Inheritance of resistance to rice yellow mottle virus disease in interspecific and intraspecific rice genotypes in Uganda.
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Rice Yellow Mottle Virus sobemovirus (RYMV) is highly infectious and variable, and several resistance-breaking strains have been identified. The most ideal and promising control measure is breeding for resistance. The objective of this study was to understand the inheritance and heritability of resistance to rice yellow mottle virus (RYMV) in interspecific and intraspecific rice varieties. Field and screen house based studies were established to evaluate nine genotypes; four were interspecific (N-1, N-4, N-6 and Naric 1) and five intraspecific (two locals K5 and K85, and three introduced WAC 116, WAC117 and Gigante). The screening of these parental materials for resistance to RYMV was done by artificial inoculation with a virulent isolate from Iganga in a screen house. The results revealed four patterns of reaction to RYMV among the cultivars: three resistant (WAC 116, WAC 117 and Naric1), four moderately resistant (N-6, N-4, N-1 and Gigante) and two susceptible (K85 and K5). A full diallel (9 x 9) was employed to generate F1 and F2 populations. Screening of these progeny revealed significant variation for RYMV resistance among genotypes. Effects of both general combining ability (GCA) and specific combining ability (SCA) were significant, indicating that both additive and non-additive effects, respectively, were important in determining resistance to RYMV. The preponderance of GCA effects (Baker‘s ratio= 77%) suggested that additive genetic effects were more important than non-additive effects. Narrow sense heritability, calculated by mid-parent offspring regression, was high (76%). These results suggest that simple selection and/or backcrossing would be the best procedures for improving strategies for breeding for resistance to RYMV. Parents Naric1, WAC 116 and WAC 117 were identified as good general combiners and could effectively be utilized in future hybrid rice breeding programmes for developing RYMV-resistant varieties. Cytoplasmic gene effects played a role in modifying resistance to RYMV with enhanced resistance when the resistant parent was used as female. This phenomenon should therefore be taken in consideration while selecting the female parents in hybridization programs. Segregation patterns generally suggested the presence of one or two genes with modifications beyond Mendelian ratios. Thus the resistance to RYMV is affected by a very complex interaction of cytoplasm and nuclear genes. The study on estimating the level of heritability of resistance to RYMV within advanced generations in crosses selected for resistance showed consistent levels of resistance in all generations. Both, variance components and parent offspring-regression methods indicated high heritability (68.7 to 98.1%) for resistance to RYMV in all generationssuggesting that the additive gene effect was more important in F3, F4 and F5 generations and that the selection for resistance to RYMV in early generations could be effective. For other traits (Tiller number, Days to flowering and maturity, and Yield hill-1), heritability was generally between 11.0 and 95.7%. The high heritability in these traits indicated that selection is practical in improving for these traits of interest. Generally all the hybrids were consistently resistant, suggesting that these crosses would be effectively used in rice breeding programmes for improvement for RYMV resistance. Crosses involving parental lines Nerica 11and Nerica12 showed high RYMV resistance implies that the two parents could be used extensively in breeding for resistance to RYMV. Moreover, parental lines Nerica 8 and Nerica 13 displayed better performance for yield hill-1 suggesting that these parents could be most suitable for rice yield improvement.