Effect of Relaxin-Like Gonad-Stimulating Peptide on Ovaries of the Sea Cucumber Holothuria leucospilota.

Relaxin-like gonad-stimulating peptide (RGP), identified in starfish as a gonadotropin-like active peptide, has been shown to have a similar effect in sea cucumbers. Chemically synthesized RGP of the black sea cucumber, Holothuria leucospilota, (Hle-RGP) induced oocyte maturation and ex vivo spawning in ovarian fragments of the same species in the reproductive season. Therefore, this study established ovarian fragments of H. leucospilota as a bioassay model to determine the RGP concentration (EC50) required for 50% ex vivo spawning. The EC50 for Hle-RGP was approximately 7 nM. However, Hle-RGP could not induce germinal vesicle breakdown (GVBD) in defolliculated immature oocytes isolated from ovaries, suggesting that follicle cells are essential for Hle-RGP-directed oocyte maturation. When the ovaries of H. leucospilota were incubated with Hle-RGP for 6 h at 18°C, activity of a maturation-inducing hormone (MIH) was found in the media. Because approximately 50 follicle cells surround an oocyte, it is probable that follicle cells stimulated by Hle-RGP produce the MIH. In contrast, neither 1-methyladenine (1-MeAde) nor RGP of the starfish Patiria pectinifera (Ppe-RGP) had any effect on oocyte maturation in H. leucospilota. These results strongly suggest that an MIH is involved in the process of oocyte maturation in sea cucumbers, similar to starfish, but the MIH in sea cucumbers is not 1-MeAde.