a not-so-nice surprise. The not-so-nice surprise was a manuscript rejected without review; the nice surprise was a new article by Elizabeth Pennisi in Science, which came out when I was somewhere between Upper South Branch Pond and Webster Outlet.

Upper South Branch Pond

Upper South Branch Pond, Baxter State Park, Maine. I spent two nights here.

The article, for which I was interviewed before Baxter, synthesizes recent work across a wide range of organisms that suggests that the evolution of multicellularity may not be as difficult a step as we often assume:

The evolutionary histories of some groups of organisms record repeated transitions from single-celled to multicellular forms, suggesting the hurdles could not have been so high. Genetic comparisons between simple multicellular organisms and their single-celled relatives have revealed that much of the molecular equipment needed for cells to band together and coordinate their activities may have been in place well before multicellularity evolved. And clever experiments have shown that in the test tube, single-celled life can evolve the beginnings of multicellularity in just a few hundred generations—an evolutionary instant.

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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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Another take on volvocine individuality

Dinah Davison & Erik Hanschen

Dinah Davison and Erik Hanschen.

A couple of weeks ago, I indulged in a little shameless self-promotion, writing about my new chapter on volvocine individuality in Biological Individuality, Integrating Scientific, Philosophical, and Historical Perspectives. Now two graduate students in the Michod lab at the University of Arizona, Erik Hanschen and Dinah Davison, have published their own take on volvocine individuality in Philosophy, Theory, and Practice in Biology (“Evolution of individuality: a case study in the volvocine green algae“). The article is open-access, and Hanschen and Davison are listed as equal contributors.

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Non-model model organisms

Jim Umen, the lead organizer of the upcoming Volvox meeting, has written a section for a new paper in BMC Biology, “Non-model model organisms.” Like all of the BMC journals, BMC Biology is open access, so you can check out the original.

The article surveys organisms that, while not among the traditional model systems, have been developed as model systems for studying particular biological questions. The paper has an unusual format, with a discrete section devoted to each species, each written by one or two of the authors. Aside from Volvox, there are sections on diatoms, the ciliates Stentor and Oxytricha, the amoeba Naeglaria, fission yeast, the filamentous fungus Ashbya, the moss Physcomitrella, the cnidarian Nematostella, tardigrades, axolotls, killifish, R bodies (a bacterial toxin delivery system), and cerebral organoids (a kind of lab-grown micro-brain).

Dr. Umen presents Volvox and its relatives as a model system for understand the evolution of traits related to the evolution of multicellularity:

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