The Climatic Variability Hypothesis and trade-offs in thermal performance in coastal and inland populations of Mimulus guttatus.

Ecologists and evolutionary biologists have long predicted that organisms in more climatically variable environments should be adapted to handle a wider range of conditions. This intuitive idea, known as the Climatic Variability Hypothesis (CVH), has gained mixed support from empirical studies. We tested the CVH in a novel system by comparing the thermal breadth of coastal and inland populations of Mimulus guttatus. To quantify thermal breadth, we performed a thermal performance experiment and built performance curves. Using these performance curves, we also evaluated evidence for a breadth-performance trade-off and the Hotter-is-Better hypothesis. We did not find support for the CVH; coastal and inland populations did not differ in thermal breadth. However, we found evidence for a breadth-performance trade-off and the Hotter-is-Better hypothesis. Surprisingly, the two most inland populations differed the most in the thermal performance traits we evaluated. Our results highlight the importance of explicitly measuring thermal performance to test explanations of species distribution patterns and the need to examine alternative mechanisms by which organisms occupy different climatic regimes.

Evolution of the ability to modulate host chemokine networks via gene duplication in human cytomegalovirus (HCMV).

Human cytomegalovirus (HCMV) is a widespread pathogen that is particularly skillful at evading immune detection and defense mechanisms, largely due to extensive co-evolution with its host. One aspect of this co-evolution involves the acquisition of virally encoded G protein-coupled receptors (GPCRs) with homology to the chemokine receptor family. GPCRs are the largest family of cell surface proteins, found in organisms from yeast to humans, and they regulate a variety of cellular processes including development, sensory perception, and immune cell trafficking. The US27 and US28 genes are encoded by human and primate CMVs, but homologs are not found in the genomes of viruses infecting rodents or other species. Phylogenetic analysis was used to investigate the US27 and US28 genes, which are adjacent in the unique short (US) region of the HCMV genome, and their relationship to one another and to human chemokine receptor genes. The results indicate that both US27 and US28 share the same common ancestor with human chemokine receptor CX3CR1, suggesting that a single host gene was captured and a subsequent viral gene duplication event occurred. The US28 gene product (pUS28) has maintained the function of the ancestral gene and has the ability to bind and signal in response to CX3CL1/fractalkine, the natural ligand for CX3CR1. In contrast, pUS27 does not bind to any known chemokine ligand, and the sequence has diverged significantly, highlighted by the fact that pUS27 currently exhibits greater sequence similarity to human CCR1. While the evolutionary advantage of the gene duplication and neofunctionalization event remains unclear, the US27 and US28 genes are highly conserved among different HCMV strains and retained even in laboratory strains that have lost many virulence genes, suggesting that US27 and US28 have each evolved distinct, important functions during virus infection.

Genetics of water use physiology in locally adapted Arabidopsis thaliana.

Identifying the genetic basis of adaptation to climate has long been a goal in evolutionary biology and has applications in agriculture. Adaptation to drought represents one important aspect of local adaptation, and drought is the major factor limiting agricultural yield. We examined local adaptation between Sweden and Italy Arabidopsis thaliana ecotypes, which show contrasting levels of water availability in their local environments. To identify quantitative trait loci (QTL) controlling water use physiology traits and adaptive trait QTL (genomic regions where trait QTL and fitness QTL colocalize), we performed QTL mapping on 374F9 recombinant inbred lines in well-watered and terminal drought conditions. We found 72 QTL (32 in well-watered, 31 in drought, 9 for plasticity) across five water use physiology traits: δ(13)C, rosette area, dry rosette weight, leaf water content and percent leaf nitrogen. Some of these genomic regions colocalize with fitness QTL and with other physiology QTL in defined hotspots. In addition, we found evidence of both constitutive and inducible water use physiology QTL. Finally, we identified highly divergent candidate genes, in silico. Our results suggest that many genes with minor effects may influence adaptation through water use physiology and that pleiotropic water use physiology QTL have fitness consequences.

Fine-scale niche structure of Neotropical forests reflects a legacy of the Great American Biotic Interchange.

The tendency of species to retain their ancestral niches may link processes that determine community assembly with biogeographic histories that span geological time scales. Biogeographic history is likely to have had a particularly strong impact on Neotropical forests because of the influence of the Great American Biotic Interchange, which followed emergence of a land connection between North and South America ~3 Ma. Here we examine the community structure, ancestral niches and ancestral distributions of the related, hyperdiverse woody plant genera Psychotria and Palicourea (Rubiaceae) in Panama. We find that 49% of the variation in hydraulic traits, a strong determinant of community structure, is explained by species' origins in climatically distinct biogeographic regions. Niche evolution models for a regional sample of 152 species indicate that ancestral climatic niches are associated with species' habitat distributions, and hence local community structure and composition, even millions of years after dispersal into new geographic regions.

Quantifying the impact of gene flow on phenotype-environment mismatch: a demonstration with the scarlet monkeyflower Mimulus cardinalis.

Geographic range margins offer testing grounds for limits to adaptation. If range limits are concordant with niche limits, range expansions require the evolution of new phenotypes that can maintain populations beyond current range margins. However, many species' range margins appear static over time, suggesting limits on the ability of marginal populations to evolve appropriate phenotypes. A potential explanation is the swamping gene flow hypothesis, which posits that asymmetrical gene flow from large, well-adapted central populations prevents marginal populations from locally adapting. We present an empirical framework for combining gene flow, environment, and fitness-related phenotypes to infer the potential for maladaptation, and we demonstrate its application using the scarlet monkeyflower Mimulus cardinalis. We grew individuals sampled from populations on a latitudinal transect under varied temperatures and determined the phenotypic deviation (PD), the mismatch between phenotype and local environment. We inferred gene flow among populations and predicted that populations receiving the most temperature- or latitude-weighted immigration would show the greatest PD and that these populations were likely marginal. We found asymmetrical gene flow from central to marginal populations. Populations with more latitude-weighted immigration had significantly greater PD but were not necessarily marginal. Gene flow may limit local adaptation in this species, but swamping gene flow is unlikely to explain its northern range limit.

Incorporating population-level variation in thermal performance into predictions of geographic range shifts.

Determining how species' geographic ranges are governed by current climates and how they will respond to rapid climatic change poses a major biological challenge. Geographic ranges are often spatially fragmented and composed of genetically differentiated populations that are locally adapted to different thermal regimes. Tradeoffs between different aspects of thermal performance, such as between tolerance to high temperature and tolerance to low temperature or between maximal performance and breadth of performance, suggest that the performance of a given population will be a subset of that of the species. Therefore, species-level projections of distribution might overestimate the species' ability to persist at any given location. However, current approaches to modeling distributions often do not consider variation among populations. Here, we estimated genetically-based differences in thermal performance curves for growth among 12 populations of the scarlet monkeyflower, Mimulus cardinalis, a perennial herb of western North America. We inferred the maximum relative growth rate (RGR(max)), temperature optimum (T(opt)), and temperature breadth (T(breadth)) for each population. We used these data to test for tradeoffs in thermal performance, generate mechanistic population-level projections of distribution under current and future climates, and examine how variation in aspects of thermal performance influences forecasts of range shifts. Populations differed significantly in RGR(max) and had variable, but overlapping, estimates of T(opt) and T(breadth). T(opt) declined with latitude and increased with temperature of origin, consistent with tradeoffs between performances at low temperatures versus those at high temperatures. Further, T(breadth) was negatively related to RGR(max), as expected for a specialist-generalist tradeoff. Parameters of the thermal performance curve influenced properties of projected distributions. For both current and future climates, T(opt) was negatively related to latitudinal position, while T(breadth) was positively related to projected range size. The magnitude and direction of range shifts also varied with T(opt) and T(breadth), but sometimes in unexpected ways. For example, the fraction of habitat remaining suitable increased with T(opt) but decreased with T(breadth). Northern limits of all populations were projected to shift north, but the magnitude of shift decreased with T(opt) and increased with T(breadth). Median latitude was projected to shift north for populations with high T(breadth) and low T(opt), but south for populations with low T(breadth) and high T(opt). Distributions inferred by integrating population-level projections did not differ from a species-level projection that ignored variation among populations. However, the species-level approach masked the potential array of divergent responses by populations that might lead to genotypic sorting within the species' range. Thermal performance tradeoffs among populations within the species' range had important, but sometimes counterintuitive, effects on projected responses to climatic change.

Evolutionary time for dispersal limits the extent but not the occupancy of species' potential ranges in the tropical plant genus Psychotria (Rubiaceae).

Explaining the diversity in geographic range sizes among species is a central goal of ecological and evolutionary studies. We tested species age as an explanation of range size variation within a group of understory shrubs in the Neotropics (Psychotria subgenus Psychotria, Rubiaceae). We distinguish between range occupancy (filling an occupied area) and range extent (maximum distances dispersed). We used Bayesian relaxed-clock dating of molecular sequence data to estimate the relative age of species, and we used species distribution modeling to predict species' potential ranges. If the range sizes of species are limited by time for dispersal, we hypothesize that older species should have (1) larger realized range occupancies and realized range extents than younger species, (2) filled a greater proportion of their potential range occupancies, and (3) colonized a greater proportion of their potential range extents. We found (1) a significant but weak positive relationship between species age versus both realized range occupancy and realized range extent, (2) no relationship between species age and filling of potential range occupancies, but (3) that older species had colonized a significantly greater proportion of their potential range extents than younger species. Our results indicate that a time-for-dispersal effect can limit the extent of ranges of species but not necessarily their occupancies.

Epidemiologia de la poliomielitis

Publicado en ingles en World Health Monograph Series 26:9