Resistance in tropical maize to the maize weevil and larger grain borer.
Mwololo, James Kyalo
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Storability of maize grain is constrained by two postharvest insect pests namely the maize weevil (Sitophilus zeamais) and larger grain borer (LGB) (Prostephanus truncatus). Host plant resistance is the most economic way to manage such pests especially for resource constrained farmers as a component of integrated pest management. Breeding for resistance to these pests in maize is dependent on good understanding of genetic mechanisms underlying the resistance. The objectives of this study were to (i) assess resistance in tropical maize to the maize weevil and larger grain borer; (ii) determine the genetic relationships in tropical maize germplasm in relation to resistance to the postharvest insect pests; and (iii) map quantitative trait loci (QTL) associated with postharvest insect pest resistance in tropical maize. Two hundred and ninety five (295) entries of maize genotypes including resistant and susceptible checks were evaluated for biophysical traits at two sites during 2010 and 2011 rain seasons. The germplasm was subsequently screened for resistance to the maize weevil and LGB. Data was collected on husk cover tip length, and grain texture in the field. Biochemical traits were analyzed on the maize grain. Harvested grain were evaluated for postharvest insect pest resistance namely grain damage, grain weight loss, and numbers of insects. Grain hardness was measured as a putative trait of resistance at harvest. Genetic relationship among accessions was determined using biophysical/bioassay and simple sequence repeat DNA (SSR) data. To map resistance loci using QTL approach, a mapping population of 203 F2:3 derived progenies was developed from a cross between susceptible and resistant inbred lines. The F2:3 progenies were crossed to a tester and the testcrosses were evaluated across six environments. The same data used for assessment of pest resistance were taken for the QTL generation. Multivariate and univariate analysis of variance for all the traits was done using the general linear model (GLM) of statistical analysis system (SAS). The insect-damage traits and protein content were correlated using a canonical correlation. The genetic relationship and distance were estimated using the NTSYS and MEGA software. Genetic mapping was done using Joinmap 4, while QTL analysis was done using PLABQTL. There were great variations in resistance levels among the genotypes to the maize weevil and LGB. The most resistant genotypes were CKPH08003, BRAZ 2451 and CKSPL10028, while the most susceptible were PH 3254, BRAZ 4 and CML 312, among the hybrids, landraces and inbred lines, respectively. Dual resistance to maize weevil and LGB was observed in the germplasm evaluated. Commercial varieties were the most susceptible among the hybrids evaluated. Percentage weight loss was the most important trait for grouping the genotypes into resistant and susceptible categories. Protein and oil contents were high in grain of resistant compared to susceptible genotypes. The resistant hybrids and inbred lines had flint texture, whereas the susceptible ones had dent kernel texture. Grain texture was positively correlated (r = 0.7) with grain hardness. High genetic divergence for resistance to maize weevil and LGB was observed among tropical maize germplasm. QTL for resistance to maize weevil and LGB were mapped to chromosomes 1, 2, 3, 5, 8, 9, that also contain loci for resistance to disease and lepidopteran insect pests, and enzymes involved in biosynthesis of cell wall and phenolic acid. The QTL for damage resistance traits were located in chromosomes 1, 5 and 9. Chromosome 1 had a common QTL linked to protein content, grain hardness and husk cover tip length. Additive genetic effects were prevalent in all detected QTL. Overall, these studies show that mechanisms of resistance to the maize weevil and LGB are similar. As such breeding for resistance to both pests is possible within a breeding programme. The results obtained in this study are helpful in understanding the genetic basis of resistance to the maize weevil, management options for the LGB and for fine mapping of QTL. There is potential for development of genotypes with dual resistance to the maize weevil and LGB.