Characterization ofEcRandRXRGene Homologs and Receptor Expression During the Molt Cycle in the Crab,Uca pugilator

Crustaceans have the remarkable ability to regenerate limbs. Unlike insects, crustaceans also continue to increase in body size with age, and new limb regeneration must occur in concert with the growth and reproductive activities of the adult. In the fiddler crab, Uca pugilator, regeneration consists of the formation of a blastema (limb primordium) from cells migrating into the site of the wound. Over a time course related to the physiology and growth cycle of the animal, these cells proliferate and differentiate into an intact miniature limb, which will then increase in size and emerge as a functional appendage with the next molting of the body exoskeleton. In arthropods, changes in gene expression mediating both growth (e.g., cuticular molting) and differentiation (e.g., insect metamorphosis) are regulated by ecdysteroids. We hypothesize that ecdysteroids and their receptors are also involved in the regulation of limb regeneration in crabs. To investigate this hypothesis, clones representing the crustacean ecdysteroid receptor have been isolated and are providing the basis for the development of nucleic acid and immunological probes to identify, at the tissue level, the pattern of receptor expression relative to changes in hormone titer during all stages of regeneration. In support of this hypothesis, we have detected the expression of the nuclear receptor genes encoding the ecdysteroid receptor at the earliest stages of blastemal development. We have also shown that receptor transcript synthesis can be influenced by external factors (retinoid exposure) which disrupt Uca blastemal differentiation. These studies allow us to identify and characterize putative ecdysteroid target tissues. They begin to address the goal of defining how a common signal (e.g., circulating hormones) can influence a variety of discrete developmental programs.