Expression patterns and evolution of urocortin and corticotropin-releasing hormone genes in a cichlid fish.

The corticotropin-releasing hormone and urocortin family of peptides consists of five members in many vertebrates: CRH (crha/crhb in teleosts), CRH2, UCN/UTS1, UCN2, and UCN3. These genes differ in expression pattern, as well as receptor affinity, allowing them to serve a wide range of functions in a variety of species. To better understand the roles of these genes in a single species, we examined their expression patterns in the cichlid fish Astatotilapia burtoni. In situ hybridization to map mRNA expression patterns of crhb, uts1, ucn2, and ucn3 in the brain revealed conserved and distinct spatial features of expression. crhb- and uts1-expressing cells were the most broadly distributed, with several areas of co-regionalization. ucn3 was less abundant but was found in discrete regions throughout the extent of the brain, with high expression in the cerebellum, while ucn2 was restricted to only a few areas. RT-PCR showed that while crhb, uts1, and ucn3 are found in several body tissues and widespread throughout the brain, ucn2 is quite restricted in the brain, and crha is only expressed in the eye. Bayesian phylogenetic analyses identified detailed relationships and novel orthologs in the urocortin family. We found evidence for a UCN2 gene loss in some reptiles. Our detailed description of the complete family of genes in the central nervous system of a model organism will inform future studies on the function of these genes in A. burtoni and provides a foundation for comparative studies with teleosts and other vertebrates.