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Cultured pearl production from Pinctada margaritifera uses the biomineralization capacities of the mantle graft, the saibo, which is usually obtained from only the middle mantle section of the donor oyster. To evaluate the potential for using other parts of the mantle, this study explores and describes the cultured pearl quality traits, pearl size, shape, surface defects and colour parameters obtained with saibo from the entire length of the mantle, comprising the four following sections: 1) posterior, 2) connection with the gills, 3) middle, (the section usually used commercially), and 4) anterior. Rates of nucleus retention and oyster mortality were also recorded and compared between sections. For this, two experimental grafts were designed and conducted in two contrasting culture sites, using 10 selected wild donor oysters in each to perform a total of 1536 grafts. Mantle section comparison revealed that the anterior section was different from the three other sections, showing: 1) the lowest nacre deposition rate in terms of weight and thickness, 2) the palest pearls, with lowest rate of the attractive overtone colour and the 3) a lower rate of pearls with lustre. For pearl circles and shape, no difference was recorded among the different mantle sections. Posterior, connection and middle sections showed similar pearl quality traits, revealing how the number of high quality saibo obtainable from the same batch of donors can easily be increased, thus benefitting the P. margaritifera pearl industry. This finding could provide significant benefits to pearl farmers and the further development of current pearl grafting practices.

The black-lipped pearl oyster, Pinctada margaritifera , produces the largest and most valuable coloured pearls in the world. Cultured pearl size remains one of the most important quality traits. Despite the great geographical area covered by pearl farms in the atolls of French Polynesia, little is known about the influence of grow-out site effects on pearl size attained. To explore the genetic and environmental impact on the size of pearls as well as the genetic × environment interaction, a uniform experimental graft was designed on two contrasting macro-geographical lagoons. Five biparental families of donor oysters were grafted and then reared at both sites. After 18 months of culture, phenotypic parameters corresponding to pearl size, i.e. nacre weight and thickness, were recorded among the harvested pearls. The expression of Pif-177 gene, a biomarker encoding protein in the aragonite nacreous layer, was analysed in the corresponding pearl sac. The results show a family effect for nacre weight and thickness on both sites, with family F058 producing the heaviest and thickest nacre, and F805 the lightest and thinnest. By contrast, inter-site comparison revealed no significant site effects for these two parameters. In addition, grow-out location did not modify the relative gene expression of Pif-177 in the pearl sac between donor families in either culture site. Both nacre weight and thickness were positively correlated with the level gene expression of Pif-177. These results suggest that pearl size parameters were not affected by the environment in the present study and this is supported by the relative gene expression of Pif-177 observed. This knowledge constitutes an initial step in the study of pearl size trait inheritance, which will be helpful in the near future for the diffusion of genetically selected donor oyster lines produced by hatchery systems throughout production sites.

The black-lip pearl oyster Pinctada margaritifera is a protandrous hermaphrodite species. Its economic value has led to the development of controlled hatchery reproduction techniques, although many aspects remain to be optimized. In order to understand reproductive mechanisms and their controlling factors, two independent experiments were designed to test hypotheses of gametogenesis and sex ratio control by environmental and hormonal factors. In one, pearl oysters were exposed under controlled conditions at different combinations of temperature (24 and 28°C) and food level (10,000 and 40,000 cells mL−1); whereas in the other, pearl oysters were conditioned under natural conditions into the lagoon and subjected to successive 17β-estradiol injections (100 μg per injection). Gametogenesis and sex ratio were assessed by histology for each treatment. In parallel, mRNA expressions of nine marker genes of the sexual pathway (pmarg-foxl2, pmarg-c43476, pmarg-c45042, pmarg-c19309, pmarg-c54338, pmarg-vit6, pmarg-zglp1, pmarg-dmrt, and pmarg-fem1-like) were investigated. Maximum maturation was observed in the treatment combining the highest temperature (28°C) and the highest microalgae concentration (40,000 cells mL−1), where the female sex tended to be maintained. Injection of 17β-estradiol induced a significant increase of undetermined stage proportion 2 weeks after the final injection. These results suggest that gametogenesis and gender in adult pearl oysters can be controlled by environmental factors and estrogens. While there were no significant effects on relative gene expression, the 3-gene-pair expression ratio model of the sexual pathway of P. margaritifera, suggest a probable dominance of genetic sex determinism without excluding a mixed sex determination mode (genetic + environmental).