RESPONSES TO 4 CYCLES OF FULL-SIB FAMILY RECURRENT SELECTION IN AN F2MAIZE POPULATION

Four cycles of full-sib family recurrent selection for machine-harvest able grain yield were conducted in the source F2 population of the mai ze (Zea mays L.) single cross A632 x Mu195. This study was designed to evaluate the responses to selection and the main effects of the chang es in the frequency of alleles with selection. The five populations pe r se (from F2 = C0 to C4), their 20 testcrosses with four inbred line testers (i.e., A632, Mu195. Oh43, and W117) and five single crosses am ong these inbred lines were compared in three field trials. Positive r esponses to selection were shown by populations per se for all investi gated traits and followed a quadratiC trend for grain yield, kernels p er plant, plant and ear height, and a linear trend for kernel weight, sowing-silking interval, and kernel moisture. Average response for gra in yield was 7.3% cycle-1. Positive linear responses to selection were also shown in the testcrosses. These findings are attributed mainly t o changes in frequencies of alleles with important additive effects. T he quadratic trend of responses shown by populations per se can be at least partly ascribed to changes in frequencies of alleles with import ant dominance effects; random genetic drift was assumed to be a factor in such changes. C4 testcrosses with Oh43 and W117 had lower grain yi eld but also greater earliness and smaller plant size than correspondi ng A632 single crosses.