Colour plasticity in the shells and pearls of animal graft model Pinctada margaritifera assessed by HSV colour quantification

Pierre-Louis Stenger,Chin-Long Ky,Serge Planes,Jérémie Vidal-Dupiol,Céline Reisser

doi:10.1038/s41598-019-43777-4

Abstract

The bivalve Pinctada margaritifera has the capacity to produce the most varied and colourful pearls in the world. Colour expression in the inner shell is under combined genetic and environmental control and is correlated with the colour of pearls produced when the same individual is used as a graft donor. One major limitation when studying colour phenotypes is grader subjectivity, which leads to inconsistent colour qualification and quantification. Through the use of HSV (Hue Saturation Value) colour space, we created an R package named ‘ImaginR’ to characterise inner shell colour variations in P. margaritifera. Using a machine-learning protocol with a training dataset, ImaginR was able to reassign individual oysters and pearls to predefined human-based phenotype categories. We then tested the package on samples obtained in an experiment testing the effects of donor conditioning depth on the colour of the donor inner shell and colour of the pearls harvested from recipients following grafting and 20 months of culture in situ. These analyses successfully detected donor shell colour modifications due to depth-related plasticity and the maintenance of these modifications through to the harvested pearls. Besides its potential interest for standardization in the pearl industry, this new method is relevant to other research projects using biological models.

Highlights

Gamut, ranging from red to yellow, green to blue or peacock to white, with all possible intermediate nuances[21,26]
Our R package, based on image analysis with HSV colour space and machine-learning approaches, validated the method used through the analysis of the influence of depth on the colour of two economically important pearl donor phenotypes: the green and the red inner shell phenotypes
Our results show that (i) the R package successfully categorized the pearl phenotypes used in this study, that (ii) cultivation environment of the donor oysters heavily influences the brightness V and the saturation S of the colour, and (iii) that the colour variation related to depth was transmitted from donors to pearls

Summary

Introduction

Gamut, ranging from red to yellow, green to blue or peacock to white, with all possible intermediate nuances[21,26]. The assessment of colour remains a subjective trait in which human quantification and qualification can be strongly biased, as visual perception of colours differs between individuals[31]. For this reason, efforts are currently being made to develop accurate and reproducible computational methods for automatic objective colour qualification and quantification[10]. The objective of the present study is the qualification and quantification of colour with a new standardized and reproducible method. We used HSV colour space to characterise colour variation in the inner shell of black-lipped oysters and pearls produced by the pearl industry. To enable a wider use of this method in scientific and private programmes, we developed an R package, ImaginR32 and made this publicly available

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Methods

Results

Conclusion