GENETIC VARIANCE AND COVARIANCE FOR PHYSIOLOGICAL TRAITS IN LOBELIA: ARE THERE CONSTRAINTS ON ADAPTIVE EVOLUTION?

Natural variation identifies new effectors of water-use efficiency in Arabidopsis

Water-use efficiency (WUE) is the ratio of biomass produced per unit of water consumed; thus, it can be altered by genetic factors that affect either side of the ratio. In the present study, we exploited natural variation for WUE to discover loci affecting either biomass accumulation or water use as factors affecting WUE. Genome-wide association studies (GWAS) using integrated WUE measured through carbon isotope discrimination (δ¹³C) of Arabidopsis thaliana accessions identified genomic regions associated with WUE. Reverse genetic analysis of 70 candidate genes selected based on the GWAS results and transcriptome data identified 25 genes affecting WUE as measured by gravimetric and δ¹³C analyses. Mutants of four genes had higher WUE than wild type, while mutants of the other 21 genes had lower WUE. The differences in WUE were caused by either altered biomass or water consumption (or both). Stomatal density (SD) was not a primary cause of altered WUE in these mutants. Leaf surface temperatures indicated that transpiration differed for mutants of 16 genes, but generally biomass accumulation had a greater effect on WUE. The genes we identified are involved in diverse cellular processes, including hormone and calcium signaling, meristematic activity, photosynthesis, flowering time, leaf/vasculature development, and cell wall composition; however, none of them had been previously linked to WUE. Thus, our study successfully identified effectors of WUE that can be used to understand the genetic basis of WUE and improve crop productivity.

How functional traits, herbivory, and genetic diversity interact in Echinacea: implications for fragmented populations

Habitat fragmentation produces small, spatially isolated populations that promote inbreeding. Remnant populations often contain inbred and outbred individuals, but it is unclear how inbreeding relative to outbreeding affects the expression of functional traits and biotic interactions such as herbivory. We measured a suite of 12 functional traits and herbivore damage on three genotypic cross types in the prairie forb, Echinacea angustifolia: inbred, and outbred crosses resulting from matings within and between remnant populations. Inbreeding significantly affected the expression of all 12 functional traits that influence resource capture. Inbred individuals had consistently lower photosynthetic rates, water use efficiencies, specific leaf areas, and had higher trichome numbers, percent C, and percent N than outbred individuals. However, herbivore damage did not differ significantly among the cross types and was not correlated with other leaf functional traits. Leaf architecture and low physiological rates of the inbred compared to outbred individuals imply poorer capture or use of resources. Inbred plants also had lower survival and fitness relative to outbred plants. Our results show that inbreeding, a phenomenon predicted and observed to occur in fragmented populations, influences key functional traits such as plant structure, physiology and elemental composition. Because of their likely role in fitness of individuals and ecological dynamics plant functional traits can serve as a bridge between evolution and community or ecosystem ecology.

Physiological performance and mating system in Clarkia (Onagraceae): does phenotypic selection predict divergence between sister species?

PREMISE OF THE STUDY

The evolution of self-fertilization often occurs in association with other floral, life history, and fitness-related traits. A previous study found that field populations of Clarkia exilis (a predominantly autogamous selfer) and its sister species, Clarkia unguiculata (a facultative outcrosser) differ in mean photosynthetic rates and instantaneous water use efficiency (WUE(i)). Here, we investigate the strength and direction of selection on these traits in multiple populations of each taxon to determine whether natural selection may contribute to the phenotypic differences between them.

METHODS

In spring 2008, we measured instantaneous gas exchange rates in nine populations during vegetative growth (Early) and/or during flowering (Late). We conducted selection gradient analyses and estimated selection differentials within populations and across pooled conspecific populations to evaluate the strength, direction, and consistency of selection on each trait early and late in the season.

KEY RESULTS

The direction and relative strength of selection on photosynthetic rates in these taxa corresponds to the phenotypic difference between them; C. exilis has higher photosynthetic rates than C. unguiculata, as well as stronger, more consistent selection favoring rapid photosynthesis throughout the growing season. Patterns of selection on transpiration, WUE(i), and the timing of flowering progression are less consistent with phenotypic differences (or lack thereof) between taxa.

CONCLUSIONS

We detected several examples where selection was consistent with the phenotypic divergence between sister taxa, but there were also numerous instances that were equivocal or in which selection did not predict the realized phenotypic difference between taxa.

Plastic Trait Integration across a CO2Gradient inArabidopsis thaliana

Shifts across environments in patterns of trait integration may govern or alter adaptive responses. Changes in resource supply rates may be an especially important cause of plasticity of trait integration because they can lead to shifts in colimitation and coregulation of traits. Traditional evolutionary genetic characterization of trait integration relies on covariance analyses. Structural equation modeling (SEM) can complement such analyses. The SEM provides insights into causal structure not possible with a covariance analysis, thereby providing mechanistic understanding of shifts in integration and suggesting likely foci of selection in changing environments. We tested for changes in trait integration by growing 35 genotypes of Arabidopsis thaliana (Brassicaceae; mouse-eared cress) from throughout the species' range in four atmospheric CO2 concentrations: 250 (past), 355 (approximately recent CO2), 530, and 710 (future) microM M(-1). SEM revealed significant shifts in the integration of N, C, and H2O use and their effects on reproductive dry mass across the CO2 gradient. The low CO2 stress of 250 microM M(-1) had the most divergent integration structures. Standardized total effects of C assimilation, water loss, and early N mass changed in sign across the C supply gradient, and the total effect of quantum yield decreased from significant to nonsignificant values across the gradient. Transpiration exhibited significant genetic variation and is thus a candidate target for selection and adaptation under novel growth CO2 concentrations. The strength of the correlation between C assimilation and transpiration declined by 19% from 250 to 710 microM M(-1), indicating a partial decoupling of their current mutual evolutionary constraint in the atmosphere of the future. Structural equation analysis of functional integration provides unique insights into the mechanisms through which changes in limiting resources can alter the nature of trait integration.