Bindiganavile S. Vivek

Validation of Consensus Quantitative Trait Loci Associated with Resistance to Multiple Foliar Pathogens of Maize

Maize production in sub-Saharan Africa incurs serious losses to epiphytotics of foliar diseases. Quantitative trait loci conditioning partial resistance (rQTL) to infection by causal agents of gray leaf spot (GLS), northern corn leaf blight (NCLB), and maize streak have been reported. Our objectives were to identify simple-sequence repeat (SSR) molecular markers linked to consensus rQTL and one recently identified rQTL associated with GLS, and to determine their suitability as tools for selection of improved host resistance. We conducted evaluations of disease severity phenotypes in separate field nurseries, each containing 410 F2:3 families derived from a cross between maize inbred CML202 (NCLB and maize streak resistant) and VP31 (a GLS-resistant breeding line) that possess complimentary rQTL. F2:3 families were selected for resistance based on genotypic (SSR marker), phenotypic, or combined data and the selected F3:4 families were reevaluated. Phenotypic values associated with SSR markers for consensus rQTL in bins 4.08 for GLS, 5.04 for NCLB, and 1.04 for maize streak significantly reduced disease severity in both generations based on single-factor analysis of variance and marker-interval analysis. These results were consistent with the presence of homozygous resistant parent alleles, except in bin 8.06, where markers were contributed by the NCLB-susceptible parent. Only one marker associated with resistance could be confirmed in bins 2.09 (GLS) and 3.06 (NCLB), illustrating the need for more robust rQTL discovery, fine-mapping, and validation prior to undertaking marker-based selection.

Genetic gain and cost efficiency of marker-assisted selection of maize for improved resistance to multiple foliar pathogens

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum, gray leaf spot (GLS) caused by Cercospora zeae-maydis and maize streak caused by maize streak Mastrevirus (MSV) are the most destructive foliar diseases limiting maize production in sub-Saharan Africa. Most foliar diseases of maize are managed using quantitative (partial) resistance, and previous studies have reported quantitative trait loci associated with host resistance (rQTL). Our objective was to compare the genetic gain and costs resulting from phenotypic, genotypic, and marker-assisted selection of partially inbred lines derived from many families for resistance to infection by three foliar pathogens. We developed a population of 410 F2:3 families by crossing inbred line CML202 with a breeding line designated VP31. These families were planted in nurseries inoculated separately with each pathogen. We conducted one cycle of early generation pedigree selection using three different procedures, phenotypic, genotypic, and marker/phenotypic index, for improvement of resistance to each pathogen. We used simple sequence repeat (SSR) markers flanking six target rQTL associated with partial resistance. Broad- and narrow-sense heritability estimates were also obtained for the F2:3 families, and selected and non-selected F2:4 families. Genetic gains resulting from the selection procedures were determined. Gene action of the candidate rQTL was determined using orthogonal contrasts. Estimates of costs based on lower boundary values indicated that the cost of marker-based selection was lower than that of phenotypic selection. Our results indicate that molecular markers linked to target rQTL can facilitate pyramiding resistance to multiple diseases during early generation pedigree selection.

Gene action determining Phaeosphaeria leaf spot disease resistance in experimental maize hybrids

Phaeosphaeria leaf spot (Phaeosphaeria maydis Henn.) has the potential to cause substantial yield losses in maize. Maize is grown by small-scale farmers without fungicides; hence there is need to breed for resistance in regionally adapted germplasm. Little information about the gene action determining Phaeosphaeria leaf spot disease (PLS) resistance in African maize germplasm is currently available. This study was therefore conducted to determine the gene action controlling resistance to PLS in African maize germplasm. Seventy-two experimental hybrids were generated in eight sets according to a North Carolina Design II mating scheme. Experimental and check hybrids were evaluated in an 8 × 8 simple lattice design during the 2003/4 season, and in an 8 × 10 α-lattice design, with two replications, during the 2004/5 seasons at the Cedara and Rattray Arnold Research Stations, in South Africa and Zimbabwe, respectively. There was significant variation among hybrids for resistance. General combining ability (GCA) due to both male and female inbred parents were highly significant (P<0.01), whereas specific combining ability effects were not significant for PLS scores, indicating that mainly additive gene action conditioned PLS resistance in experimental hybrids. Significant differences between male and female GCA variances, suggested the importance of cytoplasmic inheritance for PLS resistance. Resistance was highly heritable indicating that selection could be used to improve the resistance in this germplasm.

The Influence of Water Stress on Yield and Related Characteristics in Inbred Quality Protein Maize Lines and Their Hybrid Progeny

Water stress is one of the factors most frequently limiting maize production, food security, and economic growth in sub-Saharan Africa. The unprecedented combination of climatic risk, declining soil fertility, the need to expand food production into more marginal areas as population pressure increases, high input costs, extreme poverty, and unavailability of credit systems, have resulted in small holder farmers in southern and eastern Africa producing maize in extremely low-input/low risk systems (Banziger and Diallo, 2004). As a consequence, crop yields are falling to very low levels and food insecurity is widespread amongst agricultural communities (Kamara et al., 2004). The development of maize germplasm able to tolerate water stress is crucial if the productivity of maize based farming systems is to be sustained or increased (Betran et al., 2003). Maize genotypes perform differently under water stress conditions due to the existence of genetic variability for tolerance to stress (Bolanos and Edmeades, 1993; Lafitte and Edmeades, 1994; Banziger et al., 2000; 2006; Diallo et al., 2004). Betran et al. (2003) observed hybrids performing well under stress and suggested the possibility of combining stress tolerance and yield potential in tropical maize hybrids. Tolerance of maize to water stress is partly related to the development of the root system, which in turn influences water and nutrient uptake by crop plants (Moll et al., 1982; Kamara et al., 2004). In general, however, the amount of grain yields recorded from maize genotypes fall with the severity of water stress (Betran et al., 2003). Breeding strategies to develop stress tolerant maize inbred lines include screening and selection of inbreds under managed stress conditions, multi-location testing of progeny in a representative sample of the target environments, and selection under high plant populations (Beck et al., 1997). Additional information from adaptive secondary traits (ears per plant, anthesis-silking interval and leaf senescence) that show differential expression between optimal and stress conditions is genetically variable and is correlated with grain yield and is commonly used to increase selection efficiency (Bolanos and Edmeades, 1993; 1996; Banziger and Lafitte, 1997). When genetic variance and

Genetic relationships and heterotic structure of quality protein maize (Zea mays L.) inbred lines adapted to eastern and southern Africa

Information on heterotic patterns of quality protein maize (QPM) inbred lines would help to design appropriate breeding strategies for QPM. The objectives of this study were to assess the genetic relationships and heterotic structure of elite QPM inbred lines adapted to eastern and southern Africa (ESA) using simple sequence repeat (SSR) markers. Thirty-five inbred lines consisting of 32 QPM and three non-QPM inbred lines were genotyped using 40 SSR markers that yielded a total of 217 alleles, with an average of 5.43 and range of 2–10 alleles per marker. The number of alleles per inbred line varied from 40 to 82, with a mean of 51.5. About 81% of the pairwise comparisons between the lines had genetic distance estimates varying from 0.250 to 0.415. The unweighted paired group method using arithmetic averages (UPGMA) clustering algorithm and model-based population structure analysis showed the presence of two to three major groups, mainly consistent with maturity groups and pedigree information. This information can facilitate effective utilization of the QPM inbred lines in breeding programs.