Variation among and within S1 progenies of maize
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Maize (Zea mays L.) breeders emphasize selection within F[subscript]2 populations of elite line crosses that have different levels of genetic relationship. Two maize F[subscript]2 populations, one of a cross of unrelated lines (B73 x Mo17) and one of a cross of related lines (B73 x B84), were used to derive two groups of unselected S[subscript]2 lines. Two S[subscript]2 lines per S[subscript]1 family were evaluated for 86 S[subscript]1 families of the unrelated line cross (172 S[subscript]2 lines) and 80 S[subscript]1 families of the related line cross (160 S[subscript]2 lines). The 332 S[subscript]2 entries were evaluated as lines per se in 1985 and 1986 in a total of three environments. Testcrosses of the S[subscript]2 lines also were evaluated during 1986 and 1987 in a total of six environments. The single-crosses of H99 x A619 and Mo17 x MBS2040 were used as testers for the unrelated and related line crosses, respectively. Objectives of this study were to: (1) estimate and compare genetic components of variance among and within S[subscript]1 families developed from unrelated and related line F[subscript]2 populations, and (2) estimate and compare genetic components of variance among and within the testcrosses of the S[subscript]1 families;Results of the line per se evaluation showed more estimates of among family variance to be significant and, in all instances, larger than the estimate of within family variance for the unrelated and related line crosses. Genotypic components of variance, heritabilities, and genetic gains suggested that selection among versus selection within families would result in a greater response to selection at the S[subscript]1 level of inbreeding. Genetic variances among lines were greater in the unrelated line cross for all traits except percentage of root lodging. Although progress from selection would be expected in both groups of lines, a greater response would occur in the unrelated line cross;Genotypic variances among and within testcross families of S[subscript]1 lines of the unrelated line cross were not significantly different from one another for grain yield, percentage of root lodging, and percentage of grain moisture. Estimates that were not significantly different from zero were obtained for among and within testcross families of the related line cross. Relatively high allelic frequencies for traits of interest in the testers may have masked the genetic variation that was present among and within the testcross families.