Extreme QTL mapping of germination speed in Arabidopsis thaliana

Seed germination is a key life history transition for annual plants and partly determines lifetime performance and fitness. Germination speed, the elapsed time for a nondormant seed to germinate, is a poorly understood trait important for plants’ competitiveness and fitness in fluctuating environments. Germination speed varied by 30% among 18 Arabidopsis thaliana populations measured, and exhibited weak negative correlation with flowering time and seed weight, with significant genotype effect ( P  < 0.005). To dissect the genetic architecture of germination speed, we developed the extreme QTL (X‐ QTL ) mapping method in A. thaliana . The method has been shown in yeast to increase QTL mapping power by integrating selective screening and bulk‐segregant analysis in a very large mapping population. By pooled genotyping of top 5% of rapid germinants from ~100 000 F 3 individuals, three X‐ QTL regions were identified on chromosomes 1, 3 and 4. All regions were confirmed as QTL regions by sequencing 192 rapid germinants from an independent F 3 selection experiment. Positional overlaps were found between X‐ QTL s and previously identified seed, life history and fitness QTL s. Our method provides a rapid mapping platform in A. thaliana with potentially greater power. One can also relate identified X‐ QTL s to the A. thaliana physical map, facilitating candidate gene identification.