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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Peer review isn’t magic

In response to Tom Sheldon’s dire warnings of the dangers of preprints, “Preprints could promote confusion and distortion,” I’ve suggested that what really promotes confusion and distortion is credulous reporters failing to apply basic journalistic standards:

Peer review isn’t a magic wand that guarantees that only solid work gets published, and it isn’t a substitute for skepticism. Reporters have a responsibility to evaluate the evidence in a paper whether it is peer reviewed or not.

A couple of recent examples are relevant. First, the claim by mathematician Michael Atiyah to have proven the Riemann Hypothesis, an immensely important number theory problem related to the distribution of prime numbers. Remember, along with promoting “confusion and distortion,” Sheldon had warned that preprints could rob journalists of “time and breathing space,” pressuring them to rush to sensationalize bad science. Reporting on Atiyah’s claim shows what utter nonsense this is.

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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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In defense of preprints

At the end of July, I criticized an opinion piece that Tom Sheldon published in Nature, “Preprints could promote confusion and distortion“:

While the article casts preprints, preprint servers, and scientists who post their work to preprint servers as potential sources of misinformation, its arguments better support the case that science reporters should act more responsibly…What irks me is that I can’t find any hint in the article that Sheldon thinks journalists share the blame when they sensationalize bad science.

The latest issue of Nature includes three more critical responses to Sheldon’s nonsense.

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Volvox inversion review

Alexey Desnitskiy from St. Petersburg State University has published a short review of the process of embryonic inversion in the genus Volvox. It is a translation, by the author, of his Russian-language paper in the journal Ontogenez (Desnitskiy, AG. 2018. Ontogenez 49:147-152). The article, in the Russian Journal of Developmental Biology, isn’t listed as open access, but it also doesn’t seem to be paywalled.

Inversion occurs during the development of all known species in the family Volvocaceae (Colemanosphaera, Eudorina, Pandorina, Platydorina, Pleodorina, Volvox, Volvulina, and Yamagishiella), where it serves to turn the embryo inside-out and get the flagella on the outer surface of the colony. The paper discusses the two distinct inversion processes found in different Volvox species:

…the inversion of “type A” and the inversion of “type B,” represented by the two species most thoroughly studied, respectively V. carteri f. nagariensis and V. globator (Hallmann, 2006; Höhn and Hallmann, 2011). The principal difference between these two types of inversion is that this process begins at the anterior pole of the embryo in the first case, while in its posterior hemisphere in the second case. Coordinated displacements of cells relative to the system of intercellular cytoplasmic bridges play, along with changes of the cell shape, an important role in the inversion process in embryos of both Volvox species. In V. globator, though, the spindle-shaped cells could be observed not in the entire embryo but only in the posterior hemisphere at the stage of its compression.

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One more test post

If you’re an email subscriber (Hi Mom), you may have noticed that the links in the emails that accompany new Fierce Roller posts have been linking to old Pharyngula posts for a while now. The problem was that the links were of the form https://freethoughtblogs.com/?p=4279 instead of https://freethoughtblogs.com/fierceroller/?p=4279. With some help from WordPress chat support, I believe that problem is now fixed. Hopefully the email accompanying this and future posts will include the right links.

What’s a DOI?

Whenever possible, I try to link to a publicly accessible version of any scientific article I report on. Sometimes this is on ResearchGate, sometimes on an author’s academic website, sometimes even a course website. The benefit is that anyone can download and read the article; the downside is that these links sometimes disappear. Publishers issue takedown notices, academics change institutions, and courses end.

For these reasons, I usually provide, in addition to a publicly accessible link, a link that I can count on to not disappear (these appear at the end of the post, under “Stable Links”). Almost always, these are DOIs. DOIs are Digital Object Identifiers, persistent handles that are permanently assigned to particular documents. Almost all academic journals assign DOIs to all of their articles (Evolutionary Ecology Research is one exception, much to my annoyance), as do the preprint servers arXiv and bioRxiv and the digital data archive Dryad.

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Jonathan Wells debunks something nobody believes

Black bear

Black bear, Glacier National Park, September 2014.

Charles Darwin speculated that whales might have evolved from bears. He was wrong, but then he didn’t have the benefit of molecular sequence data, detailed morphological comparisons, and sophisticated methods of phylogenetic inference. We’ve known for at least 50 years that cetaceans (whales, dolphins, and porpoises) are most closely related to ungulates, specifically even-toed ungulates (artiodactyls). The current consensus is that the closest living relatives of cetaceans are hippopotamuses. Not everyone agrees with this specific relationship, but no one really doubts that whales are closely related to ungulates.

You wouldn’t learn that from reading Discovery Institute Senior Fellow Jonathan Wells’ recent post, “From Bears to Whales: A Difficult Transition.”

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