Quantitative trait loci affecting delta13C and response to differential water availibility in Arabidopsis thaliana.

Phenotypic plasticity is an important response mechanism of plants to environmental heterogeneity. Here, we explored the genetic basis of plastic responses of Arabidopsis thaliana to water deficit by experimentally mapping quantitative trait loci (QTL) in two recombinant inbred populations (Cvi x Ler and Ler x Col). We detected genetic variation and significant genotype-by-environment interactions for many traits related to water use. We also mapped 26 QTL, including six for carbon isotope composition (delta13C). Negative genetic correlations between fruit length and fruit production as well as between flowering time and branch production were corroborated by QTL colocalization, suggesting these correlations are due to pleiotropy or physical linkage. Water-limited plants were more apically dominant with greater root:shoot ratios and higher delta13C (higher water-use efficiency) when compared to well-watered plants. Many of the QTL effects for these traits interacted significantly with the irrigation treatment, suggesting that the observed phenotypic plasticity is genetically based. We specifically searched for epistatic (QTL-QTL) interactions using a two-dimensional genome scan, which allowed us to detect epistasis regardless of additive genetic effects. We found several significant QTL-QTL interactions including three that exhibited environmental dependence. These results provide preliminary evidence for proposed genetic mechanisms underlying phenotypic plasticity.

Testing for stress-dependent inbreeding depression in Impatiens capensis (Balsaminaceae).

The relevance of inbreeding depression to the persistence of plant populations can depend upon whether stress magnifies inbreeding depression for fitness-related traits. To examine whether drought stress exacerbates inbreeding depression in gas exchange traits and biomass, we grew selfed and outcrossed progeny of inbred lines from two populations of Impatiens capensis in a greenhouse experiment under water-limited and moist soil conditions. Drought stress did not magnify the degree of inbreeding depression for any of the traits measured. In fact, in one population there was a trend for stronger inbreeding depression under well-watered, benign conditions. Furthermore, significant inbreeding depression for carbon assimilation rate and stomatal conductance was only detected in the lines from one population. In contrast, inbreeding depression for biomass was detected within both populations and differed among lines. Drought stress exerted significant selection on physiological traits, favoring increased carbon assimilation rates and decreased stomatal conductance in drought-stressed plants. Patterns of selection did not differ between inbred and outcrossed plants but did differ marginally between populations. Thus, estimates of selection were not biased by the mixed mating system per se, but may be biased by combining individuals from populations with different histories of selection and inbreeding.