GENETIC ANALYSIS OF DROUGHT TOLERANT EARLY MATURING WHITE MAIZE (Zea mays L.) INBREDS WITH STRIGA RESISTANCE GENES FROM Zea diploperennis
Recurrent drought and parasitism by Striga hermonthica Del. Benth constitute the two most important stresses limiting maize (Zea mays L.) production and productivity in sub-Saharan Africa (SSA). Yield losses can reach up to 85% when the two stresses occur simultaneously in the field. The use of resistant varieties is more sustainable, economical, and efficient for African farmers. Several early (90-95 days to maturity) Striga resistant maize hybrids have been
commercialized in the sub-region. However, the levels of resistance are not as high as desired as they still support Striga emergence thus increasing the Striga seed bank in the soil each season. The International Institute of Tropical Agriculture (IITA) has developed new early maturing maize inbreds containing novel Striga resistance genes from Zea diploperennis. Knowledge and understanding of the mode of gene action conferring Striga resistance and drought tolerance in these new early maturing maize inbreds would be invaluable in developing hybrids adapted to both stresses in the sub-region. The objectives of this study were to: (i) determine the genetic diversity and reaction of these early maturing maize inbred lines under Striga infestation and drought environments, (ii) determine the mode of inheritance of Striga resistance in an early maturing inbred line containing resistance genes from Zea diploperennis, (iii) determine the combining abilities for grain yield and other agronomic traits and heterotic groups of 30 drought tolerant, early maturing inbreds with the Striga resistance genes, (iv) identify high yielding and stable hybrids under Striga-infested, drought and optimal growing conditions (v) examine the inter-trait relationship of early maturing maize hybrids under the Striga infestation and drought. Genetic diversity among 36 early maturing inbred lines was assessed using 8145 SNP markers. The cluster analysis and population structure analysis separated the inbred lines into four distinct groups based on their genetic distance indicating high level of genetic variability among the lines. Using the base indices for selection, 22% of the inbred lines combined resistance to Striga and tolerance to drought. Generation mean analysis was used to study the inheritance of resistance to Striga in the early maturing maize inbred line, TZdEI 352, containing genes from Zea diploperennis to facilitate its effective use in resistance breeding programmes in SSA. Only models that incorporated epistasis in addition to additive and dominance gene effects were adequate in explaining variation in the six generations studied. Epistasis played an important role in Striga resistance genes from Zea diploperennis in tropical maize. One hundred and fifty hybrids derived from crosses involving the 30 inbreds utilizing North Carolina Design II plus six hybrid checks were evaluated under artificial Striga infestation at Mokwa and Abuja, drought at Ikenne, Bagauda, Minjibir and optimal environments at Ikenne, Mokwa and Abuja, in 2013 and 2015. Significant GCA and SCA effects for grain yield and most measured traits were detected under the three research conditions. The higher values of GCA over SCA obtained for grain yield, flowering traits, plant and ear heights, husk cover, Striga damage and number of emerged Striga plants at 8 and 10 weeks after planting under Striga infested and optimal environments, indicated that they were controlled by additive gene action. The non-additive gene action was more important than the additive gene action for days to silking, anthesis-silking interval, ear height, stalk and root lodging, ears per plants, ear and plant aspects while additive gene action was more important for grain yield, plant height, husk cover, and stay green characteristics under drought environments. There were no maternal effects in the expression of the traits either under Striga infestation, drought or optimal growing environments. Inbreds TZdEI 268, TZdEI 352 and TZdEI 173 had superior positive GCA-male and GCA-female effects for grain yield and negative GCA-male and GCA-female effects for Striga damage and number of emerged Striga plants under Striga infestation indicating that they contributed to higher grain yield in their hybrids and could be used to improve tropical germplasm for Striga resistance. The lines TZdEI 492 and TZdEI 378 with outstanding positive GCA effects for grain yield under drought environments could be used to improve tropical germplasm for drought tolerance. The inbred lines were classified into four heterotic groups across the research environments using GCA effects of multiple traits. The inbred lines classified into each heterotic group may be recombined to form populations that could be improved through recurrent selection. Grain yield ranged from 1134 kg ha-1 for TZEI 26 x TZEI 5 to 5362 kg ha-1 for TZdEI 173 x TZdEI 280 under Striga infestation, 579 kg ha-1 for TZdEI 314 x TZdEI 378 to 3601 kg ha-1 for TZdEI 479 x TZdEI 260 under drought and 2376 kg ha-1 for TZdEI 82 x TZdEI 71 to 7769 kg ha-1 for TZdEI 260 x TZdEI 396 under optimal conditions. The additive main effects and multiplicative interaction analysis identified TZdEI 173 x TZdEI 280, TZdEI 173 x TZdEI 492, TZdEI 441 x TZdEI 260, TZdEI 82 x TZdEI 260, TZdEI 71 x TZdEI 396, TZdEI 396 x TZdEI 131, TZdEI 396 x TZdEI 264, TZdEI 98 x TZdEI 352, TZdEI 157 x TZdEI 352, TZEI 18 x TZdEI 357, TZdEI 268 x TZdEI 378, TZdEI 157 x TZdEI 280, TZdEI 492 x TZdEI 441 and TZEI 60 x TZEI 5 as the highest yielding and stable hybrids with combined Striga resistance and drought tolerance genes. Also, they had reduced Striga emergence and host plant damage. These hybrids should be tested in multi-location and on-farm trials to confirm the consistency in performance and promoted for release and commercialization in the Striga endemic areas with short duration of rainfall in West and Central Africa to contribute to increased maize productivity, poverty alleviation and reduced Striga seed bank in the soil. Striga resistant and drought tolerant hybrids with outstanding performance across stress environments could be obtained through accumulation of favorable alleles for stress tolerance in parental lines. Ear aspect was identified as the most reliable secondary trait for indirect selection for grain yield under both Striga-infested and drought.