Grégory Charrier

Parallel evolution of local adaptation and reproductive isolation in the face of gene flow.

Parallel evolution of similar phenotypes provides strong evidence for the operation of natural selection. Where these phenotypes contribute to reproductive isolation, they further support a role for divergent, habitat‐associated selection in speciation. However, the observation of pairs of divergent ecotypes currently occupying contrasting habitats in distinct geographical regions is not sufficient to infer parallel origins. Here we show striking parallel phenotypic divergence between populations of the rocky‐shore gastropod, Littorina saxatilis, occupying contrasting habitats exposed to either wave action or crab predation. This divergence is associated with barriers to gene exchange but, nevertheless, genetic variation is more strongly structured by geography than by ecotype. Using approximate Bayesian analysis of sequence data and amplified fragment length polymorphism markers, we show that the ecotypes are likely to have arisen in the face of continuous gene flow and that the demographic separation of ecotypes has occurred in parallel at both regional and local scales. Parameter estimates suggest a long delay between colonization of a locality and ecotype formation, perhaps because the postglacial spread of crab populations was slower than the spread of snails. Adaptive differentiation may not be fully genetically independent despite being demographically parallel. These results provide new insight into a major model of ecologically driven speciation.

Variation in Size and Growth of the Great Scallop Pecten maximus along a Latitudinal Gradient

Understanding the relationship between growth and temperature will aid in the evaluation of thermal stress and threats to ectotherms in the context of anticipated climate changes. Most Pecten maximus scallops living at high latitudes in the northern hemisphere have a larger maximum body size than individuals further south, a common pattern among many ectotherms. We investigated differences in daily shell growth among scallop populations along the Northeast Atlantic coast from Spain to Norway. This study design allowed us to address precisely whether the asymptotic size observed along a latitudinal gradient, mainly defined by a temperature gradient, results from differences in annual or daily growth rates, or a difference in the length of the growing season. We found that low annual growth rates in northern populations are not due to low daily growth values, but to the smaller number of days available each year to achieve growth compared to the south. We documented a decrease in the annual number of growth days with age regardless of latitude. However, despite initially lower annual growth performances in terms of growing season length and growth rate, differences in asymptotic size as a function of latitude resulted from persistent annual growth performances in the north and sharp declines in the south. Our measurements of daily growth rates throughout life in a long-lived ectothermic species provide new insight into spatio-temporal variations in growth dynamics and growing season length that cannot be accounted for by classical growth models that only address asymptotic size and annual growth rate.

The Effect of Multiple Paternity on Genetic Diversity of Small Populations during and after Colonisation

Genetic variation within and among populations is influenced by the genetic content of the founders and the migrants following establishment. This is particularly true if populations are small, migration rate low and habitats arranged in a stepping-stone fashion. Under these circumstances the level of multiple paternity is critical since multiply mated females bring more genetic variation into founder groups than single mated females. One such example is the marine snail Littorina saxatilis that during postglacial times has invaded mainland refuge areas and thereafter small islands emerging due to isostatic uplift by occasional rafting of multiply mated females. We modelled effects of varying degrees of multiple paternity on the genetic variation of island populations colonised by the founders spreading from the mainland, by quantifying the population heterozygosity during both the transient colonisation process, and after a steady state (with migration) has been reached. During colonisation, multiple mating by males increased the heterozygosity by in comparison with single paternity, while in the steady state the increase was compared with single paternity. In the steady state the increase of heterozygosity due to multiple paternity is determined by a corresponding increase in effective population size. During colonisation, by contrast, the increase in heterozygosity is larger and it cannot be explained in terms of the effective population size alone. During the steady-state phase bursts of high genetic variation spread through the system, and far from the mainland this led to short periods of high diversity separated by long periods of low diversity. The size of these fluctuations was boosted by multiple paternity. We conclude that following glacial periods of extirpation, recolonization of isolated habitats by this species has been supported by its high level of multiple paternity.

Genetic structure of a commercially exploited bivalve, the great scallop Pecten maximus, along the European coasts

The great scallop Pecten maximus is harvested in several European countries and fisheries targeting this species are severely regulated by fishing quotas. In addition, hatchery-based population enhancement has been developed in some countries to provide alternative or complementary production. The genetic structure of wild populations of P. maximus and the potential impact of aquaculture on the genetic diversity of this species remains poorly documented. In this study, we explored the genetic structure of P. maximus using 12 microsatellite markers, considering 14 populations sampled from Galicia (Spain) to the North of Norway, and one population of Pecten jacobaeus (L., 1758) from the Lion Gulf (Mediterranean Sea). Results indicated a clear differentiation between Norwegian and Atlantic (from Ireland to Spain) populations, but very little to no difference between populations within these two groups. A decrease of the genetic diversity was observed with latitude. No significant reduction of the genetic diversity was observed in the Bay of Brest, where hatchery-based population enhancement has been performed intensively since 1983. Our results are discussed in the light of the inferred recent evolutionary history, phylogeography and connectivity of populations in Europe, and of the phenotypic variability reported in previous studies between northern and southern populations.

The Story of a Hitchhiker: Population Genetic Patterns in the Invasive Barnacle Balanus(Amphibalanus) improvisus Darwin 1854

Understanding the ecological and evolutionary forces that determine the genetic structure and spread of invasive species is a key component of invasion biology. The bay barnacle, Balanus improvisus (= Amphibalanus improvisus), is one of the most successful aquatic invaders worldwide, and is characterised by broad environmental tolerance. Although the species can spread through natural larval dispersal, human-mediated transport through (primarily) shipping has almost certainly contributed to the current global distribution of this species. Despite its worldwide distribution, little is known about the phylogeography of this species. Here, we characterize the population genetic structure and model dispersal dynamics of the barnacle B. improvisus, and describe how human-mediated spreading via shipping as well as natural larval dispersal may have contributed to observed genetic variation. We used both mitochondrial DNA (cytochrome c oxidase subunit I: COI) and nuclear microsatellites to characterize the genetic structure in 14 populations of B. improvisus on a global and regional scale (Baltic Sea). Genetic diversity was high in most populations, and many haplotypes were shared among populations on a global scale, indicating that long-distance dispersal (presumably through shipping and other anthropogenic activities) has played an important role in shaping the population genetic structure of this cosmopolitan species. We could not clearly confirm prior claims that B. improvisus originates from the western margins of the Atlantic coasts; although there were indications that Argentina could be part of a native region. In addition to dispersal via shipping, we show that natural larval dispersal may play an important role for further colonisation following initial introduction.

Responses of juvenile European flounder (Platichthys flesus) to multistress in the Vilaine estuary, during a 6-month survey

Physiological and genetic responses of age 0+ Platichthys flesus were investigated in the eutrophicated and moderately contaminated Vilaine estuary, during a 6-month survey. The main objective of this study was to explore the biological responses of fishes during their juvenile period in an estuarine system in order to detect a possible selective pressure induced by the environmental stress. Our results showed a general convergence in physiological responses along the survey: an increase in genotoxicity was associated with an increase in mRNA expression of ATPase and betaine homocysteine methyltransferase. These results could suggest an increase of cellular damage, energetic request, and detoxification rate related to the growing exposure time to stress. Considering the aging of the cohort, the genetic characteristics of the Vilaine flounder cohort came closer to the one observed in a highly stressed system, the Seine estuary, suggesting a potential selective pressure mainly induced by the chemical stress.

Development of new microsatellite markers derived from expressed sequence tags for the great scallop (Pecten maximus)

Nine microsatellites were developed from expressed sequence tags sequences for the great scallop (Pecten maximus), and were tested in three natural populations. These markers displayed low to moderate levels of variability. On average across all populations, the number of alleles per locus ranged from 1.333 to 8.667, and the observed and expected heterozygosities ranged from 0.029 to 0.703 and 0.028 to 0.711, respectively. All loci conformed to Hardy–Weinberg equilibrium, except PmGC20 in one single population. These new loci might be very useful for the genetic monitoring of the possible impact of aquaculture on the genetic diversity of native scallop populations.

Large and fine scale population structure in European hake (Merluccius merluccius) in the Northeast Atlantic

Recently, there have been reports of increased abundance and landings of European hake in the northern part of the species range. Biological studies are however scarce and information about finer scale population structure important for stock assessments and fishery management is largely lacking. Here, we report on a population genetic study using neutral and outlier SNP loci assessing population structure in hake in the north-eastern parts of its range in the Atlantic. Hake samples from localities along the west coast of Norway, the Kattegat, the northern North Sea, and one locality in the Bay of Biscay were analysed using 53 SNPs, six of which were outliers potentially influenced by natural selection. We detected small-scale structure among northern samples, all of which were also distinct from Bay of Biscay hake, with the exception of a few individuals from the North Sea and the coast of Norway who clustered genetically together with Bay of Biscay hake. Our findings suggest that the present management unit of a single northern stock of hake is not biologically correct, and that there is more detail in the fine-scale population structure indicating that independent population dynamics could be expected in response to fishing patterns or changing environmental conditions.

New set of candidate gene SNPs and microsatellites to disentangle selective and neutral processes shaping population responses of European flounder (Platichthys flesus) to anthropogenic stress and contrasted environments

Abstract Six candidate gene SNP assays and 12 new microsatellites were developed in the European flounder (Platichthys flesus), in order to provide new tools to (1) further investigate signatures of selection suspected on candidate genes, and (2) better depict the neutral evolutionary background shaping the differentiation of populations. All markers were successfully tested in two populations from contrasted estuaries, and should be helpful to better understand the possible adaptive responses of flounder populations to anthropogenic pressures (e.g. global warming, eutrophication, pollutions), over the whole distribution range of the species.