-Individual Pinctada margaritifera molluscs were collected from the Takapoto atoll (Tuamotu Archipelago, French Polynesia) and used to produce ten first generation full-sib families in a hatchery system, following artificial breeding protocols. After three years of culture, these progenies were transferred to Rangiroa atoll (Tuamotu Archipelago, French Polynesia) and tested for their potential as graft donors. A large-scale grafting experiment of 1500 grafts was conducted, in which a single professional grafter used ten individual donor oysters from each of the ten families, grafting 15 recipient oysters from each donor. The recipient oysters were all obtained from wild spat collection in Ahe (Tuamotu Archipelago, French Polynesia). After 18 months of culture, 874 pearls were harvested. Highly significant donor family effects were found for nucleus retention, nacre thickness, nacre weight, pearl colour darkness and visually-perceived colour (bodycolor and overtone), pearl shape categories, surface defects and lustre, the last two of which are components of the Tahitian classification grade. No significant difference was recorded between the ten G1 families for the absence or presence of rings. The progenies could be ranked from "best" (i.e., the donor whose grafts produced the greatest number of grade A pearls) to the "worst". Some progenies had extreme characteristics: family B presented the greatest number of pearls with lustre (98%) and a high proportion of dark gray to black with green overtone pearls (70%). These results have important implications for the selective breeding of donor pearl oysters: it may be possible to reach a point where specific donor lines whose grafts produce pearls with specific quality traits could be identified and maintained as specific breeding lines.
Highlights ► Correlation between pearl grade with colour and shape exist in P. margaritifera. ► Grade A pearls was related to green overtone, round shape and uncircled pearls. ► Pearls in reject class was related to more grey bodycolor and circled pearls. ► Selection for green/peacock pearl increase the grade A pearls rate.
Molluscs display a vast range of shell colours both between and within species. However, only a few species show colour variation in their soft tissues. In French Polynesia, the pearl oyster Pinctada margaritifera has three tissue morphotypes: the black wild-type and two rare mutations: white albino and orange mantle. Phenotypic transmission is known to occur from these phenotypes when they are used as graft donors for pearl production, leading to multicoloured and white pearls from black and albino mantle grafts, respectively. The present study furthers this knowledge by examining the phenotypic association between the orange mantle tissue morphotype and hard tissues: shells and cultured pearls. Based on a large experimental graft, shell colour quantification and pearl qualification showed that the orange morphotype is associated with light-coloured shells and pearls. Expression analysis of some candidate genes previously identified in the white mantle mutant, tested here on both graft and pearl sac tissues from orange mantle donors, confirmed the involvement of genes associated with shell matrix protein (shem4) and the melanin biosynthesis pathway (zinc). This study provides fundamental information on the mechanism behind mantle tissue colour in P. margaritifera and its association with biomineralisation and pigmentation processes that will be potentially valuable in future selection programs.
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