Genetic Selection for High Body Weight Reshapes Gastrointestinal Structure and Microbiota in Lueyang Black-Boned Chickens

Genetic selection for high growth performance has markedly improved poultry production efficiency. However, gastrointestinal responses to selection for high body weight (HBW) remain poorly characterized. In this study, ten chickens from each of the primitive and HBW lines of Lueyang Black-boned chickens were monitored for growth performance from 1 to 180 days of age, and gastrointestinal morphology and microbial composition were compared at 180 days. Gastrointestinal structural traits were analyzed using t-tests, whereas microbial profiles were evaluated using non-parametric statistical methods. Compared with the primitive line, the HBW line exhibited: 1) Growth performance: no differences in body weight at 1 and 45 days, but significantly higher at 120 (39.82%) and 180 days (29.17%) (P < 0.05), along with a significantly lower feed conversion ratio during 45-120 days (10.17%) (P < 0.05); 2) Serum parameters: increased serum biomarkers associated with lipid transport (P < 0.05), with upward trends in glucose and uric acid concentrations (P < 0.10); 3) Organ morphology: increased weights of the crop (35.93%), gizzard (30.90%), duodenum (46.07%), cecum (33.80%), pancreas (32.74%), and liver (38.11%) (P < 0.05), while glandular stomach weight remained unchanged; and elevated villus height-to-crypt depth ratios in the duodenum (130.18%) and ileum (40.95%) (P < 0.05). 4) Microbial alterations: Segment-specific microbial alterations indicated an adaptive pattern characterized by enhanced nutrient utilization but attenuated immune-related functions. Specifically, enriched nutrient-associated taxa included crop Enterococcus and Bacillus, duodenal Lactobacillus, ileal Enterococcus, and cecal Bacteroidales. In contrast, immune-related functional shifts were characterized by crop xenobiotic biodegradation pathways, suppressed duodenal Salmonella- and immune-related signaling pathways, altered ileal alanine, aspartate, and glutamate metabolism, and cecal isoquinoline alkaloid and peptidoglycan biosynthesis pathways. Collectively, this study demonstrates that genetic selection for HBW has driven enhanced nutrient utilization in the gastrointestinal tract while attenuating microbiota-related immune functions, thereby providing a theoretical basis for precision nutrition strategies in poultry.