Efeito do processo dispersivo em subpopulações de tamanho reduzido de milho (Zea mays L.). / Dispersive process effect in reduced size subpopulations of maize (Zea mays L.).

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

The composite GN-04 was used in the present work with the objetives: a) evaluation of the effect of the dispersive process in subpopulations of maize with different effective sizes, and submitted to divergent selection for higth and low yield; b) evaluation of the combining ability and heterotic potential of the subpopulations; and c) to estimate components of the genetic variance in the composite GN-04, using chain crosses. A pollination block of the base population was planted for the development of three different progenies: S1 progenies obtained by selfing, full-sib progenies obtained by plant-to-plant crosses, and chain crosses following the scheme: 1 x 2, 2 x 3 e 3 x 1. The numbers of progenies were: 77 S1´s, 75 full-sibs and 72 chains represented by three crosses in each chain. The progenies from each type were evaluated in different experiments in two locations following the randomized complete block design with three replications. The following traits were analysed: plant height (AP), ear height (AE), ear length (CE), ear diameter (DE), ear weight (PE) and prolificacy (PR). Observed means of PE were used for selection of 10 full-sib progenies, 10 S1´s progenies and 10 complete chains, each group being represented by five high yielding and five low yielding progenies or chains. The recombination of the selected progenies or chains, individually, led to the development of subpopulations with different effective sizes: N1, N2 and N3, corresponding to inbreeding rates of 50%, 25% and 17% for S1´s, full-sibs and chain crosses, respectively. The subpopulations were later evaluated based on their performance per se, in the topcrosses and in diallel crosses. Results showed that the reduced size of the subpopulations led to changes in the means of the evaluated traits, which were smaller or higher than the original means as a consequence of genetic drift. On the average, all traits showed a decrease in the mean as a consequence of inbreeding depression. The most depressive trait was PE and the depression effects in subpopulations with N3 and N1 were 7.17% and 22.94% in the positive selection and 16.85% and 55.91% in the negative selection. The divergent selection among subpopulations led to a genetic differentiation sufficient for the expression of heterosis in crosses between subpopulations in the same level of effective size. The estimates of the additive genetic variance in the composite GN-04 were obtained from the analysis of chain crosses. The estimates for the traits PE, CE, DE, PR, AP and AE were, respectively: 128.00 (g/pl)², 1.490 (cm/esp.)², 0.1704 (cm/esp.)², 0.0200 (esp./pl)², 256.55 (cm/pl)² and 136.80 (cm/pl)². Negative estimates of the dominance variance were obtained for the traits CE, DE, AP and AE; for the traits PE and PR, the estimates were 185.00 (g/pl)² and 0.0188 (esp./pl)², respectively. The mating design based on chain crosses with size 3 was considered feasible and easy to be used and can be a reliable alternative as compared with other more complex designs.

ASSUNTO(S)

vegetable populations endogamia milho genetics drift populações vegetais inbreeding deriva gênica maize heterosis heterose

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