ALIMATU SADIA OSUMAN
GENOME-WIDE ASSOCIATION AND COMBINING ABILITY STUDIES OF GRAIN YIELD AND OTHER TRAITS OF EARLY MATURING MAIZE INBRED LINES UNDER COMBINED HEAT AND DROUGHT STRESS
ABSTRACT
Addressing the malnutrition and food insecurity in sub-Saharan Africa (SSA) as a results of recurrent drought and elevaled temperature, there is the need to develop “climate smart”, maize varieties that can withstand the harsh conditions prevailing within the sub-region. To achieve this goal, the International Institute of Tropical Agriculture Maize Improvement Program (IITA-MIP) has developed inbred lines with inherent tolerance to heat stress (HS), drought (DT) and combined heat and drought (CHD) stress conditions. The purpose of this study were to: (a) assess the performance of yellow and white early maturing inbred lines under terminal drought and CHD conditions (b) determine the genetic diversity of selected DT and CHD tolerant early maturing white and yellow endosperm inbred lines using single nucleotide polymorphism (SNP) markers, (c) identify marker-traits associated with combined heat and drought stress, (d) determine the combining ability and heterotic groups of selected early maturing white and yellow endosperm maize inbred lines under heat stress, terminal drought and combined heat and drought stress environments, and (e) identify stable and high yielding hybrids under CHD stress. The genetic diversity of the inbred lines was examined using the Nei’s genetic distance method and the model-based structure analysis and consistently revealed four distinct clusters. One hundred and fifty hybrids generated from 30 inbred lines using the North Carolina Design II (NCDII) were assessed under terminal drought, heat stress and CHD stresses in Nigeria and Ghana from 2018 to 2019. Of the best 20 hybrids selected under CHDS using the base-index, five hybrids (TZEI 31 x TZEI 188, TZEI 240 x TZEI 1517, TZEI 56 x TZEI 1496, TZEI 752 x TZEI 7 and TZEI 56 x TZEI 1517) out-yielded the best commercial hybrid check (TZEI 86 x TZEI 60) in grain yield. Additionally, seven hybrids (TZEI 1013 x TZEI 240, TZEI 135 x TZEI 501, TZEI 1013 x TZEI 56, TZEI 135 x TZEI 182, TZEI 498 x TZEI 502, TZEI 480 x TZEI 135, and TZEI 10 x TZEI17) performed better than the best commercial hybrid check under heat stress. Under terminal drought stress, TZEI 17 x TZEI 8 was found to be the best performing hybrid with a yield of 4.58 kg/ha. The stable hybrids across the test environments were TZEI 135 x TZEI 501 and TZEI 501 x TZEI 528 while the best performing hybrid under HS environments were TZEI 240 x TZEI 1517 and TZEI 135 x TZEI 501. Across the test environments TZEI 135 x TZEI 182 was identified as the best single cross hybrid. The outstanding hybrids should be extensively evaluated to confirm the consistency of superior performance for subsequent release to farmers in SSA. The Genome-wide association studies (GWAS) identified 117 significant (P<0.001) marker trait associations (MTAs). Combined heat and drought (CHD) condition identified the highest (66) Marker trait association (MATs). Under terminal drought condition, 27 Single Nucleutide Polymorphism (SNP) were identified with these SNP accounting for about 12.53 to 64.79% variation of the phenotypic variance. Four SNPs identified on Chromosomes (Chr) 3, 4, 5 and 6 associated with flowering traits and husk cover could be targeted for screening for earliness which is a key driver of drought stress and heat stress tolerance as well as husk cover for pest and insect tolerance. The highest number of SNPs found on Chr 1 which was linked to grain yield under CHD shows the important role Chr 1 plays in regulation of the trait (yield) under CHD environments. The highest numbers of SNPs identified under CHD were linked with grain yield and these SNPs were distributed across the 10 chromosomes with Chr 4 having five SNPs indicating it as a hot spot for grain yield. The additive genetic effects were more essential as compared to the (non-additive effects for grain yield and most measured triats under combined heat and drought stress, heat stress, terminal drought and across environments. Four distinct heterotic groups were identified using the SNP-based DArT markers and were identified as the most efficient method than the heterotic groping based on general combining ability of multiple triats (HGCAMT). TZEI 10 and TZEI 17 were identified as the best male and female testers for heterotic group I and TZEI 135 was identified as best male and female for heterotic group. These three inbreds would be very useful for classifying other inbred lines into heterotic groups.