Across-environment genetic correlations and the frequency of selective environments shape the evolutionary dynamics of growth rate in Impatiens capensis.

Trade-offs can exist within and across environments, and constrain evolutionary trajectories. To examine the effects of competition and resource availability on trade-offs, we grew individuals of recombinant inbred lines of Impatiens capensis in a factorial combination of five densities with two light environments (full light and neutral shade) and used a Bayesian logistic growth analysis to estimate intrinsic growth rates. To estimate across-environment constraints, we developed a variance decomposition approach to principal components analysis, which accounted for sample size, model-fitting, and within-RIL variation prior to eigenanalysis. We detected negative across-environment genetic covariances in intrinsic growth rates, although only under full-light. To evaluate the potential importance of these covariances, we surveyed natural populations of I. capensis to measure the frequency of different density environments across space and time. We combined our empirical estimates of across-environment genetic variance-covariance matrices and frequency of selective environments with hypothetical (yet realistic) selection gradients to project evolutionary responses in multiple density environments. Selection in common environments can lead to correlated responses to selection in rare environments that oppose and counteract direct selection in those rare environments. Our results highlight the importance of considering both the frequency of selective environments and the across-environment genetic covariances in traits simultaneously.

Diversification of phytochrome contributions to germination as a function of seed-maturation environment.

Environmental conditions during seed maturation influence germination, but the genetic basis of maternal environmental effects on germination is virtually unknown. Using single and multiple mutants of phytochromes, it is shown here that different phytochromes contributed to germination differently, depending on seed-maturation conditions. Arabidopsis thaliana wild-type seeds that were matured under cool temperatures were intensely dormant compared with seeds matured at warmer temperature, and this dormancy was broken only after warm seed-stratification followed by cold seed-stratification. The warm-cold stratification broke dormancy in fresh seeds but not in dry after-ripened seeds. Functional PHYB and PHYD were necessary to break cool-induced dormancy, which indicates a previously unknown and ecologically important function for PHYD. Disruption of PHYA in combination with PHYD (but not PHYB) restored germination to near wild-type levels, indicating that PHYA contributes to the maintenance of cool-induced dormancy on a phyD background. Effects of seed-maturation temperature were much stronger than effects of seed-maturation photoperiod. PHYB contributed to germination somewhat more strongly in seeds matured under short days, whereas PHYD contributed to germination somewhat more strongly in seeds matured under long days. The variable contributions of different phytochromes to germination as a function of seed-maturation conditions reveal further functional diversification of the phytochromes during the process of germination. This study identifies among the first genes to be associated with maternal environmental effects on germination.

A new role for phytochromes in temperature-dependent germination.

Germination timing is a fundamental life-history trait, as seedling establishment predicates realized fitness in the wild. Light and temperature are two important cues by which seeds sense the proper season of germination. Using Arabidopsis thaliana, we provide evidence that phytochrome-mediated germination pathways simultaneously respond to light and temperature cues in ways that affect germination. Phytochrome mutant seeds were sown on agar plates and allowed to germinate in lit, growth chambers across a range of temperatures (7 degrees C to 28 degrees C). phyA had an important role in promoting germination at warmer temperatures, phyE was important to germination at colder temperatures and phyB was important to germination across a range of temperatures. Different phytochromes were required for germination at different temperatures, indicating a restriction or even a potential specialization of individual phytochrome activity as a function of temperature. This temperature-dependent activity of particular phytochromes reveals a potentially novel role for phytochrome pathways in regulating the seasonal timing of germination.

Phytochrome mediates germination responses to multiple seasonal cues.

We identified a new role of phytochrome in mediating germination responses to seasonal cues and thereby identified for the first time a gene involved in maternal environmental effects on germination. We examined the germination responses of a mutant, hy2-1, which is deficient in the phytochrome chromophore. The background genotype, Landsberg erecta (Ler), lacked dormancy in most treatments, while hy2-1 required cold stratification for germination in a manner that resembled a more dormant ecotype, Columbia (Col). Unlike Col, hy2-1 was not induced into dormancy by warm stratification. Therefore, the down-regulation of phytochrome-mediated germination pathways results in sensitivity to cold, but we found no evidence that reduced phytochrome activity enables the warm-induction of dormancy. Cool temperatures during seed maturation induced dormancy. The hy2-1 mutants did not overcome this dormancy, indicating that phytochrome-mediated pathways are required to break cold-induced dormancy. Ler did not respond to post-stratification temperature, but hy2-1 did respond, suggesting phytochrome pathways are involved in germination responses to temperature. In summary, phytochromes mediate dormancy and germination responses to seasonal cues experienced both during seed maturation and after dispersal. Phytochromes therefore appear to be involved in mediating seasonal germination timing, a trait of great ecological importance and one that is under strong natural selection.

Maternal effects of drought stress and inbreeding in Impatiens capensis (Balsaminaceae).

Maternal effects can have substantial impacts on plant fitness and plant populations. Stressful environmental conditions can cause a maternal plant to inadequately provision its progeny, resulting in poor seedling growth, low reproductive success, and decreased competitive ability. Maternal effects consist of environmental and genetic load components, but the interactions between these two components have rarely been considered. To determine the effects of maternal drought stress and maternal inbreeding on progeny biomass (a fitness correlate) and physiological responses to drought stress, we conducted a greenhouse experiment with genetic lines from two populations (mesic site vs. dry site) of the herbaceous annual Impatiens capensis (Balsaminaceae). Seeds were collected from cleistogamous flowers of inbred or outcrossed maternal plants that were subject to either a drought or control treatment. These seeds were grown into juvenile plants that were also subject to either a drought stress or a control treatment. Plants from the mesic site had significantly reduced biomass from maternal drought stress, while plants from the dry site maintained biomass despite adverse maternal environmental conditions. Juvenile plants of both populations had reduced biomass only as a result of maternal inbreeding. Interestingly, inbreeding depression was more apparent when maternal environmental conditions were benign.

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.

Mechanisms of selection for drought stress tolerance and avoidance in Impatiens capensis (Balsaminaceae).

For longer lived annual plants, high water-use efficiency (WUE) and low stomatal conductance are hypothesized to confer a fitness advantage under drought stress. To directly test the adaptive significance of WUE and stomatal conductance under drought stress, inbred lines of Impatiens capensis were grown in two field environments (watered and not-watered), in a year of unusual early-season drought. In contrast to the results from a previous study of late-season drought in the same system, selection was detected for lower WUE, increased stomatal conductance, and early flowering time. These findings suggest that early-season drought conditions may select for drought avoidance traits such as low WUE and early reproduction, whereas later drought selects for tolerance traits such as high WUE.

Genetic variance for diapause expression and associated life histories in Drosophila melanogaster.

The dipteran Drosophila melanogaster can express a form of reproductive quiescence or diapause when exposed to low temperature and shortened photoperiod. Among natural populations in the eastern United States, the frequency of lines that express reproductive diapause in the laboratory varies substantially and predictably with latitudinal origin. The goals of the present study were twofold: (1) to examine the impact of genetic variance for diapause expression on multiple traits associated with organismal fitness; and (2) to evaluate the potential for fitness trade-offs between diapause and nondiapause phenotypes that may result in the observed cline. Even prior to diapause entry or expression, inbred lines that express and do not express reproductive diapause in laboratory assays were constitutively distinct for life span, age-specific mortality rates, fecundity profiles, resistance to cold and starvation stress, lipid content, development time, and egg-to-adult viability. Furthermore, estimates of genetic correlations based on line means revealed significant differentiation for genetic variance/covariance matrices between diapause and nondiapause lines. The data indicate the potential for life-history trade-offs associated with variation for the diapause phenotype. The observed cline in diapause incidence in the eastern United States may be generated by these tradeoffs and the associated spatial and/or temporal variation in relative fitness of these two phenotypes in natural populations.

Frequency and microenvironmental pattern of selection on plastic shade-avoidance traits in a natural population of Impatiens capensis.

The frequency and predictability of different selective environments are important parameters in models for the evolution of plasticity but have rarely been measured empirically in natural populations. We used an experimental phytometer approach to examine the frequency, predictability, and environmental determinants of heterogeneous selection on phytochrome-mediated shade-avoidance responses in a natural population of the annual plant Impatiens capensis. The strength and direction of selection on shade-avoidance traits varied substantially on a fine spatial scale. The shade-avoidance phenotype had high relative fecundity in some microsites but was disadvantageous in other microsites. Local seedling density proved to be a surprisingly poor predictor of microenvironmental variation in the strength and direction of selection on stem elongation in this study population. At least some of this unpredictability resulted from microenvironmental variation in water availability; the shade-avoidance phenotype was more costly in dry microsites. Thus, environmental heterogeneity in resource availability can affect the relative costs and benefits of expressing shade-avoidance traits independent of local seedling density, the inductive environmental cue. Theory predicts that these conditions may promote local genetic differentiation in reaction norms in structured populations, as observed in I. capensis.

Natural selection on light response curve parameters in the herbaceous annual, Impatiens capensis.

We tested for genetic variation in light response curves and their acclimation to sun versus shade in recombinant inbred lines (RILs) of the annual species Impatiens capensis derived from a cross between sun and shade populations. We exposed replicates of 49 RILs to experimentally manipulated light levels (open versus shade) in a greenhouse and measured photosynthetic light response curves, height, biomass, and reproduction. Plants were taller in the shade treatment, but we were unable to detect differences between light treatments (i.e., acclimation) in the maximal rate of photosynthesis, the light compensation point, or the quantum efficiency of photosynthesis. Genotypic selection analyses indicated that higher maximal rates of carbon assimilation and higher light compensation points (typical of sun-acclimated light curves) were favored by natural selection in both light treatments. Thus, it appears that the pattern of selection on photosynthetic parameters may not depend on light environment in this species.

The adaptive evolution of plasticity: phytochrome-mediated shade avoidance responses.

Many plants display a characteristic suite of developmental "shade avoidance" responses, such as stem elongation and accelerated reproduction, to the low ratio of red to far-red wavelengths (R:FR) reflected or transmitted from green vegetation. This R:FR cue of crowding and vegetation shade is perceived by the phytochrome family of photoreceptors. Phytochrome-mediated responses provide an ideal system for investigating the adaptive evolution of phenotypic plasticity in natural environments. The molecular and developmental mechanisms underlying shade avoidance responses are well studied, and testable ecological hypotheses exist for their adaptive significance. Experimental manipulation of phenotypes demonstrates that shade avoidance responses may be adaptive, resulting in phenotypes with high relative fitness in the environments that induce those phenotypes. The adaptive value of shade avoidance depends upon the competitive environment, resource availability, and the reliability of the R:FR cue for predicting the selective environment experienced by an induced phenotype. Comparative studies and a reciprocal transplant experiment with Impatiens capensis provide evidence of adaptive divergence in shade avoidance responses between woodland and clearing habitats, which may result from population differences in the frequency of selection on shade avoidance traits, as well as differences in the reliability of the R:FR cue. Recent rapid progress in elucidating phytochrome signaling pathways in the genetic model Arabidopsis thaliana and other species now provides the opportunity for studying how selection on shade avoidance traits in natural environments acts upon the molecular mechanisms underlying natural phenotypic variation.