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Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln 68583-0915
* Corresponding author (krussell3{at}unl.edu)
NB[RFS]1_8 (GP-489, PI 636476), NB[RFS]2_8 (GP-490, PI 636477), NB[RFS]3_8 (GP-491, PI 636478), NB[S1]1_8 (GP-498, PI 636485), NB[S1]2_8 (GP-499, PI 636486), NB[S1]3_8 (GP-500, PI 636487), NS[RFS]1_8 (GP-492, PI 636479), NS[RFS]2_8 (GP-493, PI 636480), NS[RFS]3_8 (GP-494, PI 636481), NS[S1]1_8 (GP-495, PI 636482), NS[S1]2_8 (GP-496, PI 636483), and NS[S1]3_8 (GP-497, PI 636484) are populations of maize (Zea mays L.) that resulted from eight cycles of replicated selection from two base populations, NB_0 (GP-510, PI 641709) and NS_0 (GP-511, PI 641710). NB_0 was derived from a synthetic that was developed by intermating 32 inbreds, and NS_0 was derived from the Iowa State Stiff Stalk synthetic (Galusha, 1999). All the populations are adapted to eastern Nebraska. Each base population was independently improved by two methods of selection, reciprocal full-sib family [RFS] selection and S1 family [S1] selection. In addition, for each base population and selection method, the selection procedure was replicated three times. This resulted in 12 populations at each cycle of selection. In the population name, the number appearing immediately after the selection method is the replicate number. This replicated selection study was initiated and directed through all eight cycles by W.A. Compton.
The selection criterion for both selection methods was a multiplicative index that equaled grain yield at 15.5% moisture x (100 – % lodged plants) x (100 – % plants with dropped ears). In each cycle of selection, 100 families were evaluated in an unreplicated test at three locations for 1 yr, and the 10 families with the highest mean index values were selected. For RFS selection, the S1 families from each population that were descended from the parents of the selected full-sib families were recombined using a chain-crossing breeding scheme. For S1 selection, the recombination procedure was identical except that remnant seed of the selected S1 families was used. Seed counts were made from all hand-pollinated ears produced in the recombination block to ensure that each selected family contributed equally to the next cycle. Other details of the selection procedures were provided by West et al. (1980).
Gains from eight cycles of selection have been evaluated in two multienvironmental tests. Averaged across replicates and methods of selection, eight cycles of selection generally increased index values and grain yield and decreased grain moisture, days to flowering, and plant height (Table 1; Galusha, 1999). When observed responses to selection were combined across replicates and tested against the variation among replicates within methods, highly significant (p < 0.01) linear increases in index values and grain yield occurred for both methods of selection in the crosses between the reciprocal populations and for S1 selection in the first generation selfed of each population but not in the populations per se for either method (Galusha, 1999). None of the differences between methods of selection in these linear responses was significant (p < 0.05). However, when responses occurring in individual replicates were considered, then whether a significant difference existed between RFS and S1 selection depended on which replicates were being compared. Responses of these two methods differed significantly in approximately 25% of the replicate comparisons for the index and grain yield. This discrepancy about the relative merits of RFS and S1 selection occurred because variation among replicates for the index was significant for the population cross, for both populations per se, and for the first generation selfed of the NB population. Aguilar-Sanmiguel (1997) also reported significant differences between at least two of the three replicate populations with RFS selection for the selection index, grain yield, and eight other agronomic traits.
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Without replication of selection experiments, the effects of genetic drift and selection are completely confounded. Such confounding can lead to erroneous conclusions about observed selection responses. Despite this important limitation of nonreplicated selection experiments, these 12 cycle 8 populations are the only known populations developed from replicated selection experiments of a cultivated crop or domesticated animal species. As such, they are a valuable genetic resource for the study of the effects of genetic drift and selection.
A 200-kernel sample of each population can be obtained from the corresponding author. It is requested that the source of these materials be acknowledged if they are used in subsequent genetic studies.
NOTES
Journal Series No. 14597. Registration by CSSA.
Received for publication August 15, 2005.
REFERENCES
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