Metapopulation patterns of additive and nonadditive genetic variance in the sea bass (Dicentrarchus labrax)

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

An investigation of links between metabolic rate and feed efficiency in European sea bass Dicentrarchus labrax

Feed efficiency (FE) is the amount of body weight gain for a given feed intake. Improving FE through selective breeding is key for sustainable finfish aquaculture but its evaluation at individual level is technically challenging. We therefore investigated whether individual routine metabolic rate (RMR) was a predictor of individual FE in the European sea bass Dicentrarchus labrax, a major species in European mariculture. The European sea bass has three genetically distinct populations across its geographical range, namely Atlantic (AT), West Mediterranean (WM), and East Mediterranean (EM). We compared FE and RMR of fish from these three populations at 18 or 24 °C. We held 200 fish (62 AT, 66 WM, and 72 EM) in individual aquaria and fed them from ad libitum down to fasting. FI was assessed for an ad libitum feeding rate and for a fixed restricted ration (1% of metabolic body weight·day-1, with metabolic body weight = body weight0.8). After being refed 12 wk in a common tank, individual RMR was measured over 36 h by intermittent flow respirometry. There was a significant effect of temperature whereby fish at 18 °C had greater mean FE (P < 0.05) and lower RMR (P < 0.001). There was also a significant effect of population, where AT fish had lower FE (P < 0.05) and greater RMR (P < 0.001) than WM and EM, at both temperatures. Despite these differences in temperature and population means, individual FE and RMR were not significantly correlated (P > 0.05). Therefore, although the results provide evidence of an association between metabolic rate and FE, RMR was not a predictor of individual FE, for reasons that require further investigation.

Advanced Aboveground Spatial Analysis as Proxy for the Competitive Environment Affecting Sapling Development

Tree saplings are exposed to a competitive growth environment in which resources are limited and the ability to adapt determines general vitality and specific growth performance. In this study we analyzed the aboveground spatial neighborhood of oak [Quercus petraea (Matt.) Liebl.] and beech (Fagus sylvatica L.) saplings growing in Germany, by using hemispherical photography and terrestrial laser scanning as proxy for the competitive pressure saplings were exposed to. The hemispherical images were used to analyze the light availability and the three-dimensional (3D) point clouds from the laser scanning were used to assess the space and forest structure around the saplings. The aim was to increase the precision with which the biomass allocation, growth, and morphology of the saplings could be predicted by including more detailed information of their environment. The predictive strength of the models was especially increased through direct neighborhood variables (e.g., relative space filling), next to the light availability being the most important predictor variable. The biomass allocation patterns within the more light demanding oak were strongly driven by the space availability around the saplings. Diameter and height growth variables of both species reacted significantly to changes in light availability, and partly also to the neighborhood variables. The leaf morphology [as leaf-area ratio (LAR)] was also driven by light availability and decreased with increasing light availability. However, the branch morphology (as mean branch weight) could not be explained for oak and the model outcome for beech was hard to interpret. The results could show that individuals of the same species perform differently under constant light conditions but differing neighborhoods. Assessing the neighborhood of trees with highly precise measurement devices, like terrestrial laser scanners, proved to be useful. However, the primary response to a dense neighborhood seemed to be coping with a reduction of the lateral light availability aboveground, rather than responding to an increase of competition belowground. The results suggest continuing efforts to increase the precision with which plant environments can be described through innovative and efficient methods, like terrestrial laser scanning.

The European sea bass: a key marine fish model in the wild and in aquaculture

The European sea bass (Dicentrarchus labrax L.) is a marine fish of key economic and cultural importance in Europe. It is now more an aquaculture than a fisheries species (>96% of the production in 2016), although modern rearing techniques date back only from the late 1980s. It also has high interest for evolutionary studies, as it is composed of two semispecies (Atlantic and Mediterranean lineages) that have come into secondary contact following the last glaciation. Based on quantitative genetics studies of most traits of interest over the past 10–15 years, selective breeding programs are now applied to this species, which is at the beginning of its domestication process. The availability of a good quality reference genome has accelerated the development of new genomic resources, including SNP arrays that will enable genomic selection to improve genetic gain. There is a need to improve feed efficiency, both for economic and environmental reasons, but this will require novel phenotyping approaches. Further developments will likely focus on the understanding of genotype‐by‐environment interactions, which will be important both for efficient breeding of farmed stocks and for improving knowledge of the evolution of natural populations. At the interface between both, the domestication process must be better understood to improve production and also to fully evaluate the possible impact of aquaculture escapees on wild populations. The latter is an important question for all large‐scale aquaculture productions.

Population-specific variations of the genetic architecture of sex determination in wild European sea bass Dicentrarchus labrax L

Polygenic sex determination (PSD) may show variations in terms of genetic and environmental components between populations of fish species exposed/adapted to different environments. The European sea bass (Dicentrarchus labrax) is an interesting model, combining both a PSD system and a genetic subdivision into an Atlantic and a Mediterranean lineage, with genetic substructures within the Mediterranean Sea. Here, we produced experimental progeny crosses (N = 927) from broodstock sampled in four wild populations (North Atlantic, NAT; Western Mediterranean, WEM; North-Eastern Mediterranean, NEM; South-Eastern Mediterranean, SEM). We found less females than males in the progeny, both in the global dataset (32.5%) and within each paternal group (from 25.1% for NEM to 39.0% for WEM), with significant variation among populations, dams, and sires. Sex, body weight (BW), and body length (BL) showed moderate heritability (0.52 ± 0.17, 0.46 ± 0.17, 0.34 ± 0.15, respectively) and sex was genetically correlated with BW and BL, with r_Asex/BW = 0.69 ± 0.12 and r_{A sex/BL} = 0.66 ± 0.13. A weighted GWAS performed both on the global dataset and within each paternal group revealed a different genetic architecture of sex determination between Atlantic and Mediterranean populations (9 QTLs found in NAT, 7 in WEM, 5 in NEM, and 4 in SEM, with a cumulated variance explained of 27.04%, 21.87%, 15.89%, and 12.10%, respectively) and a more similar genetic architecture among geographically close populations compared to geographically distant populations, consistent with the hypothesis of a population-specific evolution of polygenic sex determination systems in different environments.

Metapopulation patterns of additive and nonadditive genetic variance in the sea bass (Dicentrarchus labrax)

Describing and explaining the geographic within‐species variation in phenotypes (“phenogeography”) in the sea over a species distribution range is central to our understanding of a variety of eco‐evolutionary topics. However, phenogeographic studies that have a large potential to investigate adaptive variation are overcome by phylogeographic studies, still mainly focusing on neutral markers. How genotypic and phenotypic data could covary over large geographic scales remains poorly understood in marine species. We crossed 75 noninbred sires (five origins) and 26 dams (two origins; each side of a hybrid zone) in a factorial diallel cross in order to investigate geographic variation for early survival and sex ratio in the metapopulation of the European sea bass (Dicentrarchus labrax), a highly prized marine fish species. Full‐sib families (N = 1,950) were produced and reared in a common environment. Parentage assignment of 7,200 individuals was performed with seven microsatellite markers. Generalized linear models showed significant additive effects for both traits and pleiotropy between traits. A significant nonadditive genetic effect was detected. Different expression of traits and distinct relative performances were found for reciprocal crosses involving populations located on each side of the main hybrid zone located at the Almeria‐Oran front, illustrating asymmetric reproductive isolation. The poor fitness performance observed for the Western Mediterranean population of sea bass is discussed as it represents the main source of seed hatchery production, but also because it potentially illustrates nonadaptive introgression and maladaptation.