Abstract
This paper reports the effects of three cycles of reciprocal recurrent selection (RRS) on the means, genetic variances, and on the genetic correlations for several traits in the IG-1 and IG-2 maize (Zea mays L.) populations. Interpopulation full-sib progenies from cycle zero (C0) and from cycle 3 (C3) of RRS were evaluated in two locations. RRS was highly effective to improve the traits according the objectives of the program: grain yield and prolificacy increased significantly, while plant height, ear height, and ear placement decreased significantly. Genetic variances for all traits decreased significantly from C0 to C3, but the genetic correlations did not change consistently across the cycles of selection. The expected responses to the fourth cycle of RRS and the probability of selecting double-crosses from C3 that outperform those from C0 showed that the decreases in the genetic variances were not great enough to limit the continued improvement of the populations as well as the use of the improved populations as sources of inbred lines to develop commercial hybrids. However, if the magnitudes of the genetic variances continue to decrease, new sources of improved germplasm should be incorporated into both populations to allow the continued improvement of the interpopulation by RRS.
Highlights
Reciprocal recurrent selection (RRS) is a cyclical breeding procedure designed to improve the cross of two populations from different heterotic groups by using both general and specific combining ability
Because of the joint effects of the selection and of the genetic drift, the genetic variability of the traits being selected could be reduced after some cycles of selection and, the selection response could be reduced to such a level that the selected populations become unfeasible as source populations for the development of commercial hybrids (Souza Jr., 1999; Souza Jr., 2001)
The IG-1 and IG-2 maize populations used in this study are early-maturing with orange flint and yellow dent kernels, respectively, and they were released by the maize breeding program of the Department of Genetics, Escola Superior de Agricultura “Luiz de Queiroz”, São Paulo University (ESALQ/USP), Piracicaba, São Paulo, Brazil
Summary
Reciprocal recurrent selection (RRS) is a cyclical breeding procedure designed to improve the cross of two populations from different heterotic groups by using both general and specific combining ability. Interpopulation half-sib or full-sib progenies are used as evaluation units and intrapopulation S1 progenies as recombination units (Comstock et al, 1949; Hallauer and Eberhart, 1970). Reported results from this breeding procedure have shown that RRS has been highly effective to improve the population crosses in maize (Zea mays L.) (Hallauer et al, 1988; Eyherabide and Hallauer, 1991; Souza Jr. and Pinto, 2000; Rezende and Souza Jr., 2000). Molecular marker studies in the populations submitted to RRS showed that there were significant losses and/or fixation of alleles because of genetic drift effects (Labate et al, 1997; Pinto et al, 2003a; Pinto et al, 2003b), and significant genetic drift effects on the means of several traits in maize populations undergoing RRS programs have been reported (Smith, 1983; Helms et al, 1989; Keeratinijakal and Lamkey, 1993)
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