Expression of POU domain genes in zebrafish development

Development in higher eukaryotes requires the sequential activation of a large number of transcription factors. One family of developmental regulators is the homeodomain family, found in plants and animals alike. These proteins are the products of "homeobox" genes. A subset of homeobox genes is the POU domain group of genes. POU domain genes encode sequence-specific DNA-binding developmental regulators found from worms to humans. Using the polymerase chain reaction and cDNA cloning, four POU domain-containing sequences were isolated from the zebrafish, Danio rerio. The expression patterns of the four genes, Brn1, Brn1.1, Brn2a, and Brn3, were determined by reverse transcription-polymerase chain reaction and whole mount in situ hybridization. Brn1 is expressed during neurulation in developing embryos. In adult zebrafish, Brn1 is expressed in the brain and the eyes. Brn1.1 is expressed in early cleavage, and declines during gastrulation. Later in development, embryos show expression of Brn1.1 in the eyes and brain. Expression of Brn1.1 in adult zebrafish is restricted to the ovary, eyes, and brain. The Brn2a transcript is expressed during gastrulation, with increasing levels later on in development. In adults, Brn2a expression is strongest in the brain. Brn3 shows a complex pattern of expression, with a large transcript detected in early embryos and the adult ovary. A smaller transcript is expressed in hatching larvae and in the adult eyes, gill, brain, and testes. All four Brain genes are expressed in the developing and adult nervous systems, but not restricted to these tissues. These genes are similar in their expression patterns to that of their the mammalian counterparts. It is likely that the Brain genes exert specific roles in the determination of various cell types in the embryonic and adult nervous systems in the zebrafish.