lpetrich

In that recent issue of PNAS, June 4, 2002 (99), 12, there is an article of an interesting gene that gets expressed in the embryo of a coral animal, Acropora millepora (Anthozoa), a cnidarian. It is a homolog of the gene "decapentaplegic" (dpp) in Drosophila fruit flies and of "Bone Morphogenetic Protein 4" (BMP-4) in Xenopus frogs (fruit-fly geneticists have had a long tradition of inventing colorful gene names). This protein specifies the dorsal side of the fruit fly and the ventral side of the frog, in accordance with Geoffroy St. Hilaire's inversion hypothesis. And can even do so cross-species; both the frog and the coral proteins have the same effect on a fly embryo as a fly one.

This protein is expressed asymmetrically in the coral-animal embryo, suggesting some vestigial bilaterality. Echinoderms are similar in that respect, having bilateral larvae and radial adults.

But the other important bilateral-animal patterning genes, the Hox genes, have never been found in cnidarians. Which deepens the mystery of how they are related; is the front-to-rear bilaterian axis really homologous to the cnidarian oral-aboral axis? (that's the tentacled end - flat/attached end axis)

However, one interesting riddle in cnidarian evolution has been resolved. Cnidarians have two possible phases, an attached polyp phase, and a free-swimming medusa (jellyfish) phase. Some cnidarians are only one phase, while others alternate phase. So which came first, the polyp or the medusa?

The answer is the polyp, as determined from molecular studies. An all-polyp group, the Anthozoa, was the first to branch in the cnidarian tree. Two other groups, the Hydrozoa and the Scyphozoa, share medusa forms and a linear mitochondrial genome, a departure from the nearly universal circularity of this genome in the animal kingdom. Applying the principle of parsimony suggests that the medusa phase and the linear mt genome were invented by the ancestor of those two groups, and not in the ancestor of all the cnidarians (would have to reverse both in Anthozoa).