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    ABSTRACT: Many autonomously replicating genetic elements exist as multiple copies within the cell. The copy number of these elements is often assumed to have important fitness consequences for both element and host, yet the forces shaping its evolution are not well understood. The 2 μm is a multicopy plasmid of Saccharomyces yeasts, encoding just four genes that are solely involved in plasmid replication. One simple model for the fitness relationship between yeasts and 2 μm is that plasmid copy number evolves as a trade-off between selection for increased vertical transmission, favouring high copy number, and selection for decreased virulence, favouring low copy number. To test this model, we experimentally manipulated the copy number of the plasmid and directly measured the fitness cost, in terms of growth rate reduction, associated with high plasmid copy number. We find that the fitness burden imposed by the 2 μm increases with plasmid copy number, such that each copy imposes a fitness burden of 0.17% (± 0.008%), greatly exceeding the cost expected for it to be stably maintained in yeast populations. Our results demonstrate the crucial importance of copy number in the evolution of yeast per 2 μm associations and pave the way for future studies examining how selection can shape the cost of multicopy elements.
    Journal of Evolutionary Biology 09/2012;
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    ABSTRACT: The ecological forces shaping adaptive radiations are of great interest to evolutionary ecologists. Here, we experimentally test the hypothesis that the diversification of a lineage should be limited in the presence of competition from another taxon. We do this by studying a model microbial adaptive radiation (the generation of phenotypic diversity in asexual lineages of the bacterium Pseudomonas fluorescens) in the presence or absence of a competitor (Pseudomonas putida). In a spatially heterogeneous environment, the competitor P. putida reduced P. fluorescens population size only slightly and had no effect on diversification. In a spatially homogeneous environment, the competitor reduced P. fuoresecens population size to a much greater extent. Again the final extent of diversification in P. fluorescens was not affected by the competitor, but early diversification was accelerated. In this environment, P. putida suppressed the growth of a common variant of P. fluorescens and directly or indirectly facilitated the growth of a rare morph. Our results suggest that competition experienced by diversifying lineages may have complex effects on adaptive radiations not fully captured by current theory.
    Evolution 06/2012; 66(6):1985-90.
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    ABSTRACT: Species lists for particular geographic areas are often used in macroecology and conservation; for example, they have been used to identify hotspots of biological diversity, and for the study of latitudinal species diversity gradients. However, there is uncertainty over the accuracy of species lists due to undiscovered species and synonymy of described species. This paper concentrates on taxonomic overdescription caused by the latter. Where bias in the amount of taxonomic overdescription occurs along a variable of interest (e.g. latitude or body size), inferences from macroecological studies may be affected. This form of taxonomic overdescription is likely to be most pronounced in speciose groups, where many species have relatively small geographic range sizes and have low numerical abundance. A good example of such a group is the wasp family Ichneumonidae. We first use taxonomic and region-specific species list data for the Ichneumonidae to estimate the probability of species validity. Then we use this estimated probability to statistically correct the region's species richness estimate using a Monte Carlo simulation approach, and examine the effect the correction has on three major macroecological patterns: the relative species richness of geographic regions, latitudinal species richness pattern, and body size. Our results indicate that although there is significant geographic variation in overdescription, the bias is not sufficient to qualitatively alter broad-scale macroecological conclusions such as hotspot identity, and the qualitative global patterns of diversity and mean body size.
    Ecography 03/2012; 35(4):333 - 340.
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    ABSTRACT: Host traits, such as migratory behavior, could facilitate the dispersal of disease-causing parasites, potentially leading to the transfer of infections both across geographic areas and between host species. There is, however, little quantitative information on whether variation in such host attributes does indeed affect the evolutionary outcome of host-parasite associations. Here, we employ Leucocytozoon blood parasites of birds, a group of parasites closely related to avian malaria, to study host-parasite coevolution in relation to host behavior using a phylogenetic comparative approach. We reconstruct the molecular phylogenies of both the hosts and parasites and use cophylogenetic tools to assess whether each host-parasite association contributes significantly to the overall congruence between the two phylogenies. We find evidence for a significant fit between host and parasite phylogenies in this system, but show that this is due only to associations between nonmigrant parasites and their hosts. We also show that migrant bird species harbor a greater genetic diversity of parasites compared with nonmigrant species. Taken together, these results suggest that the migratory habits of birds could influence their coevolutionary relationship with their parasites, and that consideration of host traits is important in predicting the outcome of coevolutionary interactions.
    Evolution 03/2012; 66(3):740-51.
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    ABSTRACT: When organisms are attacked by multiple natural enemies, the evolution of a resistance mechanism to one natural enemy will be influenced by the degree of cross-resistance to another natural enemy. Cross-resistance can be positive, when a resistance mechanism against one natural enemy also offers resistance to another; or negative, in the form of a trade-off, when an increase in resistance against one natural enemy results in a decrease in resistance against another. Using Drosophila melanogaster, an important model system for the evolution of invertebrate immunity, we test for the existence of cross-resistance against parasites and pathogens, at both a phenotypic and evolutionary level. We used a field strain of D. melanogaster to test whether surviving parasitism by the parasitoid Asobara tabida has an effect on the resistance against Beauveria bassiana, an entomopathogenic fungus; and whether infection with the microsporidian Tubulinosema kingi has an effect on the resistance against A. tabida. We used lines selected for increased resistance to A. tabida to test whether increased parasitoid resistance has an effect on resistance against B. bassiana and T. kingi. We used lines selected for increased tolerance against B. bassiana to test whether increased fungal resistance has an effect on resistance against A. tabida. We found no positive cross-resistance or trade-offs in the resistance to parasites and pathogens. This is an important finding, given the use of D. melanogaster as a model system for the evolution of invertebrate immunity. The lack of any cross-resistance to parasites and pathogens, at both the phenotypic and the evolutionary level, suggests that evolution of resistance against one class of natural enemies is largely independent of evolution of resistance against the other.
    PLoS ONE 01/2012; 7(12):e53002.
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    ABSTRACT: The relationship between biodiversity and the rapidly expanding research and policy field of ecosystem services is confused and is damaging efforts to create coherent policy. Using the widely accepted Convention on Biological Diversity definition of biodiversity and work for the U.K. National Ecosystem Assessment we show that biodiversity has key roles at all levels of the ecosystem service hierarchy: as a regulator of underpinning ecosystem processes, as a final ecosystem service and as a good that is subject to valuation, whether economic or otherwise. Ecosystem science and practice has not yet absorbed the lessons of this complex relationship, which suggests an urgent need to develop the interdisciplinary science of ecosystem management bringing together ecologists, conservation biologists, resource economists and others.
    Trends in Ecology & Evolution 09/2011; 27(1):19-26.
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    ABSTRACT: Relationships between hosts and parasites represent complex co-evolving systems that can vary both temporally and spatially. This variation may result in different phylogeographic outcomes, ranging from highly geographically structured parasite populations comprised of specialist lineages that are locally abundant but have restricted global occupancy to geographically unstructured parasite populations consisting of widespread parasites. Here, we present results from a large biogeographic study of the Leucocytozoon blood parasites of two nonmigrant bird species, conducted at nine sites across Europe. The aim was to determine whether the parasite lineages of the two hosts were phylogeographically structured across Europe. Employing molecular methods, we found a large diversity of parasites, and although overall prevalence varied greatly, the parasites were not genetically structured. Several measures of local parasite abundance were associated with the number of sites that the lineage occurred in, which is consistent with the macroecological phenomenon of the abundance-occupancy relationship. Taken together, our results show that parasite dispersal is somewhat uncoupled to that of the host in this system: we suggest that broad host and/or vector preference may play an important role in determining the distribution of these parasites and in affecting host-parasite coevolution in this system.
    Molecular Ecology 08/2011; 20(18):3910-20.
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    ABSTRACT: Evaluating the relative importance of ecological drivers responsible for natural population fluctuations in size is challenging. Longitudinal studies where most individuals are monitored from birth to death and where environmental conditions are known provide a valuable resource to characterize complex ecological interactions. We used a recently developed approach to decompose the observed fluctuation in population growth of the red deer population on the Isle of Rum into contributions from climate, density and their interaction and to quantify their relative importance. We also quantified the contribution of individual covariates, including phenotypic and life-history traits, to population growth. Fluctuations in composition in age and sex classes ((st)age structure) of the population contributed substantially to the population dynamics. Density, climate, birth weight and reproductive status contributed less and approximately equally to the population growth. Our results support the contention that fluctuations in the population's (st)age structure have important consequences for population dynamics and underline the importance of including information on population composition to understand the effect of human-driven changes on population performance of long-lived species.
    Proceedings of the Royal Society B: Biological Sciences 06/2011; 279(1727):394-401.
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    ABSTRACT: Distributions of mutation fitness effects from evolution experiments are available in an increasing number of species, opening the way for a vast array of applications in evolutionary biology. However, comparison of estimated distributions among studies is hampered by inconsistencies in the definitions of fitness effects and selection coefficients. In particular, the use of ratios of Malthusian growth rates as 'relative fitnesses' leads to wrong inference of the strength of selection. Scaling Malthusian fitness by the generation time may help overcome this shortcoming, and allow accurate comparison of selection coefficients across species. For species reproducing by binary fission (neglecting cellular death), ln2 can be used as a correction factor, but in general, the growth rate and generation time of the wild-type should be provided in studies reporting distribution of mutation fitness effects. I also discuss how density and frequency dependence of population growth affect selection and its measurement in evolution experiments.
    Biology letters 04/2011; 7(2):210-3.
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    ABSTRACT: There is increasing evidence that litter decomposition is faster beneath the plant species it was derived from, an effect called home-field advantage (HFA). Adaptation of soil biota to decompose the litter that they encounter most often has been proposed as the main mechanism to explain HFA. However, there is little direct evidence supporting this assumption and the contribution of different decomposer groups to the HFA is unknown. Furthermore, the large observed variation in the strength of the HFA may be linked to litter quality, with increasing HFA for more recalcitrant substrates. To test the relationship between HFA, litter quality and soil fauna, we performed a field litter bag experiment, with different mesh sizes and reciprocal litter transplants, across a successional gradient varying in litter quality inputs and soil fauna composition. The results show that the HFA was stronger in sites dominated by more recalcitrant litter inputs and provide evidence that a range of soil fauna size classes contribute to this effect. Our findings help explain the lack of HFA in ecosystems with simple litters and in studies where experimental artefacts (e.g. fine mesh sizes) affect the contribution from certain classes of soil fauna.
    Oikos 02/2011; 120(9):1366 - 1370.
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