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Abstract
Objectives of this study were to: (1) assess the genetic differences among Iowa Stiff Stalk Synthetic and seven populations derived from it at the University of Nebraska, and (2) estimate genetic parameters $\rm (a\sb{j}, d\sb{j}, h\sb{jj\sp\prime},$ and aa$\sb{\rm jj\sp\prime}$) and their proportional contribution to the total sums of squares due to differences among the eight populations and derived relatives.
Eight Stiff Stalk Synthetic corn populations and derived generations were evaluated and estimates made of genetic effects from: (1) parents per se and their testcrosses; (2) parents, parents selfed and F$\sb1$'s; (3) parents, parents selfed, F$\sb1$'s, F$\sb1$'s random mated and backcrosses. Parents, testcrosses, parents selfed and F$\sb1$ crosses were evaluated in four environments, but F$\sb1$'s random mated and backcrosses were evaluated in only two.
The analysis proposed by Eberhart and Gardner was used for the estimation of genetic effects in numbers (2) and (3) above. The latter allowed the inclusion of additive by additive epistasis.
NSS(5) and NS(B)RF(5), selected for yield index were highest yielding as populations per se, as selfed progenies, and on the average in interpopulations crosses. However, NSCT-15 and NSCT-FT-15 selected for cold and freeze tolerance yielded as well in testcrosses to Mo.17. Furthermore, NSCT-FT-15 exhibited higher heterosis in inter-population crosses.
NSCT-FT-15, NS(B)RF(5) and NSS(5) had the least inbreeding depression and highest cumulative additive effects suggesting an increase of favorable alleles. In general, populations with little or no selection for yield showed greatest inbreeding depression, greatest cumulative intrapopulation dominance effects, and lowest yields.
Selection for cold and freeze tolerance resulted in intermediate yielding populations but seemed to contribute something to heterosis, both in testcrosses and in interpopulation crosses.
Of the total sums of squares among entry means for yield, dominance effects explained 68 and 50% and additive effects 26 and 40%, for the models without and with epistasis, respectively. Interpopulation heterosis and epistasis were small and generally non-significant.
