SWAMP: Sliding Window Alignment Masker for PAML.

With the greater availability of genetic data, large genome-wide scans for positive selection increasingly incorporate data from a range of sources. These data sets may be derived from different sequencing methods, each of which has potential sources of error. Sequencing errors, compounded by alignment errors, greatly increase the number of false positives in tests for adaptive evolution. Genome-wide analyses often fail to fully address these issues or to provide sufficient detail on postalignment masking/filtering. Here, we introduce a Sliding Window Alignment Masker for Phylogenetic Analysis by Maximum Likelihood (SWAMP) that scans multiple-sequence alignments for short regions enriched with unreasonably high rates of nonsynonymous substitutions caused, for example, by sequence or alignment errors. SWAMP prevents their inclusion in downstream evolutionary analyses and therefore increases the reliability of downstream analyses. It is able to effectively mask short stretches of erroneous sequence, particularly prevalent in low-coverage genomes, which may not be detected by existing methods based on filtering by sitewise conservation or alignment confidence. SWAMP offers a flexible masking approach, and the user can apply different masking regimens to specific branches or sequences in the phylogeny allowing the stringency of masking to vary according to branch length, expected divergence levels, or assembly quality. We exemplify SWAMPs effectiveness on a dataset of 6,379 protein-coding genes from primate species, including data of variable quality. Full reporting of the software parameters will further improve the reproducibility of genome-wide analyses, as well as reduce false-positive rates.

Insights into hominid evolution from the gorilla genome sequence.

Gorillas are humans' closest living relatives after chimpanzees, and are of comparable importance for the study of human origins and evolution. Here we present the assembly and analysis of a genome sequence for the western lowland gorilla, and compare the whole genomes of all extant great ape genera. We propose a synthesis of genetic and fossil evidence consistent with placing the human-chimpanzee and human-chimpanzee-gorilla speciation events at approximately 6 and 10 million years ago. In 30% of the genome, gorilla is closer to human or chimpanzee than the latter are to each other; this is rarer around coding genes, indicating pervasive selection throughout great ape evolution, and has functional consequences in gene expression. A comparison of protein coding genes reveals approximately 500 genes showing accelerated evolution on each of the gorilla, human and chimpanzee lineages, and evidence for parallel acceleration, particularly of genes involved in hearing. We also compare the western and eastern gorilla species, estimating an average sequence divergence time 1.75 million years ago, but with evidence for more recent genetic exchange and a population bottleneck in the eastern species. The use of the genome sequence in these and future analyses will promote a deeper understanding of great ape biology and evolution.

PhyloSim - Monte Carlo simulation of sequence evolution in the R statistical computing environment.

BACKGROUND: The Monte Carlo simulation of sequence evolution is routinely used to assess the performance of phylogenetic inference methods and sequence alignment algorithms. Progress in the field of molecular evolution fuels the need for more realistic and hence more complex simulations, adapted to particular situations, yet current software makes unreasonable assumptions such as homogeneous substitution dynamics or a uniform distribution of indels across the simulated sequences. This calls for an extensible simulation framework written in a high-level functional language, offering new functionality and making it easy to incorporate further complexity. RESULTS: PhyloSim is an extensible framework for the Monte Carlo simulation of sequence evolution, written in R, using the Gillespie algorithm to integrate the actions of many concurrent processes such as substitutions, insertions and deletions. Uniquely among sequence simulation tools, PhyloSim can simulate arbitrarily complex patterns of rate variation and multiple indel processes, and allows for the incorporation of selective constraints on indel events. User-defined complex patterns of mutation and selection can be easily integrated into simulations, allowing PhyloSim to be adapted to specific needs. CONCLUSIONS: Close integration with R and the wide range of features implemented offer unmatched flexibility, making it possible to simulate sequence evolution under a wide range of realistic settings. We believe that PhyloSim will be useful to future studies involving simulated alignments.

Assigning strains to bacterial species via the internet.

BACKGROUND: Methods for assigning strains to bacterial species are cumbersome and no longer fit for purpose. The concatenated sequences of multiple house-keeping genes have been shown to be able to define and circumscribe bacterial species as sequence clusters. The advantage of this approach (multilocus sequence analysis; MLSA) is that, for any group of related species, a strain database can be produced and combined with software that allows query strains to be assigned to species via the internet. As an exemplar of this approach, we have studied a group of species, the viridans streptococci, which are very difficult to assign to species using standard taxonomic procedures, and have developed a website that allows species assignment via the internet. RESULTS: Seven house-keeping gene sequences were obtained from 420 streptococcal strains to produce a viridans group database. The reference tree produced using the concatenated sequences identified sequence clusters which, by examining the position on the tree of the type strain of each viridans group species, could be equated with species clusters. MLSA also identified clusters that may correspond to new species, and previously described species whose status needs to be re-examined. A generic website and software for electronic taxonomy was developed. This site http://www.eMLSA.net allows the sequences of the seven gene fragments of a query strain to be entered and for the species assignment to be returned, according to its position within an assigned species cluster on the reference tree. CONCLUSION: The MLSA approach resulted in the identification of well-resolved species clusters within this taxonomically challenging group and, using the software we have developed, allows unknown strains to be assigned to viridans species via the internet. Submission of new strains will provide a growing resource for the taxonomy of viridans group streptococci, allowing the recognition of potential new species and taxonomic anomalies. More generally, as the software at the MLSA website is generic, MLSA schemes and strain databases for other groups of related species can be hosted at this website, providing a portal for microbial electronic taxonomy.

PhyloWidget: web-based visualizations for the tree of life.

UNLABELLED: PhyloWidget is a web-based tool for the visualization and manipulation of phylogenetic tree data. It can be accessed online or downloaded as a standalone application. A simple URL-based API allows databases to easily link to and customize PhyloWidget for interactively viewing medium- to large-sized trees. AVAILABILITY: PhyloWidget is available for online use or download at http://www.phylowidget.org/.