Phenotypic responses of chickens to long-term selection for high or low antibody titers to sheep red blood cells.

A long-term bidirectional selection experiment was conducted to study antibody response to SRBC. Lines, high antibody selection (HAS) and low antibody selection (LAS), originating from the same White Leghorn base population had undergone 37 generations of selection for either high or low antibody response 5 d after a single intravenous injection of 0.1 mL of a 0.25% suspension of SRBC antigen. Subpopulations, where selection was relaxed, were maintained as contemporaries with the selected lines from generations 16 to 24 [high antibody relaxed (HAR) and low antibody relaxed (LAR)] and 24 to 37. Body weights were obtained at 4, 24, and 38 wk of age and at the onset of lay (BW at first egg). Also measured were age in days to first egg, percentages of hen-day ovulations and normal egg production, and percentages of normal and defective eggs from total ovulation (PNE and PDE). Selection lead to a large divergence in antibody titers between the selected lines, with a plateau reached in line LAS. Line HAS and HAR females displayed higher antibody titers, lower BW4, and matured at older ages than those from LAS and LAR (P < 0.05). Correlations between BW at 4 wk and antibody titers were different between the selected lines, being positive in line LAS and negative in line HAS. Quadratic regression models fit well with antibody titers, BW4, and PNE, with limiting values for these traits calculated based on regression curves. For line HAS, plots showed that an increased tendency of antibody titers was followed by decreased BW4 and increased PNE. For line LAS, however, antibody titers and BW4 decreased in parallel while PNE increased. It appears that at the phenotypic level there was a resource balance between immune response, growth, and reproductive traits, which during long-term selection, individuals altered their dynamic of resource allocations to satisfy certain needs.