Heritability estimates and selection response for resistance to Streptococcus agalactiae in red tilapia Oreochromis spp.

This study presents results of selection response to Streptococcosis agalactiae (SA) resistance in a commercial stock of red tilapia (Oreochromis spp.) after one generation of selection. Families were produced from 93 males and 128 females using a nested design (1 male: 2 females). At 60 days post-hatch, 30 fish from each family were challenged by intraperitoneal injection of bacterial solution (SA strain AQSA01) at the 96 h LD50 concentration (1 x 10(9) CFU mL(-1)) and observed for 14 days. Disease-resistance was measured as a binary trait (dead/alive) and as survival time, or the number of days from challenge initiation until death. Animal and sire-dam models were applied with body weight fitted as a covariate, whereas tank and generation were treated as fixed effects. Data from 128 full-sib and 35 half-sib families was used to estimate variance components using ASREML version 4.1. Heritability estimates for the base generation (GO) were highest, 0.25 +/- 0.12 for binary data on the liability scale and 0.20 +/- 0.08 for days until death. Breeding candidates in GO, selected from the top 10 high-ranking families based on their estimated breeding values (EBVs), were used to produce 25 full- and six half-sib families. When data from GO and G1 were combined, heritability estimates were 0.29 +/- 0.11 and 0.27 +/- 0.08 for threshold sire-dam and animal models, respectively. The realized genetic gain was 2.53 days for survival time, equivalent to about a 58% improvement from 4.3 days in the base generation. In terms of binary response, the probability that the G1 fish survived to the end of challenge period increased 10% from the base generation. This study demonstrates a significant selection potential for increased disease-resistance to S. agalactiae in red tilapia.