Bernard Chevassus

Production of second generation triploid and tetraploid rainbow trout by mating tetraploid males and diploid females - Potential of tetraploid fish.

SummaryFirst generation tetraploids were produced by hydrostatic pressure treatment before the first cleavage and raised until the adult stage. Their survival and growth were severely depressed when compared to the diploid control: after two years, no ovulated females were found although males produced sperm at 1 and 2 years of age and were mated individually with diploid females. The progenies were consistently normal with high survival rates. They were found to be almost all triploids by karyology, which failed to detect a significant rate of aneuploidies. However, the fertilizing ability of tetraploid males was always low (0 to 97% of the control; average 40%). Several arguments presented here support the hypothesis that diploid spermatozoas, which are wider than haploid ones, would be frequently blocked during their penetration through the micropyle canal. Second generation tetraploids were produced after such matings by heat shocks, causing the retention of the second polar body. Their survival and growth were much more satisfactory than in the first generation, although still lower than in diploid and triploid controls issuing from diploid parents. Performances of second generation triploids were comparable to those of diploids, and slightly better than those of conventional triploids issuing from diploid parents. 94.5% of the second generation tetraploids were male.

Genetics of resistance to disease in fishes.

Abstract The first part of this paper deals with the identification and exploitation of genetic variations in resistance to diseases. At the interspecific level, hybridization may in some cases allow the exploitation of the character of resistance of one of the parents. Polyploid hybrids represent a particular advantage for this approach. Variations between populations have been treated in many papers. However, the studies are generally limited to F1 crosses and do not allow analysis of genetic determinism of the observed variations. Significant variations are also observed between individuals of one and the same population. Several mass selection results using this variability have been developed. The second part deals with disease resistance mechanisms. At the quantitative level, the studies concern the variability of antibody levels, neutralizing serum and mucus activities and resistance to stress; several cases of correlation between these characters and resistance to diseases are presented. The Mendelian approach is particularly developed in the field of histocompatibility genetics. However, the link between this variability and disease resistance remains to be established. The final discussion deals with the respective advantages of quantitative, Mendelian and molecular biology approaches in order to improve disease resistance in fish.

Production of viable hybrids in salmonids by triploidization

Rainbow trout eggs were fertilized with milt collected from various salmanids (brown trout Salmo trutta ; brook trout Salvelinus lontinaliv ; coho salmon Oncorhynchus kisutch) and heat-shocked few minutes later in order to prevent the second polar body extrusion. This operation resulted in high survival rates after 161 days : respectively 49.2 p. 100, 66.2 p. 100 and 12.5 p. 100 of the diploid rainbow trout control (basis 100 p. 100) ; the triploid hybrid constitution of these fish was ascertained by karyological and biochemical investigations.

Early development and survival in brown trout (Salmo trutta fario L.): indirect effects of selection for growth rate and estimation of genetic parameters

Abstract Some components of the response to selection for growth in the PROSPER program on brown trout (Salmo trutta fario L.) (+30% in three generations) were investigated. A correlated response on growth during yolk-sac resorption was looked for, and we estimated the genetic parameters on growth-related traits at the end of endogenous feeding. For this purpose, a diallel cross between selected and control fish was designed, including 200 full-sib families (20 males×10 females), each of them being represented by four individually recorded fish. A positive-correlated effect of selection was seen on survival and percent weight gain at swim-up, and a negative one was shown on dry weight and dry matter content. No effect was seen on final wet weight. A putative interpretation is that selected individuals have a higher development rate, inducing a higher consumption of yolk reserves by the end of the experiment, and therefore, a decrease in wet weight, dry weight and dry matter content. Significant sire heritabilities were seen for final wet weight and percent weight gain (hs2=0.28±0.13 in both cases). For all traits except percent weight gain, maternal effects were strong, and an important part of them was related to the initial weight of eggs. Significant dominance effects were reported for wet weight, dry weight and percent weight gain.

Enhanced individual selection for selecting fast growing fish: the "PROSPER" method, with application on brown trout (Salmo trutta fario)

Growth rate is the main breeding goal of fish breeders, but individual selection has often shown poor responses in fish species. The PROSPER method was developed to overcome possible factors that may contribute to this low success, using (1) a variable base population and high number of breeders (Ne > 100), (2) selection within groups with low non-genetic effects and (3) repeated growth challenges. Using calculations, we show that individual selection within groups, with appropriate management of maternal effects, can be superior to mass selection as soon as the maternal effect ratio exceeds 0.15, when heritability is 0.25. Practically, brown trout were selected on length at the age of one year with the PROSPER method. The genetic gain was evaluated against an unselected control line. After four generations, the mean response per generation in length at one year was 6.2% of the control mean, while the mean correlated response in weight was 21.5% of the control mean per generation. At the 4th generation, selected fish also appeared to be leaner than control fish when compared at the same size, and the response on weight was maximal (≈130% of the control mean) between 386 and 470 days post fertilisation. This high response is promising, however, the key points of the method have to be investigated in more detail.

Setting up a strain-testing design for the seabass, Dicentrarchus labrax: a simulation study

The seabass, Dicentrarchus labrax L., is a leading species in European marine aquaculture, but its genetic characteristics necessary to set up breeding programmes are still unknown. New tools, such as genomic markers, now allow to carry out strain testings and estimations of genetic parameters in reasonable scale experimental facilities. However, because of the high cost of genotyping, the total number of fish to be measured will be limited and mating designs (type of cross, number of sires and dams) must be optimised. An optimisation trial is done in this study by simulation of a quantitative trait for two different strains of fish according to a polygenic model with additive, dominance, maternal and residual effects. The phenotypic coefficient of variation and the difference between crosses means (D=10%) were also included in the model. For technical limitations due to the species, the mating design chosen was a factorial cross between sires of both strains with dams from one strain (topcross). Different combinations of size of progeny sample (500–4000), number of sires (10–120/strain) and dams (4–16) were studied. Various mixed models (SAS®, proc Mixed, option REML) were used to estimate D, the power for detecting D, heritability (h2) and their standard error (S.E.) over 1000 repetitions. Incomplete factorials were also tested as well as the effect of differential survival between dam families. In strictly additive models (no dominance or maternal effects), D and h2 estimates were unbiased. The number of dams did not noticeably affect the precision of estimates, whatever the number of sires. A joint optimisation of sample size and number of sires led to a consensus value of 40 sires/strain with a sample size of 2000 progenies, which gave good precisions for D and h2. It was also shown that a mixed model (fixed strain effect+random sire effect) gave more accurate results than a simple fixed model (strain effect only). The inclusion of maternal and dominance effects did not markedly bias nor affect the precision of D and h2 estimates in a full factorial design, but could lower the precision (up to 40% increase S.E.) in incomplete factorial design. Variance in dam family sizes (due to differential survivals) did not affect the results in full factorials when set at values observed in seabass. It is then concluded that a factorial cross of 40 sires/strain with 8 dams and a sample size of 2000 progenies (on average 1000/strain) is appropriate for comparing seabass strains (power >0.8 for detecting a 20% difference between strains means) and jointly estimating heritability.

Early life history of triploid rainbow trout (Salmo gairdneri Richardson)

Abstract A study of the early life history of triploid rainbow trout ( Salmo gairdneri Richardson) showed that the mean embryonic development time was significantly shorter in triploid fish (381°C × days) than in the control group (384°C × days; P P P

Optimization of factorial mating designs for inference on heritability in fish species

Abstract Microsatellites allow pedigrees to be redrawn in groups of mixed families. However, genotyping costs are still high and experiments deserve new optimizations based on total number of genotyped offspring. Using stochastic simulations, this paper compares and optimizes three factorial or partly factorial mating designs for inference on heritability: FF: full factorial: s sires, each mated with s dams ( s 2 families) FD: s sires, each mated with two dams (2 s families) BH: described by Berg and Henryon [A comparison of mating designs for inference on genetic parameters in fish. Proceedings of the Sixth World Congress on Genetic Applied to Livestock Production, Armidale, Australia, 11–16th January, 27, 115.]: s sires mated with s dams: sire 1 with dams 1, 2; sire 2 with dams 2, 3…sire S with dams 1, S (2 s families). A quantitative trait is simulated according to a strictly additive, polygenic model. Two levels for number of genotyped offspring (NO=300 or 1000) and three levels for true heritability ( h 2 =0.1, 0.25 and 0.5) are considered. For each NO– h 2 combination, all possible couples number of families (NF)/family size were studied to find the one giving the most precise estimation of heritability. Standard deviation of heritability was calculated over 5000 repetitions in each situation. In most cases, FF designs are less interesting than FD and BH ones. FD designs are more precise than BH designs, except for NO=1000 and h 2 =0.5. Optimum family size is similar for FD and BH designs and both NOs: 3–5 offspring per family for h 2 =0.5, 5–8 offspring per family for h 2 =0.25 and 12–20 offspring per family for h 2 =0.1.

Survival growth and sexual maturation of the tiger trout hybrid (Salmo trutta ♀×Salvelinus fontinalis ♂)

Abstract Brown trout ♀ × brook trout ♂ hybrids, together with parental species controls from the same breeders, were tested up to the age of 3 years at four different fish farms. Aside from egg and newborn alevin mortalities, the survival rate of the hybrids was similar to that of brown trout while that of brook trout was much lower. Hybrid growth was about intermediate between those of brown trout (inferior) and brook trout (superior). Sexual maturation was severely impaired in the hybrids, which resulted in high dressing rates: females had practically no ovarian development and male gonadogenesis, although limited, caused some morbidity during the spawning season. The feasibility of hybrid fish culture using genetic improvements is discussed in the light of these results.

Comparison of growth performances of three French strains of common carp (Cyprinus carpio) using hemi-isogenic scaly carp as internal control

Three strains of mirror carp originating from important regions of carp culture in France (Brenne, Dombes, Forez) were compared for growth at 5 weeks, 1 summer and 2 summers of age in ponds using a paternal cross on the same Dombes dams. An internal scaly control was produced by crossing an homozygous inbred strain on the same dams and mixed with the tested strains in all ponds in a proportion of 1:3. No between-strain differences in weight were seen at any time when taking into account the weight of the internal control. However, the internal control weight at 2 summers of age was lower than that of the tested strains (P<0.05). The use of the internal control divides the number of replicates needed to have a good statistical power by 5 at 5 weeks and by 8 at 2 summers, when compared to separate testing without control. No genotype by environment or strain-control interactions were detected. The validity and optimisation of the use of an internal control in strain testing are discussed.