The Cellular Retinol-Binding Protein Genes Are Duplicated and Differentially Transcribed in the Developing and Adult Zebrafish (Danio rerio)

There are single copies of the genes encoding the cellular retinol-binding protein type I and II (CRBPI and CRBPII) in the human and rodent genomes. We have identified duplicate genes for both CRBPI and CRBPII in the zebrafish (Danio rerio) genome (rbp1b and rbp2b). The zebrafish rbp1b and rbp2b have conserved gene structures, amino acid sequence similarities, gene phylogenies, and syntenic relationships with their mammalian orthologs and zebrafish paralogs, rbp1a and rbp2a. Like the mammalian genes for CRBPI and CRBPII, the zebrafish rbp1b and rbp2b genes are closely linked on a single linkage group. Comparative analysis suggests that the duplicate genes of rbp1 and rbp2 in the zebrafish genome may have arisen by chromosomal or whole-genome duplication. During embryonic development, rbp1b transcripts were detected in the gall bladder of 5-day postfertilization (5 dpf) larvae. The rbp2b mRNA was abundant in the developing liver through 48 hours postfertilization (48 hpf) to 5 dpf. Using reverse transcription-polymerase chain reaction (RT-PCR), rbp1b transcripts were detected in the ovary, and rbp2b mRNA was observed predominantly in the adult liver. Tissue section in situ hybridization and emulsion autoradiography localized rbp1b mRNA to primary oocytes within the zebrafish ovary. The differential mRNA distribution patterns of the rbp1a, rbp1b, rbp2a, and rbp2b genes in the developing and adult zebrafish suggest that shuffling of subfunctions among duplicate copies of paralogous genes may be a mechanism for the retention of duplicated genes in vertebrates.