TSINDI ALBERT
GENETIC STUDIES OF SOYBEAN (Glycine max (L.) Merrill) FOR EARLINESS AND ADAPTATION TO SELECTED SOUTHERN AFRICA ENVIRONMENTS
ABSTRACT
Soybean is the most cultivated grain legume worldwide due to its high oil and protein contents. Its production and popularity is increasing among small holder farmers in Africa. However, its production in Southern Africa is threatened by its photoperiod sensitivity to latitudinal changes and climate change which is shortening the growing seasons. The development of early maturing and high yielding varieties will help mitigate the effects of climate change for increased production of soybeans. The current research was aimed at developing early maturing and high yielding soybean genotypes adapted to Southern African environments. The specific objectives were to: (i) determine genetic diversity and population structure among temperate and tropical soybean lines using single nucleotide polymorphism (SNP) markers; (ii) determine the gene action controlling maturity, yield and other quantitative traits in soybean and (iii) assess the maturity and adaptation of the developed F4 populations in different selected Southern African environments. A total of 210 genotypes from USA, Zimbabwe, South Africa and Malawi were genotyped using 500 SNP markers to assess the genetic diversity within the germplasm. There was very low level of genetic diversity among the 210 lines with implications for the breeding strategy. Low fixation index (FST) value of 0.06 was observed, indicating low genetic differences among genotypes. This suggests high genetic exchange among different lines due to global germplasm sharing. Inference based on three tools viz. the Evanno method, silhouette plots and unweighted pair group method using arithmetic average (UPMGA) phylogenetic tree showed the existence of three sub-populations. The first sub-cluster was composed of three genotypes (Nitchuu 47, Tara and Tousan), the second cluster had two genotypes (Forrest and Fowler), while 205 of the genotypes constituted the third cluster. The third cluster revealed low variation among most genotypes suggesting common ancestors. The 210 accessions were then genotyped at the E9 locus to determine the maturity alleles at this locus for each genotype. Eight genotypes were identified carrying E9 alleles that condition early maturity. The eight genotypes were used as males in crosses to five lines from Zimbabwe used as females in a North Carolina design II mating scheme which was later used to generate the general combining ability (GCA) and specific combining ability (SCA) data. The F4 progenies of 37 successful crosses were evaluated together with 13 parents, across 11 locations in Zimbabwe and Zambia using a 10x5 Alpha Lattice experimental design with three replications. Plots comprised four rows of 3 m length, spaced at 0.45 m inter-row and in-row spacing of 0.05 m. The data for the quantitative traits was subjected to analysis of variance in Analysis of Genetic Designs in R (AGD-R) Version 5.0. Significant differences were observed among genotypes for the quantitative traits, indicating genetic variation among the F4 populations. Both GCA and SCA effects were highly significant for yield, maturity dates and some quantitative traits such as oil and protein content. This indicates that both additive and non-additive effects were important for governing these traits. However, the GCA effects were preponderant, indicating that additive gene action was predominant for controlling all the traits. This indicates that improvements of these traits can be achieved through selection. At least three lines, two used as males (lines 7 and 4) and one as female (line 13), displayed high general combining ability effects and contributed significantly to early maturity. Broad sense heritability and narrow sense heritability effects were high which suggests that selection for early maturity, high yield and other recorded traits from the current populations will be effective. Thirty-seven tropical x temperate F4 populations, and 13 checks were evaluated across eight sites in Zimbabwe and Zambia to determine the maturity periods and their adaptation. Genotype, environment and genotype by environment interactions were significant for all traits measured, signifying differences in performance of genotypes across environments. Grain yield had significant positive correlations with days to flowering (0.59), days to maturity (0.68), 100 seed weight (0.13), plant height (0.29), number of pods (0.84) and number of branches (0.99). Number of pods per plant, 100 seed weight, pod height and number of branches per plant had the highest direct effects on grain yield. Genotype G26 and G15 were identified as superior and ideal genotypes which were early, adapted and moderately stable across environments maturing in 109 and 112 days respectively and all yielding above 4000kg/ha. Test environments were divided into four mega environments and Mpongwe was the most discriminating and representative. Selections from advanced generations of G26 and G15 is most likely to isolate early maturing and high yielding lines.