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BackgroundCultured pearls are unique gems produced by living organisms, mainly molluscs of the Pinctada genus, through the biomineralization properties of pearl sac tissue. Improvement of P. margaritifera pearl quality is one of the biggest challenges that Polynesian research has faced to date. To achieve this goal, a better understanding of the complex mechanisms related to nacre and pearl formation is essential and can now be approached through the use of massive parallel sequencing technologies. The aim of this study was to use RNA-seq to compare whole transcriptome expression of pearl sacs that had producing pearls with high and low quality. For this purpose, a comprehensive reference transcriptome of P. margaritifera was built based on multi-tissue sampling (mantle, gonad, whole animal), including different living stages (juvenile, adults) and phenotypes (colour morphotypes, sex).ResultsStrikingly, few genes were found to be up-regulated for high quality pearls (n = 16) compared to the up-regulated genes in low quality pearls (n = 246). Biomineralization genes up-regulated in low quality pearls were specific to prismatic and prism-nacre layers. Alternative splicing was further identified in several key biomineralization genes based on a recent P. margaritifera draft genome.ConclusionThis study lifts the veil on the multi-level regulation of biomineralization genes associated with pearl quality determination.

Grafting associates two distinct genotypes, each of which maintains its own genetic identity throughout the life of the grafted organism. Grafting technology is well documented in the plant kingdom, but much less so in animals. The pearl oyster, Pinctada margaritifera, produces valuable pearls as a result of the biomineralisation process of a mantle graft from a donor inserted together with a nucleus into the gonad of a recipient oyster. To explore the respective roles of donor and recipient in pearl formation, a uniform experimental graft was designed using donor and recipient oysters monitored for their growth traits. At the same time, phenotypic parameters corresponding to pearl size and quality traits were recorded. Phenotypic interaction analysis demonstrated: 1) a positive correlation between recipient shell biometric parameters and pearl size, 2) an individual donor effect on cultured pearl quality traits. Furthermore, the expressions of biomineralisation biomarkers encoding proteins in the aragonite or prismatic layer showed: 1) higher gene expression levels of aragonite-related genes in the large donor phenotype in the graft tissue, and 2) correlation of gene expression in the pearl sac tissue with pearl quality traits and recipient biometric parameters. These results emphasize that pearl size is mainly driven by the recipient and that pearl quality traits are mainly driven by the donor.

International audience; The mollusc Pinctada margaritifera is the top economic aquaculture species in French Polynesia (export value) and forms the basis of the black pearl industry. Mass production of this unique gem, produced by a living organism, relies on a surgical operation requiring tissue from a donor pearl oyster to be grafted, together with a nucleus made of shell, into the gonad of a receiving pearl oyster. This technique is performed by expert grafters, whose work constitutes the first step influencing pearl farm production yield. This study makes the first report of effects mediated by individual grafters and by the season of grafting on three rates scored at 45 days post operation: nucleus retention, nucleus rejection and pearl oyster mortality. These results were obtained in a very large scale grafting experiment, designed and conducted in a single culture site in the atoll of Arutua (Tuamotu Archipelago) and involving a total of 52,910 grafts performed by ten professional grafters, during two contrasting seasons: autumn (four grafting experiments) and spring (three grafting experiments). Statistical analysis using linear mixed-effect model for both univariate and multivariate analysis was performed. The results show a very highly significant season effect (p < 0.001), with a higher rate of nucleus retention in autumn than spring: 91.1 versus 79.4 %, respectively. A very highly significant season effect was also recorded for nucleus rejection rate (p < 0.001), which was greater in spring than in autumn: 12.6 versus 7.7 %. Oyster mortality rate was up to six times higher in spring than in autumn (p < 0.001). Within the two seasons, there was no significant difference between the grafting experiments for any of the three variables, adding robustness to the inter-season results. Taking into account this seasonal variation effect, grafter skill had a significant impact on the three variables (p < 0.001), making it possible to rank grafters from most to least efficient. Aside from skill differences between grafters, grafting in autumn may significantly increase graft success. Water temperature and its indirect consequences could be one of the most important environmental factors implicated in overall efficiency. These findings may help the potential development of an annual grafting schedule according to season/ lagoon temperature as a way to maximize production yield for the black-lipped pearl oyster industry in French Polynesia.

Cultured pearl nacre deposition rate in Pinctada margaritifera oysters was studied to examine its influence on Tahitian classification grade and colour and to further our understanding of using selected donor oysters produced in a hatchery system for the improvement of these traits. A large-scale grafting experiment using 150 farmed donor oysters was realised in Rangiroa atoll (French Polynesia). After 18 months of culture, 874 pearls were harvested and 7 pearls quality traits were recorded: (1) nacre deposition rate assessed by nacre thickness and weight, (2) classification grade, lustre and surface defects, and (3) colour assessed by pearl colour darkness level and visual colour categories. Nacre deposition rate significantly influence pearl grade, surface defects, darkness level and colour categories, but not the lustre. Grade A pearls had the thickest and heaviest nacre on average, compared with grade D pearls and rejects. In addition, pearls without any surface defect had thicker and heavier nacre. Pearls with low darkness were the thinnest and the lightest. Pearls of aubergine–peacock secondary colours were the thickest and the heaviest. These results highlight the importance of rapid nacre deposition rate for increasing the production of grade A pearls, with less surface defects and dominance of a black tone colour. Fast nacre deposition may, however, have a negative impact on other pearl quality traits, such as pearl lustre. This study demonstrates the importance of examining relationships between pearl quality traits in a way that provides solid information that can be directly used for breeding and genetic selection programmes.

In coffee trees, chlorogenic acids (CGA) mainly includes three classes: the caffeoylquinic acids (CQA), the dicaffeoylquinic acids (diCQA) and the ferulylquinic acids (FQA), each class containing three isomers). An interspecific cross between Coffea pseudozanguebariae (PSE) With low CGA content and C. liberica var dewevrei (DEW) high CGA content was investigated for chlorogenic acid (CGA) contents in F1 and back-cross hybrids. The aim was to analyse CGA inheritance and relationships between CGA classes and isomers. Results constitute a model for the genetic improvement of C. canephora using interspecific crosses with PSE. CGA inheritance: 1. The importance of environmental effects (years + interactions) varies from 22% (5-CQA) to 65% (3,4-diCQA). 2. Additivity is found for 5-CQA, 3,4-diCQA and 4,5-diCQA isomers. 3. Additivity is verified, but after data transformation, for 3-CQA, 3,5-diCQA, and 5-FQA. Relationship between CGA isomers: 1. Relationships between CGA isomers are curvilinear or curvilinea. 2. The shape of the relation explains the use of transformation for additivity test. 3. The presence and the importance of the relation agree with our current knowledge on CGA pathways.