The influence of genetic architecture on responses to selection under drought in rice.

Accurately predicting responses to selection is a major goal in biology and important for successful crop breeding in changing environments. However, evolutionary responses to selection can be constrained by such factors as genetic and cross-environment correlations, linkage, and pleiotropy, and our understanding of the extent and impact of such constraints is still developing. Here, we conducted a field experiment to investigate potential constraints to selection for drought resistance in rice (Oryza sativa) using phenotypic selection analysis and quantitative genetics. We found that traits related to drought response were heritable, and some were under selection, including selection for earlier flowering, which could allow drought escape. However, patterns of selection generally were not opposite under wet and dry conditions, and we did not find individual or closely linked genes that influenced multiple traits, indicating a lack of evidence that antagonistic pleiotropy, linkage, or cross-environment correlations would constrain selection for drought resistance. In most cases, genetic correlations had little influence on responses to selection, with direct and indirect selection largely congruent. The exception to this was seed mass under drought, which was predicted to evolve in the opposite direction of direct selection due to correlations. Because of this indirect effect on selection on seed mass, selection for drought resistance was not accompanied by a decrease in seed mass, and yield increased with fecundity. Furthermore, breeding lines with high fitness and yield under drought also had high fitness and yield under wet conditions, indicating that there was no evidence for a yield penalty on drought resistance. We found multiple genes in which expression influenced both water use efficiency (WUE) and days to first flowering, supporting a genetic basis for the trade-off between drought escape and avoidance strategies. Together, these results can provide helpful guidance for understanding and managing evolutionary constraints and breeding stress-resistant crops.

Rapid-cycling Brassica rapa evolves even earlier flowering under experimental drought.

PREMISE: Changes in climate can impose selection on populations and may lead to rapid evolution. One such climatic stress is drought, which plant populations may respond to with escape (rapid growth and early flowering) or avoidance (slow growth and efficient water-use). However, it is unclear if drought escape would be a viable strategy for populations that already flower early from prior selection. METHODS: In an experimental evolution study, we subjected rapid-cycling Brassica rapa (RCBr), which was previously selected for early flowering, to four generations of experimental drought or watered conditions. We then grew ancestral and descendant populations concurrently under drought and watered conditions to assess evolution, plasticity, and adaptation. RESULTS: The RCBr populations that evolved under drought had earlier flowering and lower water-use efficiency than the populations that evolved under watered conditions, indicating evolutionary divergence. The drought descendants also had a trend of earlier flowering compared to ancestors, indicating evolution. Evolution of earlier flowering under drought followed the direction of selection and increased fitness and was consistent with studies in natural and experimental populations of this species, suggesting adaptive evolution. CONCLUSIONS: We found rapid adaptive evolution of drought escape in RCBr and little evidence for constraints on flowering time evolution, even though RCBr already flowers extremely early. Our results suggest that some populations may harbor sufficient genetic variation for evolution even after strong selection has occurred. Our study also illustrates the utility of combining artificial selection, experimental evolution, and the resurrection approach to study the evolution of functional traits.

Rapid, parallel evolution of field mustard (Brassica rapa) under experimental drought.

Climate change is driving evolutionary and plastic responses in populations, but predicting these responses remains challenging. Studies that combine experimental evolution with ancestor-descendant comparisons allow assessment of the causes, parallelism, and adaptive nature of evolutionary responses, although such studies remain rare, particularly in a climate change context. Here, we created experimental populations of Brassica rapa derived from the same natural population and exposed these replicated populations to experimental drought or watered conditions for four generations. We then grew ancestors and descendants concurrently, following the resurrection approach. Experimental populations under drought showed rapid evolution of earlier flowering time and increased specific leaf area, consistent with a drought escape strategy and observations in natural populations. Evolutionary shifts followed the direction of selection and increased fitness under drought, indicative of adaptive evolution. Evolution to drought also occurred largely in parallel among replicate populations. Further, traits showed phenotypic plasticity to drought, but the direction and effect size of plasticity varied. Our results demonstrate parallel evolution to experimental drought, suggesting that evolution to strong, consistent selection may be predictable. Broadly, our study demonstrates the utility of combining experimental evolution with the resurrection approach to investigate responses to climate change.

Costs of reproduction under experimental climate change across elevations in the perennial forb Boechera stricta.

Investment in current reproduction can reduce future fitness by depleting resources needed for maintenance, particularly under environmental stress. These trade-offs influence life-history evolution. We tested whether climate change alters the future-fitness costs of current reproduction in a large-scale field experiment of Boechera stricta (Brassicaceae). Over 6 years, we simulated climate change along an elevational gradient in the Rocky Mountains through snow removal, which accelerates snowmelt and reduces soil water availability. Costs of reproduction were greatest in arid, lower elevations, where high initial reproductive effort depressed future fitness. At mid-elevations, initial reproduction augmented subsequent fitness in benign conditions, but pronounced costs emerged under snow removal. At high elevation, snow removal dampened costs of reproduction by prolonging the growing season. In most scenarios, failed reproduction in response to resource limitation depressed lifetime fecundity. Indeed, fruit abortion only benefited high-fitness individuals under benign conditions. We propose that climate change could shift life-history trade-offs in an environment-dependent fashion, possibly favouring early reproduction and short lifespans in stressful conditions.

Review: Plant eco-evolutionary responses to climate change: Emerging directions.

Contemporary climate change is exposing plant populations to novel combinations of temperatures, drought stress, [CO2] and other abiotic and biotic conditions. These changes are rapidly disrupting the evolutionary dynamics of plants. Despite the multifactorial nature of climate change, most studies typically manipulate only one climatic factor. In this opinion piece, we explore how climate change factors interact with each other and with biotic pressures to alter evolutionary processes. We evaluate the ramifications of climate change across life history stages,and examine how mating system variation influences population persistence under rapid environmental change. Furthermore, we discuss how spatial and temporal mismatches between plants and their mutualists and antagonists could affect adaptive responses to climate change. For example, plant-virus interactions vary from highly pathogenic to mildly facilitative, and are partly mediated by temperature, moisture availability and [CO2]. Will host plants exposed to novel, stressful abiotic conditions be more susceptible to viral pathogens? Finally, we propose novel experimental approaches that could illuminate how plants will cope with unprecedented global change, such as resurrection studies combined with experimental evolution, genomics or epigenetics.

Climate change alters plant-herbivore interactions.

Plant-herbivore interactions have evolved in response to coevolutionary dynamics, along with selection driven by abiotic conditions. We examine how abiotic factors influence trait expression in both plants and herbivores to evaluate how climate change will alter this long-standing interaction. The paleontological record documents increased herbivory during periods of global warming in the deep past. In phylogenetically corrected meta-analyses, we find that elevated temperatures, CO2 concentrations, drought stress and nutrient conditions directly and indirectly induce greater food consumption by herbivores. Additionally, elevated CO2 delays herbivore development, but increased temperatures accelerate development. For annual plants, higher temperatures, CO2 and drought stress increase foliar herbivory. Our meta-analysis also suggests that greater temperatures and drought may heighten florivory in perennials. Human actions are causing concurrent shifts in CO2 , temperature, precipitation regimes and nitrogen deposition, yet few studies evaluate interactions among these changing conditions. We call for additional multifactorial studies that simultaneously manipulate multiple climatic factors, which will enable us to generate more robust predictions of how climate change could disrupt plant-herbivore interactions. Finally, we consider how shifts in insect and plant phenology and distribution patterns could lead to ecological mismatches, and how these changes may drive future adaptation and coevolution between interacting species.

Rapid evolutionary changes in gene expression in response to climate fluctuations.

There is now abundant evidence of rapid evolution in natural populations, but the genetic mechanisms of these changes remain unclear. One possible route to rapid evolution is through changes in the expression of genes that influence traits under selection. We examined contemporary evolutionary gene expression changes in plant populations responding to environmental fluctuations. We compared genome-wide gene expression, using RNA-seq, in two populations of Brassica rapa collected over four time points between 1997 and 2014, during which precipitation in southern California fluctuated dramatically and phenotypic and genotypic changes occurred. By combining transcriptome profiling with the resurrection approach, we directly examined evolutionary changes in gene expression over time. For both populations, we found a substantial number of differentially expressed genes between generations, indicating rapid evolution in the expression of many genes. Using existing gene annotations, we found that many changes occurred in genes involved in regulating stress responses and flowering time. These appeared related to the fluctuations in precipitation and were potentially adaptive. However, the evolutionary changes in gene expression differed across generations within and between populations, indicating largely independent evolutionary trajectories across populations and over time. Our study provides strong evidence for rapid evolution in gene expression, and indicates that changes in gene expression can be one mechanism of rapid evolutionary responses to selection episodes. This study also illustrates that combining resurrection studies with transcriptomics is a powerful approach for investigating evolutionary changes at the gene regulatory level, and will provide new insights into the genetic basis of contemporary evolution.

Two decades of evolutionary changes in Brassica rapa in response to fluctuations in precipitation and severe drought.

As climate changes at unprecedented rates, understanding population responses is a major challenge. Resurrection studies can provide crucial insights into the contemporary evolution of species to climate change. We used a seed collection of two Californian populations of the annual plant Brassica rapa made over two decades of dramatic precipitation fluctuations, including increasingly severe droughts. We compared flowering phenology, other drought response traits, and seed production among four generations, grown under drought and control conditions, to test for evolutionary change and to characterize the strength and direction of selection. Postdrought generations flowered earlier, with a reduced stem diameter, and lower water-use efficiency (WUE), while intervening wet seasons reversed these adaptations. There was selection for earlier flowering, which was adaptive, but delayed flowering after wet years resulted in reduced total seed mass, indicating a maladaptive response caused by brief wet periods. Furthermore, evolutionary changes and plastic responses often differed in magnitude between populations and drought periods, suggesting independent adaptive pathways. While B. rapa rapidly evolved a drought escape strategy, plant fitness was reduced in contemporary generations, suggesting that rapid shifts in flowering time may no longer keep up with the increasing severity of drought periods, especially when drought adaptation is slowed by occasional wet seasons.

Using the resurrection approach to understand contemporary evolution in changing environments.

The resurrection approach of reviving ancestors from stored propagules and comparing them with descendants under common conditions has emerged as a powerful method of detecting and characterizing contemporary evolution. As climatic and other environmental conditions continue to change at a rapid pace, this approach is becoming particularly useful for predicting and monitoring evolutionary responses. We evaluate this approach, explain the advantages and limitations, suggest best practices for implementation, review studies in which this approach has been used, and explore how it can be incorporated into conservation and management efforts. We find that although the approach has thus far been used in a limited number of cases, these studies have provided strong evidence for rapid contemporary adaptive evolution in a variety of systems, particularly in response to anthropogenic environmental change, although it is far from clear that evolution will be able to rescue many populations from extinction given current rates of global changes. We also highlight one effort, known as Project Baseline, to create a collection of stored seeds that can take advantage of the resurrection approach to examine evolutionary responses to environmental change over the coming decades. We conclude that the resurrection approach is a useful tool that could be more widely employed to examine basic questions about evolution in natural populations and to assist in the conservation and management of these populations as they face continued environmental change.

Lower plasticity exhibited by high- versus mid-elevation species in their phenological responses to manipulated temperature and drought.

BACKGROUND AND AIMS: Recent global changes, particularly warming and drought, have had worldwide repercussions on the timing of flowering events for many plant species. Phenological shifts have also been reported in alpine environments, where short growing seasons and low temperatures make reproduction particularly challenging, requiring fine-tuning to environmental cues. However, it remains unclear if species from such habitats, with their specific adaptations, harbour the same potential for phenological plasticity as species from less demanding habitats. METHODS: Fourteen congeneric species pairs originating from mid and high elevation were reciprocally transplanted to common gardens at 1050 and 2000 m a.s.l. that mimic prospective climates and natural field conditions. A drought treatment was implemented to assess the combined effects of temperature and precipitation changes on the onset and duration of reproductive phenophases. A phenotypic plasticity index was calculated to evaluate if mid- and high-elevation species harbour the same potential for plasticity in reproductive phenology. KEY RESULTS: Transplantations resulted in considerable shifts in reproductive phenology, with highly advanced initiation and shortened phenophases at the lower (and warmer) site for both mid- and high-elevation species. Drought stress amplified these responses and induced even further advances and shortening of phenophases, a response consistent with an 'escape strategy'. The observed phenological shifts were generally smaller in number of days for high-elevation species and resulted in a smaller phenotypic plasticity index, relative to their mid-elevation congeners. CONCLUSIONS: While mid- and high-elevation species seem to adequately shift their reproductive phenology to track ongoing climate changes, high-elevation species were less capable of doing so and appeared more genetically constrained to their specific adaptations to an extreme environment (i.e. a short, cold growing season).

Novel microsatellite markers for the high-alpine Geum reptans (Rosaceae).

PREMISE OF THE STUDY: Geum reptans reproduces by outcrossing or by the formation of stolons. Sexual and clonal reproduction are not exclusive and occur mostly simultaneously. We developed novel microsatellite primers for this species, which will be used in a study about local adaptation, phenotypic plasticity, and random molecular divergence of alpine plants. • METHODS AND RESULTS: Twelve microsatellite primer sets were developed for G. reptans, of which nine were polymorphic. Initially, the forward primers had an M13 tail, and the allelic signals of each locus were amplified using a single fluorescent-labeled M13 forward sequence. In the running phase, a multiplex PCR assay was developed using different fluorophore-labeled forward primers. Two to 11 alleles were found per locus, depending on the studied population. • CONCLUSIONS: Identical multilocus genotypes (i.e., clonal offspring) were not found because individuals in our sampling were at least 4 m distant from each other. F ST-Q ST analysis will be applied to detect selection processes in populations of G. reptans across the Alps.

Biomechanical responses of aquatic plants to aerial conditions.

BACKGROUND AND AIMS: Wetlands are impacted by changes in hydrological regimes that can lead to periods of low water levels. During these periods, aquatic plants experience a drastic change in the mechanical conditions that they encounter, from low gravitational and tensile hydrodynamic forces when exposed to flow under aquatic conditions, to high gravitational and bending forces under terrestrial conditions. The objective of this study was to test the capacity of aquatic plants to produce self-supporting growth forms when growing under aerial conditions by assessing their resistance to terrestrial mechanical conditions and the associated morpho-anatomical changes. METHODS: Plastic responses to aerial conditions were assessed by sampling Berula erecta, Hippuris vulgaris, Juncus articulatus, Lythrum salicaria, Mentha aquatica, Myosotis scorpioides, Nuphar lutea and Sparganium emersum under submerged and emergent conditions. The cross-sectional area and dry matter content (DMC) were measured in the plant organs that bear the mechanical forces, and their biomechanical properties in tension and bending were assessed. KEY RESULTS: All of the species except for two had significantly higher stiffness in bending and thus an increased resistance to terrestrial mechanical conditions when growing under emergent conditions. This response was determined either by an increased allocation to strengthening tissues and thus a higher DMC, or by an increased cross-sectional area. These morpho-anatomical changes also resulted in increased strength and stiffness in tension. CONCLUSIONS: The capacity of the studied species to colonize this fluctuating environment can be accounted for by a high degree of phenotypic plasticity in response to emersion. Further investigation is however needed to disentangle the finer mechanisms behind these responses (e.g. allometric relations, tissue make-up), their costs and adaptive value.

New microsatellite markers for Anthyllis vulneraria (Fabaceae), analyzed with Spreadex gel electrophoresis.

PREMISE OF THE STUDY: New microsatellite primers were developed for the diploid herb Anthyllis vulneraria. These primers will be used in upcoming studies focusing on random genetic variation, local adaptation, and phenotypic plasticity in alpine plants. • METHODS AND RESULTS: The new primers were adjusted to separate PCR amplicons (70 to 170 bp) on precast Spreadex gels using horizontal gel electrophoresis. No capillary sequencer was needed. Three to twelve alleles were found per locus depending on the population studied. • CONCLUSIONS: Our preliminary results showed that the three studied alpine populations are predominantly outcrossing, but include variable levels of self-fertilization.