Genetic variability and responses to selection for the spotted stem borer, chilo partellus (swinhoe) resistance in a maize (zea mays L.) population

Abstract

The maj or obj ecti ves of this study were to determine genetic variability for parameters of resistance to Partellus and other desired agronomic traits, construct and test appropriate selection procedures, and estimate heritabilities, predict both direct and correlated responses to selection. The study involved a white grained parental population named ICZ3 from which 163 S2 lines and 150 ~ testcrosses were obtained and evaluated according to the study objectives so far stated. Genetic variability and predicted responses to S2 and S2 testcross selection methods were studied in the ICZ3 population. Evaluation of progress after one cycle of selection for grain yield, parameters of resistance to ~hilo partellus and agronomic traits were investigated. The results of determining the genetic variability for each of the three parameters of resistance (leaf feeding, dead heart and stem tunnelling) and grain yield in each progeny type revealed high levels of variability for effective selection to be practised. Using a 9 - point rating scale for leaf feeding, populations derived from S2 and testcross progenies s'howed a difference of 5.00-6.33 and 5.00- 6.17, 3.14%-3.29% and 2.56%-3.14% for dectd heart, 27.97%-28.52% and 19.46%-23.92% for stem tunnelling and 5.65-11.10 and 7.85-8.60 t/ha for grain yield for S2 and testcross progenies, respectively. After one cycle of selection the S2 and testcross progenies exhibited a reduction In leaf damage rating of 0.01-0.11 and 0.05-0.08 per cycle corresponding to a decrease of 0.40%-4.38% and 1.99%-3.19% per cycle relative to the source population (Co) respectively. Stem tunnel length decreased by 0.15%- 1.81 % per cycle in S2 progenies and 0.13%-1.76% per cycle in the test cross hybrids corresponding to 1.15% - 13.92% and 1.00% - 13.54% per cycle relative to Co. Grain yield increased by 5.29% to 24.23% per cycle and plant height by 4% per cycle of selection. Genetic variances and heritability estimates were moderate to high for the S2 progenies with the heritability estimates being higher for the agronomic traits compared to the resistance parameters. Predicted direct response for grain yield, parameters of resistance and rank summation index were generally higher in the S2 selections as opposed to the test crosses. Generally more gain/cycle of selection was achieved through single trait selections than by index selection. Predicted correlated response in grain yield assuming selection was done for the parameters of resistance were smaller than predicted direct responses to selection for grain yield. In all cases, there was poor correspondence between predicted and observed gains to selection. A breeding procedure involving recurrent selection for parameters of resistance (leaf feeding, dead heart and stem tunnelling) and selection indices should aid in combining C. Partellus resistance with desirable agronomic traits.