Host genetic regulation of fermentation-related cecal microbial taxa is associated with loin muscle deposition in meat rabbits

Loin muscle weight is an important indicator of carcass yield in meat rabbit production, but the host genetic and intestinal microbial factors associated with its variation remain poorly understood. Because the cecum is the primary site of hindgut fermentation in rabbits, we integrated whole-genome resequencing, cecal transcriptome profiling, and cecal and rectal 16S rRNA sequencing data from 321 Kangda meat rabbits, with rectal microbiome data used as a downstream comparative reference. Compared with the rectum, the cecum contained a richer microbial community, with more ASVs and genera and significantly higher microbial diversity, whereas predicted metabolic functions were largely conserved between the two segments. Loin muscle weight showed moderate SNP-based heritability (h{superscript 2} = 0.39), and the cecal microbiome explained a smaller but detectable proportion of phenotypic variation (m{superscript 2} = 0.13). Multi-strategy microbial screening identified 19 candidate cecal genera associated with loin muscle weight, with Methanosphaera showing the strongest negative association. Host GWAS prioritized candidate loci near MEX3C and TCF4 on chromosome 10, and integration of cecal cis-eQTL and GWAS summary statistics further prioritized GJB3 as a candidate gene associated with loin muscle weight. Consistent with the cecum-centered model, host genetic relatedness was weakly but significantly correlated with cecal microbial similarity, whereas no such global association was observed for the rectal microbiome. Microbial GWAS and SMR analyses further prioritized a cecal MRAP2-Methanobrevibacter association as the main host-regulated microbial signal, while rectal analyses identified distinct segment-specific signals, including SULF1-Roseburia. These findings suggest that host genetic variation may be linked to loin muscle deposition partly through cecal gene expression and fermentation-related cecal microbial taxa, with the rectal microbiome providing comparative evidence for hindgut segment specificity. This study provides candidate host and microbial targets for future functional validation and microbiome-informed nutritional strategies to improve carcass traits in meat rabbits.