Genetic variation in lytic activities of blood serum from Nile tilapia andgenetic associations with survival and body weight

A study was carried out on lysozyme activity and spontaneous haemolytic (SH ) activity of blood serum from 388 individuals of Nile tilapia (Oreochromis niloticus) coming from 42 full-sib groups within 21 paternal half-sib grou ps. The lysozyme activity was measured at both 15 degrees C and 30 degrees C incubation temperatures, whereas SH activity was measured at 30 degrees C incubation temperature, Significant variation in lysozyme activity was det ected between half-sib groups at 30 degrees C incubation temperature, but n ot at 15 degrees C incubation temperature and not for SH activity. Signific ant variation was found between full-sib groups for all parameters. The est imated heritabilities of lysozyme activity were relatively high (0.6-0.7) a t 30 degrees C incubation temperature and intermediate (about 0.3) at 15 de grees C incubation temperature. The heritability estimate of SH activity wa s zero according to estimates based both on the sire component of variance and on variance components from an individual animal model, whereas the est imate based on the dam component of variance was about 0.7 and highly signi ficant, Survival and growth were recorded after a grow-out period of 120 da ys in individuals from a parallel set of samples from the same sib families . Significant negative correlations were found between least-squares means in the parallel sib groups for lysozyme activity and survival rates for bot h half-sib groups (r = -0.53; P = 0.01) and full-sib groups (r = -0.32; P = 0.04). No significant correlations were found between SH activity and surv ival rate nor between lytic activities and body weight at harvest. (C) 1999 Elsevier Science B.V. All rights reserved.