Molluscan insulin-related peptide (MIP) and Crassostrea gigas insulin receptor-related receptor (CIR) expression in the germ cells of male and female Pacific oyster (Crassostrea gigas)

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The insulin-like growth factor (IGF) system is known to be involved in the reproduction of vertebrates and more recently, in invertebrates. In this study, the role played by the members of the IGF system namely, molluscan insulin-related peptide (MIP) and Crassostrea gigas insulin receptor-related receptor (CIR) were investigated concerning the reproductive cycle maturation of male and female Pacific oysters (C. gigas). A total of 30 samples were collected from the natural growing ground of oysters on Geoje Island monthly from March to September 2021. These samples were subjected to morphological measurements including body condition index (CI) calculation. Histological analysis was used to identify gonadal stages. MIP, IGF binding protein complex acid labile subunit (IGFBP_ALS), and CIR expression were detected in the gonads of oysters through reverse transcription-polymerase chain reaction , and fluorescence of MIP and CIR was visualized through Fluorescence in situ hybridization (FISH) analysis. The oyster had, on average, a shell length of 66.34 ± 1.69 mm, shell height of 123.27 ± 3.23 mm, shell width of 40.11 ± 1.15 mm, whole weight of 166.77 ± 6.08 g, and flesh weight of 25.20 ± 1.04 g. The CI increased during the developing stages (early and late active), peaked at the ripe stage, and decreased towards the spent stages. Gonad maturation stages were similar in both sexes with developing stages during the spring season, spawning in the summer, and spent stages towards autumn. High expression of MIP was observed in female gonads during the ripe stage (May and August) while, in male gonads, during the late active stage (April; p < 0.05). No major changes in the gene expression pattern of IGFBP_ALS and CIR across different gonad stages (p > 0.05). Image visualization of the gonadal tissues through FISH analysis showed that MIP and CIR had the same fluorescent expression pattern as their gene expression. The findings illustrated that members of the IGF system have sex-specific and gonad maturation-specific expression in the germ cell, however, additional investigations are necessary to determine the role played by the IGF system in the regulation of gonadal maturation of oysters.

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