1. Thesis Introduction
6.4 Discussion
6.4.3 Gene action
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gene actions. The high SCA effects of the cross combinations involving low x low combiners could be due to dominance and dominance x dominance type of gene action. Such specific crosses can be exploited for heterosis breeding.
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Fashingabo, Gakire, Intsinzi, and Ndamirabahinzi as they all had significant and high negative GCA effects.
The importance of estimating GCA and SCA effects is not only about identification of parental lines to be involved in selection programmes, but also understanding gene effects governing the expression of specific traits. From the genetic point of view GCA effects measure additive gene action while SCA effects indicate non-additive effects (Bradshaw, 2016). In the present study, the mean squares associated with GCA were highly significant for all the traits, whereas SCA was not significant only for LS. This is an indication of the presence of both additive and non- additive gene effect in the mechanisms of expression of these traits associated with resistance to sheath rot of rice. Non-significant SCA for lesion size suggests that non- additive effect of genes were less important, and consequently, it would be useful to consider genotypes with inherently smaller lesions in further crossing programmes aimed at developing progenies with resistance to ShR. However, reports by Reif et al. (2007) suggest that in the absence of epistasis, GCA seems predominant over SCA and the relevance of dominance effects tends to decrease. Similarly, a relatively large SCA/GCA ratio implies the presence of dominance and epistatic gene effects. In this regards, the ratios of GCA/SCA in this study were all greater that one, suggesting additive effects were most predominant than non-additive ones. The involvement of mostly additive gene effects in the mechanism of resistance to sheath rot was not unique for this study only, as, this was also reported by Chauhan and Bhatt (1986.) and Srinivasachary et al. (2002).
Although, very little information on mode of inheritance of resistance to sheath rot of rice is available in literature, predominance of additive effects is common to most of the rice diseases.
This was reported for blast (Roumen, 1994; Mulbah et al., 2015), rice yellow mottle virus (Munganyinka et al., 2015), bacterial blight (Jeung et al., 2006) and rice sheath brown rot (Sthapit, 1995). With the predominance of additive effects, recurrent selection should be useful in improving sheath rot resistance related traits as according to (Hallauer, 2007), once additive gene effects are important, breeding methods that emphasize on GCA should be used for improving targeted traits. Also breeding methods based on phenotypic selection would be effective. Since additive genes are largely fixable, unlike non-additive genes (Dabholkar, 2006), the best combiners identified in this study are potential candidates for use in breeding
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programmes aimed at improvement of levels of resistance to rice sheath rot, as also suggested by Mulbah et al. (2015). Therefore selecting the best progenies as parents for the next generation would likely lead to future gains.
On the other hand, predominance of additive gene effects was also revealed by narrow sense heritability levels obtained in this study. Since heritability is a measure of the heritable portion of variability, higher heritability values of quantitative traits are useful as they provide the basis of selection for phenotypic performance (Girish et al., 2006). A high narrow sense heritability is an indication that the expression of targeted trait is mainly due to the additive gene effects (Brown et al., 2014). Heritability estimates ranging from 63.7 to 89.2% for both broad and narrow sense were observed for LS, AUDPC and PE. These high levels of heritability might also reflect the environmental conditions in which the trial was established. The trial site is a continuously irrigated scheme with less variability of environmental conditions, especially rainfall and temperatures.
However, even if little information is available in literature concerning inheritance of resistance of sheath rot, various reports identified a strong connection between sheath of rice and panicle exsertion (Vinod et al., 1990; Lalan Sharma et al., 2013; Hittalmani et al., 2016). In the absence of information on heritability for sheath rot based traits, one can use information on panicle exsertion. High heritability estimates for panicle exertion corroborates results of a number of authors, including Sellammal et al. (2014) and are in contrast with those of Girish et al. (2006), Cruz et al. (2008) and Neelima et al. (2015) who reported moderate estimates of heritability.
High estimates of heritability observed for all three traits, indicated these traits could be selected in controlled environments in a recurrent selection programme. Based on heritability estimates of sheath rot resistance components, resistance to this disease can be achieved through mass selection or any other methods based on progeny testing. This is in accordance with (Lopes and Boiteux, 2012), who suggested that the selection of resistance traits that display high heritability and simple genetic control, mainly of the additive type, might be achieved through the individual performance or, rather, the performance per se of inbred lines or populations. Notably, if there is a significant maternal effect, then there will be a difference in the selection of the female parent for a particular crossing. For this study, nevertheless, maternal effects were not significant.
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Concepts of the proportion of dominant and recessive genes occurring in a group of parents, as well as the degree and direction of dominance have been clearly elaborated by Viana et al.
(2001). The level of dominance estimated in this study were all between 0-1 except for PE when estimated based on male parents. This indicates partial dominance of genes involved in resistance to sheath rot according to Chahal and Gosal (2002). The evidence of predominance of additive genetic effect on inheritance of resistance to ShR paves the way for a possibility of improving the resistance by introgression of resistance genes through recurrent selection or series of backcrossing.