Extraordinary genome stability in the ciliate Paramecium tetraurelia

Darwinism for the Genomic Age: Connecting Mutation to Diversification

A growing body of evidence suggests that rates of diversification of biological lineages are correlated with differences in genome-wide mutation rate. Given that most research into differential patterns of diversification rate have focused on species traits or ecological parameters, a connection to the biochemical processes of genome change is an unexpected observation. While the empirical evidence for a significant association between mutation rate and diversification rate is mounting, there has been less effort in explaining the factors that mediate this connection between genetic change and species richness. Here we draw together empirical studies and theoretical concepts that may help to build links in the explanatory chain that connects mutation to diversification. First we consider the way that mutation rates vary between species. We then explore how differences in mutation rates have flow-through effects to the rate at which populations acquire substitutions, which in turn influences the speed at which populations become reproductively isolated from each other due to the acquisition of genomic incompatibilities. Since diversification rate is commonly measured from phylogenetic analyses, we propose a conceptual approach for relating events of reproductive isolation to bifurcations on molecular phylogenies. As we examine each of these relationships, we consider theoretical models that might shine a light on the observed association between rate of molecular evolution and diversification rate, and critically evaluate the empirical evidence for these links, focusing on phylogenetic comparative studies. Finally, we ask whether we are getting closer to a real understanding of the way that the processes of molecular evolution connect to the observable patterns of diversification.

Low Base-Substitution Mutation Rate in the Germline Genome of the Ciliate Tetrahymena thermophil

Mutation is the ultimate source of all genetic variation and is, therefore, central to evolutionary change. Previous work on Paramecium tetraurelia found an unusually low germline base-substitution mutation rate in this ciliate. Here, we tested the generality of this result among ciliates using Tetrahymena thermophila. We sequenced the genomes of 10 lines of T. thermophila that had each undergone approximately 1,000 generations of mutation accumulation (MA). We applied an existing mutation-calling pipeline and developed a new probabilistic mutation detection approach that directly models the design of an MA experiment and accommodates the noise introduced by mismapped reads. Our probabilistic mutation-calling method provides a straightforward way of estimating the number of sites at which a mutation could have been called if one was present, providing the denominator for our mutation rate calculations. From these methods, we find that T. thermophila has a germline base-substitution mutation rate of 7.61 × 10 - 12 per-site, per cell division, which is consistent with the low base-substitution mutation rate in P. tetraurelia. Over the course of the evolution experiment, genomic exclusion lines derived from the MA lines experienced a fitness decline that cannot be accounted for by germline base-substitution mutations alone, suggesting that other genetic or epigenetic factors must be involved. Because selection can only operate to reduce mutation rates based upon the "visible" mutational load, asexual reproduction with a transcriptionally silent germline may allow ciliates to evolve extremely low germline mutation rates.

Genome-Wide Biases in the Rate and Molecular Spectrum of Spontaneous Mutations in Vibrio cholerae and Vibrio fischeri

The vast diversity in nucleotide composition and architecture among bacterial genomes may be partly explained by inherent biases in the rates and spectra of spontaneous mutations. Bacterial genomes with multiple chromosomes are relatively unusual but some are relevant to human health, none more so than the causative agent of cholera, Vibrio cholerae Here, we present the genome-wide mutation spectra in wild-type and mismatch repair (MMR) defective backgrounds of two Vibrio species, the low-%GC squid symbiont V. fischeri and the pathogen V. cholerae, collected under conditions that greatly minimize the efficiency of natural selection. In apparent contrast to their high diversity in nature, both wild-type V. fischeri and V. cholerae have among the lowest rates for base-substitution mutations (bpsms) and insertion-deletion mutations (indels) that have been measured, below 10^-3/genome/generation. Vibrio fischeri and V. cholerae have distinct mutation spectra, but both are AT-biased and produce a surprising number of multi-nucleotide indels. Furthermore, the loss of a functional MMR system caused the mutation spectra of these species to converge, implying that the MMR system itself contributes to species-specific mutation patterns. Bpsm and indel rates varied among genome regions, but do not explain the more rapid evolutionary rates of genes on chromosome 2, which likely result from weaker purifying selection. More generally, the very low mutation rates of Vibrio species correlate inversely with their immense population sizes and suggest that selection may not only have maximized replication fidelity but also optimized other polygenic traits relative to the constraints of genetic drift.

Evolution of the Insertion-Deletion Mutation Rate Across the Tree of Life

Mutations are the ultimate source of variation used for evolutionary adaptation, while also being predominantly deleterious and a source of genetic disorders. Understanding the rate of insertion-deletion mutations (indels) is essential to understanding evolutionary processes, especially in coding regions, where such mutations can disrupt production of essential proteins. Using direct estimates of indel rates from 14 phylogenetically diverse eukaryotic and bacterial species, along with measures of standing variation in such species, we obtain results that imply an inverse relationship of mutation rate and effective population size. These results, which corroborate earlier observations on the base-substitution mutation rate, appear most compatible with the hypothesis that natural selection reduces mutation rates per effective genome to the point at which the power of random genetic drift (approximated by the inverse of effective population size) becomes overwhelming. Given the substantial differences in DNA metabolism pathways that give rise to these two types of mutations, this consistency of results raises the possibility that refinement of other molecular and cellular traits may be inversely related to species-specific levels of random genetic drift.

The repeatability of genome-wide mutation rate and spectrum estimates

Over the last decade, mutation studies have grown in popularity due to the affordability and accessibility of whole genome sequencing. As the number of species in which spontaneous mutation has been directly estimated approaches 20 across two domains of life, questions arise over the repeatability of results in such experiments. Five species were identified in which duplicate mutation studies have been performed. Across these studies the difference in estimated spontaneous mutation rate is at most, weakly significant (p < 0.01). However, a highly significant (p < 10(-5)), threefold difference in the rate of insertions/deletions (indels) exists between two recent studies in Schizosaccharomyces pombe. Upon investigation of the ancestral genome sequence for both studies, a possible anti-mutator allele was identified. The observed variation in indel rate may imply that the use of indel markers, such as microsatellites, for the investigation of genetic diversity within and among populations may be inappropriate because of the assumption of uniform mutation rate within a species.

Direct estimate of the rate of germline mutation in a bird

The fidelity of DNA replication together with repair mechanisms ensure that the genetic material is properly copied from one generation to another. However, on extremely rare occasions when damages to DNA or replication errors are not repaired, germline mutations can be transmitted to the next generation. Because of the rarity of these events, studying the rate at which new mutations arise across organisms has been a great challenge, especially in multicellular nonmodel organisms with large genomes. We sequenced the genomes of 11 birds from a three-generation pedigree of the collared flycatcher (Ficedula albicollis) and used highly stringent bioinformatic criteria for mutation detection and used several procedures to validate mutations, including following the stable inheritance of new mutations to subsequent generations. We identified 55 de novo mutations with a 10-fold enrichment of mutations at CpG sites and with only a modest male mutation bias. The estimated rate of mutation per site per generation was 4.6 × 10(-9), which corresponds to 2.3 × 10(-9) mutations per site per year. Compared to mammals, this is similar to mouse but about half of that reported for humans, which may be due to the higher frequency of male mutations in humans. We confirm that mutation rate scales positively with genome size and that there is a strong negative relationship between mutation rate and effective population size, in line with the drift-barrier hypothesis. Our study illustrates that it should be feasible to obtain direct estimates of the rate of mutation in essentially any organism from which family material can be obtained.

The Rate and Spectrum of Spontaneous Mutations in Mycobacterium smegmatis, a Bacterium Naturally Devoid of the Postreplicative Mismatch Repair Pathway

Mycobacterium smegmatis is a bacterium that is naturally devoid of known postreplicative DNA mismatch repair (MMR) homologs, mutS and mutL, providing an opportunity to investigate how the mutation rate and spectrum has evolved in the absence of a highly conserved primary repair pathway. Mutation accumulation experiments of M. smegmatis yielded a base-substitution mutation rate of 5.27 × 10⁻¹⁰ per site per generation, or 0.0036 per genome per generation, which is surprisingly similar to the mutation rate in MMR-functional unicellular organisms. Transitions were found more frequently than transversions, with the A:T→G:C transition rate significantly higher than the G:C→A:T transition rate, opposite to what is observed in most studied bacteria. We also found that the transition-mutation rate of M. smegmatis is significantly lower than that of other naturally MMR-devoid or MMR-knockout organisms. Two possible candidates that could be responsible for maintaining high DNA fidelity in this MMR-deficient organism are the ancestral-like DNA polymerase DnaE1, which contains a highly efficient DNA proofreading histidinol phosphatase (PHP) domain, and/or the existence of a uracil-DNA glycosylase B (UdgB) homolog that might protect the GC-rich M. smegmatis genome against DNA damage arising from oxidation or deamination. Our results suggest that M. smegmatis has a noncanonical Dam (DNA adenine methylase) methylation system, with target motifs differing from those previously reported. The mutation features of M. smegmatis provide further evidence that genomes harbor alternative routes for improving replication fidelity, even in the absence of major repair pathways.

Hidden genetic variation in the germline genome of Tetrahymena thermophila

Genome architecture varies greatly among eukaryotes. This diversity may profoundly affect the origin and maintenance of genetic variation within a population. Ciliates are microbial eukaryotes with unusual genome features, such as the separation of germline and somatic genomes within a single cell and amitotic division. These features have previously been proposed to increase the rate of molecular evolution in these species. Here, we assessed the fitness effects of genetic variation in the two genomes of natural isolates of the ciliate Tetrahymena thermophila. We find more extensive genetic variation in fitness in the transcriptionally silent germline genome than in the expressed somatic genome. Surprisingly, this variation is not primarily deleterious, but has both beneficial and deleterious effects. We conclude that Tetrahymena genome architecture allows for the maintenance of genetic variation that would otherwise be eliminated by selection. We consider the effect of selection on the two genomes and the impacts of reproductive strategies and the mechanism of sex determination on the structure of this variation.

Diverse phenotypic and genetic responses to short-term selection in evolving Escherichia coli populations

Beneficial mutations fuel adaptation by altering phenotypes that enhance the fit of organisms to their environment. However, the phenotypic effects of mutations often depend on ecological context, making the distribution of effects across multiple environments essential to understanding the true nature of beneficial mutations. Studies that address both the genetic basis and ecological consequences of adaptive mutations remain rare. Here, we characterize the direct and pleiotropic fitness effects of a collection of 21 first-step beneficial mutants derived from naïve and adapted genotypes used in a long-term experimental evolution of Escherichia coli. Whole-genome sequencing was able to identify the majority of beneficial mutations. In contrast to previous studies, we find diverse fitness effects of mutations selected in a simple environment and few cases of genetic parallelism. The pleiotropic effects of these mutations were predominantly positive but some mutants were highly antagonistic in alternative environments. Further, the fitness effects of mutations derived from the adapted genotypes were dramatically reduced in nearly all environments. These findings suggest that many beneficial variants are accessible from a single point on the fitness landscape, and the fixation of alternative beneficial mutations may have dramatic consequences for niche breadth reduction via metabolic erosion.

Programmed Rearrangement in Ciliates: Paramecium

Programmed genome rearrangements in the ciliate Paramecium provide a nice illustration of the impact of transposons on genome evolution and plasticity. During the sexual cycle, development of the somatic macronucleus involves elimination of ∼30% of the germline genome, including repeated DNA (e.g., transposons) and ∼45,000 single-copy internal eliminated sequences (IES). IES excision is a precise cut-and-close process, in which double-stranded DNA cleavage at IES ends depends on PiggyMac, a domesticated piggyBac transposase. Genome-wide analysis has revealed that at least a fraction of IESs originate from Tc/mariner transposons unrelated to piggyBac. Moreover, genomic sequences with no transposon origin, such as gene promoters, can be excised reproducibly as IESs, indicating that genome rearrangements contribute to the control of gene expression. How the system has evolved to allow elimination of DNA sequences with no recognizable conserved motif has been the subject of extensive research during the past two decades. Increasing evidence has accumulated for the participation of noncoding RNAs in epigenetic control of elimination for a subset of IESs, and in trans-generational inheritance of alternative rearrangement patterns. This chapter summarizes our current knowledge of the structure of the germline and somatic genomes for the model species Paramecium tetraurelia, and describes the DNA cleavage and repair factors that constitute the IES excision machinery. We present an overview of the role of specialized RNA interference machineries and their associated noncoding RNAs in the control of DNA elimination. Finally, we discuss how RNA-dependent modification and/or remodeling of chromatin may guide PiggyMac to its cognate cleavage sites.

The properties of spontaneous mutations in the opportunistic pathogen Pseudomonas aeruginosa

Background

Natural genetic variation ultimately arises from the process of mutation. Knowledge of how the raw material for evolution is produced is necessary for a full understanding of several fundamental evolutionary concepts. We performed a mutation accumulation experiment with wild-type and mismatch-repair deficient, mutator lines of the pathogenic bacterium Pseudomonas aeruginosa, and used whole-genome sequencing to reveal the genome-wide rate, spectrum, distribution, leading/lagging bias, and context-dependency of spontaneous mutations.

Results

Wild-type base-pair mutation and indel rates were ~10⁻¹⁰ and ~10⁻¹¹ per nucleotide per generation, respectively, and deficiencies in the mismatch-repair system caused rates to increase by over two orders of magnitude. A universal bias towards AT was observed in wild-type lines, but was reversed in mutator lines to a bias towards GC. Biases for which types of mutations occurred during replication of the leading versus lagging strand were detected reciprocally in both replichores. The distribution of mutations along the chromosome was non-random, with peaks near the terminus of replication and at positions intermediate to the replication origin and terminus. A similar distribution bias was observed along the chromosome in natural populations of P. aeruginosa. Site-specific mutation rates were higher when the focal nucleotide was immediately flanked by C:G pairings.

Conclusions

Whole-genome sequencing of mutation accumulation lines allowed the comprehensive identification of mutations and revealed what factors of molecular and genomic architecture affect the mutational process. Our study provides a more complete view of how several mechanisms of mutation, mutation repair, and bias act simultaneously to produce the raw material for evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2244-3) contains supplementary material, which is available to authorized users.

High mutational rates of large-scale duplication and deletion in Daphnia pulex

Knowledge of the genome-wide rate and spectrum of mutations is necessary to understand the origin of disease and the genetic variation driving all evolutionary processes. Here, we provide a genome-wide analysis of the rate and spectrum of mutations obtained in two Daphnia pulex genotypes via separate mutation-accumulation (MA) experiments. Unlike most MA studies that utilize haploid, homozygous, or self-fertilizing lines, D. pulex can be propagated ameiotically while maintaining a naturally heterozygous, diploid genome, allowing the capture of the full spectrum of genomic changes that arise in a heterozygous state. While base-substitution mutation rates are similar to those in other multicellular eukaryotes (about 4 × 10(-9) per site per generation), we find that the rates of large-scale (>100 kb) de novo copy-number variants (CNVs) are significantly elevated relative to those seen in previous MA studies. The heterozygosity maintained in this experiment allowed for estimates of gene-conversion processes. While most of the conversion tract lengths we report are similar to those generated by meiotic processes, we also find larger tract lengths that are indicative of mitotic processes. Comparison of MA lines to natural isolates reveals that a majority of large-scale CNVs in natural populations are removed by purifying selection. The mutations observed here share similarities with disease-causing, complex, large-scale CNVs, thereby demonstrating that MA studies in D. pulex serve as a system for studying the processes leading to such alterations.

Genome-Wide Estimates of Mutation Rates and Spectrum in Schizosaccharomyces pombe Indicate CpG Sites are Highly Mutagenic Despite the Absence of DNA Methylation

We accumulated mutations for 1952 generations in 79 initially identical, haploid lines of the fission yeast Schizosaccharomyces pombe, and then performed whole-genome sequencing to determine the mutation rates and spectrum. We captured 696 spontaneous mutations across the 79 mutation accumulation (MA) lines. We compared the mutation spectrum and rate to a recently published equivalent experiment on the same species, and to another model ascomycetous yeast, the budding yeast Saccharomyces cerevisiae. While the two species are approximately 600 million years diverged from each other, they share similar life histories, genome size and genomic G/C content. We found that Sc. pombe and S. cerevisiae have similar mutation rates, but Sc. pombe exhibits a stronger insertion bias. Intriguingly, we observed an increased mutation rate at cytosine nucleotides, specifically CpG nucleotides, which is also seen in S. cerevisiae. However, the absence of methylation in Sc. pombe and the pattern of mutation at these sites, primarily C → A as opposed to C → T, strongly suggest that the increased mutation rate is not caused by deamination of methylated cytosines. This result implies that the high mutability of CpG dinucleotides in other species may be caused in part by a methylation-independent mechanism. Many of our findings mirror those seen in the recent study, despite the use of different passaging conditions, indicating that MA is a reliable method for estimating mutation rates and spectra.

The Spontaneous Mutation Rate in the Fission Yeast Schizosaccharomyces pombe

The rate at which new mutations arise in the genome is a key factor in the evolution and adaptation of species. Here we describe the rate and spectrum of spontaneous mutations for the fission yeast Schizosaccharomyces pombe, a key model organism with many similarities to higher eukaryotes. We undertook an ∼1700-generation mutation accumulation (MA) experiment with a haploid S. pombe, generating 422 single-base substitutions and 119 insertion-deletion mutations (indels) across the 96 replicates. This equates to a base-substitution mutation rate of 2.00 × 10(-10) mutations per site per generation, similar to that reported for the distantly related budding yeast Saccharomyces cerevisiae. However, these two yeast species differ dramatically in their spectrum of base substitutions, the types of indels (S. pombe is more prone to insertions), and the pattern of selection required to counteract a strong AT-biased mutation rate. Overall, our results indicate that GC-biased gene conversion does not play a major role in shaping the nucleotide composition of the S. pombe genome and suggest that the mechanisms of DNA maintenance may have diverged significantly between fission and budding yeasts. Unexpectedly, CpG sites appear to be excessively liable to mutation in both species despite the likely absence of DNA methylation.

Genetics: Feedforward loop for diversity

DNA-sequence analysis suggests that genetic mutations arise at elevated rates in genomes harbouring high levels of heterozygosity — the state in which the two copies of a genetic region contain sequence differences.

Delimiting Species Boundaries within a Paraphyletic Species Complex: Insights from Morphological, Genetic, and Molecular Data on Paramecium sonneborni (Paramecium aurelia species complex, Ciliophora, Protozoa)

The demarcation of boundaries between protist species is often problematic because of the absence of a uniform species definition, the abundance of cryptic diversity, and the occurrence of convergent morphology. The ciliates belonging to the Paramecium aurelia complex, consisting of 15 species, are a good model for such systematic and evolutionary studies. One member of the complex is P. sonneborni, previously known only from one stand in Texas (USA), but recently found in two new sampling sites in Cyprus (creeks running to Salt Lake and Oroklini Lake near Larnaca). The studied Paramecium sonneborni strains (from the USA and Cyprus) reveal low viability in the F1 and F2 generations of interstrain hybrids and may be an example of ongoing allopatric speciation. Despite its molecular distinctiveness, we postulate that P. sonneborni should remain in the P. aurelia complex, making it a paraphyletic taxon. Morphological studies have revealed that some features of the nuclear apparatus of P. sonneborni correspond to the P. aurelia spp. complex, while others are similar to P. jenningsi and P. schewiakoffi. The observed discordance indicates rapid splitting of the P. aurelia-P. jenningsi-P. schewiakoffi group, in which genetic, morphological, and molecular boundaries between species are not congruent.

Background Mutational Features of the Radiation-Resistant Bacterium Deinococcus radiodurans

Deinococcus bacteria are extremely resistant to radiation, oxidation, and desiccation. Resilience to these factors has been suggested to be due to enhanced damage prevention and repair mechanisms, as well as highly efficient antioxidant protection systems. Here, using mutation-accumulation experiments, we find that the GC-rich Deinococcus radiodurans has an overall background genomic mutation rate similar to that of E. coli, but differs in mutation spectrum, with the A/T to G/C mutation rate (based on a total count of 88 A:T → G:C transitions and 82 A:T → C:G transversions) per site per generation higher than that in the other direction (based on a total count of 157 G:C → A:T transitions and 33 G:C → T:A transversions). We propose that this unique spectrum is shaped mainly by the abundant uracil DNA glycosylases reducing G:C → A:T transitions, adenine methylation elevating A:T → C:G transversions, and absence of cytosine methylation decreasing G:C → A:T transitions. As opposed to the greater than 100× elevation of the mutation rate in MMR(-) (DNA Mismatch Repair deficient) strains of most other organisms, MMR(-) D. radiodurans only exhibits a 4-fold elevation, raising the possibility that other DNA repair mechanisms compensate for a relatively low-efficiency DNA MMR pathway. As D. radiodurans has plentiful insertion sequence (IS) elements in the genome and the activities of IS elements are rarely directly explored, we also estimated the insertion (transposition) rate of the IS elements to be 2.50 × 10(-3) per genome per generation in the wild-type strain; knocking out MMR did not elevate the IS element insertion rate in this organism.

The Rate and Molecular Spectrum of Spontaneous Mutations in the GC-Rich Multichromosome Genome of Burkholderia cenocepacia

Spontaneous mutations are ultimately essential for evolutionary change and are also the root cause of many diseases. However, until recently, both biological and technical barriers have prevented detailed analyses of mutation profiles, constraining our understanding of the mutation process to a few model organisms and leaving major gaps in our understanding of the role of genome content and structure on mutation. Here, we present a genome-wide view of the molecular mutation spectrum in Burkholderia cenocepacia, a clinically relevant pathogen with high %GC content and multiple chromosomes. We find that B. cenocepacia has low genome-wide mutation rates with insertion-deletion mutations biased toward deletions, consistent with the idea that deletion pressure reduces prokaryotic genome sizes. Unlike prior studies of other organisms, mutations in B. cenocepacia are not AT biased, which suggests that at least some genomes with high %GC content experience unusual base-substitution mutation pressure. Importantly, we also observe variation in both the rates and spectra of mutations among chromosomes and elevated G:C > T:A transversions in late-replicating regions. Thus, although some patterns of mutation appear to be highly conserved across cellular life, others vary between species and even between chromosomes of the same species, potentially influencing the evolution of nucleotide composition and genome architecture.

Dissecting genetic and environmental mutation signatures with model organisms

Deep sequencing has impacted on cancer research by enabling routine sequencing of genomes and exomes to identify genetic changes associated with carcinogenesis. Researchers can now use the frequency, type, and context of all mutations in tumor genomes to extract mutation signatures that reflect the driving mutational processes. Identifying mutation signatures, however, may not immediately suggest a mechanism. Consequently, several recent studies have employed deep sequencing of model organisms exposed to discrete genetic or environmental perturbations. These studies exploit the simpler genomes and availability of powerful genetic tools in model organisms to analyze mutation signatures under controlled conditions, forging mechanistic links between mutational processes and signatures. We discuss the power of this approach and suggest that many such studies may be on the horizon.

Maintenance and Loss of Duplicated Genes by Dosage Subfunctionalization

Whole-genome duplications (WGDs) have contributed to gene-repertoire enrichment in many eukaryotic lineages. However, most duplicated genes are eventually lost and it is still unclear why some duplicated genes are evolutionary successful whereas others quickly turn to pseudogenes. Here, we show that dosage constraints are major factors opposing post-WGD gene loss in several Paramecium species that share a common ancestral WGD. We propose a model where a majority of WGD-derived duplicates preserve their ancestral function and are retained to produce enough of the proteins performing this same ancestral function. Under this model, the expression level of individual duplicated genes can evolve neutrally as long as they maintain a roughly constant summed expression, and this allows random genetic drift toward uneven contributions of the two copies to total expression. Our analysis suggests that once a high level of imbalance is reached, which can require substantial lengths of time, the copy with the lowest expression level contributes a small enough fraction of the total expression that selection no longer opposes its loss. Extension of our analysis to yeast species sharing a common ancestral WGD yields similar results, suggesting that duplicated-gene retention for dosage constraints followed by divergence in expression level and eventual deterministic gene loss might be a universal feature of post-WGD evolution.

Amitotic chromosome loss predicts distinct patterns of senescence and non-senescence in ciliates

Over time and repeated asexual divisions, many ciliate species display the characteristics of senescence, reduced fecundity and increased mortality. Their only path to recovery is sexual conjugation or autogamy. While more traditional models of cellular aging have been proposed, one of the most accepted explanations relies on the faulty mechanism by which ciliates duplicate their somatic nucleus, a process referred to as amitosis. Amitosis involves the random segregation of chromosomes with no consideration for homology. Over subsequent divisions, chromosome copy numbers will fluctuate until an entire chromosome is lost, resulting in death. Via simulations of this process, we find that senescence and death via chromosome loss is not the only possible result of amitosis. Random chromosome loss is less damaging to populations than previously thought, and strict adherence to the model predicts that Paramecium tetraurelia would not senesce. A combination of the reciprocal nature of amitosis and lethal selection against low-copy number chromosomes is responsible for this startling prediction. Additionally, our results provide an alternate explanation to recent evidence for selection on chromosome copy number in Tetrahymena thermophila and peculiar patterns of senescence in Tetrahymena pyriformis.

Asymmetric Context-Dependent Mutation Patterns Revealed through Mutation-Accumulation Experiments

Despite the general assumption that site-specific mutation rates are independent of the local sequence context, a growing body of evidence suggests otherwise. To further examine context-dependent patterns of mutation, we amassed 5,645 spontaneous mutations in wild- type (WT) and mismatch-repair deficient (MMR(-)) mutation-accumulation (MA) lines of the gram-positive model organism Bacillus subtilis. We then analyzed>7,500 spontaneous base-substitution mutations across B. subtilis, Escherichia coli, and Mesoplasma florum WT and MMR(-) MA lines, finding a context-dependent mutation pattern that is asymmetric around the origin of replication. Different neighboring nucleotides can alter site-specific mutation rates by as much as 75-fold, with sites neighboring G:C base pairs or dimers involving alternating pyrimidine-purine and purine-pyrimidine nucleotides having significantly elevated mutation rates. The influence of context-dependent mutation on genome architecture is strongest in M. florum, consistent with the reduced efficiency of selection in organisms with low effective population size. If not properly accounted for, the disparities arising from patterns of context-dependent mutation can significantly influence interpretations of positive and purifying selection.

Exploring the Relationships between Mutation Rates, Life History, Genome Size, Environment, and Species Richness in Flowering Plants

A new view is emerging of the interplay between mutation at the genomic level, substitution at the population level, and diversification at the lineage level. Many studies have suggested that rate of molecular evolution is linked to rate of diversification, but few have evaluated competing hypotheses. By analyzing sequences from 130 families of angiosperms, we show that variation in the synonymous substitution rate is correlated among genes from the mitochondrial, chloroplast, and nuclear genomes and linked to differences in traits among families (average height and genome size). Within each genome, synonymous rates are correlated to nonsynonymous substitution rates, suggesting that increasing the mutation rate results in a faster rate of genome evolution. Substitution rates are correlated with species richness in protein-coding sequences from the chloroplast and nuclear genomes. These data suggest that species traits contribute to lineage-specific differences in the mutation rate that drive both synonymous and nonsynonymous rates of change across all three genomes, which in turn contribute to greater rates of divergence between populations, generating higher rates of diversification. These observations link mutation in individuals to population-level processes and to patterns of lineage divergence.

Mutation rate, spectrum, topology, and context-dependency in the DNA mismatch repair-deficient Pseudomonas fluorescens ATCC948

High levels of genetic diversity exist among natural isolates of the bacterium Pseudomonas fluorescens, and are especially elevated around the replication terminus of the genome, where strain-specific genes are found. In an effort to understand the role of genetic variation in the evolution of Pseudomonas, we analyzed 31,106 base substitutions from 45 mutation accumulation lines of P. fluorescens ATCC948, naturally deficient for mismatch repair, yielding a base-substitution mutation rate of 2.34 × 10⁻⁸ per site per generation (SE: 0.01 × 10⁻⁸) and a small-insertion-deletion mutation rate of 1.65 × 10⁻⁹ per site per generation (SE: 0.03 × 10⁻⁹). We find that the spectrum of mutations in prophage regions, which often contain virulence factors and antibiotic resistance, is highly similar to that in the intergenic regions of the host genome. Our results show that the mutation rate varies around the chromosome, with the lowest mutation rate found near the origin of replication. Consistent with observations from other studies, we find that site-specific mutation rates are heavily influenced by the immediately flanking nucleotides, indicating that mutations are context dependent.

Differential retention and divergent resolution of duplicate genes following whole-genome duplication

The Paramecium aurelia complex is a group of 15 species that share at least three past whole-genome duplications (WGDs). The macronuclear genome sequences of P. biaurelia and P. sexaurelia are presented and compared to the published sequence of P. tetraurelia. Levels of duplicate-gene retention from the recent WGD differ by >10% across species, with P. sexaurelia losing significantly more genes than P. biaurelia or P. tetraurelia. In addition, historically high rates of gene conversion have homogenized WGD paralogs, probably extending the paralogs’ lifetimes. The probability of duplicate retention is positively correlated with GC content and expression level; ribosomal proteins, transcription factors, and intracellular signaling proteins are overrepresented among maintained duplicates. Finally, multiple sources of evidence indicate that P. sexaurelia diverged from the two other lineages immediately following, or perhaps concurrent with, the recent WGD, with approximately half of gene losses between P. tetraurelia and P. sexaurelia representing divergent gene resolutions (i.e., silencing of alternative paralogs), as expected for random duplicate loss between these species. Additionally, though P. biaurelia and P. tetraurelia diverged from each other much later, there are still more than 100 cases of divergent resolution between these two species. Taken together, these results indicate that divergent resolution of duplicate genes between lineages acts to reinforce reproductive isolation between species in the Paramecium aurelia complex.

Genome-wide analysis of genetic and epigenetic control of programmed DNA deletion

During the development of the somatic genome from the Paramecium germline genome the bulk of the copies of ∼45 000 unique, internal eliminated sequences (IESs) are deleted. IES targeting is facilitated by two small RNA (sRNA) classes: scnRNAs, which relay epigenetic information from the parental nucleus to the developing nucleus, and iesRNAs, which are produced and used in the developing nucleus. Why only certain IESs require sRNAs for their removal has been enigmatic. By analyzing the silencing effects of three genes: PGM (responsible for DNA excision), DCL2/3 (scnRNA production) and DCL5 (iesRNA production), we identify key properties required for IES elimination. Based on these results, we propose that, depending on the exact combination of their lengths and end bases, some IESs are less efficiently recognized or excised and have a greater requirement for targeting by scnRNAs and iesRNAs. We suggest that the variation in IES retention following silencing of DCL2/3 is not primarily due to scnRNA density, which is comparatively uniform relative to IES retention, but rather the genetic properties of IESs. Taken together, our analyses demonstrate that in Paramecium the underlying genetic properties of developmentally deleted DNA sequences are essential in determining the sensitivity of these sequences to epigenetic control.

Emergence of resistance to atovaquone-proguanil in malaria parasites: insights from computational modeling and clinical case reports

The usefulness of atovaquone-proguanil (AP) as an antimalarial treatment is compromised by the emergence of atovaquone resistance during therapy. However, the origin of the parasite mitochondrial DNA (mtDNA) mutation conferring atovaquone resistance remains elusive. Here, we report a patient-based stochastic model that tracks the intrahost emergence of mutations in the multicopy mtDNA during the first erythrocytic parasite cycles leading to the malaria febrile episode. The effect of mtDNA copy number, mutation rate, mutation cost, and total parasite load on the mutant parasite load per patient was evaluated. Computer simulations showed that almost any infected patient carried, after four to seven erythrocytic cycles, de novo mutant parasites at low frequency, with varied frequencies of parasites carrying varied numbers of mutant mtDNA copies. A large interpatient variability in the size of this mutant reservoir was found; this variability was due to the different parameters tested but also to the relaxed replication and partitioning of mtDNA copies during mitosis. We also report seven clinical cases in which AP-resistant infections were treated by AP. These provided evidence that parasiticidal drug concentrations against AP-resistant parasites were transiently obtained within days after treatment initiation. Altogether, these results suggest that each patient carries new mtDNA mutant parasites that emerge before treatment but are killed by high starting drug concentrations. However, because the size of this mutant reservoir is highly variable from patient to patient, we propose that some patients fail to eliminate all of the mutant parasites, repeatedly producing de novo AP treatment failures.

Insights into three whole-genome duplications gleaned from the Paramecium caudatum genome sequence

Paramecium has long been a model eukaryote. The sequence of the Paramecium tetraurelia genome reveals a history of three successive whole-genome duplications (WGDs), and the sequences of P. biaurelia and P. sexaurelia suggest that these WGDs are shared by all members of the aurelia species complex. Here, we present the genome sequence of P. caudatum, a species closely related to the P. aurelia species group. P. caudatum shares only the most ancient of the three WGDs with the aurelia complex. We found that P. caudatum maintains twice as many paralogs from this early event as the P. aurelia species, suggesting that post-WGD gene retention is influenced by subsequent WGDs and supporting the importance of selection for dosage in gene retention. The availability of P. caudatum as an outgroup allows an expanded analysis of the aurelia intermediate and recent WGD events. Both the Guanine+Cytosine (GC) content and the expression level of preduplication genes are significant predictors of duplicate retention. We find widespread asymmetrical evolution among aurelia paralogs, which is likely caused by gradual pseudogenization rather than by neofunctionalization. Finally, cases of divergent resolution of intermediate WGD duplicates between aurelia species implicate this process acts as an ongoing reinforcement mechanism of reproductive isolation long after a WGD event.

Fitness is strongly influenced by rare mutations of large effect in a microbial mutation accumulation experiment

Our understanding of the evolutionary consequences of mutation relies heavily on estimates of the rate and fitness effect of spontaneous mutations generated by mutation accumulation (MA) experiments. We performed a classic MA experiment in which frequent sampling of MA lines was combined with whole genome resequencing to develop a high-resolution picture of the effect of spontaneous mutations in a hypermutator (ΔmutS) strain of the bacterium Pseudomonas aeruginosa. After ∼644 generations of mutation accumulation, MA lines had accumulated an average of 118 mutations, and we found that average fitness across all lines decayed linearly over time. Detailed analyses of the dynamics of fitness change in individual lines revealed that a large fraction of the total decay in fitness (42.3%) was attributable to the fixation of rare, highly deleterious mutations (comprising only 0.5% of fixed mutations). Furthermore, we found that at least 0.64% of mutations were beneficial and probably fixed due to positive selection. The majority of mutations that fixed (82.4%) were base substitutions and we failed to find any signatures of selection on nonsynonymous or intergenic mutations. Short indels made up a much smaller fraction of the mutations that were fixed (17.4%), but we found evidence of strong selection against indels that caused frameshift mutations in coding regions. These results help to quantify the amount of natural selection present in microbial MA experiments and demonstrate that changes in fitness are strongly influenced by rare mutations of large effect.

Accumulation of spontaneous mutations in the ciliate Tetrahymena thermophila

Knowledge of the rate and fitness effects of mutations is essential for understanding the process of evolution. Mutations are inherently difficult to study because they are rare and are frequently eliminated by natural selection. In the ciliate Tetrahymena thermophila, mutations can accumulate in the germline genome without being exposed to selection. We have conducted a mutation accumulation (MA) experiment in this species. Assuming that all mutations are deleterious and have the same effect, we estimate that the deleterious mutation rate per haploid germline genome per generation is U=0.0047 (95% credible interval: 0.0015, 0.0125), and that germline mutations decrease fitness by s=11% when expressed in a homozygous state (95% CI: 4.4%, 27%). We also estimate that deleterious mutations are partially recessive on average (h=0.26; 95% CI: -0.022, 0.62) and that the rate of lethal mutations is <10% of the deleterious mutation rate. Comparisons between the observed evolutionary responses in the germline and somatic genomes and the results from individual-based simulations of MA suggest that the two genomes have similar mutational parameters. These are the first estimates of the deleterious mutation rate and fitness effects from the eukaryotic supergroup Chromalveolata and are within the range of those of other eukaryotes.

Mutation rates: how low can you go?

Two recent reports combine mutation accumulation and whole-genome sequencing to measure mutation rates in microbes with unusual genome sizes and life cycles. The results are broadly consistent with the hypothesis that genetic drift plays a role in shaping genomic mutation rates across a wide range of taxa.

The rate and effects of spontaneous mutation on fitness traits in the social amoeba, Dictyostelium discoideum

We performed a mutation accumulation (MA) experiment in the social amoeba Dictyostelium discoideum to estimate the rate and distribution of effects of spontaneous mutations affecting eight putative fitness traits. We found that the per-generation mutation rate for most fitness components is 0.0019 mutations per haploid genome per generation or larger. This rate is an order of magnitude higher than estimates for fitness components in the unicellular eukaryote Saccharomyces cerevisiae, even though the base-pair substitution rate is two orders of magnitude lower. The high rate of fitness-altering mutations observed in this species may be partially explained by a large mutational target relative to S. cerevisiae. Fitness-altering mutations also may occur primarily at simple sequence repeats, which are common throughout the genome, including in coding regions, and may represent a target that is particularly likely to give fitness effects upon mutation. The majority of mutations had deleterious effects on fitness, but there was evidence for a substantial fraction, up to 40%, being beneficial for some of the putative fitness traits. Competitive ability within the multicellular slug appears to be under weak directional selection, perhaps reflecting the fact that slugs are sometimes, but not often, comprised of multiple clones in nature. Evidence for pleiotropy among fitness components across MA lines was absent, suggesting that mutations tend to act on single fitness components.

The distribution of recombination repair genes is linked to information content in bacteria

The concept of a 'proteomic constraint' proposes that the information content of the proteome exerts a selective pressure to reduce mutation rates, implying that larger proteomes produce a greater selective pressure to evolve or maintain DNA repair, resulting in a decrease in mutational load. Here, the distribution of 21 recombination repair genes was characterized across 900 bacterial genomes. Consistent with prediction, the presence of 17 genes correlated with proteome size. Intracellular bacteria were marked by a pervasive absence of recombination repair genes, consistent with their small proteome sizes, but also consistent with alternative explanations that reduced effective population size or lack of recombination may decrease selection pressure. However, when only non-intracellular bacteria were examined, the relationship between proteome size and gene presence was maintained. In addition, the more widely distributed (i.e. conserved) a gene, the smaller the average size of the proteomes from which it was absent. Together, these observations are consistent with the operation of a proteomic constraint on DNA repair. Lastly, a correlation between gene absence and genome AT content was shown, indicating a link between absence of DNA repair and elevated genome AT content.

Convergent evolution of heat-inducibility during subfunctionalization of the Hsp70 gene family

BACKGROUND

Heat-shock proteins of the 70 kDa family (Hsp70s) are essential chaperones required for key cellular functions. In eukaryotes, four subfamilies can be distinguished according to their function and localisation in different cellular compartments: cytosol, endoplasmic reticulum, mitochondria and chloroplasts. Generally, multiple cytosol-type Hsp70s can be found in metazoans that show either constitutive expression and/or stress-inducibility, arguing for the evolution of different tasks and functions. Information about the hsp70 copy number and diversity in microbial eukaryotes is, however, scarce, and detailed knowledge about the differential gene expression in most protists is lacking. Therefore, we have characterised the Hsp70 gene family of Paramecium caudatum to gain insight into the evolution and differential heat stress response of the distinct family members in protists and to investigate the diversification of eukaryotic hsp70s focusing on the evolution of heat-inducibility.

RESULTS

Eleven putative hsp70 genes could be detected in P. caudatum comprising homologs of three major Hsp70-subfamilies. Phylogenetic analyses revealed five evolutionarily distinct Hsp70-groups, each with a closer relationship to orthologous sequences of Paramecium tetraurelia than to another P. caudatum Hsp70-group. These highly diverse, paralogous groups resulted from duplications preceding Paramecium speciation, underwent divergent evolution and were subject to purifying selection. Heat-shock treatments were performed to test for differential expression patterns among the five Hsp70-groups as well as for a functional conservation within Paramecium. These treatments induced exceptionally high mRNA up-regulations in one cytosolic group with a low basal expression, indicative for the major heat inducible hsp70s. All other groups showed comparatively high basal expression levels and moderate heat-inducibility, signifying constitutively expressed genes. Comparative EST analyses for P. tetraurelia hsp70s unveiled a corresponding expression pattern, which supports a functionally conserved evolution of the Hsp70 gene family in Paramecium.

CONCLUSIONS

Our analyses suggest an independent evolution of the heat-inducible cytosol-type hsp70s in Paramecium and in its close relative Tetrahymena, as well as within higher eukaryotes. This result indicates convergent evolution during hsp70 subfunctionalization and implies that heat-inducibility evolved several times during the course of eukaryotic evolution.

Genetic hitchhiking and the dynamic buildup of genomic divergence during speciation with gene flow

A major issue in evolutionary biology is explaining patterns of differentiation observed in population genomic data, as divergence can be due to both direct selection on a locus and genetic hitchhiking. "Divergence hitchhiking" (DH) theory postulates that divergent selection on a locus reduces gene flow at physically linked sites, facilitating the formation of localized clusters of tightly linked, diverged loci. "Genome hitchhiking" (GH) theory emphasizes genome-wide effects of divergent selection. Past theoretical investigations of DH and GH focused on static snapshots of divergence. Here, we used simulations assessing a variety of strengths of selection, migration rates, population sizes, and mutation rates to investigate the relative importance of direct selection, GH, and DH in facilitating the dynamic buildup of genomic divergence as speciation proceeds through time. When divergently selected mutations were limiting, GH promoted divergence, but DH had little measurable effect. When populations were small and divergently selected mutations were common, DH enhanced the accumulation of weakly selected mutations, but this contributed little to reproductive isolation. In general, GH promoted reproductive isolation by reducing effective migration rates below that due to direct selection alone, and was important for genome-wide "congealing" or "coupling" of differentiation (F(ST)) across loci as speciation progressed.

Drift-barrier hypothesis and mutation-rate evolution

Mutation dictates the tempo and mode of evolution, and like all traits, the mutation rate is subject to evolutionary modification. Here, we report refined estimates of the mutation rate for a prokaryote with an exceptionally small genome and for a unicellular eukaryote with a large genome. Combined with prior results, these estimates provide the basis for a potentially unifying explanation for the wide range in mutation rates that exists among organisms. Natural selection appears to reduce the mutation rate of a species to a level that scales negatively with both the effective population size (N(e)), which imposes a drift barrier to the evolution of molecular refinements, and the genomic content of coding DNA, which is proportional to the target size for deleterious mutations. As a consequence of an expansion in genome size, some microbial eukaryotes with large N(e) appear to have evolved mutation rates that are lower than those known to occur in prokaryotes, but multicellular eukaryotes have experienced elevations in the genome-wide deleterious mutation rate because of substantial reductions in N(e).