Diversity of Mycobacterium tuberculosis across evolutionary scales

Diversity of Mycobacterium tuberculosis across evolutionary scales
Mary B O’Neill, Tatum D Mortimer, Caitlin S Pepperell
doi: http://dx.doi.org/10.1101/014217

Tuberculosis (TB) is a global public health emergency. Increasingly drug resistant strains of Mycobacterium tuberculosis (M.tb) continue to emerge and spread, highlighting the adaptability of this pathogen. Most studies of M.tb evolution have relied on ‘between-host’ samples, in which each person with TB is represented by a single M.tb isolate. However, individuals with TB commonly harbor populations of M.tb numbering in the billions. Here, we use analyses of M.tb diversity found within and between hosts to gain insight into the adaptation of this pathogen. We find that the amount of M.tb genetic diversity harbored by individuals with TB is similar to that of global between-host surveys of TB patients. This suggests that M.tb genetic diversity is generated within hosts and then lost as the infection is transmitted. In examining genomic data from M.tb samples within and between hosts with TB, we find that genes involved in the regulation, synthesis, and transportation of immunomodulatory cell envelope lipids appear repeatedly in the extremes of various statistical measures of diversity. Polyketide synthase and Mycobacterial membrane protein Large (mmpL) genes are particularly notable in this regard. In addition, we observe identical mutations emerging across samples from different TB patients. Taken together, our observations suggest that M.tb cell envelope lipids are targets of selection within hosts. These lipids are specific to pathogenic mycobacteria and, in some cases, human-pathogenic mycobacteria. We speculate that rapid adaptation of cell envelope lipids is facilitated by functional redundancy, flexibility in their metabolism, and their roles mediating interactions with the host.

Bayesian priors for tree calibration: Evaluating two new approaches based on fossil intervals

Bayesian priors for tree calibration: Evaluating two new approaches based on fossil intervals
Ryan W Norris, Cory L Strope, David M McCandlish, Arlin Stoltzfus
doi: http://dx.doi.org/10.1101/014340

Background: Studies of diversification and trait evolution increasingly rely on combining molecular sequences and fossil dates to infer time-calibrated phylogenetic trees. Available calibration software provides many options for the shape of the prior probability distribution of ages at a node to be calibrated, but the question of how to assign a Bayesian prior from limited fossil data remains open. Results: We introduce two new methods for generating priors based upon (1) the interval between the two oldest fossils in a clade, i.e., the penultimate gap (PenG), and (2) the ghost lineage length (GLin), defined as the difference between the oldest fossils for each of two sister lineages. We show that PenG and GLin/2 are point estimates of the interval between the oldest fossil and the true age for the node. Furthermore, given either of these quantities, we derive a principled prior distribution for the true age. This prior is log-logistic, and can be implemented approximately in existing software. Using simulated data, we test these new methods against some other approaches. Conclusions: When implemented as approaches for assigning Bayesian priors, the PenG and GLin methods increase the accuracy of inferred divergence times, showing considerably more precision than the other methods tested, without significantly greater bias. When implemented as approaches to post-hoc scaling of a tree by linear regression, the PenG and GLin methods exhibit less bias than other methods tested. The new methods are simple to use and can be applied to a variety of studies that call for calibrated trees.

Genetics of intra-species variation in avoidance behavior induced by a thermal stimulus in C. elegans

Genetics of intra-species variation in avoidance behavior induced by a thermal stimulus in C. elegans
doi: http://dx.doi.org/10.1101/014290

Individuals within a species vary in their responses to a wide range of stimuli, partly as a result of differences in their genetic makeup. Relatively little is known about the genetic and neuronal mechanisms contributing to diversity of behavior in natural populations. By studying animal-to-animal variation in innate avoidance behavior to thermal stimuli in the nematode Caenorhabditis elegans, we uncovered genetic principles of how different components of a behavioral response can be altered in nature to generate behavioral diversity. Using a thermal pulse assay, we uncovered heritable variation in responses to a transient temperature increase. Quantitative trait locus mapping revealed that separate components of this response were controlled by distinct genomic loci. The loci we identified contributed to variation in components of thermal pulse avoidance behavior in an additive fashion. Our results show that the escape behavior induced by thermal stimuli is composed of simpler behavioral components that are influenced by at least six distinct genetic loci. The loci that decouple components of the escape behavior reveal a genetic system that allows independent modification of behavioral parameters. Our work sets the foundation for future studies of evolution of innate behaviors at the molecular and neuronal level.

Partitioning heritability by functional category using GWAS summary statistics

Partitioning heritability by functional category using GWAS summary statistics
Hilary Kiyo Finucane, Brendan Bulik-Sullivan, Alexander Gusev, Gosia Trynka, Yakir Reshef, Po-Ru Loh, Verneri Anttilla, Han Xu, Chongzhi Zang, Kyle Farh, Stephan Ripke, Felix Day, ReproGen Consortium, Schizophrenia Working Group of the Psychiatric Genetics Consortium, RACI Consortium, Shaun Purcell, Eli Stahl, Sara Lindstrom, John R.B. Perry, Yukinori Okada, Soumya Raychaudhuri, Mark Daly, Nick Patterson, Benjamin M. Neale, Alkes L. Price
doi: http://dx.doi.org/10.1101/014241

Recent work has demonstrated that some functional categories of the genome contribute disproportionately to the heritability of complex diseases. Here, we analyze a broad set of functional elements, including cell-type-specific elements, to estimate their polygenic contributions to heritability in genome-wide association studies (GWAS) of 17 complex diseases and traits spanning a total of 1.3 million phenotype measurements. To enable this analysis, we introduce a new method for partitioning heritability from GWAS summary statistics while controlling for linked markers. This new method is computationally tractable at very large sample sizes, and leverages genome-wide information. Our results include a large enrichment of heritability in conserved regions across many traits; a very large immunological disease-specific enrichment of heritability in FANTOM5 enhancers; and many cell-type-specific enrichments including significant enrichment of central nervous system cell types in body mass index, age at menarche, educational attainment, and smoking behavior. These results demonstrate that GWAS can aid in understanding the biological basis of disease and provide direction for functional follow-up.

Genetic Variation, Not Cell Type of Origin, Underlies Regulatory Differences in iPSCs

Genetic Variation, Not Cell Type of Origin, Underlies Regulatory Differences in iPSCs
Courtney L Kagan, Nicholas E Banovich, Bryan J Pavlovic, Kristen Patterson, Irene Gallego Romero, Jonathan K Pritchard, Yoav Gilad
doi: http://dx.doi.org/10.1101/013888

The advent of induced pluripotent stem cells (iPSCs) revolutionized Human Genetics by allowing us to generate pluripotent cells from easily accessible somatic tissues. This technology can have immense implications for regenerative medicine, but iPSCs also represent a paradigm shift in the study of complex human phenotypes, including gene regulation and disease. Yet, an unresolved caveat of the iPSC model system is the extent to which reprogrammed iPSCs retain residual phenotypes from their precursor somatic cells. To directly address this issue, we used an effective study design to compare regulatory phenotypes between iPSCs derived from two types of commonly used somatic precursor cells. We show that the cell type of origin only minimally affects gene expression levels and DNA methylation in iPSCs. Instead, genetic variation is the main driver of regulatory differences between iPSCs of different donors.

Rates of karyotypic evolution in Estrildid finches differ between island and continental clades

Rates of karyotypic evolution in Estrildid finches differ between island and continental clades
Daniel M Hooper, Trevor D Price
doi: http://dx.doi.org/10.1101/013987

Reasons why chromosomal rearrangements spread to fixation and frequently distinguish related taxa remain poorly understood. We used cytological descriptions of karyotype to identify large pericentric inversions between species of Estrildid finches (family Estrildidae) and a time-dated phylogeny to assess the genomic, geographic, and phylogenetic context of karyotype evolution in this group. Inversions between finch species fixed at an average rate of one every 2.26 My. Inversions were twice as likely to fix on the sex chromosomes compared to the autosomes, possibly a result of their repeat density, and inversion fixation rate for all chromosomes scales with range size. Alternative mutagenic input explanations are not supported, as the number of inversions on a chromosome does not correlate with its length or map size. Inversions have fixed 3.3× faster in three continental clades than in two island chain clades, and fixation rate correlates with both range size and the number of sympatric species pairs. These results point to adaptation as the dominant mechanism driving fixation and suggest a role for gene flow in karyotype divergence. A review shows that the rapid karyotype evolution observed in the Estrildid finches appears to be more general across birds, and by implication other understudied taxa.

MultiMeta: an R package for meta-analysing multi-phenotype genome-wide association studies

MultiMeta: an R package for meta-analysing multi-phenotype genome-wide association studies
Dragana Vuckovic, Paolo Gasparini, Nicole Soranzo, Valentina Iotchkova
doi: http://dx.doi.org/10.1101/013920

Summary: As new methods for multivariate analysis of Genome Wide Association Studies (GWAS) become available, it is important to be able to combine results from different cohorts in a meta-analysis. The R package MultiMeta provides an implementation of the inverse-variance based method for meta-analysis, generalized to an n-dimensional setting. Availability: The R package MultiMeta can be downloaded from CRAN Contact: dragana.vuckovic@burlo.trieste.it