Gene Action and Heterotic Response of Elite Maize Varieties under Varying Growing Conditions in Rain-Forest Agro-Ecology of Nigeria

Abstract

Several maize breeding programs use open-pollinated varieties (OPVs) as a source of genetic variability and favourable alleles of target traits. Drought tolerant and provitamin A (PVA) maize hybrids are needed to address malnutrition and food insecurity in sub-Saharan Africa (SSA). In response to this need, OPVs with drought tolerant and PVA genetic backgrounds were developed through the effort of the International Institute of Tropical Agriculture Maize Improvement Programme (IITA-MIP) for hybrid development and commercialization especially among the small holder rural farmers. The objectives of the study were to (i) assess genetic variability among 14 elite maize OPVs using genetic method, (ii) determine the mode of gene action controlling grain yield and other agronomic traits (iii) classify the maize varietal parents into distinct heterotic groups, and (iv) analyse yield stability of the parents and their hybrids across contrasting environments. Ninety-one hybrids generated from 14 elite OPV parents in a diallel mating design plus three checks, were evaluated under marginal rainfall, drought, and optimal environments in Nigeria from 2017 to 2018. The experiment was laid out in a randomized incomplete block design (9 x 12 alpha lattice) with three replications in each environment. The combined analysis of variance showed wide genetic variability among the evaluated genotypes. Out of the 91 hybrids, 41 yielded above average (2.31 t ha-1). Based on threshold score of 3 for both ear and plant aspects, about 18% of the hybrids could be selected. From the sequential path coefficient analysis, plant aspect and ear aspect were considered important selection indices for improvement aimed at developing high yielding OPVs in the sub-Saharan region. Non-additive genetic effects were more important than additive for grain yield and other agronomic traits across all the the research environments. The classification of some of the parents with similar genetic backgrounds together in the same group in each and across test environments indicated that the grouping of the OPVs was based largely on their pedigree. Using HSGCA method, TZL Comp - 3 C3 DT (P6) and White DT STR SYN/IWD C3 SYN F2 (P9) were consistently grouped differently under all the test environments suggesting their heterotic potential. The genotypes were divided into three groups, containing either the parents, hybrids or checks, for estimating the stability variance and grain yield. The difference between the lowest and highest yielding environment was 3.2 t ha-1, while the repeatability of the grain yield trials ranged from 35% for marginal rainfall condition at Ile-Ife to 80% for optimal condition at Umudike. The average grain yield of the hybrids was significantly higher than that of the parents and the check varieties. The hybrids were also more stable than both the parents and the checks. This study established wide genetic variability among the parental varieties and preponderance of non-additive variance over additive variance for inheritance of most traits. A number of high yielding and stable variety hybrids were identified which are potential candidate for release to farmers. Molecular markers approach to this work is recommended for the future in order to validate the phenotypic results that have been achieved in this present study.