Submitted to: Maydica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/1995
Publication Date: N/A
Citation: Interpretive Summary: Genetic variation is the raw material of selection. In order for maize populations to respond to selection, adequate genetic variation must be present. Long-term selection can decrease the genetic variation in a population by changing the frequency of the genes controlling the trait of interest or by using small sample sizes. The objective of this study was to determine if long-term selection for grain yield had reduced the available genetic variation for unselected traits in the Iowa Stiff Stalk Synthetic (BSSS) maize population. Two long-term selection programs initiated in BSSS were studied. Eleven generations of selection had been completed in one program and 13 generations of selection had been completed in the other program. There were few significant changes in genetic variation for the traits studied. This finding suggests that maize has a tremendous reservoir of genetic variance and that much remains to be learned about how this variation is maintained. These results suggest that future selection progress should be maintained at current rates and that genetic variance in maize synthetics is not a limiting factor in achieving selection response. These results will be of particular interest to maize breeders designing breeding programs and population biologists interested in the depletion of genetic variation.
Technical Abstract: Recurrent selection is a cyclical breeding procedure that focuses on improving the mean performance of a population by increasing the frequency of favorable alleles and maintaining adequate genetic variability for continued selection response. The Iowa Stiff Stalk Synthetic (BSSS) maize (Zea mays L.) population has undergone continuous recurrent selection for more than 50 years as the base population for two independent selection programs. This study was designed to estimate the mean performance and important genetic parameters in BSSS before and after seven cycles of half-sib (HS) progeny selection, six cycles of S2-progeny selection, and 11 cycles of reciprocal recurrent selection (RRS). A Design II (factorial) mating design was constructed to give direct estimates of additive and dominance variance in the individual populations. Progenies from each of the four populations were grown for two years in a randomized incomplete block experiment. The relative effectiveness of the three selection methods for improving the mean performance for grain yield of the BSSS populations per se was 0.4% cycle**-1 for HS selection, 1.1% cycle**-1 for S2-progeny selection, and 1.4% cycle**-1 for RRS. There were few changes in variance components after selection for most of the traits. In general, additive variance accounted for the largest portion of the total genetic variance for all traits except grain yield. The importance of dominance variance for grain yield implies that interpopulation and testcross selection methods may give the greatest direct response to selection. The estimates of heritability and corresponding estimates of additive genetic variance indicate that continued response to selection is expected for each selection method.