Résumé
Crassostrea gigas originated from the Pacific coast of Asia but was
introduced into several European countries in the early 1970s. Natural
populations have now spread the length of the western seaboard of Europe.
To elucidate the demographic and selective processes at play during this
rapid expansion, genome-scan analysis was performed on different
populations. High diversities and low differentiation were observed
overall, but significant genetic differentiation was found among
newly-established populations and between the newly-established northern
group and a nearly panmictic group composed of southern European
populations and a population from Japan. Loss of genetic diversity was
also seen in the north, likely caused by founder events during
colonisation. The few strongly supported outlier loci revealed a genetic
structure uncorrelated with the north/south differentiation, but grouping
two samples from the Danish Fjords (northern group) and one from the Dutch
Scheldt estuary (southern group) with the one from Japan. These findings
might reflect: (i) parallel adaptation to similar environmental pressures
(Fjord-like environment) within each of the two groups or (ii) a footprint
of a secondary introduction of an alternative genomic background
maintained by multifarious isolation factors. Our results call for a
closer examination of adaptive genetic structure in the area of origin.