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Title:   Author  item
                                            WARDYN, BRANDON - AGRELIANT GENETICS, LLCitem
                                            Edwards, Jodeitem
                                            LAMKEY, KENDALL - ISU
  Submitted to: Crop Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 9/1/2006
                                    Publication  Date: 3/1/2007
                                    Citation: Wardyn, B.M., Edwards, J.W., Lamkey, K.R. 2007. The genetic structure of a maize synthetic:  the role of dominance. Crop Science. 47:467-476.Interpretive Summary: Inbreeding depression, which is the difference in performance between hybrids and their inbred parents, is an important phenomenon to the hybrid seed industry because of its direct impact on the cost of producing hybrid seed.  Hybrid seed is produced on inbred lines, which almost always have greatly reduced yields compared to hybrids produced from them.  It has been very difficult for corn breeders to improve both inbred and hybrid yields simultaneously and it has been very difficult to predict performance of hybrids based on observations of inbred lines.  The present study addresses these issues by obtaining estimates of genetic parameters that allow geneticists to quantify the relationship between inbred and hybrid performance.  This study suggests that in fact there is a positive genetic correlation between inbred and hybrid performance for several traits studied, but the correlation between inbred performance and hybrid performance is much smaller for grain yield than for all other traits.  This provides a genetic model that is consistent with what corn breeders have observed.  Furthermore, the study provides evidence that continuous selection for hybrid performance (13 generations of selection) within a single population has changed the genetic properties of the population in such a way that it will be much more difficult to derive inbred lines with high performance and possibly, the correlation between inbred and hybrid performance has decreased.  Such results demonstrate that continuous selection for hybrid performance within a closed population may have unintended consequences.  This research will benefit breeders and scientists who operate and develop hybrid breeding programs.  The results will also benefit basic scientists who are studying the genetic mechanisms of selection response.
                                Technical Abstract: In selection programs, the covariance between parents and offspring largely determines the success of selection.  We have estimated the variances and covariances between noninbred individuals and both their inbred and outbred progeny in the non-stiff stalk maize population BSCB1(R)C13.  Estimation of these variances and covariances has allowed us to estimate the genetic covariance parameters for BSCB1(R)C13.  Previous estimates of genetic covariance parameters in maize have been used to describe the ineffectiveness of inbred progeny selection in the stiff stalk population BS13.  Our estimates in BSCB1(R)C13 indicated that the dominance variance was larger than the additive variance for grain yield whereas the additive variance was larger than the dominance variance for all other traits.  Negative estimates of the covariance between additive and homozygous dominance deviations were found for all traits with the exception of traits associated with reproductive maturity, suggesting a negative relationship between inbred and outbred performance.  A direct result of this genetic variance structure was the correlation between genotypic values and breeding values was lower for grain yield than any other trait.  Our results were similar to previous results found in the stiff stalk maize population BS13.  Thus it appears that similar genetic variance structures have been formed by selection in two maize populations that differ in their genetic background.


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