Abstract:

Hybridization among ten pure breeding lines of soybean was made in 1997 to study the inheritance of pod shattering in soybean. Lines Nam 2, Roan, Duiker, TGx 1448-2E and GC 81090-2E were resistant, AGS 292 and TGm 737P susceptible while Kabanyolo 1, Nam 1 and Samsoy 1 were intermediate to pod shattering. Preliminary evaluations for pod shattering among F2 segregating populations were carried out in oven conditions at 60°C for 12 hours, 80°C for 6 hours and 80°C for 12 hours. Scales of 1 - 5 and 0-10 were used to group segregating populations into their shattering phenotypes. The 1 - 5 scale was modified into 1-3 scale in which 1 was regarded as resistant, 2 intermediate and 3 susceptible. The pods of parental lines, Fi, F2, F3 and F« progenies were assessed for shattering using an oven set at 80°C for 12 hours. A Chi square test was used to determine the departure of the observed frequencies from the expected ones. Maternal influence on the inheritance of pod shattering was determined using crosses of TGm 737P x Duiker, TGm 737P x Roan and AGS 292 x TGx 1448-2E and their reciprocals. The original Castle-Wright’s method used by Zeng et al. (1990) was used to estimate the number of loci and Bjarco and Line (1987) method was used to determine the number of genes. The half dialleimating design was used to estimate genetic parameters and to detect presence of non-allelic interaction of genes affecting pod shattering following analyses by Hayman (1954) and Griffing (1956). season, 1998 to determine amount of seed yield lost in the field due to pod shattering and effect of G X E interactions. Weather parameters were recorded at the locations starting from physiological maturity stage. Basing on the number of shattered and unshattered pods, an extrapolation model was developed and applied to determine the amount of soybean seed yield lost in the field due to pod shattering. The phenotype of Fi plants was closer to that of susceptible parents indicating partial dominance and the distribution of F2 population was not discrete and could not fit Mendelian ratios. Based on a 1 - 3 scale, the distribution best fitted the 1:3:12 genetic model suggesting pod-shattering trait to be conditioned by 2 genes and classical dominant epistasis. The ratios obtained from the single plant progenies advanced to F3 and F« were in agreement with expected ratios of 16:0:0, 0:16:0, 0:0:16, 0:4:12, 4:0:12, 4:12:0 and 1:3:12 for resistant: intermediate: and susceptible phenotypic ratios respectively, confirming dominant epistasis. The improvement made on the oven method to evaluate resistance to pod shattering was effective in terms of reduced time for the experiment to be accomplished and in identification of consistent genetic ratios. There was no significant difference between reciprocal crosses of susceptible and resistant varieties suggesting absence of maternal effects. The parent offspring regression technique indicated that heritability in the narrow sense was 0.73 while estimates from Hayman’s and Griffings’ diallei was 0.70. The mean number of segregating genes was estimated to be 2.21 at F2 and 2.24 at F3, and the mean number of loci for ten different crosses was 1.65. Hayman’s diallei analysis indicated significant variation of Wr + Vr and Wr Vr over arrays suggesting epistatic gene action. Similarly results from a joint regression coefficient over replications were significantly (P < 0.05) different from unity and zero which further indicated presence of non-allelic interaction of genes. The intercept was positive suggesting partial dominance. Varieties AGS 292 and TGm 737P contained more dominant alleles while Nam 2 and Roan contained more recessive alleles than the other parental lines. Both general combining ability (GCA) and specific combining ability (SCA) effects were significant (P < 0.05) and the ratio of GCA to SCA was significant (P < 0.05) indicating that additive gene effect was important for the shattering trait. Parental lines with high negative GCA effects were Nam 2, Roan and TGx 1448-2E qualifying them as suitable parents for transferring resistance to pod shattering in their progenies and subsequent selected lines. Yield losses in susceptible and intermediate susceptible varieties ranged from 57 - 175 kg ha-1 and 0 - 186 kg ha-1 respectively depending on genotype, location, season and harvesting date. Delay of harvesting for 14 days from harvesting maturity stage does not lead to significant yield loss due to pod shattering when intermediate varieties are grown as opposed to susceptible ones which start shattering at R8 maturity stage. It was demonstrated that the developed extrapolation method was accurate in estimation of field yield loss due to pod shattering in soybean and produced estimates, which were not significantly different from actual yield loss. The resistant variety did not shatter even when harvested after 21 days. Due to significant influence of genotype, location and season on pod shattering in soybean, it is recommended that selections for resistance to pod shattering be carried out in different agro- ecological zones over several seasons to cater for the genotype x environment interactions.