Heterotic grouping of tropical early-maturing maize inbred lines based on combining ability in Striga-infested and Striga-free environments and the use of SSR markers for genotyping

Abstract

Identification of heterotic groups among inbreds is crucial to the success of a maize (Zea mays L.) hybrid breeding program. The objectives of this study were to determine the combining ability of 28 early maturing inbreds, classify them into heterotic groups, identify suitable testers under Striga-infested and Striga free environments and to assess the efficiency of three heterotic grouping methods. Three hundred and seventy-eight hybrids derived from diallel crosses of 28 early inbreds along with two checks were evaluated in Striga-infested and Striga-free environments for 2 years at two locations in Nigeria. Twenty-seven of the 28 inbreds were genotyped with 46 polymorphic simple sequence repeats (SSR) markers previously mapped on the 10 maize chromosomes. Analysis of variance revealed highly significant (P<0.01) general combining ability (GCA) and specific combining ability (SCA) mean squares for most traits under test conditions. Based on the SCA effects and heterotic group's specific and general combining ability (HSGCA) for grain yield, four and five heterotic groups were identified under Striga-infested and Striga-free environments, respectively. Two inbreds could not be classified into any of the four groups under Striga-infestation. The SSR markers revealed a wide genetic variability among the inbred lines as the genetic distance ranged between 0.21 and 0.68. Four heterotic groups were identified based on genetic distance (GD) derived from the SSR analysis. Correlation analyses showed that grouping using GD was more consistent with grouping based on SCA under Striga-free than under Striga-infested environments. The HSGCA method was the most effective in classifying early maturing maize inbreds under Striga-infested and Striga free environments. A total of 4 and 8 inbred testers were identified under Striga-infested and Striga-free environments, respectively. These inbred testers are invaluable resources for tropical maize breeding programs.