The choanoflagellate genome and metazoan evolution

Blogging on Peer-Reviewed Research

What are the key innovations that led to the evolution of multicellularity, and what were their precursors in the single-celled microbial life that existed before the metazoa? We can hypothesize at least two distinct kinds of features that had to have preceded true multicellularity.

  • The obvious feature is that cells must stick together; specific adhesion molecules must be present that link cells together, that aren’t generically sticky and bind the organism to everything. So we need molecules that link cell to cell. Another feature of multicellular animals is that they secrete extracellular matrix, a feltwork of molecules outside the cells to which they can also adhere.

  • A feature that distinguishes true multicellular animals from colonial organisms is division of labor — cells within the organism specialize and follow different functional roles. This requires cell signaling, in which information beyond simple stickiness is communicated to cells, and signal transduction mechanisms which translate the signals into different patterns of gene activity.

These are features that evolved over 600 million years ago, and we need to use a comparative approach to figure out how they arose. One strategy is to pursue breadth, cast the net wide, and examine divergent forms, for instance by
comparing multicellular plants and animals. This approach leads to an understanding of universal properties, of how general programs of multicellular development work. Another is to go deep and examine closer relatives to find the step by step details of our specific lineage, and that’s exactly what is being done in a new analysis of the choanoflagellate genome.

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The genome is not a computer program

The author of All-Too-Common Dissent has found a bizarre creationist on the web; this fellow, Randy Stimpson, isn’t at all unusual, but he does represent well some common characteristics of creationists in general: arrogance, ignorance, and projection. He writes software, so he thinks we have to interpret the genome as a big program; he knows nothing about biology; and he thinks his expertise in an unrelated field means he knows better than biologists. And he freely admits it!

I am not a geneticist or a molecular biologist. In fact, I only know slightly more about DNA than the average college educated person. However, as a software developer I have a vague idea of how many bytes of code is needed to make complex software programs. And to think that something as complicated as a human being is encoded in only 3 billion base pairs of DNA is astounding.

Wow. I know nothing about engine repair, but if I strolled down to the local garage and tried to tell the mechanics that a car was just like a zebrafish, and you need to throw a few brine shrimp in the gas tank now and then, I don’t think I would be well-received. Creationists, however, feel no compunction about expressing comparable inanities.

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We’re having a lunar eclipse tonight!

Check out Phil for the details. For us Minnesotans, the events start around 8pm, should reach totality around 9, and should be over around 11.

Here’s the good news: we’re supposed to have clear skies!

And now the bad news: it’s about -27°F outside right now, and while we’re supposed to warm up a little today, it’s still going to be about -15°F tonight. Don’t let your eyeballs freeze while you’re doing your moongazing.

Beelzebufo: best frog name ever

Blogging on Peer-Reviewed Research

It means “devil toad,” and it was a 10 pound monster that lived 70 million years ago, in what is now Madagascar. It’s huge, and judging by its living cousins, was a voracious predator. If it were alive today, it would probably be eating your cats and puppies.

In other words, this was an awesome toad, and I wish I had one for a pet.

Here’s what it looks like, with some very large extant toads for comparison.

i-e5b6bbd646907dd5b6e43f29f30c0653-beelzebufo.jpg
Beelzebufo ampinga, Late Cretaceous of Madagascar. (A) Skull
reconstruction showing parts preserved (white areas, Left) and distribution of
pit-and-ridge ornament (stippling, Right). (B) Skeletal reconstruction and
inferred body outline of average-sized (skull width, 200 mm; SVL, 425 mm)
adult female B. ampinga based mainly on Lepidobatrachus asper. White
areas indicate parts represented by fossil specimens. For size comparison,
dorsal view silhouettes of Ceratophrys aurita (the largest extant ceratophryine) (C), and Mantidactylus guttulatus (the largest extant Malagasy frog) (D),
are shown. cp, crista parotica; fm, foramen magnum; frp, frontoparietal; mx,
maxilla; n, nasal; pmx, premaxilla; qj, quadratojugal; qu, quadrate; sq, squamosal. (Scale bars: 50 mm.)
i-4ad5439499d731bfc005d4c6572bade4-deviltoad.jpg

There are some biogeographical puzzles associated with this beast. It’s found in Madagascar, but it’s closest extant relatives are South American…and since frogs and toads do a poor job of crossing salt water, that implies the existence of land bridges between those continents around the Cretaceous. It’s not a major puzzle, though, although some of the news reports I’ve seen play up the concern, as if it were a significant controversy. As the authors explain,

We suggest that extant ceratophryines are remnants
of a Gondwanan hyloid clade that once ranged from at least
South America to Indo-Madagascar. Whether this clade was
more broadly distributed and on which Gondwanan landmass it
originated cannot be determined on current evidence. However,
as the Late Cretaceous fauna of the Maevarano Fm,
including its ceratophryine anuran, bears little resemblance to
that of modern Madagascar, major biotic changes clearly occurred on the island in the intervening period. When and how the ancestors of the endemic mantellid and microhylid anurans arrived on Madagascar remains controversial,
but there is general agreement that these frogs did not diversify
significantly until the Paleogene. Their radiation
has been linked, at least in part, to the expansion of rainforests,
but may also have been facilitated by the extinction of archaic
faunal elements, including ceratophryines.

It was a diverse, widespread group once upon a time, and it’s not at all challenging to report that the continents have shifted in 70 million years. It’s just very cool that anurans achieved the status of charismatic megafauna*, once upon a time.

*For a generous definition of “mega”.


Evans SE, Jones MEH, Krause DW (2008) A giant frog with South American affinities from the Late Cretaceous of Madagascar. Proc Nat Acad Sci USA 105(8):2951-2956.

Plant and animal development compared

Blogging on Peer-Reviewed Research

Since I wrote about the wacky creationist who couldn’t wrap his mind around the idea that plants and animals are related, and since I generally do a poor job of discussing that important kingdom of the plants (I admit it, I’m a metazoan bigot…but I do try to overcome my biases), I thought I’d briefly mention an older review by Elliot Meyerowitz that compares developmental processes in plants and animals. The main message is that developmental processes, the mechanisms that assemble the multicellular whole, are very different in the two groups and are non-homologous, but don’t get confused: the basic cellular processes are homologous, and there’s no doubt that we are related. The emphasis in this paper, though, is the evidence that plants and animals independently evolved multicellular developmental strategies. There is some convergence, but the tools in the toolbox are different.

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The controversy expands

Our little grievance with a certain paper in Proteomics has made it to the attention of the Chronicle of Higher Ed. Some of the new info (there isn’t a lot) is right here:

Michael J. Dunn, the editor of Proteomics and a professor at University College Dublin’s Conway Institute of Biomolecular and Biomedical Research, told The Chronicle that “it’s not our policy to promote creationism” and added that the journal might retract the article.

As was the case with a less-well-known journal that inadvertently published a creationist paper (The Chronicle, September 24, 2004), the paper in Proteomics had passed peer review. In fact, one of the reviewers for the mitochondria paper, said Mr. Dunn, “does a lot of reviewing for our journal.”

Maybe they ought to lighten that reviewer’s workload, because something slipped by him or her. Or maybe, as some have speculated, something got inserted after the review.