Transcriptomic Analysis of the Pacific Oyster: Studying Local Adaptation of Recently Settled Populations in Northern Europe

Originating from northeastern Asia, the Pacific oyster Crassostrea gigas has been introduced, mainly for aquaculture purpose, into a large number of countries. This species was imported into Europe in the early 70’s and its now cultivated all over Europe - from Norway in the north to Portugal and the Mediterranean Sea. Although highly variable, its invasive nature has been reported in an increasing number of coastal areas, including northern Europe. Local genetic adaptation might play a role in this recent phenomenon but remains to be investigated. In this context, we set up a common garden experiment based on the comparison of progenies of wild Pacific oysters originating from France (populations settled since the massive introduction of the species in the 70’s) and Denmark (recently settled populations). Individuals, resulting from crosses within and between populations, were characterized using histological, biochemical and transcriptomic approaches in order to assess the genetic basis of observed phenotypic variance of these traits. A gene wide expression profiling was performed using a Agilent 60-mer 4x44K custom microarray containing 31,918 C. gigas contigs on 60 oyster gonads per progeny. Quantitative histology and biochemical analyses of carbohydrates and lipids were also performed on the same samples. Phenotypic variation among progenies allowed estimating Qst and eQst, identifying traits and genes likely to have evolved under the influence of directional selection due to local adaptation.