Extending the productive lifespan of laying hens beyond the conventional 70 weeks offers a promising strategy to improve the sustainability and economic efficiency of egg production, while addressing ethical and environmental concerns. However, maintaining high production performance in aging hens requires understanding the genetic basis of lipid metabolism at advanced ages. Lipid metabolism is central to reproductive function in poultry, not only because yolk formation demands substantial lipid investment, but also because lipids provide precursors for steroid hormones and signaling molecules that regulate ovulation and egg production. However, abdominal fat weight (AFW), and yolk percentage (YP), despite being key indicators of lipid dynamics, remain poorly characterized from a genetic perspective at advanced production stages.
In this study, genomic architecture of lipid-related traits (AFW, YP) and their genetic parameters with production traits (egg weight, EW; laying rate, LR) were investigated in hens at 90 weeks of age. Liver weight (LW) was included as this organ is the main site of lipid synthesis. These analyses were performed by using an experimental population of up to 7,000 Rhode Island Red hens genotyped with 60k SNP arrays imputed on 600k. Genome-wide association studies (GWAS) were followed by linkage disequilibrium (LD)-based QTL refinement. Considering the hypothesis of causal genetic variants affecting hepatic gene expression, GWAS results were integrated with liver expression quantitative trait loci (eQTL) detected at the same age, enabling the identification of candidate genes whose expression may mediate genetic effects on phenotypes.
Heritability estimates for AFW and YP were high (0.48 and 0.51) and comparable to values reported in broilers and layers. Positive genetic correlations of AFW and YP with LR (0.24 and 0.27) suggest that moderate lipid reserves might sustain laying performance at advanced ages, contrasting with negative correlations in younger hens. Conversely, negative correlations of AFW and YP with EW ( 0.16 to 0.54), point to a trade-off between lipid allocation to yolk and abdominal fat reserve and overall egg mass. LW was strongly correlated to YP (0.53) likely reflecting the high contribution of liver-derived lipids to yolk formation, whereas correlations with AFW and LR are only moderate (0.19 and 0.27).
GWAS identified several QTLs associated with lipid-related traits. A new locus on chromosome 8 was detected for both AFW and YP, although LD analysis suggested distinct underlying causal variants. Known QTLs for abdominal fat percentage were confirmed on chromosomes 2 and 28. LD-based refinement reduced QTL intervals, improving candidate gene prioritization. For AFW QTL integration with liver eQTLs highlighted several promising regulatory candidate genes, including CYB5A and GDF15 at the chromosome 2 and 28 and for YP the gene OSBPL9 on chromosome 8.
These results demonstrate that substantial genetic variability for lipid-related traits persists at advanced ages and are correlated with key production traits. New candidate genes were identified through LD based QTL refinement and hepatic eQTL integration. These findings provide a genomic basis for breeding strategies aimed at improving metabolic efficiency in laying hens during extended production cycles.
This project is funded by the European program GEroNIMO N°101000236 and ANR ‘EFFICACE’ program.
