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Maize is unique among cereal grasses because of its monoecious flowering habit. Male flowers are normally restricted to the tassel that terminates the primary shoot, whereas female flowers occur as ears at the terminal nodes of lateral branches. We observed Ki14, a tropical maize inbred that produces an ear tipped by a staminate (male) spike under certain environmental conditions, such as long daylengths. Recombinant inbred lines derived from the cross between temperate line B97, which was never observed to produce a staminate ear tip, and Ki14 segregated for the trait under long daylengths. Some progeny lines that had even longer staminate tips than Ki14 were male fertile. We mapped three QTL controlling staminate ear tip using a two-part (binomial plus normal) model. A major QTL on chromosome 3 had a large effect on penetrance of the trait (whether a line would produce staminate ear tips or not) as well as its severity (the length of the staminate tip). This QTL seems to be linked to, but at a distinct position from, a previously mapped QTL controlling the proportion of staminate florets in ears in progeny from crosses between maize and teosinte. Two additional QTL affecting staminate ear tip severity overlapped with QTL controlling photoperiod response previously mapped in this population. Alleles conferring photoperiod sensitivity for delayed flowering at these QTL seem to enhance the production of staminate ear tips under long daylengths.

Southern leaf blight (SLB) is a foliar necrotrophic disease of maize (Zea mays L.) caused by the ascomycete fungus Cochliobolus heterostrophus (Drechs.) Drechs. It is particularly important in warm humid parts of the world where maize is cultivated, such as the southern Atlantic coast area of the United States and parts of India, Africa, and Western Europe. Quantitative trait loci (QTL) for resistance to SLB disease caused by C. heterostrophus race O were identifi ed in three maize recombinant inbred populations assessed in two environments: Clayton, NC, in the summer and Homestead, FL, in the winter. The three populations were derived from the crosses B73 × CML254, CML254 × B97, and B97 × Ki14. Each of these populations was derived from a cross between a temperate maize line (B73 or B97) and a tropical maize line (Ki14 or CML254). Quantitative trait loci were identifi ed by separate analysis of each population and by joint connected and disconnected analyses of all the populations. The most signifi cant QTL identifi ed were on chromosomes 3, 8, 9, and 10. Joint analysis led to more precise position estimates than separate analysis in each case. Results are discussed in the context of previous SLB QTL analysis studies and a recent fl owering time QTL study that used the same populations. The chromosome 8 and 9 QTL colocalized with previously identifi ed fl owering time QTL, which suggested that the perceived effect on SLB resistance at these QTL may have been mediated through an effect on fl owering time.

Tropical maize (Zea mays L.) represents a valu- able genetic resource containing unique alleles not present in elite temperate maize. The strong delay in fl owering in response to long daylength photoperiods exhibited by most tropical maize hinders its incorporation into temperate maize breeding programs. We tested the hypothesis that diverse tropical inbreds carry alleles with similar effects at four key photoperiod response quantitative trait loci (QTL) previously identifi ed in maize. Four tropical maize inbreds were each crossed and backcrossed twice to the temper- ate recurrent parent B73 to establish four sets of introgression lines. Evaluation of these lines under long daylengths demonstrated that all four QTL have signifi cant effects on fl owering time or height in these lines, but the functional allelic effects varied substantially across the tropical donor lines. At the most important pho- toperiod response QTL on chromosome 10, one tropical line allele even promoted earlier fl ower- ing relative to the B73 allele. Signifi cant allelic effect differences among tropical founders were also demonstrated directly in an F 2 population derived from the cross of Ki14 and CML254. The chromosome 10 photoperiod response QTL position was validated in a set of heterogeneous inbred families evaluated in fi eld tests and in controlled environments.

Variation in maize for response to photoperiod is related to geographical adaptation in the species. Maize possesses homologs of many genes identified as regulators of flowering time in other species, but their relation to the natural variation for photoperiod response in maize is unknown. Candidate gene sequences were mapped in four populations created by crossing two temperate inbred lines to two photoperiod-sensitive tropical inbreds. Whole-genome scans were conducted by high-density genotyping of the populations, which were phenotyped over 3 years in both short- and long-day environments. Joint multiple population analysis identified genomic regions controlling photoperiod responses in flowering time, plant height, and total leaf number. Four key genome regions controlling photoperiod response across populations were identified, referred to as ZmPR1–4. Functional allelic differences within these regions among phenotypically similar founders suggest distinct evolutionary trajectories for photoperiod adaptation in maize. These regions encompass candidate genes CCA/LHY, CONZ1, CRY2, ELF4, GHD7, VGT1, HY1/SE5, TOC1/PRR7/PPD-1, PIF3, ZCN8, and ZCN19.

Southern leaf blight (SLB), gray leaf spot (GLS), and northern leaf blight (NLB) are all important foliar diseases impacting maize production. The objectives of this study were to identify quantitative trait loci (QTL) for resistance to these diseases in a maize recombinant inbred line (RIL) population derived from a cross between maize lines Ki14 and B73, and to evaluate the evidence for the presence genes or loci conferring multiple disease resistance (MDR). Each disease was scored in multiple separate trials. Highly significant correlations between the resistances and the three diseases were found. The highest correlation was identified between SLB and GLS resistance (r = 0.62). Correlations between resistance to each of the diseases and time to flowering were also highly significant. Nine, eight, and six QTL were identified for SLB, GLS, and NLB resistance, respectively. QTL for all three diseases colocalized in bin 1.06, while QTL colocalizing for two of the three diseases were identified in bins 1.08 to 1.09, 2.02/2.03, 3.04/3.05, 8.05, and 10.05. QTL for time to flowering were also identified at four of these six loci (bins 1.06, 3.04/3.05, 8.05, and 10.05). No disease resistance QTL was identified at the largest-effect QTL for flowering time in bin 10.03.

Appropriate selection of parents for the development of mapping populations is pivotal to maximizing the power of quantitative trait loci detection. Trait genotypic variation within a family is indicative of the family's informativeness for genetic studies. Accurate prediction of the most useful parental combinations within a species would help guide quantitative genetics studies. We tested the reliability of genotypic and phenotypic distance estimators between pairs of maize inbred lines to predict genotypic variation for quantitative traits within families derived from biparental crosses. We developed 25 families composed of ~200 random recombinant inbred lines each from crosses between a common reference parent inbred, B73, and 25 diverse maize inbreds. Parents and families were evaluated for 19 quantitative traits across up to 11 environments. Genetic distances (GDs) among parents were estimated with 44 simple sequence repeat and 2303 single-nucleotide polymorphism markers. GDs among parents had no predictive value for progeny variation, which is most likely due to the choice of neutral markers. In contrast, we observed for about half of the traits measured a positive correlation between phenotypic parental distances and within-family genetic variance estimates. Consequently, the choice of promising segregating populations can be based on selecting phenotypically diverse parents. These results are congruent with models of genetic architecture that posit numerous genes affecting quantitative traits, each segregating for allelic series, with dispersal of allelic effects across diverse genetic material. This architecture, common to many quantitative traits in maize, limits the predictive value of parental genotypic or phenotypic values on progeny variance.

The transition from the vegetative to reproductive development is a critical event in the plant life cycle. The accurate prediction of flowering time in elite germplasm is important for decisions in maize breeding programs and best agronomic practices. The understanding of the genetic control of flowering time in maize has significantly advanced in the past decade. Through comparative genomics, mutant analysis, genetic analysis and QTL cloning, and transgenic approaches, more than 30 flowering time candidate genes in maize have been revealed and the relationships among these genes have been partially uncovered. Based on the knowledge of the flowering time candidate genes, a conceptual gene regulatory network model for the genetic control of flowering time in maize is proposed. To demonstrate the potential of the proposed gene regulatory network model, a first attempt was made to develop a dynamic gene network model to predict flowering time of maize genotypes varying for specific genes. The dynamic gene network model is composed of four genes and was built on the basis of gene expression dynamics of the two late flowering id1 and dlf1 mutants, the early flowering landrace Gaspe Flint and the temperate inbred B73. The model was evaluated against the phenotypic data of the id1 dlf1 double mutant and the ZMM4 overexpressed transgenic lines. The model provides a working example that leverages knowledge from model organisms for the utilization of maize genomic information to predict a whole plant trait phenotype, flowering time, of maize genotypes.