Department of Biology
Adaptive divergence in two coexisting stickleback species
Author(s): Raeymaekers, JAM, Chaturvedi, A, Konijnendijk, N, Maes, G, Volckaert, F
Natural populations are at least partially adapted to local environmental conditions. The level of adaptation is promoted by divergent selection on ecologically relevant traits, but hampered by gene flow. The relative strength of both forces depends on genomic architecture as well as dispersal abilities, both of which might differ between species. In this study, we tested for divergent selection and gene flow in two coexisting and phylogenetically related species, the three- and nine-spined stickleback. Populations occur across very similar environmental clines in biotic and abiotic conditions (salinity, oxygen concentration, predation, parasites and prey), and were sampled within exactly the same spatial matrix. In each species we assessed gene flow between populations from contrasting environments and determined divergence in ecologically relevant traits (body shape, armour, spleen size, and trophic morphology). An analyses of genome-wide differentiation was used to identify the genomic basis of adaptation. Based on these data we compared the relative strength of divergent selection and gene flow in both species, as well as the biological functions of the genes under selection. We discuss how these relationships explain how both stickleback species maintain partially overlapping habitats.
School of Biosciences
Age related changes in fitness traits in a wild field cricket population
Author(s): Rodríguez-Muñoz, R, Tregenza, T
Evidence for senescence in nature has been found in a growing number of vertebrate species, particularly birds and mammals, and more recently in a few invertebrates. Insects have proved very suitable for the study of senescence in laboratory studies but their mobility and small size make them very difficult to follow in their natural environment. We have extended the study of insect ageing into the wild, by using up to 90 video cameras to continuously monitor a natural field cricket population. We tagged individuals immediately after they became adult and followed them until their death was either inferred or directly observed. We recorded mating and fighting success, singing activity and movement over the entire adult life of individual crickets for five consecutive generations. We examine changes in behaviour over the adult lifespan and relationships with reproductive success. This allows us to determine whether crickets senesce in our population in terms of both survival probability and changes in behavioural measures such as activity, and to ask questions about interactions between ageing and sexual selection. Our model system provides an excellent opportunity for the study of the selective pressures acting on life history and ageing in wild insects.
Clinal selection in Drosophila: what have we learnt, where are we going?
Author(s): Hoffmann, AA
Clines have traditionally been used to investigate patterns of selection on phenotypes and genotypes involved in climatic adaptation. More recently, clines are being used to test for changes in patterns of adaptation across time. Modern genomic and transcriptomic approaches are now being applied to understand patterns of clinal adaptation at a fine level and to allow new sets of questions to be considered and a new level of understanding to be reached. Here I focus on recent Drosophila work to overview the opportunities and challenges provided by these new approaches and how they can be integrated into the more traditional work. I also consider the ways in which experimental work in field and laboratory settings can be used to further understand patterns of selection.
Drawing a natural fitness landscape in space and time: temporal adaptation of soil bacterial communities
Author(s): Kraemer, SA, Kassen, R
Microbial communities are fundamental for ecosystem function, yet only little is known about their in situ ecology and evolution. How are such communities structured in space and time and which environmental factors drive their adaptation? We obtained soil bacterial isolates from three sites in a spatially structured design every month for eight months, representing a complete growth season. In a fully factorial transplant experiment we measured fitness of all isolates in media mirroring the environmental conditions in the soil sample they were isolated from (their “home” soil) and compared it to their growth rates in media representing soil at different spatial and temporal distances, thus not only describing the spatial fitness landscape for each isolate but also how the shape of this landscape changes over time. In comparison to growth under “home” soil conditions, growth rates were steadily declining in media representing future soil conditions, indicating temporal adaption of isolates. Moreover, fitness increased in media representing past conditions, providing evidence for past selection for successful growth. These findings were unaffected by limiting our analysis to isolates with vigorous growth rates or to soil obtained from different geographical sites. Spatial structuring, either at large (kilometer) or small (meter) scales, did not significantly influence bacterial fitness, indicating a large role of dispersal in soil bacterial biogeography at these scales. Lastly, we correlated environmental factors such as nutrient ion availability, mean temperature and pH with the obtained fitness landscapes to deduce key factors influencing bacterial temporal adaptation in nature.
Department of Aquatic Ecology
Evidence for cost of sex and parasite-mediated selection in a natural population of co-existing sexual and asexual snails
Author(s): Paczesniak, D, Neiman, M, Klappert, K, Kopp, K, Liljeroos, K, Lively, CM, Adolfsson, S, Jokela, J
One of the main hypotheses for why sexual reproduction is so common despite the cost of producing males proposes that negative frequency-dependence selection by co-evolving parasites provides an advantage for rare genotypes. However, this mechanism fails to predict advantage to sex when asexual assemblages are diverse.
The challenge to sex posed by a diverse set of asexual lineages depends on the extent to which they realize their theorized two-fold advantage. Here, we used field-based comparisons of the rate of population growth between obligate sexual vs. multiple lineages of obligate asexual Potamopyrgus antipodarum, a New Zealand freshwater snail. We also used a population genetics approach to evaluate whether the temporal changes of population genetic structure follow patterns expected under parasite-mediated selection: faster clonal turnover in locations where parasite pressure is higher.
The reproductive output of asexual lineages measured using experimental enclosures (cages anchored to the bottom of the lake) over a course of one year was as high as that of the best sexual families, which implies the cost of sex. We also found that the reproductive output of asexual lineages depends on the habitat, which implies that environmental heterogeneity may select for habitat-specific clonal assemblages.
The genetic structure of the asexual population changed significantly over a 4-year period (4-8 generations) in shallow and mid-water habitats (high parasite pressure), but not in the deep habitat (low parasite pressure).
Our results show that the fitness of many asexual lineages is high enough to impose the cost of sex and that clonal turnover is faster in high infection sites. While the latter result is consistent with the parasite hypothesis for the maintenance of sex, the high relative fitness of many asexual lineages suggests that other mechanisms are needed to explain the persistence of sex in the face of a diverse array of asexual competitors.
Department of Life Sciences
Evolutionary changes in the sexual reproductive system: the case study of the invasive tristylous Oxalis pes-caprae
Author(s): Castro, S, Ferrero, V, Costa, J, Castro, M, Navarro, L, Loureiro, J
The establishment and spread of invasive species depends on different evolutionary and ecological pressures, with reproduction being a key factor for success. Among other strategies, vegetative propagation has been correlated with invasion potential. Because clonality affects the spatial distribution of genets and its flowers, clonal species are expected to have increased rates of self-pollination. However, selfing might be advantageous as it enables to overcome the lack of compatible mates or scarcity of pollinators during invasion. Under this scenario, in self-incompatible species, selection may favor the breakdown of the incompatibility system, as plants with some levels of compatibility would have advantage in low density conditions and be able to establish new populations after dispersal (Baker’s law). Oxalis pes-caprae is a tristylous species with heteromorphic incompatibility; in most invaded areas, strong founder events lead to the introduction of the 5x short-styled morph only, leading to an exclusively asexual mode of reproduction. Yet, recent studies have shown scattered sexual reproduction and the emergence of new floral morphs in the western Mediterranean basin. Our objective was to evaluate the rates of self- and morph-incompatibility across this invaded area, and compare it with the native range, in order to assess changes in the reproductive system. Controlled hand-pollinations were made in invasive and native populations and plant reproductive success was quantified. The incompatibility system varied among invasive populations and floral morphs; overall a breakdown in several populations across the invaded range was observed, mostly through a partial breakdown in morph-incompatibility of S-morph, but also in self-incompatibility system of M- and L-morphs. Because reproductive strategies determine the demography and genetics of invasive populations, the results obtained are important to understand the evolutionary dynamics operating during invasion.
Department of Biology
Evolutionary potential of arrival date on breeding grounds: selection, heritability and microevolution
Author(s): Tarka, M, Hansson, B, Hasselquist, D
In migratory species, timing of arrival on the breeding grounds in spring is crucial for the reproductive success. Thus, in the face of a changing environment, it is important to be able to adjust the timing accordingly. However, little is known about the genetic background and hence evolutionary potential of arrival date. We have used a multi-level approach to investigate the evolutionary potential of arrival date combining data from a multigenerational pedigree and novel migration tracking techniques of a natural population of great reed warblers (Acrocephalus arundinaceus). We found that selection favours early arrival both in male and female great reed warblers, and that the trait is both repeatable and heritable. Further, arrival date in the population has advanced during the two decades of this study, a pattern that is in accordance with the response attributed to climate change reported in other migrant birds but also in accordance to the directional selection acting on the trait. Tracking the full migratory annual cycle of individual great reed warblers show that departure date from the wintering site determines arrival date and that spring migration is faster than autumn migration, corroborating the selection for earlier arrival. Our study is a first step towards dissecting the genetic and environmental factors that contribute to shape arrival date in long-distance migrant birds. Such analyses are essential if we want to understand how migratory species are able to cope with a rapidly changing environment.
Institute of Evolutionary Biology
Gene mapping reveals why sexually-selected genetic variation is maintained in a wild mammal population.
Author(s): Johnston, SE, Gratten, J, Berenos, C, Pilkington, JG, Clutton-Brock, TH, Pemberton, JM, Slate, J
Sexual selection is assumed to be a source of strong and sustained directional selection in the wild, and is expected to lead to a decrease in underlying genetic variation. However, there is often considerable genetic variation in sexually-selected traits in wild populations and consequently, this phenomenon has become a long-discussed paradox in the field of evolutionary biology. In wild Soay sheep, large horns confer an advantage in strong intra-sexual competition, yet both sexes have an inherited polymorphism for horn type and have substantial genetic variation in their horn size. We pinpointed the causal genetic mechanisms influencing horn development using genome-wide association studies (GWAS) and fine-scale SNP genotyping. This allowed us to test the most commonly proposed explanations for the maintenance of sexually-selected genetic variation, such as genic capture (“good genes”), sexually antagonistic selection, and trade-offs between sexual and non-sexual fitness (i.e. reproductive success vs. survival). I present the evidence for and against these mechanisms in the Soay sheep population, and discuss the applications of these findings to other wild populations.
Genetic and ecological consequences of individual generalization in plant-pollinator systems
Author(s): Valverde, J, Lorite, J, Corral, JM, Sharbel, T, Jiménez, N, Gómez, JM, Perfectti, F
Most theoretical approaches assume that, in pollination generalist systems, all individual plants interact with random subsets of the overall pollinator pool. In contrast, we think that the generalization degree of a given individual plant could be non random, but related with some intrinsic and extrinsic factors, such as its spatial location within the population, its microenvironment and its phenotype and genotype. Plants exhibiting different values for those factors would attract different subset of pollinators, and plants showing similar values of those traits would share similar subgroups of pollinators. Since pollinator species of a generalist plant vary in their foraging behaviour, per-visit effectiveness, and floral constancy, individual plants could receive a biased sample of pollen that could affect their reproductive success. To test this idea, we have thoroughly sampled a population of Erysimum mediohispanicum (Brassicaceae) in Sierra Nevada Mountains (Spain). This species is an extremely generalist plant, being pollinated by more than 180 insect species in the study area, ranging from 30-40 pollinator species per population. For every plant in the population, we determined the diversity and identity of insects visiting its flowers. In addition, we also quantified the micro-environment (irradiance, soil water content, top soil nutrients), the pollination-related phenotypic traits (flower number, flowering stalk height, stigma exertion, and corolla size, color, and shape), and the multilocus genotype (using cpDNA haplotypes and nuclear microsatellites) of each plant. Finally, to estimate individual fitness, we quantified the number and genetic diversity of the seeds produced per plant. Our goal in this study is to evaluate how the individual interaction with subsets of pollinators may affect the quantity and quality of plants' progeny.
Department of Zoology
How social structure affects gene flow in a wild passerine population
Author(s): Radersma, R, Garroway, CJ, Santure, AW, De Cauwer, I, Slate, J, Sheldon, BC
Gene flow is strongly affected by the spatial distribution of individuals, the variability of the physical environment and social processes such as dispersal, resource competition and territoriality. Here we investigate the contributions of those forces to gene flow in foraging flocks of great tits (Parus major) for three consecutive winters. We used a total of 85602 visits of flocks to 60 feeding tables which recorded the identity of 1711 birds by radio frequency identification technology. Of those birds 962 were genotyped based on 4701 autosomal single-nucleotide polymorphisms (SNPs). For 87% of the visits we were able to genotype at least one individual. We used asymmetric eigenvector maps (AEMs) to partition the contributions of space and social structure to the allele frequencies of all 4701 SNPS in the feeding flocks while taking the previous positions of individuals into account. We were able to explain 58% of the variance in allele frequencies with AEMs. This study shows that space and social structure have a substantial effect on the distribution of alleles over subpopulations and therefore on gene flow. To our knowledge this is the first study to investigate the effect of social structure and space to gene flow at such a fine scale while accounting for previous location of the individuals. Therefore it contributes to the understanding of how social behaviour affects evolution. Next we will extract Moran’s eigenvector maps (MEMs) for the spatial locations of feeding tables and use partial redundancy analysis (AEMs as explanatory variables while controlling for MEMs) to investigate how social structure affects allele frequencies, while controlling for space. We will use variance partitioning to quantify the relative contributions of space and social structure, determine which alleles have large effects on the AEMs and check whether those alleles are in linkage disequilibrium with candidate genes or are known to correlate to environmental variables.