4.4 Discussion
4.4.2 Genetic effects
66 infestation, and this is associated with non-additive genes. Consequently, the role of SCA and GCA depends on the set of germplasm and the environment sampled.
The hybrids 5057/CML206 and MP709/CML206 had positive and significant SCA effects for grain yield despite being derived from parents with negative and significant GCA effects for the same trait under nematode infestation. This implies that poor general combiners can produce cross combinations with good SCA for traits controlled by overdominance genetic effects, which was the case for grain yield. For this reason, sum of all SCA effects was not equal to zero since it was a purely non-additive case. Dominance deviation SCA can only be equal to zero in a purely additive case (Kiekens et al., 2006).
Hybrids MP709/CML444 and MP709/CML395 had negative and significant reciprocal effects for grain yield under nematode infestation. These can be explained by the negative maternal effects observed for grain yield in parent MP709 when used as a female parent. However, this might be due to the failure of the inbred MP709 to adequately adapt to the local environmental conditions, resulting in low yields in some of its hybrid combinations (Chapter 5). Notably, hybrid MP709/CML312 had positive and significant non-maternal effects for grain yield. Therefore, grain yield for this hybrid combination is influenced by the interaction of nuclear and cytoplasmic genes under nematode infestation.
67 For plant height and grain yield, the dominance was largely unidirectional since it was significant for b1. The array means were higher than the parental means for both of these traits, which indicates that dominance is in the direction of tall plants with high grain yield.
Similarly, Stuber (1994) reported increased plant height in F1 hybrids to be due to complementary effects of dominant alleles at two loci, one affecting node number and the other affecting internode length leading to heterosis. Likewise, dominant genes and epistasis affect the improvement of grain yield since they are the genetic basis of heterosis (Falconer, 1981).
Asymmetry in gene distribution (b2) was significant for P. zeae densities. Therefore, some parents namely MP709, 5057, CML206 and CML444 contained more dominant alleles for P.
zeae resistance than CML395 and CML312. Residual dominance effects (b3) were also significant for P. zeae densities, which confirms that some dominance for P. zeae resistance is peculiar to individual F1 crosses, resulting from epistasis or failure of assumptions.
Presence of maternal effects for grain yield confirms results obtained earlier in the current study using Griffing’s analysis. The maternal effects obtained in Griffing’s analysis were a result of inbred line MP709, which led to greater nematode susceptibility when used as a female parent in two of the crosses. The non-maternal effects for P. zeae densities observed in Hayman analysis imply that nuclear and cytoplasmic genes are interacting in the inheritance of P. zeae resistance.
The Wr/Vr regression gives a measure of the adequacy of the model, average dominance, and the distribution of dominant and recessive genes (Hayman, 1954). For plant height, Wr/Vr regression over the three sites was significantly (P < 0.001) different from zero with a regression coefficient not significantly different from unity. All the parents were closer to the regression line, an indication of absence of epistatic effects. The regression line intercepted the Wr axis far below the origin, which signifies that overdominance effects were important in the inheritance of plant height under nematode pressure. Parents MP709, CML395, CML312 and CML444 contributed most dominant genes for plant height since they were clustered closer to the origin of the regression line (Fig. 4.1). The dominant genes were specifically associated with an increase in plant height which is reflected by the large array means compared to the parental means. These parents also had low variances and covariances, a characteristic of dominant alleles (Hayman, 1954; Kearsey and Pooni, 1996). Parent CML206 was in the middle and 5057 at the extreme end of the regression line. Therefore, parent CML206 contributed both dominant and recessive genes whereas parent 5057 had
68 recessive genes associated with reduced plant height under nematode pressure. The very low Wr-Vr value but high Wr+Vr value for parent 5057 was further confirmation of recessive genes controlling inheritance of reduced plant height in this parent.
Overdominance was similarly observed for the number of root lesions, P. zeae densities and Meloidogyne spp. densities since the respective regression lines were below the origin.
However, for Meloidogyne spp., the regression coefficient was not significantly different from zero and had a low R2 value hence the data was not statistically valid at P = 0.05.
Nevertheless, parents CML312 and CML206 contributed most of the dominant genes for susceptibility and resistance to Meloidogyne spp., respectively. Lordello and Lordello (1992) attributed resistance to M. javanica to dominant genes prevalent in IAC Ip365-4-1 maize parental line. Parents MP709 and CML395 contributed both dominant and recessive genes with MP709 favouring resistance whereas CML395 genes were inclined towards susceptibility to Meloidogyne spp. Parent 5057 contributed most of the recessive genes associated with susceptibility to Meloidogyne spp. However, since parent CML444 deviated highly from the regression line it probably has epistatic genes associated with susceptibility to Meloidogyne spp. According to Jink (1954), high deviation of some genotypes from the slope of one for a particular trait is a result of genic interactions.
For the number of root lesions, the regression coefficient was significant at P = 0.005.
Parents MP709, CML206 and CML444 contributed most of the dominant genes towards reduced number of root lesions since they were closer to the origin, whereas parent CML395 contributed both dominant and recessive genes (Fig. 4.2). Parent 5057 had most of the recessive genes since it was furthest from the origin. However, this contradicts earlier findings by Oyekanmi (2007) that this parent is resistant to P. zeae since root lesions are a result of P. zeae damage. Probably the tropical conditions in Uganda had a negative impact on performance of this parent in terms of number of root lesions. Nevertheless, all the parents were closer to the regression line, which indicates absence of epistatic effects. The six array means for number of root lesions were lower than the parent means over the six arrays, which was further proof that genes for enhancing reduced number of root lesions were dominant over genes for susceptibility.
For P. zeae densities, regression of Wr/Vr was significantly different (P = 0.05) from zero.
Parent MP709, 5057, CML206 and CML444 contributed most of the dominant genes towards resistance since they were closer to the origin of the graph, whereas CML312 and CML395 had most of the recessive genes associated with susceptibility to P. zeae (Fig. 4.3).
Resistance to P. zeae by parent 5057 corroborates findings by Oyekanmi (2007) whereas
69 resistance to P. zeae by MP709 and CML206 is consistent with findings from preliminary screening studies in the current study (Table 4.1; Appendix 4.1). Parent CML444 was tolerant to P. zeae in the preliminary studies (Table 4.1; Appendix 1). The preliminary studies were, however, based on a single site compared to the final evaluation trials (three sites), which might be the reason for the differences in ranking.
Grain yield inheritance under nematode pressure was controlled by overdominance gene effects since the regression line intercepted the Wr axis far below the origin (Fig. 4.5). Nawar et al. (1997) similarly recorded the presence of overdominance in the inheritance of grain yield and other traits such as ear length and plant height, but not under nematode pressure.
On the graph, the array points were scattered along the regression line which indicates genetic diversity among the parents for grain yield. Parent CML444 contributed most dominant genes for high grain yield, whereas MP709, CML395 and 5057 had similar frequency of both dominant and recessive genes. Parents CML206 and CML312 had most recessive genes for reduced grain yield under nematode pressure. Nearness of parents CML444 and 5057 to the regression line may suggest that these two parents were entirely free from non-allelic interaction and linkage. For grain yield, all the array means were higher than the parent means over the six arrays, suggesting that genes for high grain yield were dominant over genes for low grain yield.
Average heterosis was positive and in the desired direction for plant height and grain yield, which supports the preponderance of overdominance and dominance genes in the inheritance of these traits. For P. zeae and number of root lesions, average heterosis was negative, which is in the desired direction. There is no information on heterosis for P. zeae and Meloidogyne spp. resistance in maize. However, heterosis for root-knot nematode resistance has previously been reported in four tomato hybrids (Rani et al., 2009). According to Betrán (2003), heterosis has been observed to be generally greater under stress conditions than under non-stress conditions. This indicates that hybrid vigour can be achieved among hybrids improved for resistance to P. zeae.
Following analysis of variance of Wr+Vr and Wr-Vr values, dominance and epistasis was not detected for plant height, number of root lesions, Meloidogyne spp. and grain yield since non-significant effects were obtained. Therefore, the observed heterosis can be attributed to overdominance genes detected in the Wr/Vr regression graphs. However, according to Hayman (1957), epistasis is likely to be detected whenever the heritability is high and a large number of families is compared. Related studies detected non-allelic interaction for Meloidogyne incognita resistance on upland cotton following analysis of variance on Wr–Vr
70 (Zhang et al., 2007). The same authors found significant variance for Wr + Vr indicating the existence of dominance effects for M. incognita resistance on upland cotton. For P. zeae, mean squares for Wr-Vr had a significant effect, which confirms presence of epistasis in the inheritance of P. zeae resistance.