Porcine relaxin, a 500 million-year-old hormone? The tunicate Ciona intestinalis has porcine relaxin

The fossil record of tunicates reaches back to the upper Cambrian period. A scidians have mobile, tadpole-like juvenile forms with a notochord, which i nspired the classification of tunicates as Urochordata, i.e., predecessors of vertebrates. The genome of the tunicate Ciona intestinalis contains a re laxin coding region that is organized like a mammalian gene, i.e., signal p eptide, B-chain domain, connecting peptide domain, followed by the A-chain domain with a stop codon after cysteine A-22, RNA-derived cDNA encodes a re laxin that is identical to the circulating form of the porcine hormone. In contrast to the porcine gene, the ascidian gene has no intron in the C-pept ide domain, and in that respect is similar to the bombyxin gene of the silk worm. During the spawning period, only enough relaxin could be extracted an d isolated from gonads of C. intestinalis for a partial sequence analysis. Remarkable as it may be, these findings suggest that relaxin is identical i n pigs, whales, and the tunicate C, intestinalis.