Purging Deleterious Mutations under Self Fertilization: Paradoxical Recovery in Fitness with Increasing Mutation Rate in Caenorhabditis elegans

Background: The accumulation of deleterious mutations can drastically reduce population mean fitness. Self-fertilization is thought to be an effective means of purging deleterious mutations. However, widespread linkage disequilibrium generated and maintained by self-fertilization is predicted to reduce the efficacy of purging when mutations are present at multiple loci. Methodology/Principal Findings: We tested the ability of self-fertilizing populations to purge deleterious mutations at multiple loci by exposing obligately self-fertilizing populations of Caenorhabditis elegans to a range of elevated mutation rates and found that mutations accumulated, as evidenced by a reduction in mean fitness, in each population. Therefore, purging in obligate selfing populations is overwhelmed by an increase in mutation rate. Surprisingly, we also found that obligate and predominantly self-fertilizing populations exposed to very high mutation rates exhibited consistently greater fitness than those subject to lesser increases in mutation rate, which contradicts the assumption that increases in mutation rate are negatively correlated with fitness. The high levels of genetic linkage inherent in self-fertilization could drive this fitness increase. Conclusions: Compensatory mutations can be more frequent under high mutation rates and may alleviate a portion of the fitness lost due to the accumulation of deleterious mutations through epistatic interactions with deleterious mutations. Th

Coexisting Cyclic Parthenogens Comprise a Holocene Species Flock in Eubosmina

Background: Mixed breeding systems with extended clonal phases and weak sexual recruitment are widespread in nature but often thought to impede the formation of discrete evolutionary clusters. Thus, cyclic parthenogens, such as cladocerans and rotifers, could be predisposed to ‘‘species problems’ ’ and a lack of discrete species. However, species flocks have been proposed for one cladoceran group, Eubosmina, where putative species are sympatric, and there is a detailed paleolimnological record indicating a Holocene age. These factors make the Eubosmina system suitable for testing the hypotheses that extended clonal phases and weak sexual recruitment inhibit speciation. Although common garden experiments have revealed a genetic component to the morphotypic variation, the evolutionary significance of the morphotypes remains controversial. Methodology/Principal Findings: In the present study, we tested the hypothesis of a single polymorphic species (i.e., mixing occurs but selection maintains genes for morphology) in four northern European lakes where the morphotypes coexist. Our evidence is based on nuclear DNA sequence, mitochondrial DNA sequence, and morphometric analysis of coexisting morphotypes. We found significant genetic differentiation, genealogical exclusivity, and morphometric differentiation for coexisting morphotypes. Conclusions: We conclude that the studied morphotypes represent a group of young species undergoing speciation wit

Activation of H-2 by halogenocarbonylbis(phosphine)rhodium(I) complexes. The use of parahydrogen induced polarisation to detect species present at low concentration

Complexes of the form RhX(CO)(PR3)2 [X = Cl, Br or I; R = Me or Ph] reacted with H2 to form a series of binuclear complexes of the type (PR3)2H2Rh(μ-X)2Rh(CO)(PR 3) [X = Cl, Br or I, R = Ph; X = I, R = Me] and (PMe3)2(X)HRh(μ-H)(μ-X)Rh(CO)(PMe3) [X = Cl, Br or I] according to parahydrogen sensitised 1H, 13C, 31P and 103Rh NMR spectroscopy. Analogous complexes containing mixed halide bridges (PPh3)2H2Rh(μ-X)(μ-Y)Rh(CO)(PPh 3) [X, Y = Cl, Br or I; X ≠ Y] are detected when RhX(CO)(PPh3)2 and RhY(CO)(PPh3)2 are warmed together with p-H2. In these reactions only one isomer of the products (PPh3)2H2Rh(μ-I)(μ-Cl)Rh(CO)(PPh 3) and (PPh3)2H2Rh(μ-I)(μ-Br)Rh(CO)(PPh 3) is formed in which the μ-iodide is trans to the CO ligand of the rhodium(I) centre. When (PPh3)2H2Rh(μ-Cl)(μ-Br)Rh(CO)(PPh 3) is produced in the same way two isomers are observed. The mechanism of the hydrogen addition reaction is complex and involves initial formation of RhH2X(CO)(PR3)2 [R = Ph or Me], followed by CO loss to yield RhH2X(PR3)2. This intermediate is then attacked by the halide of a precursor complex to form a binuclear species which yields the final product after PR3 loss. The (PPh3)2H2Rh(μ-X)2Rh(CO)(PPh 3) systems are shown to undergo hydride self exchange by exchange spectroscopy with rates of 13.7 s-1 for the (μ-Cl)2 complex and 2.5 s-1 for the (μ-I)2 complex at 313 K. Activation parameters indicate that ordering dominates up to the rate determining step; for the (μ-Cl)2 system ΔH‡ = 52 ± 9 kJ mol-1 and ΔS‡ = -61 ± 27 J K-1 mol-1. This process most likely proceeds via halide bridge opening at the rhodium(III) centre, rotation of the rhodium(III) fragment around the remaining halide bond and bridge re-establishment. If the triphenylphosphine ligands are replaced by trimethylphosphine distinctly different reactivity is observed. When RhX(CO)(PMe3)2 [X = Cl or Br] is warmed with p-H2 the complex (PMe3)2(X)HRh(μ-H)(μ-X)Rh(CO)(PMe3) [X = Cl or Br] is detected which contains a bridging hydride trans to the rhodium(I) PMe3 ligand. However, when X = I, the situation is far more complex, with (PMe3)2H2Rh(μ-I)2Rh(CO)(PMe 3) observed preferentially at low temperatures and (PMe3)2(I)HRh(μ-H)(μ-I)Rh(CO)(PMe3) at higher temperatures. Additional binuclear products corresponding to a second isomer of (PMe3)2(I)HRh(μ-H)(μ-I)Rh(CO)(PMe3), in which the bridging hydride is trans to the rhodium(I) CO ligand, and (PMe3)2HRh(μ-H)(μ-I)2Rh(CO)(PMe 3) are also observed in this reaction. The relative stabilities of related systems containing the phosphine PH3 have been calculated using approximate density functional theory. In each case, the (μ-X)2 complex is found to be the most stable, followed by the (μ-H)(μ-X) species with hydride trans to PH3. © The Royal Society of Chemistry 1999

Higher rates of sex evolve in spatially heterogeneous environments

The evolution and maintenance of sexual reproduction has puzzled biologists for decades(1,2). Although this field is rich in hypotheses(3-5), experimental evidence is scarce. Some important experiments have demonstrated differences in evolutionary rates between sexual and asexual populations(6-8); other experiments have documented evolutionary changes in phenomena related to genetic mixing, such as recombination(9,10) and selfing(11). However, direct experiments of the evolution of sex within populations are extremely rare (but see ref. 12). Here we use the rotifer, Brachionus calyciflorus, which is capable of both sexual and asexual reproduction, to test recent theory(13-15) predicting that there is more opportunity for sex to evolve in spatially heterogeneous environments. Replicated experimental populations of rotifers were maintained in homogeneous environments, composed of either high-or low-quality food habitats, or in heterogeneous environments that consisted of a mix of the two habitats. For populations maintained in either type of homogeneous environment, the rate of sex evolves rapidly towards zero. In contrast, higher rates of sex evolve in populations experiencing spatially heterogeneous environments. The data indicate that the higher level of sex observed under heterogeneity is not due to sex being less costly or selection against sex being less efficient; rather sex is sufficiently advantageous in heterogeneous environments to overwhelm its inherent costs(2). Counter to some alternative theories(16,17) for the evolution of sex, there is no evidence that genetic drift plays any part in the evolution of sex in these populations

Host-Pathogen Coevolution: The Selective Advantage of Bacillus thuringiensis Virulence and Its Cry Toxin Genes.

Reciprocal coevolution between host and pathogen is widely seen as a major driver of evolution and biological innovation. Yet, to date, the underlying genetic mechanisms and associated trait functions that are unique to rapid coevolutionary change are generally unknown. We here combined experimental evolution of the bacterial biocontrol agent Bacillus thuringiensis and its nematode host Caenorhabditis elegans with large-scale phenotyping, whole genome analysis, and functional genetics to demonstrate the selective benefit of pathogen virulence and the underlying toxin genes during the adaptation process. We show that: (i) high virulence was specifically favoured during pathogen-host coevolution rather than pathogen one-sided adaptation to a nonchanging host or to an environment without host; (ii) the pathogen genotype BT-679 with known nematocidal toxin genes and high virulence specifically swept to fixation in all of the independent replicate populations under coevolution but only some under one-sided adaptation; (iii) high virulence in the BT-679-dominated populations correlated with elevated copy numbers of the plasmid containing the nematocidal toxin genes; (iv) loss of virulence in a toxin-plasmid lacking BT-679 isolate was reconstituted by genetic reintroduction or external addition of the toxins. We conclude that sustained coevolution is distinct from unidirectional selection in shaping the pathogen's genome and life history characteristics. To our knowledge, this study is the first to characterize the pathogen genes involved in coevolutionary adaptation in an animal host-pathogen interaction system.peerReviewe

The Influence of Perceptions of Fairness on Performance Appraisal Reactions

Perceptions of fairness, Performance appraisal reactions, Two-factor justice model, Agent-system justice model,