The adoption of the doubled haploid (DH) technology in tropical maize {Zea mays L.) breeding programs is lagging behind that of temperate programs due to a lack of tropical haploid inducers and reliable Information on the performance of temperate inducers under nontemperate conditions. The objective of this study was to determine the in vivo haploid induction ability of three temperate inducers crossed to a diverse set of tropical maize source germplasm under tropical conditions. Three experiments were conducted employing inducers as male parents to pollinate 120 source germplasm in three environments in Mexico. Haploid induction rates (HIR) obtained under field conditions were determined with two different haploid identification systems. Highly significant genotypic differences were detected among inducers and source germplasm for HIR but no interactions were observed between the two factors. Mean HIR under tropical conditions were similar to those reported for evaluations under temperate conditions indicating that temperate inducers can be employed for initiation of DH breeding programs in the tropics. Misclassification of diploids as haploids resulted in inflated HIR, particularly in highly variable source germplasm such as landraces or when expression of the identification marker was weak. We conclude that induction of haploidy is not a limiting factor for DH line production in tropical maize, but there is a need for the development of well-adapted tropical inducers.
The genetic basis of heterosis for grain yield and its components was investigated at the single- and two-locus levels using molecular markers with an immortalized F(2) (IF(2)) population, which was developed by pair crosses among recombinant inbred lines (RILs) derived from the elite maize hybrid Yuyu22. Mid-parent heterosis of each cross in the IF(2) population was used to map heterotic quantitative trait loci. A total of 13 heterotic loci (HL) were detected. These included three HL for grain yield, seven for ear length, one for ear row number and two for 100-kernel weight. A total of 143 digenic interactions contributing to mid-parent heterosis were detected at the two-locus level involving all three types of interactions (additive x additive = AA, additive x dominance = AD or DA, dominance x dominance = DD). There were 25 digenic interactions for grain yield, 36 for ear length, 31 for ear row number and 51 for 100-kernel weight. Altogether, dominance effects of HL at the single-locus level as well as AA interactions played an important role in the genetic basis of heterosis for grain yield and its components in Yuyu22.
For in vivo production of doubled haploid (DH) lines in maize, the rate of haploid induction is of crucial importance. Maternal haploid induction depends primarily on the inducer used as a pollinator. However, the source germplasm used as a maternal parent and the environmental conditions for induction may also influence haploid induction and these aspects have not been examined in tropical maize so far. The objectives of our study were to (i) monitor the variation for haploid induction rate (HIR) among diverse source germplasm in tropical maize, (ii) determine the relative importance of general (GCA) and specific (SCA) combining abilities for HIR, and (iii) investigate the influence of summer and winter seasons and genotype 9 season interactions on this trait. Ten inbreds were mated in a half diallel design. The resulting 45 F 1 single crosses were pollinated with the haploid inducer hybrid RWS 9 UH400 during the summer 2008 and winter 2009 seasons in a lowland tropical environment in Mexico. HIR of the single crosses averaged over seasons ranged from 2.90 to 9.66% with an overall mean of 6.74%. Mean HIR was significantly (P \ 0.01) higher during the winter (7.37%) than summer season (6.11%). Significant (P \ 0.01) variation was observed due to GCA effects of parental inbreds of single crosses but not for SCA, GCA 9 season and SCA 9 season interactions. Our study underpins that a higher HIR in tropical maize can be obtained by selecting appropriate source germplasm and undertaking pollination under favorable environmental conditions.
Ear rots caused by Fusarium spp. are a major concern in many maize (Zea mays L.)–growing regions of the world. Our objectives were (i) to evaluate a set of European maize inbreds for resistance to ear rots and mycotoxin concentrations, (ii) to estimate variance components and heritabilities, and (iii) to compute correlations among resistance traits. Forty‐two inbreds were evaluated for resistance to F. graminearum in four environments, and 21 inbreds were evaluated for resistance to F. verticillioides in three environments, under artificial inoculation. Data were recorded on severity of Gibberella (AGER) and Fusarium ear rots and accumulation of deoxynivalenol (DON), zearalenone, and fumonisins. Artificial inoculation was effective particularly for Gibberella ear rot. Genotypic and genotype × environment interaction variances were generally significant. Heritability estimates were moderate to high. Disease severity had strong correlations with respective mycotoxin concentrations. Selection for resistance is expected to have favorable correlated response for mycotoxins particularly with respect to AGER and DON (r p = 0.94). We recommend conducting initial selection on the basis of visual ratings and evaluate the selected elite material for mycotoxin concentrations.
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