An experimental comparison of selection alternatives to plateaued response.

Three alternative selection methods for extending selection limits or breaking response plateaus were compared over ten generations in a replicated model experiment using two unrelated populations of Drosophila melanogaster that no longer responded to purebred selection for high egg number, a heterotic polygenic trait. The three methods were: (1) reciprocal recurrent selection (RRS) with selection within each of the plateaued populations based solely on crossbred performance, (2) a modification of reciprocal recurrent selection (MRRS) with selection within each population based on both purebred and crossbred performance, and (3) purebred selection within a new synthetic population formed by crossing the two plateaued populations.--Conflicting estimates were obtained for heritability of purebred egg number in each of the plateaued populations. The realized heritability values and estimates from diallel analyses indicated an absence of additive genetic variation for both populations; however, estimates from conventional intraclass correlation methods were positive. The diallel analyses revealed significant amounts of nonadditive gene effects for purebred egg number in each population, while the significant gene effects for crossbred egg numbers were additive. Estimates of the genetic correlation between purebred and crossbred egg number were negative (-0.85 +/- 0.68 and -0.32 +/- 0.25) for the two base populations.--All three alternatives to continued purebred selection gave significant responses, with the average gain per generation from MRRS being significantly superior to the other two methods. Observed purebred and crossbred responses under RRS were in agreement with quantitative genetic theory. Such was not the case for MRRS, which suggested the possibility of major gene segregation.--Evidence supporting a negative genetic correlation between purebred and crossbred performance and the possibility of overdominance is presented and discussed.