Temporal and spatial expression of insulin-like peptide (insl5a and insl5b) paralog genes during the embryogenesis of Danio rerio

Relaxin (RLN) and insulin (INSL)-like peptides are member of the INSL/RLN superfamily, which are encoded by seven genes in humans and can activate the G-protein coupled receptor RXFP 1-4. These peptides evolved from a common ancestor, RLN3-like gene. Two rounds of whole genome duplication (WGD) in early vertebrate evolution, together with an additional WGD in the teleost lineage, caused an expansion of RLN genes set in the genome of Danio rerio. In particular, six RLN genes are present: a single copy of rln and insl3 genes, and two paralogs for the rln3 gene (rln3a and rln3b), and the insl5 gene (insl5a and insl5b). We have already reported the presence of rln3a and rln3b genes in the developing zebrafish brain, as well as the expression of rln gene in the developing zebrafish brain and extraneural territories, such as thyroid gland and pancreas. Here, we report for the first time the expression of the two parologs genes for insl5, insl5a, and insl5b in D. rerio embryonic development. The corresponding transcripts of both the paralogs are present in all embryonic stages analyzed by RT-qPCR. In situ hybridization analyses showed a restricted signal in intestinal cells and the pancreatic region at 72 hpf for insl5a, while at 96 hpf both genes are expressed in specific intestinal cells. Furthermore, in adult zebrafish intestine tissue, in situ hybridation experiments showed that insl5a transcript is specifically localized in the goblet cells, while insl5b transcript is in enteroendocrine cells. These data revealed a high degree of gene expression pattern conservation for such genes in vertebrate evolution