Evolutionary Analysis of the Insulin-Relaxin Gene Family from the Perspective of Gene and Genome Duplication Events
Abstract: Paralogs arise by duplications and belong to families. Ten paralogs (insulin; IGF-1 and -2; INSL3-6 and 3-relaxins) constitute the human insulin-relaxin family. The aim of this study was to outline the duplications that gave rise to the vertebrate insulin-relaxin genes and the chromosomal regions in which they reside. Neurotrophin and Trk-receptor families with more than 300, otherwise unrelated, families had paralogs in the regions hosting insulin/relaxin genes, defining two quadruplicate paralogy-regions, namely: insulin/IGF and INSL/relaxin paralogons. Thereby, the localization of insulin/relaxins in human shows that these regions were formed during two genome duplications at the stem of the vertebrates.We characterized insulin-like genes (INS-L1, -L2 and -L3) in the Ciona intestinalis genome, a species that split from the chordate lineage before the genome duplications. Conserved synteny between the Ciona region hosting the INS-Ls and two human paralogons as well as linkage of the actual paralogons, suggest that a segmental duplication gave rise to the entire region prior to the genome duplications. Synteny together with gene and protein structures demonstrate that INS-L1 is orthologous to the vertebrate INSLs/relaxins, INS-L2 to insulins and INS-L3 to IGFs. This indicates that pro-orthologs of the insulin-relaxin family were formed before Ciona. Our analysis also implies that the INSL/relaxin ancestor switched receptor from tyrosine kinase- to GPCR-type. This probably occurred after the Ciona-stage, but before the genome duplications.Using genes residing within the analyzed human paralogons that were present in a chromosomal region in the Ciona-human ancestor, we identified 37 segments with conserved synteny between the Drosophila melanogaster and human genomes. Orthologs residing in Ciona-, sea urchin- and the fly syntenic segments imply that such segments approximate an ancestral region from which the human paralogons originated.To conclude, the human paralogons are remnants of genome duplications that in addition to segmental- and single duplications, shaped the extant vertebrate genomes. Using the quadruplicate paralogy-regions we were able to deduce duplication events of the insulin-relaxin genes and their chromosomal regions.
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