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Prof. Dr.
Yvonne Willi
Department of Environmental Sciences
Profiles & Affiliations
Projects & Collaborations
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Causes of decline in rare plants of calcareous grasslands and recommendations for their management
Research Project  | 2 Project Members

Calcareous grasslands are among the most species-rich habitats of Europe. Their occurrence has declined since the beginning of the green revolution as many of them were transformed into intensively farmed meadows or fields. The region of northwestern Switzerland has still a remarkable number of remnants, though census data collected since the 1950s document the decline or local extinction in a number of flowering plants in them, especially in the more fragmented remnants. This trend suggests that despite the currently high protection status calcareous grasslands have and management efforts to maintain species diversity, many plants do not benefit from these conservation measures.

Overall objectives. The goal of the study is to assess the change in the vegetation of the remnants over time and understand the important drivers, including eutrophication, climate change, and management schemes. Furthermore, the project will assess why some outcrossing, insect-pollinated plants have continued declining. For some of them, the project will evaluate the contribution of mate limitation versus pollinator limitation. Outcrossing in plants is often determined by the genetic factor of self-incompatibility that leads to the avoidance both of self-pollination and pollination between close relatives. If plants of a species are rare, there may not be enough unrelated compatible mates. Another potential problem may be that terrestrial insects, many of which are important pollinators, have declined in the past decades, and those that remain may not provide sufficient pollination services.

Specific aims. The proposed research will focus on over 50 grassland remnants in the greater region of Basel that have been surveyed intensively over the last 70 years. The specific aims are:

  (1) To add to the time-serious of surveying the 50 remnants and analyse the change in plant composition in more detail. So far, the time serious includes 1950, 1985, 1996 and 2016. By repeating the census in 2024, we will be able to assess change in plant composition, link it to eutrophication and climate change, and pinpoint the management efforts that have had a positive effect on rare specialist plants.

  (2) Based on 5 selected plant species that have declined strongly over the past 70 years, we will assess the contribution of mate and pollinator limitation. We will test whether reproductive success is lower because of incompatible mates and/or a low abundance of effective pollinators.

Impact. Taken as a whole, this research should pinpoint the causes of the decline of rare plants in protected calcareous grasslands and provide guidelines for managing them adequately, to assure the promotion and long-term survival of plant diversity specialized to this type of habitat.

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Evolutionary constraints to the climate niche of species
Research Project  | 3 Project Members

Limits of species ranges are often associated with particular climatic conditions or isotherms. Further evidence that climate is indeed a main driver of species’ distributions is that many species have undergone range shifts towards higher latitudes or elevations under recent global warming. However, there are also species that have gained little terrain at the cool end of distributions and retracted strongly from the warm end. As evolutionary biologists we need to answer the pressing question: What constrains the evolution of the climate niche at distribution boundaries? The short answer is: We do not know.

Overall objectives. I argue that the breakthrough will come from connecting predictions of recent eco-evolutionary models with the genetic architecture of range-limiting traits. This novel framework will be used to test the suggested environmental, demographic and genetic determinants of elevational range limits within a set of plants in the Swiss Alps. My focus on elevational distribution limits is motivated by the fact that temperature is linked with both elevational and geographic range limits in many species worldwide, but elevational gradients are easier to study than latitudinal gradients because they are short.

Specific aims. The proposed research will assess the potential determinants of range limits in 6 plant species with different elevational ranges. Assessments will focus on two mountain areas extending from 600 m to 3000 m over a few km. The first 3 studies are aimed at parameter estimation:

(1) Fine-scale distribution modelling will reveal the range-limiting climatic variables and how rapidly they change along the gradient.

(2) Sequencing of individuals will allow the estimation of load due to dispersal, drift-effective population size, and signatures of past climate adaptation.

(3) Crossing experiments will be performed to (a) calculate the genetic architecture of traits involved in coping with the most limiting aspects of climate, and (b) estimate genotypic selection on these traits at four elevations.

In study 4, we will verify the importance of parameters estimated in studies 1-3 using spatially explicit eco-evolutionary simulations, comparing predicted with observed distributions.

Impact. Taken as a whole, this research should pinpoint the causes of distribution limits and resolve the evolutionary enigma of constraints to the climate niche.

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Genetic constraints at species' range limits
Research Project  | 2 Project Members
Species commonly have fairly distinct limits to spatial occurrences. The evolutionary causes of these range limits may be manifold. Often, small population size, genetic drift opposing selection or genetic constraints reducing evolvability may be involved. Previous macroevolutionary work on elevational range limits in Brassicaceae detected the importance of trade-offs involving the speed of growth under heat, acquisition capacity (SLA) and plant size to be important (Maccagni & Willi 2022). The current research builds upon these results. We investigate whether the same type of trade-offs exist on a microevolutionary scale, within species. In 6 species of Brassicaceae, we compare the genetic architecture of traits across populations of the elevational gradient. Furthermore, we assess the role of dispersal on range limits by using population genomics tools.
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Understanding the genotype-phenotype-fitness maps at species' range limits
Research Project  | 3 Project Members

The causes of geographic range limits are unresolved. What we know so far is that roughly half of current range limits are due to dispersal limitation while the other half are due to constraints on adaptation to environmental conditions at range edges. Simple evolutionary principles suggest that limits to adaptation must stem from either the shape of environmental gradients imposing selection or the genetics of traits on which selection acts. Hence, natural selection is the link between the two types of limitation. This is the focus of this research proposal; the goal is to study the selection regime at range edges, the shape of change in the selection regime from within the range to beyond the range edge, and the targets of selection: the traits, pathways and genes involved. Hypotheses: Theory is not specific about the shape of environmental change causing range limits, but all models agree that steep changes in environmental gradients can attract range limits. The main hypothesis to be tested is that steep environmental gradients, known from niche modelling, translate into selection intensity on the genome and on traits at the current range limit. For genetic limits to adaptation, theory predicts that genetic variation for key traits is low throughout the distribution or that multiple traits are strongly correlated in specific ways. Therefore, the hypotheses for genetic limitations are that selection at the edge acts on traits with generally low genetic variation or that multivariate selection is antagonistic to the genetic correlation matrix. Procedure: I propose a large scale selection experiment and assessment of phenotype, genotype and fitness at four range edges of the North American Arabidopsis lyrata. At all edges, three sites on a transect will be included - one within the range but close to the edge, one at the edge and one beyond the edge. Previous work shows that these edges are characterized by steep climatic gradients. The plant material will stem from one large outcrossing population from the current center of distribution, which will first be propagated in the greenhouse for one generation to produce seeds of 300 unrelated families. I will profile the genotype of each family by whole-genome sequencing of both parental plants, revealing family allele frequencies for each single-nucleotide polymorphism (SNP) site. In the greenhouse (N = 300 seed families . 5 stress treatments . 6 plants) and at each selection site (N = 12 sites . 300 seed families . 8 tubs), we will collect data on growth trajectory, phenology, survival and reproductive output. We will (1) estimate G-matrices of growth and phenology under different niche-relevant stress factors (greenhouse study and outdoors), (2) associate tolerance traits assessed in the greenhouse and performance outdoors with SNPs - producing genotype-phenotype-fitness maps, (3) link selection intensities with environmental conditions outdoors and (4) study antagonistic genetic interactions in traits and in selection. Importance: Although natural selection is a key process in all areas of evolutionary ecology, few studies have estimated how selection varies along environmental gradients. Changes in selection at range limits have never been investigated in nature. This project will address these major gaps. Important answers can be expected in regard to the relation between environmental gradients and selection, the relative importance of selection versus genetic constraints in adaptive evolution, and why adaptation to environmental change can fail.

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Evolutionary dynamics of drift load and its role in species distribution limits
Research Project  | 4 Project Members
Why species have geographically restricted distributions is an important unresolved question in biology. Two evolutionary hypotheses about causes of distribution limits have so far attracted little attention in empirical research. The first is how relevant ecological factors change at distribution edges. Theory predicts that if conditions change steeply, populations are unable to adapt. The second hypothesis is that populations at edges of distribution are small and isolated and therefore accumulate deleterious mutations, which lowers their growth rate. The study organism is Arabidopsis lyrata , which has both northern and southern distribution limits that are set by environmental conditions. Steepness of environmental clines will be assessed in a niche-modeling context. Drift load will be assessed based on SNP variation in coding regions, and tested for a geographic pattern of increased load at the distribution margin. Transplant experiments will be set up to assess the impact of load on growth rate.
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The genetic basis of evolutionary constraints
Research Project  | 3 Project Members
Background: A major question in evolutionary biology and ecology is why species have spatially and ecologically restricted distributions. Why do species not evolve the ability to occur everywhere? In this proposal, I argue that the most plausible answer is that the capacity to tolerate or resist stress can evolve only at the cost of reduced fitness or enhanced susceptibility to other kinds of stress. These trade-offs arise because the fitness and tolerance/resistance traits have a common genetic basis; this prevents selection from breaking the trade-off, and results in limits to distributions. This project will be the first to describe the genomic basis of key genetic correlations that define the trade-offs involved in distribution limits. Hypothesis: The fact that the southern and northern edges of plant distributions often follow climate isoclines suggests that climate factors are an important determinant of distributions. I hypothesize that tolerance or physiological resistance to abiotic stress constrains species distributions because of genetic trade-offs among tolerance/resistance traits - or between them and fitness - caused by pleiotropic effects of genes or physical linkage. This implies that such genetic correlations are robust across populations and environmental conditions. Our work will focus on Arabidopsis lyrata, a plant for which the full range of genomic tools is available. Procedure: We currently have seed and maternal DNA from 50+ populations of A. lyrata covering most of the species distribution in North America (14° latitude, from North Carolina to Canada). A first step is to link single nucleotide polymorphisms with climate conditions across all populations, using RAD-tag sequencing. This will detect the functional genes associated with climate variation, suggest traits undergoing adaptation, and highlight genetic correlations (trade-offs) that potentially constrain the distribution. Next, an association study will reveal the genes and genomic regions linked to traits identified by the first step, and especially those responsible for traits involved in trade-offs. I expect that genetic correlations will be determined by pleiotropy and tight linkage with suppressed recombination. Importance: This study will integrate phenotypic and physiological approaches to abiotic stress resistance with genomic analysis of resistance traits and their genetic correlations. The results will address fundamental questions in ecology (What determines species distributions?), in evolutionary biology (How do trade-offs limit evolution?), in evo-devo (Are there general constraints on development?), and in physiology (Which resistance traits are independent?). Plant and animal breeders are also keenly interested in trade-offs, how they function, how they can be overcome, and at what cost.