Comparative immune response and disease resistance against Aeromonas hydrophila in Catla catla, Labeo rohita and their intergeneric hybrids

Hybridization is cost effective as a method to improve genetics through disease resistance and immunity. This study compares the innate immunity and survival of Catla catla (CC), Labeo rohita (RR), and their intergeneric reciprocals CR (C. catla♀ × L. rohita♂); RC (L. rohita♀ × C. catla♂) following infection with Aeromonas hydrophila. Fingerlings from all four cross-types (CC, RR, CR, and RC) were infected intraperitoneally with A. hydrophila at 2.24 × 10⁷ CFU/g. Parameters such as hematological, immunological, and genetic expressions were analyzed, and survival was followed for 20 days. The RC hybrids had significantly higher levels of red blood cells, hemoglobin, hematocrit, and mean corpuscular hemoglobin concentration compared to other groups. Higher platelets were observed in the CR group. After infection with bacteria, high concentrations of total proteins, immunoglobulin, lysozyme activity, phagocytic activity, phagocytic index, and respiratory burst activity were observed in RC hybrids, followed by CR, RR, and CC. Similarly, survival rates were significantly higher in RC (66.7%) and CR (60%), when compared with CC (46.7%). The expression level of immune-related genes (lysozyme C, lysozyme G, and TNF-α) was found to be significantly elevated in RC hybrids within the first 6–12 h after infection, with peak expression levels 2.5 to 3.8 times higher than in parental species. The pairwise correlations among these genes were all positive, ranging from r = 0.75-0.93, with the highest correlation observed in RC. In both reciprocal crosses, there was heterosis for most of the immune characteristics tested, although the RC cross performed better than the CR cross for most of these traits. Based on the parameters evaluated, the RC hybrid exhibited the strongest responses among the evaluated hematological, serum immune, gene-expression, and survival parameters.