GENETICS Divergent Selection for Ascites Incidence in Chickens

ABSTRACT Chicken lines that were either resistant or susceptible to ascites syndrome were developed by using a hypobaric chamber to induce the disease. Birds were reared in a hypobaric chamber that simulated high alti-tude by operating under a partial vacuum, which thereby lowered the partial pressure of oxygen. Ascites mortality data from birds reared under hypobaric chamber condi-tions were used to select siblings to be used for breeding. The response to selection for the susceptible (SUS) and resistant (RES) lines of chickens was very rapid from the base population, which exhibited an incidence of ascites of 75.3%. Extremes in the incidence of ascites were ob-served in generation 8, with line SUS exhibited an average incidence of ascites of 95.1%, and in generation 9, with line RES exhibited an average incidence of ascites of 7.1%. The incidence of ascites in the relaxed line remained rela-tively stable and currently has a general incidence of ascites of 60%. The heritability estimates ± SE for ascites were estimated to be 0.30 ± 0.05 and 0.55 ± 0.05 for lines SUS and RES, respectively. Changes in the incidence o