Review on Breeding and Reproduction of European aquaculture species
European seabass (Dicentrarchus labrax L.)
Hervé Chavanne (Istituto Spallanzani, Italy), Béatrice Chatain (Ifremer, France), Pierrick Haffray (SYSAAF, France)
1. General information on production and breeding
History of the domestication
The predominant rearing system for the seabass has for a long time been traditional extensive production (e.g. “Vallicoltura” in Italy). This system is characterized by the collection of juveniles from the wild and their release into semi-artificial lagoons for growing. In the face of strong competition between the on-growers for fish seed and the diminution of natural resources, small scale production units appeared in France and in Italy in the early 80s to provide alternative sources of fry. The diffusion of controlled reproduction techniques and husbandry methods allowed higher survival and led to a progressive increase in production volumes, based essentially on wild broodstock. The first breeding programmes began in France and Israel in the 90s and have been followed by a few initiatives in Greece, Spain and Italy. Today, a high proportion of seabass fingerlings are produced from wild broodfish or F1 animals, selected animals resulting from 1 to 3 generations of broodstock selection.
Seabass production reached a total of 88,500 tonnes in 2006, with high percentages coming from Greece (38%) and Turkey (33%). The number of fingerlings produced in 2006 was 350 million, with a production peak of 375 million in 2005 .
Distribution area of wild populations (red) and production countries (green).
Source: FAO Fishbase, FEAP.
Biological features of interest for breeding practices
Main commercial data on weight and age at harvest and slaughtering performances
Rearing temperatures play a fundamental role in seabass growth. Considering that there is a 5-6°C difference between the extremes of annual mean culture temperature (from the Atlantic coast to the eastern Mediterranean or even the Red Sea), the culture duration for a given weight is highly variable among rearing sites.
Almost half of the European market volume is represented by portion fish of 300/400 g, followed by 27% of 400/600 g fish and 17% of 200/300 g fish . Fish above one kilo represent a niche market (1%). This figure differs among countries, with Spain and Turkey producing almost no fish under 300 g or above 800 g respectively, and 20% of French production represented by fish above 800 g.
Maturation may represent a problem for meat quality when the final product has been reared more than 3 years, in particular for portion fish exceeding 800 g grown in low rearing temperatures.
Generation interval for males and females
Age at first maturation is highly variable among individuals for both sexes in seabass. Precocious males mature soon after sexual differentiation during the first year (average of 50 g) and the whole male population completes maturation during the second year. Females reach puberty between the second and third year .
Viral nervous necrosis, caused by nodavirus, is an infectious neuropathology of the central nervous system and retina, producing vacuolation and cell necrosis. In most cases it induces 100% mortality. Three bacterioses, caused by Photobacterium piscicida, Vibrio anguillarum, and Flexibacter maritimus, bring about high losses at young stages and chronic mortality in adults . The ectoparasite Cryptocaryon, which develops under warmer temperature conditions, affects the gills and skin.
2. Genetic variability of the species
Wild genetic resources available
The seabass Dicentrarchus labrax is one of the marine fish species most studied for its population structure. Numerous studies based on molecular markers [4-12] have allowed the population to be divided into three main genetic groups: the north-eastern Atlantic ocean, the western Mediterranean and the eastern Mediterranean. Overlapping zones are located at the Almeria-Oran oceanic front (separation between Atlantic and western Mediterranean) and at the Siculo-Tunisian strait (separation between western and eastern Mediterranean basins). The Atlantic and western Mediterranean groups are quite homogeneous populations, even if a recent study indicates that there is probably a more complex structure of populations around the British Isles . The eastern group is more heterogeneous and structured into subpopulations that reflect the different basins of the region.
Based on allozyme variation , differences in allele frequency are observed between samples from lagoons and the open sea. Analysis of the distribution of nuclear markers between different populations revealed that among 13 non-neutral allozymes, 6 loci were implicated in the differentiation between open sea and lagoon samples. A genetic divergence between habitats was thus suggested .
Differences between wild and/or domesticated populations
Some of the cultured populations analysed using molecular markers reveal a significant reduction in genetic variability, while others are in Hardy Weinberg equilibrium indicating that they are still largely outbred and in open contact with wild genitors [4, 10, 14].
Few strain testing experiments have been conducted on seabass. Gorshkov et al , comparing domesticated and wild strains in one tropical environment (Red Sea, Israel), reported significant differences for traits of economic interest (survival, growth, body composition). Advances on wild population comparison are expected from a current European project  that focuses on performance evaluation of existing wild seabass populations in different farming conditions.
Interaction between wild and domesticated stocks
The potential genetic impact of escapees was reviewed in a previous EU project GENIMPACT . One of the more important aspects is that the main development of this industry is taking place in the geographic area (east Mediterranean) that presents the lowest biomass of wild seabass and is probably the most sensitive to impact from escapement. The hypothesis of possible contamination of wild eastern populations from such farming has already been advanced by . The introduction of non autochthonous populations occurs through the exchange of commercial fry across Europe that can then escape during the on-growing phases (no escape monitoring is made for seabass). This phenomenon may be further enhanced when farmers rely on non-local breeders . However, this last argument was not considered as justified by the GENIMPACT working group, who sustained that the effects of phenotypic changes resulting from just a few generations of selection (4-5) are equivalent between local and non local stocks. GENIMPACT concluded that knowledge on the genetic structure of a wild population and its variation with time (season, year) should first be reinforced. Secondly, a better description of seabass biology and ecology (migration behaviour, prey dependency, reproduction time, etc…) is necessary to evaluate the risks associated with escapements, depending on the size of the escaped fish (eggs, fry, portion size, juveniles), the quantity of escaped fish, their genetic make-up, the localisation of the escapement and finally the time of year (season) when escapement occurs. Sterilization by triploidisation or hybridisation has been suggested as a means of limiting the genetic impact of escapements. Some concerns have also been raised about what impact seabass selected for sex determinism might have on wild populations .
Fecundity and main reproductive features
Seabass is a fractional spawner (3-4 depositions over the reproductive season) that matures under the control of temperature and photoperiod. Fertilized egg diameter ranges from 1.2 to 1.4 mm and female fertility averages 200,000 eggs per kg body weight. Sperm release and egg deposition can be enhanced by hormonal induction (GnRH and LHRH agonists). Different hormone delivery systems have been described that allow either a synchronisation of females three days after treatment or a long term enhancement of milt production for males and multiple spawns for females , but none of these systems are officially registered in the EU. The short time available for stripping between ovulation and the point when eggs become overripe, and the difficulty in finding multiple females at the same egg development stage, strongly constrain the scale of crossing schemes in the seabass. Large crossing schemes are nevertheless currently practiced in research organisms in France [20-23], the alternative solution being to limit the number of families produced per mating scheme.
Seabass semen is characterized by a high sperm concentration (up to 60 x 109 spz.ml-1) and a short duration of sperm motility (<1 min, ). Different semen cryopreservation protocols have proved to be efficient on seabass [24, 25]. They differ in the cryoprotectant added to the extender (dimethyl sulfoxide or ethylene glycol) and in the time during which the semen needs to be exposed to the medium required before freezing. A commercial product developed for salmonids gives also good results on seabass (CryoFish - IMV product).
Some attempts have been made to constitute semen cryobanks of wild, domesticated and selected seabass populations with variable degrees of success, but only quite limited quantities of semen are presently available (principal stocks are located in research organisms, others at some breeding companies).
Genetic and environmental sex determination, sexual dimorphism
Seabass is a gonochoric species. Sexual dimorphism for growth exists and is characterized by a growth advantage of females over males. Stronger at early stages (70%), the growth difference tends to stabilize at 20-30% after the second year [15, 26-30].
Sex determination in seabass has been the object of much research activity (see review of ). The reason is that a heavily skewed sex ratio appears in the species under culture conditions, with between 70% and 99% of offspring typically being male. Sex in seabass is determined by both genetic and environmental factors, the main environmental factor being temperature [29, 30, 32-35].
Production of monosex populations can be achieved using exogenous steroid treatments that lead to complete sex reversal [36, 37].
The hypothesis of simple sex determinism was excluded by results from crosses involving sex-reversed fish. Interpreting sex as a threshold trait,  advanced the hypothesis of a polygenic component for sex determinism and concluded that the genetic component of sex is essentially additive (as also suggested by  and ), that it is linked to growth, and that it is of the same magnitude as the environmental component controlled by temperature .
Sex differentiation is observed first in females and occurs between 128 and 250 days post fertilization , equivalent to 65 to 155 mm total length . Long-term exposure to low or high temperature has a masculinizing effect. Conversely, the shift of temperature from low to high (15 to 20°C) during the labile period (18 to 60 mm total length) allows a significant increase in the proportion of females in the progeny. There is still a need for a reproducible temperature regime protocol that would result in a sex ratio strongly skewed in favour of females.
Genetic variability per trait
The estimated heritability of sex is high (0.62 ± 0.12 ). A relatively moderate genetic correlation is reported between sex and weight (r= 0.50 ± 0.09). Once corrected for the sex dimorphism on weight (females heavier than males), a positive relation is maintained between weight and the proportion of females per family, reinforcing the idea that identical or linked genes participate in growth and sex determinism.
Parental effects on growth, survival and condition factor were identified by  at very early stages, but these tended to decrease with age (parental effect on growth disappears by the age of 4 months). Similarly,  found significant maternal effects on early development but these effects tended to disappear in bigger fish . This short-term maternal influence can be explained by the relative small size of seabass eggs (around 1 mm diameter).
Dupont-Nivet et al.  found high heritability values for body weight, ranging from 0.52 ± 0.07 to 0.64 ± 0.07 in different rearing conditions, and reported similar values for length. Identical results were obtained by .
Quality traits (muscle lipid contents, visceral fat index) have shown to be variable with sex . Recent advances on quality traits  provide genetic parameters on gutted yield, fillet fat content and fillet yield, highlighting a real selection potential for these traits.
To date, genetic knowledge on seabass immune regulatory genes responsible for resistance to pathogens is relatively poor, but is rapidly improving .
Genetic correlations and undesirable side effects
Due to the positive correlation between sex and weight, an evolution of the sex ratio under farming conditions is expected to occur across the generations of selection , leading to an equilibrated sex ratio in 7-8 generations.
As expected, weight and length display high genetic correlations (above 0.9 ). However, correlations between these traits and the condition factor K change with rearing conditions, suggesting a shift in selection criteria according to breeding site. While under certain conditions the choice of length or weight may have no impact on the shape of the fish (no genetic correlation with K), under other conditions such a choice may lead to “leaner” or “belly” animals (negative or positive genetic correlation between K and length or weight, respectively).
Saillant et al  found a high genetic correlation of growth trait breeding values at various ages (mean across age groups = 0.70), suggesting that genetic values are stable within the age range sampled and may thus be estimated early in the growing phase (from 90 g).
The modulation of sex ratio of different families raised under variable temperature profiles allowed  to evidence, for both sexes, a genotype-temperature interaction in seabass sex determination. Such an interaction may correspond to differential sensitivity of genotypes to temperature, which suggests a means for the progressive elimination of environmental sex determination through selection of broodfish insensitive to the masculinizing effects of temperature.
Moderate to low G*E interactions for growth were found in two different studies. Saillant et al  estimated interactions for body weight through correlation of breeding values between extreme environments (ranging from 0.01 ± 0.26 to 0.51 ± 0.19). The environmental variation was stronger than that encountered in farming conditions and the limited number of families available for the estimation (10) resulted in large standard errors. An identical approach at different farming sites showed higher genetic correlations (ranging from 0.75 ± 0.06 to 0.98 ± 0.02) in a design involving a large number of families (253) .
Low G*E interactions have been measured for quality traits tested in variable farming conditions .
Genetic responses, progresses and control lines
Until now, no data has been available on genetic responses in seabass, but the current EU project COMPETUS  will provide the first estimates of responses derived from different breeding strategies.
A line composed of “golden” skinned animals has been isolated and is currently being studied by Ifremer.
Dominance and intraspecific crossing
5. Polyploidisation and monosexing and hybrids
Triploid induction and performances
Following the first studies on triploidy induction in the seabass [42, 43], cold shock and pressure treatments have been optimised for the production of pure gynogenetic offspring (meiogynogenesis) and 100% triploidy . Triploid growth performance is comparable or lower (in general by 20%) than that of diploids whereas some qualitative traits can be superior (e.g. Carcass yield is superior, condition factor and gonado-somatic index are inferior) [45, 46]. Triploids are sterile as in salmonids: the females present rudimentary ovaries and the males develop testes with non functional spermatozoa. According to the two studies that exist on triploid performance, it is clear that triploid females grow better than triploid males and the same as diploid males. So, according to these studies, farming of all-female triploid stocks would not modify productivity. Results such as these are scarce and have only been obtained in land based facilities (raceways, tanks), under non limiting oxygenation capacity. The next priority should be the evaluation of triploid performances in sea cages, and under different temperature conditions and farming practices, to assess their potential for the industry.
Triploid survival, migration, growth performances and the mating behaviour of triploid males in particular, are unknown in the wild. The use of triploids in production to limit genetic effects of escapement should therefore be based on all-female triploid stocks to avoid the potentially negative behaviour of triploid males.
Tetraploid induction and performances
Tetraploid seabass induced by pressure shock have only short-term survival, as no live individuals were found after two months . Peruzzi and Chatain  produced variable levels (7–95%) of tetraploid larvae at hatching.
Gynogenesis, androgenesis and mitotic clone performances
Although survival of meiogynogens at hatching is only about half that of controls, they grow similarly and display the same onset of puberty and reproductive potential at adulthood, with good quality eggs and normal sperm release in terms of volume, quality and fertilization capability .
The haploid genome of seabass consists of 24 chromosomes, weighs 0.78 pg and contains approximately 763 Mb. A first generation microsatellite linkage map containing 175 markers distributed over 25 linkage groups and covering 815 cM (Kosambi function) is now available . A large insert BAC library has a 13x genome coverage . A draft of a second generation linkage map, including 369 microsatellite and AFLP markers, is now available. EST resources in excess of 30,000 sequence traces have been generated. Functional genomic analysis in relation to reproductive biology and stress physiology are in progress. All these resources bring European seabass into the group of the top ten genome-rich fish species. Additional genomic resources such as EST sequences, macro- and micro-arrays of various sorts, a third generation linkage map, a physical BAC and radiation hybrid map, will become available in the near future .
Tools to evaluate population genetic variability
Analysis of the genetic structure of wild and farmed seabass populations first relied on mtDNA and allozyme variation [4, 5, 7]. These studies were followed by the analysis of microsatellite markers [6, 9-11]. Lemaire et al. , comparing wild populations using nuclear and mitochondrial markers, revealed strong discrepancies of genetic variation estimated according to the marker used and discussed the possible origins of these differences (drift, mutation, migration, reproduction). Today, with the development of seabass genomic resources, a large range of new markers is available .
Genetic markers for genealogical traceability
Microsatellite markers in seabass were first developed by  and their use in breeding was illustrated by . Their use in parentage assignment allows breeding designs with communal rearing environments to be employed. Such an approach has been used successively by several authors [18, 20-23] and some breeding companies.
The large number, variability, stability and low genotype error associated with SNPs make these markers potentially useful for fingerprinting, but their high cost is slowing their dissemination. Screening for new SNPs in the large seabass EST data bank is underway .
QTL and Marker Assisted Selection
Gene mapping in seabass is in its infancy as just 8 microsatellites have been linked with encoding genes, but considerable progress is expected in the coming years . A putative QTL associated with morphometric traits has been identified on linkage group 1 (LG1), from a sample of approximately 400 fish analysed. A suggestive linkage affecting body weight was also found on the same region. The candidate gene Peptide Y however, mapped on LG1, did not show any association with the studied traits .
Selective breeding experiments are underway to explore the response to sex ratio selection, and will provide material for QTL research in coming years . Other studies are searching for QTLs associated with stress and disease resistance using SNP and microsatellite markers, and screening for new SNPs in promoter regions in order to better identify candidate genes .
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