GENETIC ANALYSES OF DROUGHT TOLERANCE IN CROSSES OF ADAPTED AND EXOTIC MAIZE (Zea mays L.) INBRED LINES, ABSTRACT
Molecular-marker-based genetic distances, pedigree information, and estimates of genetic effects are often combined to select parents of superior high yielding hybrids. The objectives of this study were to:
(i) determine the genetic diversity and distances among the maize inbred lines with simple sequence repeat (SSR) makers; (ii) assess the genetic effects controlling drought tolerance in a set of adapted (IITA) and exotic (CIMMYT) drought tolerant maize inbred lines and identify superior single-cross hybrids under managed stress conditions; (iii) assign another set of exotic and adapted inbred lines into
heterotic groups and identify exotic lines for improvement of adapted germplasm; and (iv) determine the importance of genotype x environment (GxE) interaction effects on the hybrids of adapted and exotic drought tolerant inbred lines. Forty-eight exotic and adapted elite drought tolerant maize inbred lines were analysed with 81 SSR markers. Twenty-four lines were selected based on genetic distances and pedigree information to generate 96 single-cross hybrids using a North Carolina Design (NCD) II mating scheme. Another set of 20 lines were crossed to two inbred line testers (1368 and 9071) to generate 41 testcrosses including the hybrid between the two testers. The two sets of hybrids were evaluated under managed drought stress and well-watered conditions in the dry seasons of 2010 and 2011 at Ikenne in Nigeria. Diniver 2000 device was used to measure volumetric water content in the drought and non-drought blocks in order to ensure that drought treatment was truly applied to the stress managed blocks in the two seasons. The two groups of hybrids were also evaluated in replicated trials in three contrasting environments (Ikenne, Bagauda, and Saminaka) across Nigeria in the main growing season of 2011. The SSR markers revealed moderate genetic diversity among the exotic and adapted maize inbreds with mean polymorphic information content (PIC) value of 0.51. The mean estimates of genetic distance among the exotic and adapted lines were higher than the mean genetic distance among adapted lines or among exotic lines, suggesting more diversity between exotic and adapted germplasm than within the group of lines from either center. The average linkage (UPGMA) cluster analysis separated the lines into two broad groups along institutional lines. The results assisted in selecting parental inbreds for making crosses in field experiments. Additive genetic effects were more preponderant than non-additive genetic effects in the inheritance of drought tolerance and yield in the two groups of hybrids studied. Among the 24 inbred lines used in single-crosses, eight had significant and positive GCA effects for grain yield under full irrigation and seven inbred lines had significant and positive GCA effects for grain yield and other important traits recorded under drought stress. Three hybrids, namely ADL47xEXL15, ADL41xEXL15, and EXL02xADL47, were among the top ten highest grain yielders under both full irrigation and drought stress environments. Among the 20 lines used in testcrosses, five exotic lines, namely EXL11, EXL12, EXL13, EXL18, and EXL22, showed outstanding performance in testcrosses and thus can be used to improve adapted germplasm for yield and drought tolerance. The two testers efficiently separated the lines into three groups, with 9071 showing more discriminating powers than 1368. Significant genotype x environment (GxE) interaction effects for the two sets of hybrids undermined selection of stable high yielding hybrids across studied environments. Therefore, selections of outstanding hybrids based on on grain yield performance were made for each environment.