Ovarian and sperm regulatory peptides regulate ovulation in the oyster Crassostrea gigas

For more than six decades, several studies have shown that genital products to entering the mantle cavity via the incurrent siphon, initiate in oyster, strong and rhythmic contractions of the adductor muscle (AM). In order to characterize the regulatory peptides capable of triggering AM contractions, we focused on the identification of putative myotropic peptides from genital products. Two experimental approaches were developed. The first one, based on a mass spectrometry screening of the male genital products, led to the identification of the tetrapeptide APGWamide. This neuropeptide was also detected in the seminal secretions of the cephalopod Sepia officinalis. In this species, APGWamide is directly involved in the oocyte transport. In Crassostrea, in vitro bioassay demonstrated that APGWamide modulates the AM contractions that insure the release of oocytes in the external medium. Exposure of oysters to a physiological concentration of APGWamide triggered repetitive shell closures. The second experimental approach was based on the monitoring of HPLC purification by a myotropic bioassay using the cuttlefish oviduct contractions as a target. The successive purification steps of the acidic extraction of ovaries from mature female oysters, led to the characterization of the hexapeptide PIESVD. When applied to mature female oysters, this peptide triggered the increase of shell closure frequency. The activity of these two regulatory peptides is the first experimental evidence of a peptidergic control of egg-laying in oyster. APGWamide and PIESVD could be used, in commercial and experimental hatcheries, for the identification of mature females to be selected for in vitro fertilization.