Diploblasts and triploblasts

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Carl Zimmer wrote on evolution in jellyfish, with the fascinating conclusion that they bear greater molecular complexity than was previously thought. He cited a recent challenging review by Seipel and Schmid that discusses the evolution of triploblasty in the metazoa—it made me rethink some of my assumptions about germ layer phylogeny, anyway, so I thought I’d try to summarize it here. The story is clear, but I realized as I started to put it together that jeez, but we developmental biologists use a lot of jargon. If this is going to make any sense to anyone else, I’m going to have to step way back and explain a collection of concepts that we’ve been using since Lankester in the 19th century.

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Stem cell bait-and-switch

I’m taking it easy here in the fabulous Van Dusen mansion, a bed and breakfast where I’m staying tonight, and I thought I’d browse through the stem cell legislation that’s being considered in the senate right now. It’s strange: one substantive bill has come up from the House, and all of a sudden two more bills have been proposed on the floor of the Senate.

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Link dump

I’ve made it to St Paul and am sucking down some caffeine before strolling over to the venue for my talk this afternoon, and I’ve got a few minutes for a quickie link dump from the mailbag. Digest these for a while…

The evolution of deuterostome gastrulation

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Do vertebrate embryos exhibit significant variation in their early development? Yes, they do—in particular, the earliest stages show distinct differences that mainly reflect differences in maternal investment and that cause significant distortions of early morphology during gastrulation. However, these earliest patterns represent workarounds, strategies to accommodate one variable (the amount of yolk in the egg), and the animals subsequently reorganize to put tissues into a canonical arrangement. Observations of gene expression during gastrulation are revealing deeper similarities that are common in all deuterostomes—not just vertebrates, but also the invertebrate chordates (tunicates and cephalochordates) and echinoderms.

What does all that mean? If you think of development as a formal dance, the earliest stages are like the prelude; everyone is getting out of their chairs around the ballroom, looking for partners and working their way towards the floor. The dispositions of the dancers are variable and somewhat chaotic, and vary from dance to dance. Once they get to their positions, however, we’re finding that not only is there a general similarity in their arrangements, but they’re all dancing to the very same tune. In this case, one of the repeated motifs in that tune is a gene, Nodal, which is active in gastrulation and shows a similar pattern in animal after animal.

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The godless are getting rambunctious

We’re getting rude, we dare to criticize the theistic evolutionists, and now Ophelia has done gone and poo-pooed the distinction between methodological and metaphysical materialism. I love it! Rise up, all ye fierce and firebreathing atheists!

Much as I’d enjoy the squeals of agony from the usual protesters, I’m going to suggest that you might be better off arguing over it at Ophelia’s. I’m doing a bit of traveling over the next two days, my access to the net might be spotty, and so I’ll probably be slow to approve any comments that our annoying spam filters might hold up.