Inheritance of Weevil Resistance in Maize

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Lebaka, Ntjapa, Gabriel
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The study of the inheritance and gene action of weevil resistance in maize was undertaken using a 12 parent half diallel design. Seven resistant inbred lines were obtained from CIMMYT-Zimbabwe and five susceptible lines were obtained from soils and crop Research Branch(SCRB), Zambia. The lines were planted and crossed at Golden Valley Agricultural Research Trust(GART) in the 1998/99 growing season. The F2 and self ed parental lines were raised at UNZA field station in the winter of 1999 under irrigation. Because of the loss of some crosses and inbred lines due to unsuccessful seed production, 8 parents and 28 crosses were used in genetic analysis. For each of these entries three replicates each of 100 grams was prepared and put in a clean glass jar of 0.5 liter volume. The jars were randomly arranged on three shelves in a controlled environment room at 25+_2degrees Celsius and at 70+_5% relative humidity. Each entry appeared once on each shelf and each shelf was treated as a block. Twenty adult unsexed maize weevils were placed in each jar containing grain and the jars were closed with cotton cloth securely tied with the rubber bands to prevent the insects from escaping. After 21 days the jars were opened and all the insects were removed. The jars were left standing in this environment for 42 days. The jars were then opened and the insects and dust were sieved through a 5 millimeter sieve. The number of adult insects emerging were counted and recorded. The weight of the remaining grain was taken and weight loss was calculated. The grain was put back into the respective jars which were laid back on the shelves to stand for another 30 days after which the jars were removed. The same procedure was repeated and data of number of insects recovered and weight loss was recorded. These were added to the previous readings to give total number of emerged insects and total weight lost for each sample. Three replicates each of 50 grams for each entry was prepared and placed in a plastic bag. This grain was for determining the grit in the grain. The grain samples were then milled at constant speed for 60 seconds in a small electric powered mill. The meal was weighed and sieved through 0.5 millimetre sieve to separate grit from the fine meal. The remaining grit was weighed and hardness was expressed as percent grit of the total meal. Data for percent grit was transformed using square root transformation and the data for number of insects was transformed using logarithmic transformation. Straight scale was used for weight loss. The analysis of weevil resistance involved using Griffing's diallel analysis method 2 assuming fixed effects. Heritability and relative dominance estimates were made under random effects model. General combining ability(GCA) and specific combining ability(SCA)effects were significant for number of insects and weight loss at both infestation periods. At 63 days infestation period,GCA accounted for 49 and 71.42% of the total genetic variation for weight loss and number of insects, respectively. Broad sense heritabilities were76.25 for weight loss and 54.09 for number of insects. Narrow sense heritabilities were 29.01 for weight loss and 15.18 for number of insects. However at 93 days infestation, GCA to SCA ratios changed for both weight loss and number of insects.GCA accounted for 69.28% for weight loss and 72.42% for number of insects. Relative dominance was 0.84 for weight loss and 0.80 for number of insects. Broad sense heritabilities were 46.66 for weight loss and 63.16 for number of insects.Narrowe heritability was 17.37 for weight loss and 24.27 for number of insects. Grain hardness did not appear to influence grain weevil resistance as judged by insignificant correlation coefficients between percent grit and weight loss and percent grit and number of insects. For percent grit both GCA and SCA were significant but SCA effects accounted for 87.16%of the total genetic variation. Relative dominance for this trait was in the range of over dominance(3.89).These findings suggested that recurrent selection for general combining ability or reciprocal recurrent selection for reduced grain weight loss as a measure of weevil resistance can be an appropriate breeding method to improve weevil resistance in maize. The same cyclic selection methods would be appropriate for reducing the number of insects after infestation which is another measure of weevil resistance in maize. Results indicate that selection for weight loss would result in faster progress at 63 days infestation while number of insects would result in faster progress at 93 days infestation period. Recurrent selection for specific combining ability would be an appropriate breeding method for grain hardness. However progress from selection would be slow and a big population would be required as narrow sense heritability was too low.
Inheritance and gene action-weevil resistance , Weevil resistance in Maize