Acoelomorph flatworms and precambrian evolution

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One of many open questions in evolution is the nature of bilaterian origins—when the first bilaterally symmetrical common ancestor (the Last Common Bilaterian, or LCB) to all of us mammals and insects and molluscs and polychaetes and so forth arose, and what it looked like. We know it had to have been small, soft, and wormlike, and that it lived over 600 million years ago, but unfortunately, it wasn’t the kind of beast likely to be preserved in fossil deposits.

We do have a tool to help us get a glimpse of it, though: the analysis of extant organisms, searching for those common features that are likely to have been present in that first bilaterian; we’re looking for the Last Common Bilaterian by finding the Least Common Denominators among living species. And one place to look is among the flatworms.

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Mmmmm, octopus balls…

This really sounds delicious.

Hand-grilled in iron molds by cooks behind a large display window, the octopus dumplings are made from wheat flour paste mixed with fish stock, spring onions and boiled octopus chunks, and drizzled with a sweet sauce, dried bonito flakes and seaweed.

I could go for some takoyaki right now. Unfortunately, the bad news is that it’s from a story about introducing cephalopods as mass-market fast food in the US. If they became popular here, kiss a lot of beautiful molluscs good bye.

I’m going to have to advocate more vegetarianism, I’m afraid. Maybe we could indulge in some octopus dumplings on a few special occasions, but we’d be better off turning fruit and vegetables into the next big food fad.

<sigh> But seafood tastes so good…

Old spiders

Two short articles in this week’s Science link the orb-weaving spiders back to a common ancestor in the Early Cretaceous, with both physical and molecular evidence. What we have is a 110-million-year-old piece of amber that preserves a piece of an orb web and some captured prey, and a new comparative study of spider silk proteins that ties together the two orb-weaving lineages, the Araneoidea and the Deinopoidea, and dates their last common ancestor to 136 million years ago.

Araneoids and Deinopoids build similar looking webs—a radial frame supporting a sticky spiral—but they differ in how they trap prey. Deinopoids spin dry fibers that they fluff into threads that adhere electrostatically to small insects; Araneoids secrete glue onto the the strand, which takes less work (no fluffing), and is much more strongly adhesive. The differences are enough to make one question whether there was a single origin of orb weavers, or whether the two groups independently stumbled on the same efficient form of architecture.

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Cephalopod gnashers

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Cephalopods can inflict a nasty bite. On their underside, at the conjunction of their arms, they have a structure called the beak which does look rather like a bird’s beak, and which can close with enough force to crush shellfish. Many also dribble toxins into the wound that can cause pain, tissue necrosis, and paralysis. They aren’t the best animals to play with.

If you think about it, though, cephalopods don’t have a rigid internal skeleton. How do they get the leverage to move a pair of sharp-edged beaks relative to one another, and what the heck are they doing with a hard beak anyway? There’s a whole paper on the anatomy of just the buccal mass, the complex of beak, muscle, connective tissue, and ganglia that powers the cephalopod bite.

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Polar lobes and trefoil embryos in the Precambrian

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The diagram above shows the early cleavages of the embryo of the scaphopod mollusc, Dentalium. You may notice a few peculiarities: the first cleavage is asymmetric, producing a cell called AB and a larger sister cell, CD. Before the second division, CD makes a large bulge, called a polar lobe, and it almost looks like it’s a three-cell stage—this is called a trefoil embryo, and can look a bit like Mickey Mouse. The second division produces an A, a B, a C, and a D cell, and there’s that polar lobe, about as large as the regular cells, so that it now resembles a 5-cell embryo. What’s going on in these animals?

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Maternal effect genes

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Maternal effect genes are a special class of genes that have their effect in the reproductive organs of the mutant; they are interesting because the mutant organism may appear phenotypically normal, and it is the progeny that express detectable differences, and they do so whether the progeny have inherited the mutant gene or not. That sounds a little confusing, but it really isn’t that complex. I’ll explain it using one canonical example of a maternal effect gene, bicoid.

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