Intimations of a Creature Minireview

" It is impossible to say how the idea first entered my brain; but once conceived, it haunted me day and night. I think it was his eye! yes, it was this! He had the eye of a vulture-"-Edgar Allan Poe, " The TellTale Heart " Strikingly different animals have more in common than meets the eye. Animal architecture is guided by many conserved regulators, among them homeobox genes that have related functions in mammals, insects, and worms. The surprising conservation of the regulators stands in stark contrast to the diversity of animal form. Our curiosity is drawn to the ancestral creatures that first used homeo-box genes for pattern formation. Hints of the properties of these creatures lie in the genome. How were the master regulators retained as the creatures diverged? How were the earliest patterns of animal development controlled? The genetic pathways that direct body patterning are remarkably durable, more enduring than the seemingly ancient geological structures around us. In some cases, a set of signaling components, such as the Ras entourage of signal transduction components, is used for more than one purpose in the same organism and for different purposes in different organisms. In other cases, a function seems conserved among diverse organisms. Homeobox genes offer three dramatic examples of functional conservation that serve to raise new questions about how evolution occurs. The Hox Gene Clusters HQX genes are a specific subset of the homeobox gene family, common to most or all animals and arranged in Each member of the Hox gene cluster is expressed in a different domain along the anterior-posterior axis. Mutations in Hox genes lead to transformations of one part of the body into a copy of another, named homeosis by Bateson (1894), or to deletions of parts of the body pattern. In animals with repeating metameres, Hox genes direct differential development of metameres. A century ago the importance of metameric body organization and its evolutionary implications were clearly recognized: ". the resemblance between individual members of a series of Repeated Parts has led to the belief that they must originally have been alike, and that they have been formed bydiffer-entiation of members originally similar " (Bateson, 1894). Bateson saw homeotic changes affecting metameres as a key kind of variation, although metameric body plans are not necessary for Hox gene function. Pioneering studies by E. Lewis, A. Garcia-Bellido, T. Kaufman, and others clarified the ability …