Academic journal article Genetics

Genetic Architecture of Flowering Time in Maize as Inferred from Quantitative Trait Loci Meta-Analysis and Synteny Conservation with the Rice Genome

Academic journal article Genetics

Genetic Architecture of Flowering Time in Maize as Inferred from Quantitative Trait Loci Meta-Analysis and Synteny Conservation with the Rice Genome

Article excerpt

ABSTRACT

Genetic architecture of flowering time in maize was addressed by synthesizing a total of 313 quantitative trait loci (QTL) available for this trait. These were analyzed first with an overview statistic that highlighted regions of key importance and then with a meta-analysis method that yielded a synthetic genetic model with 62 consensus QTL. Six of these displayed a major effect. Meta-analysis led in this case to a twofold increase in the precision in QTL position estimation, when compared to the most precise initial QTL position within the corresponding region. The 62 consensus QTL were compared first to the positions of the few flowering-time candidate genes that have been mapped in maize. We then projected rice candidate genes onto the maize genome using a synteny conservation approach based on comparative mapping between the maize genetic map and japonica rice physical map. This yielded 19 associations between maize QTL and genes involved in flowering time in rice and in Arabidopsis. Results suggest that the combination of meta-analysis within a species of interest and synteny-based projections from a related model plant can be an efficient strategy for identifying new candidate genes for trait variation.

MAIZE (Zea mays L.) was domesticated from the Central America native Teosinte. It was then gradually adapted to temperate climates, up to the cool regions of America and then northern Europe. This acclimatization was made possible mainly by an adaptation of maize flowering time to the local climatic features. Flowering time and related traits such as plant height and total leaf number are determined mainly by the timing of the transition from vegetative to reproductive development made by the shoot apical meristem of maize (IRISH and NELSON 1991). Use of molecular markers allowed the detection, since the late 1980s, of an increasing number of quantitative trait loci (QTL) controlling these traits. Besides studies addressing flowering time for its direct interest for maize adaptation to temperate climates (RAGOT et al. 1995), this trait is also frequently scored as a component of yield (MECHIN et al. 2001), drought stress (VELDBOOM and LEE 1996), or pest resistance (BOHN et al. 2000). A large body of QTL information is therefore presently available for flowering time in maize.

As opposed to other traits such as kernel characteristics, only few mutations affecting flowering time have been identified in maize, so that knowledge of the genetic control of this trait in maize remains relatively poor. The best known gene, INDETERMINATE1 (ID1), was cloned from a mutation where the apical vegetative meristem failed to be converted into a reproductive meristem (COLASANTI et al. 1998). The ID1 gene encodes a zinc finger transcription factor. The locus id1 was mapped to chromosome 1L. Two other mutants, delayed flowering1 (dlf1) and leafy1 (lfy1 ), have shown specific albeit weak effect on the floral transition. Last, a recessive mutation of the EARLY PHASE CHANGE (EPC) gene reduced the duration of the juvenile vegetative phase, thus causing an early flowering (VEGA et al. 2002). Still, no dramatic effect on the number of leaves was observed. The epc mutation was mapped on chromosome 8.

On the other hand, the genetics and molecular biology of the floral transition have been most extensively studied in Arabidopsis thaliana. Almost 80 genes involved in the timing of flowering are cloned and described for this species. Genetic, molecular, and physiological analyses led to the elaboration of a model of the genetic interactions between these genes (KOORNNEEF et al. 1998; BLAZQUEZ 2000). Four genetic signaling pathways that promote flowering have been identified: the photoperiodic, autonomous, vernalization (transient exposure to low temperatures soon after germination), and gibberellins (GA) pathways. Briefly, photoreceptors, such as phytochromes (PHYA-E ) and cryptochromes (CRY1-2), are involved in the perception of day length and interact with an endogenous circadian clock to initiate flowering signals under long days (MILLAR 2004). …

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