3.4 Discussion
3.4.2 Estimates of genetic effects
Genetic estimates are discussed only in crosses where there is a clear separation of P1 and P2 based on LSD value at 5% level of probability. The genetic effects were therefore estimated only under high N conditions, which obeyed the assumption of contrasting parents as a cardinal assumption of GMA for both crosses.
Generations of cross T20 x C58
With cross T20 x C58, only the additive genetic effects conditioned the LCC character at all the grain filling stages under high N regime. Predominance of additive effects implied that recurrent selection (RS) or any form of cyclic selection could be effective for the character at all growth stages. This appears to support the idea that the genetic potential of a genotype is well exploited under ideal production conditions. This may suggest further that only inbred lines could be generated at all grain-filling stages from segregating generations for the LCC character under high N. Furthermore, significant differences for only additive genetic effects for LCC character under HN contradicts Elings et al. (1997), Bänziger and Lafitte (1997), Bänziger et al. (2000) and Worku et al. (2007), who reported a preponderance of additive genetic effects in secondary traits, especially under low N. The LCC character has been associated with the physiological maturity of genotypes in that the greater the time it takes to reach maturity, the more the LCC character becomes relevant. On the other hand, shorter maturing genotypes may lose LCC earlier and faster under similar conditions. In short- season areas, therefore, the high LCC character would translate to high yield (Tollenaar and Daynard, 1978a, b), as opposed to long season or varieties with extended growth. This suggests that early-maturing or short-season cultivars are source-limited, such that extended LCC may increase DMA and so result in improved grain yield.
Generations of cross T20 x NG8
Both fixable (additive, and additive x additive) and non-fixable (dominance, additive x dominance, and dominance x dominance) genetic effects for cross T20 x NG8 were preponderant under HN. This may suggest that reciprocal recurrent selection (RRS) could be
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employed for the LCC character. Epistasis was observed for cross T20 x NG8, and it could be defined as any interaction between genes at non-homologous loci (Sprague et al., 1962). A greater preponderance of epistatic interactions in cross T20 x NG8 than in cross T20 x C58 supported the hypothesis that epistasis is real in maize, although it is specific to the cross, trait and environment, as opposed to many researchers who ignore such phenomenon. This further suggests that positive epistasis could have contributed significantly to heterosis, which was common in cross T20 x NG8. The milk stage, which was earmarked as the most sensitive grain-filling stage, was more important for cross T20 x NG8 in HN (Table 3.4b) but this was not the case for cross T20 x C58, implying the effects of the genotypes and N. This stage (i.e. 14 days after the 50% silking) has been also considered by maize physiologists as a linear grain-filling stage. Significant additive effects for the milk stage in both crosses and significant dominance effects for only cross T20 x NG8 may demonstrate that the genetic effects for this trait are not easy to determine but it can simply be inferred that the trait is only estimable under high N. Generally, the R2 values were slightly above 50% and about 70% for crosses T20 x C58 and T20 x NG8, respectively suggesting that the genetic effects for LCC character would be more easily detected in and estimated for the latter, rather than the former cross.
In both crosses and growth stages for high N regime the sign of the genetic effects refer to the relative position of the parents to the mid-parent for the case of dominance effects including associated epistatic effects. In short, it refers to heterosis (Shashkumar et al., 2010) and for the case of the present study, high LCC values are desired, and vice versa. For the dominance effects, the sign is related to relative positions of F1 from the mid-parent, and this was consistent with results from the mean separations. With regards to the additive genetic effects and related epistatic effects, the signs imply which parent was chosen as superior or inferior for the trait of interest (Azizi et al., 2006). The positive sign observed for additive genetic effects per growth stage would therefore imply that the choice of parents was appropriate during mating since the individual growth stages for the LCC character were considered as separate traits. And the converse would have been the case if negative signs for the additive effects were observed. Furthermore, in standard regression terms, the signs would be associated with regression coefficients (β) for individual effects, whereby either sign depends on the trait and breeding objective. Positive β is desired for traits where
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positive values are wanted, whereas negative β values are relevant where the trait should be decreased.
The preponderance of epistasis in T20 x NG8 under HN (Table 3.5b) contradicts the findings by Perkins and Jinks. (1971), Jinks et al., (1973), Wolf and Hallauer (1977) and Ceballos et al. (1998), who reported that epistasis is more common in stressed conditions than in stress- free areas. These researchers added that the inherent phenomenon of epistasis is about specificity such that at/or near the extremes of the normal distribution curves of genotype responses, specific genetic interactions are expected compared to the middle part of the curves. The absence of duplicate epistasis for the LCC character would increase the response to selection (Iqbal and Nadeem, 2003). However, at these growth stages, heterosis breeding could still be effective due to the predominance of non-fixable effects (Table 3.5b).
Viana (2006) reported that the additive x additive and additive x dominance to be inestimable, such that their relative importance is difficult to assess. This assertion could further confirm the unknowns on the inheritance of the LCC character in maize. The results of the present study appear not to be wholly in support of other research because no systematic work has been done to quantify the LCC character across grain-filling stages under high and low regimes of N in tropical maize.
Generally, the present study suggests that only RS procedures can be effective for the LCC character under high N conditions for population T0 x C58. The preponderance of both additive and non-additive genetic effects for all growth stages demonstrates the efficacy of RRS for the LCC character for population T20 x NG8. Therefore, the inheritance of the LCC can be determined only under high N, although there are genotypic differences under N regimes, thus affecting the inheritance of the LCC character. The extended LCC (SG trait) that has been studied in many crops but not so much in maize was under the control of additive genetic effects in both crosses but the dominance effects were also statistically significant in one cross. Therefore, the inheritance of the SG trait is not fully understood but an inference can be made that it may be estimated only under ideal N conditions.
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