Abstract
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 across 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 colonization. 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 the following: (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.
Highlights
During the last century, many species were intentionally translocated beyond their natural geographical ranges for commercial reasons, including cultivation
We report the study of C. gigas populations sampled across Europe and in the native range of the species using a variety of molecular markers and aim to document their genetic variability and population structure and investigate their adaptive response using a genome-scan approach
Genetic diversity within populations Genotypes at almost 500 AFLP loci were automatically generated for twelve populations using GeneMapper (R) software
Summary
Many species were intentionally translocated beyond their natural geographical ranges for commercial reasons, including cultivation. Some of these species were able to spread autonomously and have caused economic and/or environmental damage (Keller and Lodge 2007), that is, they became invasive. The species failed to settle in northern Europe until the 1990s, when populations were reported to extend as far north as Norway. This expansion may have been the result of (i) natural dispersion, (ii) an increased ability of transplanted stocks to reproduce locally or (iii) a combination of both these processes. In Norway, populations can be found in the shallow waters of the southern and western coasts (Wrange et al 2010)
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