John R. Finnerty


www.stellabase.org


My research program addresses a fundamental question in evolutionary biology that has puzzled evolutionists since the time of Darwin--namely, how can we explain the tremendous diversity of life forms. For example, modern animals display about thirty-five discrete body plans, each distinctly different from the others, with little evidence of intermediate forms, or historical continuity, even in the fossil record. My research looks for evidence of evolutionary continuity between the body plans of distantly related animals through studying the genes that applying modern molecular techniques and computer analyses to the study of two very ancient and relatively simple groups of animals, the Cnidaria and the Ctenophora. This approach relies on the premise that in order to recognize important evolutionary changes, we must first recognize what is ancient and conserved.

Most modern animals are members of a large evolutionary assemblage known as the Bilateria. Bilaterians can trace their heritage to a single common ancestor which existed some 600 million years ago. This ancestral bilaterian possessed a body that was bilaterally symmetrical, i.e. you could distinguish its head from its tail, its belly from its back, and its left from its right. Furthermore, the ancestral bilaterian possessed three main tissue layers, known as endoderm, mesoderm, and ectoderm.

The Cnidaria and Ctenophora diverged from the main branch of animal evolution prior to the origin of the bilateria. The Cnidaria comprises tentacled organisms with unique stinging cells known as cnidocytes (e.g., hydras, sea anemones, corals, and jellyfishes). The Ctenophora comprises marine animals that locomote by means of ciliary bands known as comb rows or ctene rows. Ctenophores are commonly referred to as comb jellies. Both of these evolutionary off-shoots lack pronounced bilateral symmetry and possess only two well-distinguished germ layers, an inner layer of endoderm and an outer layer of ectoderm.

Compared to the Bilateria, the Cnidaria and Ctenophora have experienced relative evolutionary stasis in terms of ecology and morphology. While bilaterians have gone on to invade the land, invent flight, attain sizes in excess of twenty tons, and establish institutes of higher learning, the diploblasts of today remain wholly aquatic, largely marine masses of ìjellyî whose morphology has changed little in the last 600 million years. Not surprisingly, bilaterians have received the lionís share of attention from developmental biologists. However, recent findings from comparative developmental genetics suggest that many of the evolutionary innovations that were critical in the bilaterian radiation are shared by the Cnidaria and Ctenophora These putative shared characters may include the establishment of a primary body axis and the invention of sensory cells, muscle cells, and nerve cells. If such traits are shared by bilaterians, ctenophores and cnidarians, it may be possible to identify ancient and conserved developmental mechanisms acting in all three distantly related lineages.

Axial patterning is the developmental process whereby an embryo establishes the orientation of its body axis (or axes) and subdivides the axis into distinct regions. Axial patterning is a critical component of ontogeny, and the modification of axial patterning mechanisms has undoubtedly been of critical importance in the evolution of animal body plans.

Among the Bilateria, a conserved set of developmental regulatory genes are known to function in patterning the anterior-posterior (AP) axis. This set include the Hox genes, the ParaHox genes, even-skipped (EVX), and empty-spiracles (EMX). The conserved role of these genes in animals as diverse as frogs and flies is believed to reflect an underlying homology i.e., all bilaterians derive from a common ancestor that possessed an AP axis and the developmental mechanisms responsible for patterning the axis.

However, the relationship between the AP axis of bilaterian animals and the oral-aboral axis of ctenophores and cnidarians, is an open question. In order to investigate the possible relationship between these axes, I have isolated axial patterning genes from the Cnidaria and the Ctenophora, including Hox genes, ParaHox genes, EVX, and EMX. Gene mapping experiments indicate that the Hox genes of cnidarians are arranged in a cluster comparable to that found in bilaterian animals. Ongoing research on axial patterning in cnidarians and ctenophores includes comparative studies of genes expression and function, evolutionary genomics, and molecular phylogenetics.