Florence Cornette

Differentiation between populations of the Portuguese oyster, Crassostrea angulata (Lamark) and the Pacific oyster, Crassostrea gigas (Thunberg), revealed by mtDNA RFLP analysis

Restriction fragment length polymorphisms of PCR-amplified mitochondrial DNA fragments were used to examine genetic differentiation between populations of the Portuguese oyster (Crassostrea angulata) and the Pacific oyster (Crassostrea gigas). The taxonomic status of C. angulata and C. gigas has often been questioned since no morphological or genetic differences had ever been observed between the two taxa. Samples identified as C. angulata, were collected from 16 sites located in southern Portugal and Spain and samples identified as C. gigas, from European and Asian sites. Of the six haplotypes observed, one was commonly found among oysters identified as C. gigas while another one was most frequent among oysters identified as C. angulata. Analysis of haplotype diversity among sites showed that samples originating from southern Portugal and Spain cluster with the Taiwanese sample. These results implicate Taiwan as the possible origin of European C. angulata populations. The ability to differentiate between these two previously indistinguishable taxa allowed us to identify a population in Portugal as mixed. These results open new perspectives for the study of characters previously described as varying between C. gigas and C. angulata, such as growth performance and disease susceptibility.

Population genomics shed light on the demographic and adaptive histories of European invasion in the Pacific oyster, Crassostrea gigas.

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.

A comparison of growth and biochemical composition of Mytilus galloprovincialis (Lmk.) and Mytilus edulis (L.) on the West coast of Cotentin, Normandy, France 1999-2000

Abstract.The blue mussel, Mytilus edulis has been reared along the Normandy coast line since the 1960s. The gonadal cycle of this mussel species shows a sharp decrease in meat quality during the winter period after spawning. This decline in meat quality is so severe that sales have to be suspended from December to July. Another species of mussel, Mytilus galloprovincialis, which is reared in the lagoons along the French shore of the Mediterranean Sea has a different spawning cycle. An experiment was undertaken to study the meat quality of M. Galloprovincialis throughout the year after the spat were transferred from the Mediterranean Sea to the Normandy coast. This species showed an immediate adaptation to the gonadal cycle of M. edulis. Despite suggestions from researchers, no interest was expressed to consider such transfers of M. galloprovincialis in the future.

Additive transcriptomic variation associated with reproductive traits suggest local adaptation in a recently settled population of the Pacific oyster, Crassostrea gigas

BackgroundOriginating from Northeast Asia, the Pacific oyster Crassostrea gigas has been introduced into a large number of countries for aquaculture purpose. Following introduction, the Pacific oyster has turned into an invasive species in an increasing number of coastal areas, notably recently in Northern Europe.MethodsTo explore potential adaptation of reproductive traits in populations with different histories, we set up a common garden experiment based on the comparison of progenies from two populations of Pacific oyster sampled in France and Denmark and their hybrids. Sex ratio, condition index and microarray gene expression in gonads, were analyzed in each progeny (n = 60).ResultsA female-biased sex-ratio and a higher condition index were observed in the Danish progeny, possibly reflecting an evolutionary reproductive strategy to increase the potential success of natural recruitment in recently settled population. Using multifarious statistical approaches and accounting for sex differences we identified several transcripts differentially expressed between the Danish and French progenies, for which additive genetic basis is suspected (showing intermediate expression levels in hybrids, and therefore additivity). Candidate transcripts included mRNA coding for sperm quality and insulin metabolism, known to be implicated in coordinated control and success of reproduction.ConclusionsObserved differences suggest that adaptation of invasive populations might have occurred during expansion acting on reproductive traits, and in particular on a female-biased sex-ratio, gamete quality and fertility.

Analysis of genome-wide differentiation between native and introduced populations of the cupped oysters Crassostrea gigas and Crassostrea angulata

Abstract The Pacific cupped oyster is genetically subdivided into two sister taxa, Crassostrea gigas and Crassostrea angulata, which are in contact in the north-western Pacific. The nature and origin of their genetic and taxonomic differentiation remains controversial due the lack of known reproductive barriers and the high degree of morphologic similarity. In particular, whether the presence of ecological and/or intrinsic isolating mechanisms contributes to species divergence is unknown. The recent co-introduction of both taxa into Europe offers a unique opportunity to test how genetic differentiation is maintained under new environmental and demographic conditions. We generated a pseudochromosome assembly of the Pacific oyster genome using a combination of BAC-end sequencing and scaffold anchoring to a new high-density linkage map. We characterized genome-wide differentiation between C. angulata and C. gigas in both their native and introduced ranges, and showed that gene flow between species has been facilitated by their recent co-introductions in Europe. Nevertheless, patterns of genomic divergence between species remain highly similar in Asia and Europe, suggesting that the environmental transition caused by the co-introduction of the two species did not affect the genomic architecture of their partial reproductive isolation. Increased genetic differentiation was preferentially found in regions of low recombination. Using historical demographic inference, we show that the heterogeneity of differentiation across the genome is well explained by a scenario whereby recent gene flow has eroded past differentiation at different rates across the genome after a period of geographical isolation. Our results thus support the view that low-recombining regions help in maintaining intrinsic genetic differences between the two species.