The strong association of prolificacy in maize with tolerance to high plant density reported in the literature by many investigators has stimulated us to study the inheritance and expression of such trait under high and low plant densities. A diallel cross among eight diverse inbred lines in number of ears plant -1 was performed in 2010 season and the resulting F1’s along with their parental lines were evaluated in 2011 season under two plant densities (20,000 and 40,000 plants fed -1 ) using a split plot design in randomized complete blocks (RCB) arrangement with three replications at two locations (Bani Sweif and Minufiya). The main objective was to obtain information on the expression of genes controlling maize prolificacy under low and high plant density. Analyses of variance indicated existence of significant differences among locations, plant densities and genotypes for all studied traits. Performance of inbred lines and F1 crosses vary with location and plant density. Parental lines L14, L17, L18 and L53 were the best in mean performance and general combining ability effects (GCA) for ears plant -1 (EPP), grain yield plant -1 (GYPP) and grain yield fed -1 (GYPF) under high and low densities. The best F1 crosses in mean performance and specific combining ability effects (SCA) under high-density were L14×L17, L14×L18 and L18×L20 for EPP and L14×L17, L18×L20 and L18×L55 for EPP and GYPP. Type of dominance controlling inheritance of EPP trait changed from complete dominance to the prolific parent under low density to complete dominance to the non-prolific parent under high density. Results indicated that to obtain a hybrid of high EPP and GYPP under high-density, at least one of its parents should be prolific, but to obtain a hybrid of high GYPF, both of its parents should be prolific. The magnitude of genetic variance (additive and dominance) and heritability in narrow-sense for the three traits EPP, GYPP and GYPF was lower under high-density than that under low-density. Both additive and dominance variances were highly significant for the three traits, but the magnitude of dominance was higher than that of additive variance. Results confirm the simple inheritance of EPP trait (one gene) as reported by some investigators under high and low density. Prolificacy may be rapidly transferred from a prolific exotic inbred to the non-prolific Egyptian inbreds used in commercial hybrids by a conversion backcrossing program. Expected genetic advance (GA) from selection ranged from 9.49% for high EPP to 22.66% for GYPP under highdensity. Estimates of GA were higher under low than under high-density, indicating that selection for high EPP is more efficient than under low-density. Correlation coefficients for means vs. GCA effects of inbreds, means vs. heterobeltiosis, means vs. SCA effects and SCA effects vs. heterobeltiosis of F1 crosses as well as parents means vs. offspring means and breeding values were estimated and discussed.

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