Genetic Analysis of Multi-Trait Stress Tolerance in Maize

Data on Striga emergence count, ear rot, stalk and root lodging were transformed as [log (counts+1)] to reduce the heterogeneity of variances. The ANOVA for the 150 hybrids generated using the NCD II pooled over sets for each research condition [15 (link)] and across the stress conditions was carried out using the version 9.4 of SAS [33 ]. The genotypic component of the source of variation was partitioned into the variation due to males (sets), females (sets), and female × male (sets) interaction. The F-tests for male (sets), female (sets) and male × female (sets) mean squares were performed using male (sets) × environment, female (sets) × environment and male × female (sets) × environment mean squares, respectively. The mean squares attributable to environment × female × male (sets) were tested using the pooled error mean squares.
The following general linear model was used for the NCD II mating design:
$${\mathbf{X}}_{\mathbf{i}\mathbf{j}\mathbf{k}\mathbf{l}}=\mathbf{\mu }+{\mathbf{m}}_{\mathbf{i}}+{\mathbf{f}}_{\mathbf{i}}+{\left(\mathbf{m}\mathbf{f}\right)}_{\mathbf{i}\mathbf{j}}+{\mathbf{p}}_{\mathbf{i}\mathbf{j}\mathbf{k}}+{\mathbf{I}}_{\mathbf{l}}+{\mathbf{\epsilon }}_{\mathbf{i}\mathbf{j}\mathbf{k}\mathbf{l}}$$
where X_ijkl = the observed value of the progeny of the i^th male crossed with j^th female in the k^th replication; μ = the overall population mean; m_i = effect of the i^th female; f_j = the effect of the j^th male mated to the i^th female; (mf)_ij = the interaction effect between the i^thfemale and the j^th male; p_ijk = the effect of the k^th progeny from the cross between i^th female and j^th male; r_l = the effect of the l^th replication; ε_ijkl = the experimental error. The general combining ability (GCA) effects for male and female within sets (GCA_m and GCA_f) and specific combining ability (SCA) for each trait were estimated according to Kearsey and Pooni [34 ] as shown below:
$$\mathrm{G}\mathrm{C}{\mathrm{A}}_{\mathrm{m}}={\mathrm{X}}_{\mathrm{m}}-\mathrm{\mu }$$
$$\mathrm{G}\mathrm{C}{\mathrm{A}}_{\mathrm{f}}={\mathrm{X}}_{\mathrm{f}}-\mathrm{\mu }$$
where, GCA_m and GCA_f = General combining ability effects of male and female parents respectively; X_m and X_f = Average performance of a line when used as a male and female in crosses, respectively and μ = Overall mean of crosses in the set.
Standard errors (SE) for testing significance of GCA_m and GCA_f estimates, for traits of genotype, were computed from the mean squares of GCA_m × environment and GCA_f × environment, respectively as follows:
$$\mathrm{S}\mathrm{E}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{G}\mathrm{C}{\mathrm{A}}_{\mathrm{m}}=\surd \mathrm{M}\mathrm{S}\mathrm{m}\times \mathrm{e}/\left(\mathrm{f}\times \mathrm{e}\times \mathrm{r}\right)$$
$$\mathrm{S}\mathrm{E}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{G}\mathrm{C}{\mathrm{A}}_{\mathrm{f}}=\surd \mathrm{M}\mathrm{S}\mathrm{f}\times \mathrm{e}/\left(\mathrm{m}\times \mathrm{e}\times \mathrm{r}\right)$$
where, MSm × e and MSf × e were the mean squares of the interaction between male and environment as well as female × environment, respectively; f, m, r, and e were the number of females, males, replicates, and environments, respectively.
A multiple trait base index (MI) that integrated grain yield with the number of emerged Striga plants, Striga damage rating, plant and ear aspects, delayed leaf senescence, anthesis-silking interval and number of ears per plant was used to select the best performing hybrids across optimal, Striga and low-N conditions [5 (link)]. The means, adjusted for block effects of each genotype for each measured variable was standardized to minimize the effects of the different scales. A positive multiple trait base index value therefore indicated tolerance/resistance of the genotype to both Striga and low-N, while negative values indicated susceptibility to the stresses. The multiple trait base index was computed as follows:
MI = (2 × YLD) + EPP–EASP–PASP—STGR–RAT1 –RAT2 –(0.5 × C01)–(0.5 × C02)
On the other hand, the base indices for Striga and Low-N were computed as STRBI = 2.0 YLD + 1.0 EPP–(RAT1 + RAT2)– 0.5 (C01 + C02) and LNBI = 2.0 YLD + EPP–STGR–ASI—PASP–EASP, respectively to select superior hybrids under the respective stress conditions.
Where: MI = Multiple trait base index
STRBI = Base index for StrigaLNBI = Base index for Low-N
YLD = grain yield across research conditions
EPP = number of ears per plant across research conditions
EASP = Ear aspect across research conditions
PASP = Plant aspect across low-N and optimal conditions
STGR = Stay green characteristic across low-N conditions
RAT1 and RAT2 = Striga damage rating at 8 and 10 WAP across Striga infested conditions
C01 and C02 = Number of emerged Striga plants at 8 and 10 WAP across Striga -infested conditions.