This study examined the gametogenic cycle of Crassostrea gigas in controlled conditions over one year, with a focus on the initiation of gametogenesis. This work analysed also the role of temperature and photoperiod in the regulation of oyster reproduction. Broodstock were maintained in natural (NC), accelerated (AC) and perpetual winter (WC) conditions of temperature and photoperiod, with feeding ad libitum. Qualitative and quantitative analyses of the reproductive pattern were performed using biometric measurement approach, sex ratio determination, histology and a gonad filling index. Each experimental treatment led to different strategies for growth and resource allocation. The gametogenic cycle, appeared entirely modulated, accelerated or delayed, by coupled temperature/photoperiod parameters. Temperature played a key role in gonial mitosis regulation. Gonia proliferation was set off and sustained by winter temperature (8-11 °C) whatever the physiological state of oysters. Maturation of germ cells appeared to be a function of temperature and could proceed at low temperature, while ripe oysters were obtained at 8 °C in winter conditioning. The three conditioning methods used in this study, allowed the production of gametes throughout the year, including in the autumnal resting period. Moreover, stocks of ripe oysters could be maintained at low temperature during several months to produce spat when desired for aquaculture production.
The oyster vasa-like gene was previously demonstrated to be specifically expressed in germline cells of adult oysters Crassostrea gigas. In the present study, this gene was used as a molecular marker to establish the developmental pattern of germline cells during oyster ontogenesis, using whole-mount in situ hybridization and real-time PCR. The Oyvlg transcripts appeared to be localized to the vegetal pole of unfertilized oocytes and maternally transmitted to embryos. At early development, these maternal transcripts were observed to segregate into a single blastomere, from the CD macromere of 2-cell stage to the 4d mesentoblast of blastula. From late blastula stage, the mesentoblast divided into two cell clumps that migrated to both sides of the larvae body and that would correspond to primordial germ cells (PGCs). Based on these results, we postulate that the germline of C. gigas is specified at early development by maternal cytoplasmic determinants including Oyvlg mRNAs, in putative PGCs that would differentiate into germinal stem cells in juvenile oysters.
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