Life-history QTLS and natural selection on flowering time in Boechera stricta, a perennial relative of Arabidopsis

Abstract

Plants must precisely time flowering to capitalize on favorable conditions. Although we know a great deal about the genetic basis of flowering phenology in model species under controlled conditions, the genetic architecture of this ecologically important trait is poorly understood in nonmodel organisms. Here, we evaluated the transition from vegetative growth to flowering in Boechera stricta, a perennial relative of Arabidopsis thaliana. We examined flowering time QTLs using 7920 recombinant inbred individuals, across seven laboratory and field environments differing in vernalization, temperature, and photoperiod. Genetic and environmental factors strongly influenced the transition to reproduction. We found directional selection for earlier flowering in the field. In the growth chamber experiment, longer winters accelerated flowering, whereas elevated ambient temperatures delayed flowering. Our analyses identified one large effect QTL (nFT), which influenced flowering time in the laboratory and the probability of flowering in the field. In Montana, homozygotes for the native allele at nFT showed a selective advantage of 6.6%. Nevertheless, we found relatively low correlations between flowering times in the field and the growth chambers. Additionally, we detected flowering-related QTLs in the field that were absent across the full range of laboratory conditions, thus emphasizing the need to conduct experiments in natural environments.

Figure 1. Comparison of environmental effects on flowering response

The first two principal component axes of flowering behavior are shown for recombinant inbred genotypes. Symbols: triangle: probability of flowering in the field; inverted triangle: flowering date in the field; squares: 4 week vernalization treatment; circles: 6 week vernalization; red: long day at 25°C; blue: long day at 18°C; black: short day at 18°C. See text for details.

Figure 2. Time until flowering differs in six growth chamber conditions

The probability of flowering (with 95% confidence intervals) for individuals in each treatment as a function of time (days) in the growth chamber during (A) season 1, and (B) season 2.

Figure 3. Plant size at flowering vs. flowering time

Relationship (with 95% confidence intervals) between the time until flowering, and plant size at flowering (A) for season one in the growth chamber experiment and (B)at the Montana field site . Family means are plotted for the Montana field site.

Figure 4. Natural selection on plant age and stage at flowering in the field

Genotypic selection analysis (and 95% confidence intervals) indicating the relationship at the Montana transplant site between relative fitness (number of fruits relativized to mean number of fruits) and plant (A) age and (B) size at flowering .

Figure 5. QTL Cartographer results from the field and growth chamber experiments

Positions of quantitative trait loci detected in the field (hatched boxes) and growth chamber (open boxes) experiments. The 1 and 2 LOD confidence intervals are indicated by the box and bars respectively. Growth chamber QTL shown here represent trait values averaged across treatments. The separate QTL for each treatment are enumerated in Supplementary Tables S8-S10.