Retrotransposon evolution in diverse plant genomes

Retrotransposon or retrotransposon-like sequences have been reported to be conserved components of cereal centromeres. Here we show that the published sequences are derived from a single conventional Ty3-gypsy family or a nonautonomous derivative. Both autonomous and nonautonomous elements are likely to have colonized Poaceae centromeres at the time of a common ancestor but have been maintained since by active retrotransposition. The retrotransposon family is also present at a lower copy number in the Arabidopsis genome, where it shows less pronounced localization. The history of the family in the two types of genome provides an interesting contrast between "boom and bust" and persistent evolutionary patterns.     

Virus-derived variation in diverse human genomes

Acquisition of genetic material from viruses by their hosts can generate inter-host structural genome variation. We developed computational tools enabling us to study virus-derived structural variants (SVs) in population-scale whole genome sequencing (WGS) datasets and applied them to 3,332 humans. Although SVs had already been cataloged in these subjects, we found previously-overlooked virus-derived SVs. We detected non-germline SVs derived from squirrel monkey retrovirus (SMRV), human immunodeficiency virus 1 (HIV-1), and human T lymphotropic virus (HTLV-1); these variants are attributable to infection of the sequenced lymphoblastoid cell lines (LCLs) or their progenitor cells and may impact gene expression results and the biosafety of experiments using these cells. In addition, we detected new heritable SVs derived from human herpesvirus 6 (HHV-6) and human endogenous retrovirus-K (HERV-K). We report the first solo-direct repeat (DR) HHV-6 likely to reflect DR rearrangement of a known full-length endogenous HHV-6. We used linkage disequilibrium between single nucleotide variants (SNVs) and variants in reads that align to HERV-K, which often cannot be mapped uniquely using conventional short-read sequencing analysis methods, to locate previously-unknown polymorphic HERV-K loci. Some of these loci are tightly linked to trait-associated SNVs, some are in complex genome regions inaccessible by prior methods, and some contain novel HERV-K haplotypes likely derived from gene conversion from an unknown source or introgression. These tools and results broaden our perspective on the coevolution between viruses and humans, including ongoing virus-to-human gene transfer contributing to genetic variation between humans.     

An array of diverse microbial genomes

Methods for comparing gene frequencies across large, epidemiologically defined bacterial collections are limited. A novel microarray technology has been developed called 'library on a slide'. In this technology, hundreds of entire microbial genomes are arrayed, rather than sequences of a single genome or sets of genes. These slides can then be probed for the presence of specific genes allowing researchers to draw inferences regarding important differences between related strains that differ in their pathogenic potential.     

Pseudomonas genomes: diverse and adaptable

Members of the genus Pseudomonas inhabit a wide variety of environments, which is reflected in their versatile metabolic capacity and broad potential for adaptation to fluctuating environmental conditions. Here, we examine and compare the genomes of a range of Pseudomonas spp. encompassing plant, insect and human pathogens, and environmental saprophytes. In addition to a large number of allelic differences of common genes that confer regulatory and metabolic flexibility, genome analysis suggests that many other factors contribute to the diversity and adaptability of Pseudomonas spp. Horizontal gene transfer has impacted the capability of pathogenic Pseudomonas spp. in terms of disease severity (Pseudomonas aeruginosa) and specificity (Pseudomonas syringae). Genome rearrangements likely contribute to adaptation, and a considerable complement of unique genes undoubtedly contributes to strain- and species-specific activities by as yet unknown mechanisms. Because of the lack of conserved phenotypic differences, the classification of the genus has long been contentious. DNA hybridization and genome-based analyses show close relationships among members of P. aeruginosa, but that isolates within the Pseudomonas fluorescens and P. syringae species are less closely related and may constitute different species. Collectively, genome sequences of Pseudomonas spp. have provided insights into pathogenesis and the genetic basis for diversity and adaptation.     

Annotation of selection strengths in viral genomes

MOTIVATION: Viral genomes tend to code in overlapping reading frames to maximize informational content. This may result in atypical codon bias and particular evolutionary constraints. Due to the fast mutation rate of viruses, there is additional strong evidence for varying selection between intra- and intergenomic regions. The presence of multiple coding regions complicates the concept of K(a)/K(s) ratio, and thus begs for an alternative approach when investigating selection strengths. Building on the paper by McCauley and Hein, we develop a method for annotating a viral genome coding in overlapping reading frames. We introduce an evolutionary model capable of accounting for varying levels of selection along the genome, and incorporate it into our prior single sequence HMM methodology, extending it now to a phylogenetic HMM. Given an alignment of several homologous viruses to a reference sequence, we may thus achieve an annotation both of coding regions as well as selection strengths, allowing us to investigate different selection patterns and hypotheses. RESULTS: We illustrate our method by applying it to a multiple alignment of four HIV2 sequences, as well as of three Hepatitis B sequences. We obtain an annotation of the coding regions, as well as a posterior probability for each site of the strength of selection acting on it. From this we may deduce the average posterior selection acting on the different genes. Whilst we are encouraged to see in HIV2, that the known to be conserved genes gag and pol are indeed annotated as such, we also discover several sites of less stringent negative selection within the env gene. To the best of our knowledge, we are the first to subsequently provide a full selection annotation of the Hepatitis B genome by explicitly modelling the evolution within overlapping reading frames, and not relying on simple K(a)/K(s) ratios.     

Occurrence and analysis of imperfect microsatellites in diverse potyvirus genomes

Simple sequence repeats (SSRs) or microsatellites are known to exhibit ubiquitous across all kingdoms of life including viruses. However, imperfections in simple sequence repeats have been analyzed in genomes of human, Escherichia coli and Human Immunodeficiency virus. The assessment of compound microsatellites in plant viral genomes is yet to be studied. Potyviruses severely affect crop plant growth and reduce economic yield in diverse cropping systems worldwide. Hence, we analyze the nature and distribution of compound microsatellites present in complete genome of 45 potyvirus species. The results indicate that compound microsatellites accounted for about 0% to 15.15% of all microsatellites and have low complexity as compared to that of prokaryotic genomes. Overall, 14% of compound microsatellites were of similar motifs and such motif duplications were observed for CA, TA and AG repeats. Among all 45 potyvirus genomes analyzed, SSR couple (AG)-x-(AC) was found to be the most abundant one. Hence it is apparent that in contrast to eukaryotes, majority of compound microsatellites in potyviruses were composed of variant motifs. We also highlight the relative frequency of different classes of compound microsatellites as well as their patterns of distribution and correlate with biology of potyviruses. Further characterization of such variation is important for elucidating the origin, mutational processes, and structure of these widely used, but incompletely understood sequences.     

Patterns of positive selection in six Mammalian genomes

Genome-wide scans for positively selected genes (PSGs) in mammals have provided insight into the dynamics of genome evolution, the genetic basis of differences between species, and the functions of individual genes. However, previous scans have been limited in power and accuracy owing to small numbers of available genomes. Here we present the most comprehensive examination of mammalian PSGs to date, using the six high-coverage genome assemblies now available for eutherian mammals. The increased phylogenetic depth of this dataset results in substantially improved statistical power, and permits several new lineage- and clade-specific tests to be applied. Of approximately 16,500 human genes with high-confidence orthologs in at least two other species, 400 genes showed significant evidence of positive selection (FDR<0.05), according to a standard likelihood ratio test. An additional 144 genes showed evidence of positive selection on particular lineages or clades. As in previous studies, the identified PSGs were enriched for roles in defense/immunity, chemosensory perception, and reproduction, but enrichments were also evident for more specific functions, such as complement-mediated immunity and taste perception. Several pathways were strongly enriched for PSGs, suggesting possible co-evolution of interacting genes. A novel Bayesian analysis of the possible "selection histories" of each gene indicated that most PSGs have switched multiple times between positive selection and nonselection, suggesting that positive selection is often episodic. A detailed analysis of Affymetrix exon array data indicated that PSGs are expressed at significantly lower levels, and in a more tissue-specific manner, than non-PSGs. Genes that are specifically expressed in the spleen, testes, liver, and breast are significantly enriched for PSGs, but no evidence was found for an enrichment for PSGs among brain-specific genes. This study provides additional evidence for widespread positive selection in mammalian evolution and new genome-wide insights into the functional implications of positive selection.     

Low-cost sequencing opens new insights into diverse plant genomes

A report on the Plant Genomes and Biotechnology: From Genes to Networks meeting, held at Cold Spring Harbor Laboratory, 30 November to 3 December 2011.     

Positive and purifying selection in mitochondrial genomes of a bird with mitonuclear discordance

Diversifying selection on metabolic pathways can reduce intraspecific gene flow and promote population divergence. An opportunity to explore this arises from mitonuclear discordance observed in an Australian bird Eopsaltria australis. Across >1500 km, nuclear differentiation is low and latitudinally structured by isolation by distance, whereas two highly divergent, parapatric mitochondrial lineages (>6.6% in ND2) show a discordant longitudinal geographic pattern and experience different climates. Vicariance, incomplete lineage sorting and sex‐biased dispersal were shown earlier to be unlikely drivers of the mitonuclear discordance; instead, natural selection on a female‐linked trait was the preferred hypothesis. Accordingly, here we tested for signals of positive, divergent selection on mitochondrial genes in E. australis. We used codon models and physicochemical profiles of amino acid replacements to analyse complete mitochondrial genomes of the two mitochondrial lineages in E. australis, its sister species Eopsaltria griseogularis, and outgroups. We found evidence of positive selection on at least five amino acids, encoded by genes of two oxidative phosphorylation pathway complexes NADH dehydrogenase (ND4 and ND4L) and cytochrome bc₁ (cyt‐b) against a background of widespread purifying selection on all mitochondrial genes. Three of these amino acid replacements were fixed in ND4 of the geographically most widespread E. australis lineage. The other two replacements were fixed in ND4L and cyt‐b of the geographically more restricted E. australis lineage. We discuss whether this selection may reflect local environmental adaptation, a by‐product of other selective processes, or genetic incompatibilities, and propose how these hypotheses can be tested in future.     

Micro-scale signature of purifying selection in Marburg virus genomes

In the seven protein-coding genes in the Marburg virus (MARV) genome, the synonymous nucleotide diversity substantially exceeded the nonsynonymous nucleotide diversity, indicating strong purifying selection. Likewise, there was evidence of purifying selection on 5'UTR and 3'UTR, where nucleotide diversity (pi) was significantly less than piS in the coding regions. Nonsynonymous polymorphic sites showed significantly reduced mean gene diversity in comparison to other polymorphic sites, indicating that purifying selection at certain slightly deleterious nonsynonymous polymorphisms is ongoing. Moreover, nonsynonymous polymorphic sites showed significantly reduced gene diversity in comparison to adjacent synonymous sites, even though the vast majority of such adjacent synonymous sites were in the same codon or an adjacent codon. Thus purifying selection, in conjunction with recombination and/or backward mutation, can act to break up linkage relationships at a micro-scale in the MARV genome. The ability of purifying selection to break up linkage between synonymous and nonsynonymous polymorphisms on such a fine scale has not been reported in any other genome.     

Analysis of distribution indicates diverse functions of simple sequence repeats in Mycoplasma genomes

Simple sequence repeats (SSRs) composed of extensive tandem iterations of a single nucleotide or a short oligonucleotide are rare in most bacterial genomes, but they are common among Mycoplasma. Some of these repeats act as contingency loci in association with families of surface antigens. By contraction or expansion during replication, these SSRs increase genetic variance of the population and facilitate avoidance of the immune response of the host. Occurrence and distribution of SSRs are analyzed in complete genomes of 11 Mycoplasma and 3 related Mollicutes in order to gain insights into functional and evolutionary diversity of the SSRs in Mycoplasma. The results revealed an unexpected variety of SSRs with respect to their distribution and composition and suggest that it is unlikely that all SSRs function as contingency loci or recombination hot spots. Various types of SSRs are most abundant in Mycoplasma hyopneumoniae, whereas Mycoplasma penetrans, Mycoplasma mobile, and Mycoplasma synoviae do not contain unusually long SSRs. Mycoplasma hyopneumoniae and Mycoplasma pulmonis feature abundant short adenine and thymine runs periodically spaced at 11 and 12 bp, respectively, which likely affect the supercoiling propensities of the DNA molecule. Physiological roles of long adenine and thymine runs in M. hyopneumoniae appear independent of location upstream or downstream of genes, unlike contingency loci that are typically located in protein-coding regions or upstream regulatory regions. Comparisons among 3 M. hyopneumoniae strains suggest that the adenine and thymine runs are rarely involved in genome rearrangements. The results indicate that the SSRs in the Mycoplasma genomes play diverse roles, including modulating gene expression as contingency loci, facilitating genome rearrangements via recombination, affecting protein structure and possibly protein-protein interactions, and contributing to the organization of the DNA molecule in the cell.     

Detecting anomalous gene clusters and pathogenicity islands in diverse bacterial genomes.

A gene in a genome is defined as putative alien (pA) if its codon usage difference from the average gene exceeds a high threshold and codon usage differences from ribosomal protein genes, chaperone genes and protein-synthesis-processing factors are also high. pA gene clusters in bacterial genomes are relevant for detecting genomic islands (GIs), including pathogenicity islands (PAIs). Four other analyses appropriate to this task are G+C genome variation (the standard method); genomic signature divergences (dinucleotide bias); extremes of codon bias; and anomalies of amino acid usage. For example, the cagA domain of Helicobacter pylori is highly deviant in its genome signature and codon bias from the rest of the genome. Using these methods we can detect two potential PAIs in the Neisseria meningitidis genome, which contain hemagglutinin and/or hemolysin-related genes. Additionally, G+C variation and genome signature differences of the Mycobacterium tuberculosis genome indicate two pA gene clusters.     

Diversifying selection and host adaptation in two endosymbiont genomes

Background  

The endosymbiont Wolbachia pipientis infects a broad range of arthropod and filarial nematode hosts. These diverse associations form an attractive model for understanding host:symbiont coevolution. Wolbachia 's ubiquity and ability to dramatically alter host reproductive biology also form the foundation of research strategies aimed at controlling insect pests and vector-borne disease. The Wolbachia strains that infect nematodes are phylogenetically distinct, strictly vertically transmitted, and required by their hosts for growth and reproduction. Insects in contrast form more fluid associations with Wolbachia. In these taxa, host populations are most often polymorphic for infection, horizontal transmission occurs between distantly related hosts, and direct fitness effects on hosts are mild. Despite extensive interest in the Wolbachia system for many years, relatively little is known about the molecular mechanisms that mediate its varied interactions with different hosts. We have compared the genomes of the Wolbachia that infect Drosophila melanogaster, w Mel and the nematode Brugia malayi, w Bm to that of an outgroup Anaplasma marginale to identify genes that have experienced diversifying selection in the Wolbachia lineages. The goal of the study was to identify likely molecular mechanisms of the symbiosis and to understand the nature of the diverse association across different hosts.     

Detecting balancing selection in genomes: limits and prospects

In spite of the long‐term interest in the process of balancing selection, its frequency in genomes and evolutionary significance remain unclear due to challenges related to its detection. Current statistical approaches based on patterns of variation observed in molecular data suffer from low power and a high incidence of false positives. This raises the question whether balancing selection is rare or is simply difficult to detect. We discuss genetic signatures produced by this mode of selection and review the current approaches used for their identification in genomes. Advantages and disadvantages of the available methods are presented, and areas where improvement is possible are identified. Increased specificity and reduced rate of false positives may be achieved by using a demographic model, applying combinations of tests, appropriate sampling scheme and taking into account intralocus variation in selection pressures. We emphasize novel solutions, recently developed model‐based approaches and good practices that should be implemented in future studies looking for signals of balancing selection. We also draw attention of the readers to the results of recent theoretical studies, which suggest that balancing selection may be ubiquitous but transient, leaving few signatures detectable by existing methods. Testing this new theory may require the development of novel high‐throughput methods extending beyond genomic scans.     

Positive selection for unpreferred codon usage in eukaryotic genomes

Background  

Natural selection has traditionally been understood as a force responsible for pushing genes to states of higher translational efficiency, whereas lower translational efficiency has been explained by neutral mutation and genetic drift. We looked for evidence of directional selection resulting in increased unpreferred codon usage (and presumably reduced translational efficiency) in three divergent clusters of eukaryotic genomes using a simple optimal-codon-based metric (K_p/K_u).     

In-silico analysis of simple and imperfect microsatellites in diverse tobamovirus genomes

An in-silico analysis of simple sequence repeats (SSRs) in 30 species of tobamoviruses was done. SSRs (mono to hexa) were present with variant frequency across species. Compound microsatellites, primarily of variant motifs accounted for up to 11.43% of the SSRs. Motif duplications were observed for A, T, AT, and ACA repeats. (AG)–(TC) was the most prevalent SSR-couple. SSRs were differentially localized in the coding region with ~ 54% on the 128 kDa protein while 20.37% was exclusive to 186 kDa protein. Characterization of such variations is important for elucidating the origin, sequence variations, and structure of these widely used, but incompletely understood sequences.