Ascidian evo-devo

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Here are three animals. If you had to classify them on the basis of this superficial glimpse, which two would you guess were most closely related to each other, and which one would be most distant from the others?

ascidian.jpg amphioxus.jpg fish_larvae.jpg

On the left is a urochordate, an ascidian, a sessile, filter-feeding blob that is anchored to rocks or pilings and sucks in sea water to extract microorganismal meals. In the middle is a cephalochordate, Amphioxus, also a filter feeder, but capable of free swimming. On the right are some fish larvae. All are members of the chordata, the deuterostomes with notochords. If you'd asked me some years ago, I would have said it's obvious: vertebrates must be more closely related to the cephalochordates—they have such similar post-cranial anatomies—while the urochordates are the weirdos, the most distant cousins of the group. Recent developments in molecular phylogenies, though, strongly suggest that appearances are deceiving and we vertebrates are more closely related to the urochordates than to the cephalochordates, implying that some interesting evolutionary phenomena must have been going on in the urochordates. We'd expect to see some conservation of developmental mechanisms because of their common ancestry, but the radical reorganization of their morphology suggests that there ought also be some significant divergence at a deep level. That makes the urochordates a particularly interesting group to examine.

Continue reading "Ascidian evo-devo" (on Pharyngula)

7 Comments

Surely you’re not saying I’m related to some sessile, filter-feeding blob?? Say it ain’t so! After all, I haven’t been sitting in front of this computer quite that long! :)

PZ, do your homework. Did you know the famous paper that originally said that urochordates and vertebrates are sister groups DID NOT INCLUDE THE HEMICHORDATES and further also showed as a result that CEPHALOCHORDATES AND ECHINODERMS are… SISTER GROUPS???!!!!. To any one with a little acquaitance of the importance of bad taxon sampling and excessively weird group couplings would have alarm lights going off. Actually, expressed doubts can already be found in the literature.

As far as I know no new study has confirmed those weird results. I think It’s a shame so many like yourself are incapable of seeing and problems and so quickly jump on the wagon of “anything that nature says”

PZ, do your homework. Did you know the famous paper that originally said that urochordates and vertebrates are sister groups DID NOT INCLUDE THE HEMICHORDATES and further also showed as a result that CEPHALOCHORDATES AND ECHINODERMS are… SISTER GROUPS???!!!!. To any one with a little acquaitance of the importance of bad taxon sampling and excessively weird group couplings would have alarm lights going off. Actually, expressed doubts can already be found in the literature.

As far as I know no new study has confirmed those weird results. I think It’s a shame so many like yourself are incapable of seeing the problems and so quickly jump on the wagon of “anything that nature says”

I presume you are referring to Delsuc et al., which acknowledges the problem, and says more data is needed to resolve the problem. Bourlat et al. directly addressed the Delsuc problem, and a) confirmed that ascidians and vertebrates belonged together, and b) resolved the echinoderms and cephalochordates in separate groups. It wouldn’t be the Blair and Hedges paper, because that one grouped the hemichordata with the echinoderms, although it also found that tunicates and vertebrates were sister groups. Vienne and Ponarotti have come to the same conclusion.

I’m willing to read the papers that suggest otherwise, but this is not an idea that has come out of a single badly flawed paper. It’s been independently confirmed a couple of times now.

It’s also not really that weird.

The bourlat paper is at least “wishy washy” about it’s supposed confirmation of the olfactoria. They focus on the fact they have proven, as we all expected, that cephalochordates+echinoderms is baloney (just why nature published the delsuc phylogeny of deuterostomes just with no hemichordates is beyond me. crapcrapcrapcrapcrap!!!!)

Look at the third bayesian tree of mitochondrial genes presented by bourlat. It actually shows cephalochordates and vertebrates together. And they say in the text:

“We note that according to our mitochondrial data set, the cepha- lochordates and vertebrates form a highly supported monophyletic group (BPP 5 100%, TF 5 96.5% and MLBP 5 91%), which fur- ther diminishes the credibility of the cephalochordate—echinoderm link”

This is not a study keen at all on emphasizing the monophyly of the olfactoria, but more keen in establishing the monophyly of chordata , contrary to delsuc’s phylogeny.

Are all the studies you say “confirm” Delsuc’s monophyly of olfactoria as wishy-washy as this one? Long branch attraction and other artifacts can repeatedly produce an equivocal relationship in different studies.

Actually, the Bourlat paper shows how better taxon sampling, by including the hemichordates ad xenoturbella, trashes delsuc’s weird cephalochordates+echinoderms. And guess what is the main differene between the mitocondrial and nuclear genes used in bourlat’s study: taxon sampling. Look at those numbers at the nodes. Muuuch better, no? So the latest phylogeny, with clearly the best taxon sampling, shows that cephalochordates are the sister group to vertebrates. Eat that!

And what about Vienne and Ponarotti? Theyr phyloegneis are full of genes but not enough taxa. For instance: They used protostomes while leaving the ambulacria out!!. That, PZ, is what we call a gaping whole in taxon sampling. No matter how many genes per family they may clone. More and more genes simply do not make up for gaping mistakes of taxon sampling. If you include the protostomes, you include the ambulacria, for Hennig’s sake!!!!

you should at least add a caution about taxon sampling on your post.

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This page contains a single entry by PZ Myers published on May 22, 2007 1:16 PM.

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