The genetic architecture of grain yield and related traits in Zea maize L. revealed by comparing intermated and conventional populations.

Using advanced intermated populations has been proposed as a way to increase the accuracy of mapping experiments. An F(3) population of 300 lines and an advanced intermated F(3) population of 322 lines, both derived from the same parental maize inbred lines, were jointly evaluated for dry grain yield (DGY), grain moisture (GM), and silking date (SD). Genetic variance for dry grain yield was significantly lower in the intermated population compared to the F(3) population. The confidence interval around a QTL was on average 2.31 times smaller in the intermated population compared to the F(3) population. One controversy surrounding QTL mapping is whether QTL identified in fact represent single loci. This study identifies two distinct loci for dry grain yield in the intermated population in coupling phase, while the F(3) identifies only a single locus. Surprisingly, fewer QTL were detected in the intermated population than the F(3) (21 vs. 30) and <50% of the detected QTL were shared among the two populations. Cross-validation showed that selection bias was more important in the intermated population than in the F(3) and that each detected QTL explained a lower percentage of the variance. This finding supports the hypothesis that QTL detected in conventional populations correspond mainly to clusters of linked QTL. The actual number of QTL involved in the genetic architecture of complex traits may be substantially larger, with effect sizes substantially smaller than in conventional populations.

Standing variation and new mutations both contribute to a fast response to selection for flowering time in maize inbreds.

BACKGROUND: In order to investigate the rate and limits of the response to selection from highly inbred genetic material and evaluate the respective contribution of standing variation and new mutations, we conducted a divergent selection experiment from maize inbred lines in open-field conditions during 7 years. Two maize commercial seed lots considered as inbred lines, F252 and MBS847, constituted two biological replicates of the experiment. In each replicate, we derived an Early and a Late population by selecting and selfing the earliest and the latest individuals, respectively, to produce the next generation. RESULTS: All populations, except the Early MBS847, responded to selection despite a short number of generations and a small effective population size. Part of the response can be attributed to standing genetic variation in the initial seed lot. Indeed, we identified one polymorphism initially segregating in the F252 seed lot at a candidate locus for flowering time, which explained 35% of the trait variation within the Late F252 population. However, the model that best explained our data takes into account both residual polymorphism in the initial seed lots and a constant input of heritable genetic variation by new (epi)mutations. Under this model, values of mutational heritability range from 0.013 to 0.025, and stand as an upper bound compare to what is reported in other species. CONCLUSIONS: Our study reports a long-term divergent selection experiment for a complex trait, flowering time, conducted on maize in open-field conditions. Starting from a highly inbred material, we created within a few generations populations that strikingly differ from the initial seed lot for flowering time while preserving most of the phenotypic characteristics of the initial inbred. Such material is unique for studying the dynamics of the response to selection and its determinants. In addition to the fixation of a standing beneficial mutation associated with a large phenotypic effect, a constant input of genetic variance by new mutations has likely contributed to the response. We discuss our results in the context of the evolution and mutational dynamics of populations characterized by a small effective population size.

Fine mapping and haplotype structure analysis of a major flowering time quantitative trait locus on maize chromosome 10.

Flowering time is a major adaptive trait in plants and an important selection criterion for crop species. In maize, however, little is known about its molecular basis. In this study, we report the fine mapping and characterization of a major quantitative trait locus located on maize chromosome 10, which regulates flowering time through photoperiod sensitivity. This study was performed in near-isogenic material derived from a cross between the day-neutral European flint inbred line FV286 and the tropical short-day inbred line FV331. Recombinant individuals were identified among a large segregating population and their progenies were scored for flowering time. Combined genotypic characterization led to delimit the QTL to an interval of 170 kb and highlighted an unbalanced recombination pattern. Two bacterial artificial chromosomes (BACs) covering the region were analyzed to identify putative candidate genes, and synteny with rice, sorghum, and brachypodium was investigated. A gene encoding a CCT domain protein homologous to the rice Ghd7 heading date regulator was identified, but its causative role was not demonstrated and deserves further analyses. Finally, an association study showed a strong level of linkage disequilibrium over the region and highlighted haplotypes that could provide useful information for the exploitation of genetic resources and marker-assisted selection in maize.