PHENOTYPIC AND GENOMIC CORRELATIONS BETWEEN FEED EFFICIENCY AND METABOLIC TRAITS IN EUROPEAN SEA BASS
Introduction
Improvement of feed efficiency is an important factor in sustainability of the finfish aquaculture industry. Improving feed efficiency via selective breeding requires an ability to identify the most efficient individuals among selection candidates. A method recently developed for the European sea bass (Dicentrarchus labrax) is based on individual rearing of fish in aquariums under restricted feeding (Besson et al., 2019). In Besson et al., (2019), we showed that individual feed efficiency in the aquariums had a significant genetic basis (genomic h² = 0.47). There is a need, however, for greater understanding of physiological mechanisms underlying individual variation in feed efficiency in fishes. It is generally assumed that growth and, more specifically, lean growth are major contributing factors to individual feed efficiency (Knap and Kause, 2018). However, growth and body composition are only expected to explain 60-87 % of individual variation in feed efficiency in fishes; the remaining variation may be caused by individual basal metabolic rate, immune response functions, physical activity, and responses to social factors. Our aim was therefore to explore the phenotypic and genomic correlations between individual feed efficiency and metabolic traits in European sea bass.
Material and Methods
First, 458 sea bass were phenotyped in aquariums to estimate their feed efficiency following the procedure of Besson et al., (2019). Briefly, the fish stayed 3 periods of 2 weeks in 10L individual aquariums. Fish were individually fed a restricted daily ration based on their body weight and corresponding to 50% of the optimal ration given by the feed manufacturer (Le Gouessant – NEO MARIN). Because feed was restricted, individual feed efficiency was estimated as the residual of the final body weight on initial body weight at the start of the experiment (rFBW).
Second, 934 sea bass (336 previously phenotyped in aquariums and 598 of their sibs) were placed, in consecutive batches, in 32 individual respirometry chambers. Measurements of oxygen uptake (MO2) were made by intermittent stopped-flow respirometry as described in McKenzie et al. (2014). The procedure was based on 15-minutes cycles comprising 8 minutes stopped-flow and 7 minutes flushing with aerated water. Water oxygen levels in the chambers were measured every ten seconds. During stopped-flow, oxygen uptake by the fish was calculated as the slope of the decrease in oxygen quantity over the 8 minutes (MO2 in mg O2 h-1). Measurements were done for 36h following 12h of acclimation allowing us to potentially estimate 144 MO2 values per fish. With all the values of MO2 obtained, we estimated the standard metabolic rate (SMR in mg O2 h-1) of the fish by averaging the lowest 10% of the MO2 measurements. After the 48h, fish were weighed to correct the SMR for the effect of metabolic body weight (BW0.8) and generate residual SMR (rSMR).
All fish were genotyped on the 57k SNP array DlabChip (Griot et al., 2021). Genomic links between fish enabled the estimation of genetic parameters of rFBW and SMR and genetic correlations between those traits using AIREMLF90 software.
Results
We found large phenotypic variation in SMR (Figure 1). The mean SMR was 354 mg O2.kg-0.8.h-1 with a CV of 27%. Additionally, despite being weak, the phenotypic correlation between rFBW and rSMR was significant (r = -0.15, F1,335 = 7.45, p-value = 0.0066). This correlation suggests that the most efficient individuals were also those with low standard metabolic rate, as expected.
Both rFBW and rSMR were heritable (Table 1) and the genomic correlation between them was strong and negative (-0.77 ± 0.41). This result supports the phenotypic correlation found between the two traits and indicates that SMR plays a role in the genetic basis of feed efficiency in sea bass.
Discussion
This study confirms that the individual efficiency under restricted feeding in aquariums is heritable. Furthermore, to our knowledge, this is the first study to phenotype several hundreds of fish for their individual oxygen consumption, allowing reliable estimation of genomic parameters for rSMR. We found that rSMR had a genetic basis and that individual feed efficiency of sea bass was partly explained by their metabolic rate. This finding is valuable for sustainable fish farming, in understanding how the physiology of fishes may relate to their robustness and efficiency.
Acknowledgements
SELFIE – Selection for Feed Efficiency project (n° P FEA 4700 18FA 100 0021) was funded by the French Government and the European Union (EMFF, European Maritime and Fisheries Fund) at the “Appels à projets Innovants” managed by the France Agrimer Office.
References
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