Increasing Structural Diversity of the Early Growth Stages in Polynesian Pearls Reveals Biological Stress Suffered by the Grafts

In Polynesian pearls produced using Pinctada margaritifera var. Cumingii, we investigated the structure of the early growth stages, from the nucleus surface up to the first deposition of the black nacre characteristic of this subspecies. Despite simultaneous grafting from the same donor oyster and similar cultivation conditions, we observed the deposition of various non-nacreous pre-nacre structures. These unusual microstructures, which precede the return to black nacre, varied from immediate deposition onto the nucleus surface to increasing delays, depending on the graft’s position in the grafting series. Given the similar biological conditions of grafting and cultivation, we suggest that, in line with recent data demonstrating genomic sensitivity to environmental conditions, alterations in the graft cells produced during the increasing waiting period were transmitted to the pearl sacs and the early growth stages of the grafted pearls.