Starfish gonadotropic hormone: Relaxin-like gonad-stimulating peptides

Relaxin-like gonad-stimulating peptide (RGP) of starfish Patiria (= Asterina) pectinifera is the first identified invertebrate gonadotropin to trigger final gamete maturation. Recently, chemical structures of RGP were identified in several species of starfish. Three kinds of RGP molecules are found in the class Asteroidea. The chemical structure of P. pectinifera RGP (PpeRGP) is conserved among starfish of the order Valvatida beyond species. In contrast, the chemical structures of RGP identified in Asterias amurensis and Aphelasterias japonica of the order Forcipulatida are quite different from that of PpeRGP. The chemical structure of RGP in A. amurensis (AamRGP) is exactly the same as that in Asterias rubens (the order Forcipulatida), Astropecten scoparius (the order Paxillosida), Astropecten polyacanthus (the order Paxillosida), and Echinaster luzonicus (the order Spinulosida). The chemical structure of Coscinasterias acutispina RGP (the order Forcipulatida) is consistent with that of A. japonica RGP (AjaRGP). In cross-experiments using P. pectinifera, A. amurensis, and A. japonica ovaries, AamRGP and AjaRGP can induce each species of ovaries. Neither AamRGP nor AjaRGP induce oocyte maturation and ovulation in the ovary of P. pectinifera, although the PpeRGP is active in ovaries of A. amurensis and A. japonica. This suggests that the AamRGP and AjaRGP partly act species specificity.