Cell and Molecular Biology
A single molecule assay to study the fitness effects of leaky gene expression in individual cells
Author(s): Baltekin, Ö, Leroy, P, Elf, J
Most of the adaptive substitutions that are revealed in evolution experiments are in regulatory sequences. In many cases, the expression levels are under directional selection. The lac operon in Escherichia coli is a good model system to investigate the fitness effects of the different regulation precisions (tight or leaky regulation) under different conditions. LacI, the repressor of the lac operon, can bind to three different operators with different binding strengths (O1>O2>O3). Yet, LacI can bind even stronger to the artificial operator sequence, Osym. Thus, the evolutionary optimal case may not necessarily be the tightest regulation of the lac operon. Our hypothesis is that the individuals with lac operon leakage will have a fitness disadvantage due to the cost of the unnecessary expression of lac operon when there is no lactose to utilize, but if lactose suddenly becomes available those individuals will benefit from previous leakage and have a higher fitness than the individuals with no leakage. Bulk growth assays can only assess fitness differences that are due to differences in genotype. Unless one looks at the single cell level it is not possible to reveal the contribution of stochastic aspects of the phenotype to observed fitness differences between isogenic populations. We do this by correlating leakage events in the lac operon of an individual E. coli in lactose free medium, with its fitness before and after the switch to lactose medium. The leakage events are quantified using single molecule fluorescence microscopy where we can count the number of lactose permease molecules per cell (Choi 2008). The leakage in expression is monitored in cells growing in a microfluidic turbidostat (Ullman 2013). Here, we can follow growth of thousands of individual cells with high time resolution and switch between different growth media. This enables us to relate minute differences in fitness to the underlying stochastic differences in gene expression.
Ageing as a trade-off mechanism: what telomeres actually tell us?
Author(s): Criscuolo, F, Reichert, S, Stier, A, Zahn, S, Massemin, S
Telomeres are chromosome ends that protect the cell’s genomic integrity. Telomeres are lost at a variable rate as a result of the imbalance between pro- (oxidative stress) and anti-erosion processes (telomerase), and once critically shortened, telomeres can trigger cell death (apoptosis). Consequently telomere function has been one of the main cell mechanisms studied in ageing biology. Recently, it has been proposed that telomeres might be critical for our understanding of life-history trade-offs in general. Telomere length is heritable and at the organism level telomere length has been shown to correlate with individual longevity, first in humans but also in wild animals, appearing as a strong proxy of individual survival. Furthermore, telomere length was recently proposed to reflect individual phenotypic quality in terms of high rates of reproduction. All these studies suggest that telomere dynamics may be one of the key genetic determinants explaining the individual variability in ageing and fitness. However, the nature of the determinants leading to changes in telomere length early in life, during development, and of the mechanisms that link telomere length to fitness remain unknown. In this talk, I will present some of the data collected by our group which is studying how telomere length may be implicated in life-history trade-offs. For this I will show how an experimental manipulation of the cost of reproduction in zebra finch may affect telomere loss of breeders, but also how this brood size manipulation has affected telomere length in chicks. In a second step, I will show how telomere lengths are varying in early life in two wild species, great and coal tits, and how these variations may be link to local adaptation to favour chick growth and survival. Finally, as a short perspective, results of a pilot study on telomerase activity will propose a hypothetical pathway by which telomere biology may influence individual fitness.
Department of Biology
Big houses, big cars, superguppies and the costs of producing sperm
Author(s): Di Nisio, A, Pilastro, AA
Trade-offs between fitness related traits are expected to generate negative genetic covariances, unless genetic variance for resource acquisition exceeds that for resource allocation. For what concerns post-copulatory sexual selection, producing competitive ejaculates is costly, however, and it can be attained at the expenses of traits involved in mate acquisition and/or survival, because resources are limited. Alternatively, under a good‐sperm scenario, males that are better equipped for winning sperm competition should also show higher survival and, more generally, perform better than poor sperm competitors. In the guppy (Poecilia reticulata) the number of sperm inseminated is the most important determinant of male fertilization success, therefore to evaluate the evolutionary consequences of male investment in sperm production we performed a bidirectional artificial selection experiment for sperm production in guppies. Contrary to expectation, males selected for high sperm production not only transferred more sperm per copulation, but also grew faster, attained a larger body size, were more colourful, sexually more active and more attractive to females. Sperm velocity and morphology was not affected, but sperm viability increased with sperm number. All the other measured male and female fitness-related traits were unaffected by sperm production. These results suggest that, at least in lab conditions, some guppy genotypes perform better in most life-history traits, acquiring more resources than others.
Department of Ecology and Evolutionary Biology
Comparing phenotypic, quantitative genetic, and genomic approaches to measuring tradeoffs in a contemporary human population
Author(s): Stearns, SC
In women born before 1940 in Framingham, Massachusetts, there was a significant negative correlation between number of children ever born and lifespan. Each additional child was associated with a reduction of about one year of life. Analysis of the 1500+ pedigrees containing 15,000+ people with an animal model that partially controlled for cultural and environmental effects yielded a large, significant, negative genetic correlation between children ever born and lifespan. A genome wide association study that looked for genes that alter the slope of the relationship between those two traits discovered several genes, some of which have previously been identified as involved in cancer. One, EOMES, has the sort of function one would expect of a gene that influences a tradeoff: its product affects many different processes. Statistical models with and without education as a covariate yielded results consistent with the idea that education is a cultural mimic of antagonistic pleiotropy: increases in level of education are associated with fewer children and longer life.
Department of Biology
Comparison of oxidative stress related enzyme activities in two Atlantic salmon (Salmo salar L) populations: Insight to the trade-offs between physiology and survival during early life stages
Author(s): Kahar, S, Vuori, KA, Vähä, J, Vasemägi, A
Stressful conditions experienced by individuals during their early development might have long-term consequences on various fitness-related traits and ultimately their survival. Moreover, oxidative stress (OS) has been identified as a significant physiological constraint affecting trade-offs between reproduction, growth and survival. Several studies have also suggested the level of OS to be heritable, yet very little is known about the relationships between OS, fitness and survival of natural populations in wild habitats. Here, we measured the activity of three OS related enzymes: glutathione reductase (GR), glutathione-S-transferase (GST) and superoxide dismutase (SOD) during early life stages in two Atlantic salmon populations (in total 52 half-sib families) and evaluated the importance of OS related enzyme activities to survival and growth of juvenile fish in multiple natural environments. We identified significant differences in measured enzyme activities, growth rates and survival both within and between populations. In addition, we found that significant heritability exists for GR in Narva (h2=0,26; 95%CI 0,09-0,55) and GST activities in Kunda (h2=0,28; 95%CI 0,13-0,61) and in Narva (h2=0,75; 95%CI 0,51-0,91) but not for SOD. We conclude that the inter-population differences in OS during early life stages may explain significant proportion of variation in fitness and survival in wild.
Crab osmoregulation and habitat diversification: does salinity drive physiological evolution?
Author(s): Faria, S, Thurman, C, McNamara, J
The colonization of fresh water and dry land reflects physiological transformations that govern water and ion movements between the body fluids and surrounding medium, axiomatically linking osmoregulatory physiology and habitat diversification. Since monophyletic groups store biological information retrievable by adding a phylogenetic component to comparative studies, we examine the evolutionary history of osmoregulation in fiddler crabs (Brachyura, Uca). For each osmoregulatory trait examined, we evaluate: (i) phylogenetic pattern, expressed by the relationship between trait variation and phylogeny, employing an autocorrelation analysis using Moran's I coefficient; (ii) the best evolutionary fitting-model, testing Brownian motion (pure random evolution) and Ornstein-Uhlenbeck (random evolution under stabilizing selection) processes; (iii) the ancestral states, to suggest the transformational series, by squared-change parsimony analyses; and (iv) we test the hypothesis of osmoregulatory evolution associated with salinity using phylogenetic regressions. The traits disclosed suggest a directional or piecewise pattern of change over time, revealing strong phylogenetic structuring: for the first pattern, the significant and positive phylogenetic signal among closely related species, particularly at the species level, falls off with phylogenetic distance; for the second pattern, there is a linear change between significant positive and negative phylogenetic signal with phylogenetic distance, showing strong physiological plasticity. The evolution of most osmoregulatory traits follows the Ornstein-Uhlenbeck model: there is significant, strong stabilizing selection that constrains the osmoregulatory history, forcing each trait toward a central value of variation. At more inclusive levels, osmoregulatory ability and salinity are associated, demonstrating a role for both salinity and shared inheritance in driving the evolution of osmoregulatory physiology in fiddler crabs.
Institute for Biodiversity and Ecosystem Dynamics
Discovering life-history trade-offs with suppression of tomato defence in the invasive spider mite Tetranychus evansi
Author(s): Knegt, B, Egas, M, Sabelis, MW
The herbivorous spider mite Tetranychus evansi is an invasive species specialized to feed on Solanaceous plants, including tomato. In its native range (South America) it is not considered as a pest, but in Africa and Southern Europe major crop losses have been recorded due to T. evansi outbreaks. Apart from predator release, two important factors appear to facilitate the spread of this species: (1) its ability to suppress tomato plant defense to below control levels, and (2) its very high population growth rate. Related species, such as the generalist two-spotted spider mite T. urticae, also have a high population growth rate, but cannot suppress tomato defence to the same extent as T. evansi.
Plants protect themselves in various ways against herbivores by producing toxins or attracting natural enemies. Therefore, suppressing these defences can benefit herbivores, and it has been shown that some herbivores indeed do so. However, T. evansi downregulates the defence of its host plant to levels that fall even below house-keeping levels of healthy unattacked plants. Downregulating host plant defence to below house-keeping levels is beneficial for herbivores, but – perhaps surprisingly - not observed before in nature. Given that herbivores do suppress plant defence but only to a limited extent, we hypothesize that suppression of host plant defense trades off with life history traits such as oviposition rate or making dense web to defend a suppressed-defence part of the host plant against competitors.
We are currently establishing T. evansi lines that differ in their ability to suppress tomato defense through crossing mites from different geographical locations and subsequent inbreeding, and through experimental evolution. Measurements of life-history characteristics can provide insight into genetic trade-offs for tomato defense suppression. In addition, competition experiments allow us to assess the relative fitness of different T. evansi strategies.
Does increased stress resistance reduce the ability to deal with biotic challenges? A test through selection experiments on Drosophila melanogaster
Author(s): Hangartner, SB, Hoffmann, AA
Humans impact ecosystems in a multitude of ways, increasingly exposing contemporary organisms to abiotic and biotic stressors. Environmental stress has strong negative impacts on biological diversity, as species can go locally extinct, if they are unable to migrate to a more benign habitat or to overcome the stressor via plastic or evolutionary adaptation. Several factors are thought to constrain adaptive evolution, such as gene flow, lack of genetic variation and genetic or functional trade-offs. Despite trade-offs being postulated as playing a central role in evolutionary theory, interactions between abiotic and biotic stress resistances have rarely been investigated in stress adaptation. This project aims to test for trade-offs between abiotic and biotic stress resistances in the well-established Drosophila melanogaster study system. Selection experiments on stress resistance provides an opportunity to study evolutionary constraints resulting from genetic trade-offs between traits. Lines that have been selected for different abiotic stressors (heat, cold and desiccation) will be tested for costs and benefits, whereas performance will be tested in outdoor cages under different biotic stressors (competition, predation, parasites) and under different climatic conditions (hot, moderate and cold days). These experiments provide a test of whether trade-offs between abiotic and biotic stress resistances are potential constraints to stress adaptation, which is crucial to better understand the evolutionary potential of contemporary populations.
Institute for Zoology
Evolution of food preferences drives pleiotropic fitness trade-offs in bacteria
Author(s): Jousset, A, Fernkorn, F, Lapouge, K, Stefan, S
Bacteria face trade-offs between life history strategies, among which the choice of a generalist or specialist resource use strategy has important effects on fitness. Bacteria bypass this problem by prioritizing nutrient utilisation. They highly specialise on preferred nutrients when available, and invest in a generalist way of life when only second-choice nutrients are left. Here we show that bacteria rapidly evolve new food preferences as an adaptation to a new substrate. We follow the expression of the small regulatory RNA crcZ in Pseudomonas fluorescens which is expressed in absence of preferred resources. It activates the translation of genes involved in alternative resource use. We show that the substrates inhibiting crcZ levels rapidly change when bacteria are confronted with second-choice subtrates, indicating that bacteria evolve new preferences. We further show that these altered food preferences affect the use of complex substrates by bacteria as well as their coexistence with competitors. Evolution of food preferences by bacteria appear therefore as efficient strategy to adapt to new conditions by readjusting the regulation of genes linked to substrate uptake and catabolism.
Department of Animal Ecology
Evolution of male reproductive ageing under differential risk of death: the role of condition-dependent mortality
Author(s): Spagopoulou, F, Chen, H, Maklakov, A
Ageing is observed in most organisms and is an important aspect of their life histories. However, how ageing evolves is not fully understood. The classic evolutionary theory of ageing posits that high extrinsic mortality leads to evolution of rapid ageing and therefore shorter intrinsic lifespan. On the other hand, according to a novel theory, when mortality is condition-dependent and surviving cohort is not a random sample of the starting population, which is likely to be the case in nature, the classic prediction does not hold and increased mortality can result in postponed onset of ageing. To increase our understanding of how different types of extrinsic mortality may influence the evolutionary trajectories of ageing, we have previously conducted an experimental evolution study, using the nematode Caenorhabditis remanei as a model system. This study allowed us to disentangle the effects of mortality rate (High and Low) and mortality source (Condition-dependent and Random) on the evolution of intrinsic lifespan. We observed reduced longevity under high random mortality, in accordance with the classic prediction. However, when mortality was condition-dependent, high mortality resulted in the evolution of increased lifespan supporting the novel theory and highlighting the importance of mortality source. Surprisingly though we found that there was no trade-off between longer lifespan and reproduction in females. In the present study we examined the possible existence of such trade-off in males. We found that males evolving under condition-dependent mortality had lower reproductive performance, suggesting a trade-off between stress resistance and reproduction. However, these males enjoyed a lower rate of reproductive ageing than males evolving under random mortality. Our study suggests that condition-environment interactions play a key role in evolution of male reproductive ageing.