Pharyngula

There are quite a few genes that are known to be highly conserved in both sequence and function in animals. Among these are the various Hox genes, which are expressed in an ordered pattern along the length of the organism and which define positional information along the anterior-posterior axis; and another is decapentaplegic (dpp) which is one of several conserved genes that define the dorsal-ventral axis. Together, these sets of genes establish the front-back and top-bottom axes of the animal, which in turn establishes bilaterality—this specifically laid out three-dimensional organization is a hallmark of the lineage Bilateria, to which we and 99% of all the other modern animal species belong.

There are some animals that don't belong to the Bilateria, though: members of the phylum Cnidaria, the jellyfish, hydra, sea anemones, and corals, which are typically radially symmetric. A few cnidarian species exhibit bilateral symmetry, though, and Finnerty et al. (2004) ask a simple question: have those few species secondarily reinvented a mechanism for generating bilateral symmetry (so that this would be an example of convergent evolution), or do they use homologous mechanisms, that is, the combination of Hox genes for A-P patterning and dpp for D-V patterning? The answer is that this is almost certainly an example of homology—the same genes are being used.

What Finnerty et al. did was to search for and stain for Hox and dpp genes in a cnidarian with bilateral symmetry, Nematostella vectensis. Here is their summary diagram:

Summary of Hox and TGFβ gene expression. (A) Provisional homology of Nematostella Hox genes based on phylogenetic analysis of homeodomains. Vertebrate Hox paralogs are numbered from 1 to 13. Arthropod Hox paralogs are named with Drosophila gene terminology (lab, labial; pb, proboscipedia; zen, zerknullt; Dfd, Deformed; scr, sex combs reduced; ftz, fushi tarazu; Antp, Antennapedia; Ubx, Ultrabithorax; abd-A, abdominalA; and AbdB, AbdominalB). (B) Gene expression along the oral-aboral and directive axes. The germ layer composition of Nematostella is shown in longitudinal section. To simplify the depiction of the primary body axis, the pharynx has been drawn as though everted. The mesenteries are not shown. Collectively, the five Hox expression domains span practically the entire oral-aboral axis. Anthox1a, anthox7, and anthox8 are restricted to one side of the directive axis. Likewise, both TGFβ genes, dpp and GDF5-like, exhibit asymmetric expression about the directive axis. Only the asymmetric aspects of their expression are shown. Dpp is expressed in the pharyngeal ectoderm on the side of the directive axis opposite the sector expressing anthox1a, anthox7, and anthox8. GDF5-like is expressed in the endoderm on the same side as anthox1a, anthox7, and anthox8.

Nematostella has perfectly good Hox genes that are expressed in a staggered anterior-posterior pattern. It's not quite as tidy as the vertebrate or athropod pattern—there's a lot of overlap, as you can see—but it's good enough to see the canonical Hox arrangement. Dpp and another gene in the same family, GDF5-like also show the typical metazoan asymmetry. That some members of the Cnidaria exhibit precisely the same molecular organization to their body plan suggests that the function for these molecules arose early in the origin of multicellular animals, and that the radially symmetrical cnidarians have secondarily lost them.

The data summarized here suggest that bilateral symmetry evolved before the split between Cnidaria and Bilateria. Both taxa exhibit bilateral symmetry. Both taxa exhibit staggered Hox expression domains along the primary body axis and asymmetric dpp expression along the secondary body axis. Homology is the most parsimonious explanation for the shared possession of these morphological and molecular traits. If we invoke homoplasy as an explanation, we must presume that one or both of these complex axial patterning systems evolved convergently in two independent evolutionary lineages.

Finnerty JR, Pang K, Burton P, Paulson D, Martindale MQ (2004) Origins of bilateral symmetry: Hox and dpp expression in a sea anemone. Science 304:1335-1337.