Quantitative trait loci underlying variation in flowering in different environmental conditions in Boechera stricta, a close relative of Arabidopsis

Background/Question/Methods

Environmental conditions influence key life history transitions in plants, such as the shift from vegetative growth to reproduction.  We know a great deal about the genetic basis of flowering phenology in model and agronomic species under controlled laboratory conditions; however, the genetic architecture of this ecologically-relevant trait is poorly understood in natural populations of non-model organisms. Boechera stricta (Brassicaceae), a close relative of Arabidopsis, occurs in undisturbed natural populations throughout the western United States.  In this study, we conducted complementary field and growth chamber experiments with B. stricta to : 1) assess selection on flowering timing; 2) investigate the influence of environmental conditions on flowering phenology and plant size at flowering; and 3) map quantitative trait loci that contribute to adaptive variation in flowering phenology.  For both experiments, we included individuals from 180 F₆ recombinant inbred lines (RILs), generated from a cross between Colorado and Montana parents.  Phenotypic data were collected from 1080 individuals of these RILs outplanted into each parental population in the field, and 5760 individuals in vernalization (duration of winter), growing season temperature, and photoperiod treatments in the growth chamber experiment.  We genotyped these RILs at 107 single nucleotide polymorphism and 62 microsatellite markers to create a linkage map for QTL analysis.

Results/Conclusions

Genetic and environmental factors strongly influenced the transition to reproduction in our field and growth chamber experiments.  The timing of this transition has clear fitness consequences, as we found strong directional selection for earlier flowering in the field experiment.  The duration of vernalization was a key predictor of flowering time in the growth chamber experiment: longer winters accelerated flowering.  Additionally, elevated ambient temperatures delayed flowering.  These results have clear implications for predicting B. stricta’s response to climate change. Shorter winters and higher growing season temperatures will likely alter the long-term flowering patterns of B. stricta, possibly resulting in a mis-match between pollinator abundance and floral development.

Our analyses identified one large effect QTL, which was stable across multiple conditions in the growth chamber experiment and the field experiment.  This locus was very close to FT (FLOWERING LOCUS T), and explained up to 27 % of the phenotypic variance in flowering time and 45% of the variance in plant size at flowering. Additionally, we found significant QTL × environment interactions for a number of important flowering time QTLs, indicating that different loci or patterns of gene expression influence flowering phenology under contrasting conditions.