1. Thesis Introduction
6.4 Discussion
6.4.2 Combining ability effects
Combining ability effect is one of the most important parameters commonly used by plant breeders to evaluate the genetic potential of materials. This is useful, especially, for efficient selection of parents for hybridization, effective and efficient selection within a segregating population, and prediction of response to selection, among others (Acquaah, 2012). In most instances, the analysis of combining ability provides reliable information on the potential of parents to produce superior progenies following hybridization, and the magnitude of additive and non-additive gene action (Shattuck et al., 1993).
Sprague and Tatum (1942), defined GCA as the average performance of a line in hybrid combination, and SCA as cases in which certain combinations are relatively better or worse
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than would be expected on the basis of the GCA of their parents. Generally, good combiner parents result in higher frequency of heterotic hybrids than poor combiners (Virmani, 2012). In this study, both GCA and SCA revealed significant differences for the traits evaluated except for the SCA for lesion size. This significance of GCA suggests that crop improvement programmes for resistance to ShR of rice should be directed towards selection of superior parents, that is, good combiners. The significance of SCA effects for AUDPC and PE suggests that gains can be achieved through hybridization emphasizing on non-additive gene effects. According to Bokmeyer et al. (2009), negative GCA and SCA effects are desirable for disease resistance, based on a scale where the highest value corresponds to more disease attack.
However, male and female parents revealed considerable variability in estimates of GCA.
Those with highest and positive scores were considered bad combiners, as positive effects for disease resistance related traits indicate increased levels of disease susceptibility. This is why genotypes such as Nyiragikara, Buryohe and Rumbuka were regarded as bad combiners as far as AUDPC and LS are concerned. Conversely, genotypes such as Ndamirabahinzi, Intsinzi, Fashingabo, Yunkeng and Yunertian were identified as good combiners for LS and AUDPC as they recorded the highest negative GCA values. Consequently, they will be considered in hybridization programmes aiming at the improvement of resistance to sheath rot, as male and female parents. The superiority of these good x combiner parents was also observed in F1 progenies due to high negative SCA effects of crosses involving the above mentioned good combiners. However, in some progenies, a number of high negative SCA effects for F1 progenies were obtained by crossing a good combiner, either as male or female parents, to a bad combiner. This is an indication that crossing a good combiner to another good combiner does not necessarily lead to desired progenies.
Some of the crosses showing high SCA effects involved parents with high x low or low x high GCA, low x low GCA or low x average GCA. The high SCA effects of such crosses might be attributed to additive x additive type of gene action and the high disease resistance potential of these crosses can be fixed in subsequent generations (Chakraborty et al., 2009). According to the same source, the crosses that originated from high general combiner parents reflecting high negative SCA effects are expected to produce useful transgressive segregants, which can be identified following simple conventional breeding techniques like pedigree method of selection.
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Conversely, high SCA effects of the crosses that resulted from high x low combining parents are attributed to additive x dominance type of gene action (Sharma et al., 2014). The high level of resistance from such crosses would be unfixable in subsequent generations but these crosses would produce desirable transgressive segregants in later generations by modifying the conventional breeding methodologies to capitalize on both additive and non-additive genetic effects (Chakraborty et al., 2009). Various investigations reported by Virmani (2012) showed evidence of high x high general combiners resulting in crosses showing low SCA effects and concluding that crosses between good general combiners did not always result in good F1 crosses.
In general parents possessing high general combining ability also possess good performance per se in crosses, although exceptions to this rule are not uncommon. However, the magnitude and direction of combining ability effects are useful concepts to take into account for parental selection in crop improvement hybridization programmes (Singh et al., 2012). In this study, crosses exhibiting high negative specific combining ability effects for AUDPC and LS were derived from parents with various types of general combining ability effects (good combiner x good combiner, good combiner x bad combiner, bad combiner x bad combiner etc).
High SCA effects of the crosses that resulted from high x low combining parents may be due to additive x dominance type of gene action. The high performance from such crosses would be unfixable in subsequent generations and therefore cannot be exploited by standard selection procedure (Chakraborty et al., 2009). However, these crosses would produce desirable transgressive segregants in later generations if efforts could be made to modify the conventional breeding methodologies to capitalize on both additive and non-additive genetic effects.
Consequently, a breeding method involving the fixable gene effects and at the same time, maintaining considerable heterozygosity for exploiting the dominance effects, may prove most efficient for the performance of the targeted trait. In this regard, recurrent selection can be considered to be the most efficient selection procedure. However, in self-pollinated crops like rice, recurrent selection in true sense is difficult to practise due to large numbers of hand emasculation and pollination. Under such a situation, biparental mating in early segregating generations might be practised to ensure higher utilization of both additive and non-additive
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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.