Relationship of the orange tissue morphotype with shell and pearl colouration in the mollusc Pinctada margaritifera

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.