Abstract
In the present study, we investigated changes in resistance against bacterial cold-water disease and in leukocyte composition along with sexual maturation in ayu. Fish were reared under long photoperiod (LP) and the natural day length (ND) during September to November. Ayu sampled before starting the experiment was used as an initial control (IN). Gonad-somatic index significantly increased in ND group, but not in LP group, compared with IN group. Cumulative survival rate after challenge with 2.7 × 104 CFU of Flavobacterium psychrophilum was significantly higher in LP group (90%) than that in ND group (0%). Gene expression levels for lymphocyte markers (IgM, IgD, CD3ε and TCRβ) in the trunk kidney were significantly lower in ND group compared with the values in IN group. Consistently, the decrease of IgM and CD8α mRNA positive cells in the trunk kidney of ND group was also shown by in situ hybridization. Further, flow cytometry revealed that percentage of B lymphocytes in the trunk kidney leukocytes were significantly decreased in ND group than in the other groups. These data suggest that sexual maturation is the major cause of the decrease in disease resistance and of the lymphocytopenia, in ayu during the spawning season.
