Agriculture driving male expansion in Neolithic Time
Chuan-Chao Wang, Yunzhi Huang, Shao-Qing Wen, Chun Chen, Li Jin, Hui Li
(Submitted on 27 Nov 2013)
The emergence of agriculture is suggested to have driven extensive human population growths. However, genetic evidence from maternal mitochondrial genomes suggests major population expansions began before the emergence of agriculture. Therefore, role of agriculture that played in initial population expansions still remains controversial. Here, we analyzed a set of globally distributed whole Y chromosome and mitochondrial genomes of 526 male samples from 1000 Genome Project. We found that most major paternal lineage expansions coalesced in Neolithic Time. The estimated effective population sizes through time revealed strong evidence for 10- to 100-fold increase in population growth of males with the advent of agriculture. This sex-biased Neolithic expansion might result from the reduction in hunting-related mortality of males.
Population genetics and substitution models of adaptive evolution
Mario dos Reis
(Submitted on 26 Nov 2013)
The ratio of non-synonymous to synonymous substitutions ω(=dN/dS) has been widely used as a measure of adaptive evolution in protein coding genes. Omega can be defined in terms of population genetics parameters as the fixation ratio of selected vs. neutral mutants. Here it is argued that approaches based on the infinite sites model are not appropriate to define ω for single codon locations. Simple models of amino acid substitution with reversible mutation and selection are analysed, and used to define ω under several evolutionary scenarios. In most practical cases ω1 can be sometimes expected for single locations at equilibrium. An example with influenza data is discussed.
The effect of linkage on establishment and survival of locally beneficial mutations
Simon Aeschbacher, Reinhard Buerger
(Submitted on 25 Nov 2013)
When organisms adapt to spatially heterogeneous environments, selection may drive divergence at multiple genes. If populations under divergent selection also exchange migrants, we expect genetic differentiation to be high at selected loci, relative to the baseline caused by migration and genetic drift. Indeed, empirical studies have found peaks of putatively adaptive differentiation. These are highly variable in length, some of them extending over several hundreds of thousands of base pairs. How can such `islands of differentiation’ be explained? Physical linkage produces elevated levels of differentiation at loci close to genes under selection. However, whether this is enough to account for the observed patterns of divergence is not well understood. Here, we investigate the fate of a locally beneficial mutation that arises in linkage to an existing migration-selection polymorphism and derive two important quantities: the probability that the mutation becomes established, and the expected time to its extinction. We find that intermediate levels of recombinations are sometimes favourable, and that physical linkage can lead to strongly elevated invasion probabilities and extinction times. We provide a rule of thumb for when this is the case. Moreover, we quantify the long-term effect of polygenic local adaptation on linked neutral variation.
Interspecific Introgressive Origin of Genomic Diversity in the House Mouse
Kevin J. Liu, Ying Song, Michael H. Kohn, Luay Nakhleh
(Submitted on 22 Nov 2013)
We report on a genome-wide scan for introgression in a eukaryote. The scan identified kilobase-to-megabase-long regions of introgressive origin involving Mus spretus in six Mus musculus domesticus chromosomes, based on genomes sampled from and near the European range of sympatry. Our analyses point to the introgression of both adaptive driver and linked passenger loci. Introgression could transfer traits, such as the discovered warfarin resistance in European M. m. domesticus, and could create new traits, as we infer using a functional network analysis. Our study sheds new light on the extent of adaptive introgession and calls for new analyses of eukaryotic genomes that explicitly account for the possibility of introgression.
Generation of high-resolution a priori Y-chromosome phylogenies using “next-generation” sequencing data
Gregory R Magoon, Raymond H Banks, Christian Rottensteiner, Bonnie E Schrack, Vincent O Tilroe, Andrew J Grierson
An approach for generating high-resolution a priori maximum parsimony Y-chromosome (chrY) phylogenies based on SNP and small INDEL variant data from massively-parallel short-read (next-generation) sequencing data is described; the tree-generation methodology produces annotations localizing mutations to individual branches of the tree, along with indications of mutation placement uncertainty in cases for which “no-calls” (through lack of mapped reads or otherwise) at particular site precludes a precise placement of the mutation. The approach leverages careful variant site filtering and a novel iterative reweighting procedure to generate high-accuracy trees while considering variants in regions of chrY that had previously been excluded from analyses based on short-read sequencing data. It is argued that the proposed approach is also superior to previous region-based filtering approaches in that it adapts to the quality of the underlying data and will automatically allow the scope of sites considered to expand as the underlying data quality improves (e.g. through longer read lengths). Key related issues, including calling of genotypes for the hemizygous chrY, reliability of variant results, read mismappings and “heterozygous” genotype calls, and the mutational stability of different variants are discussed and taken into account. The methodology is demonstrated through application to a dataset consisting of 1292 male samples from diverse populations and haplogroups, with the majority coming from low-coverage sequencing by the 1000 Genomes Project. Application of the tree-generation approach to these data produces a tree involving over 120,000 chrY variant sites (about 45,000 sites if singletons are excluded). The utility of this approach in refining the Y-chromosome phylogenetic tree is demonstrated by examining results for several haplogroups. The results indicate a number of new branches on the Y-chromosome phylogenetic tree, many of them subdividing known branches, but also including some that inform the presence of additional levels along the trunk of the tree. Finally, opportunities for extensions of this phylogenetic analysis approach to other types of genetic data are examined.
Computational inference beyond Kingman’s coalescent
Jere Koskela, Paul Jenkins, Dario Spano
(Submitted on 22 Nov 2013)
Full likelihood inference under Kingman’s coalescent is a computationally challenging problem to which importance sampling (IS) and the product of approximate conditionals (PAC) method have been applied successfully. Both methods can be expressed in terms of families of intractable conditional sampling distributions (CSDs), and rely on principled approximations for accurate inference. Recently, more general Λ- and Ξ-coalescents have been observed to provide better modelling fits to some genetic data sets. We derive families of approximate CSDs for finite sites Λ- and Ξ-coalescents, and use them to obtain “approximately optimal” IS and PAC algorithms for Λ-coalescents, yielding substantial gains in efficiency over existing methods.
Comment on “TopHat2: accurate alignment of transcriptomes in the presence of insertions, deletions and gene fusions” by Kim et al.
Alexander Dobin, Thomas R Gingeras
In the recent paper by Kim et al. (Genome biology, 2013. 14(4): p. R36) the accuracy of TopHat2 was compared to other RNA-seq aligners. In this comment we re-examine most important analyses from this paper and identify several deficiencies that significantly diminished performance of some of the aligners, including incorrect choice of mapping parameters, unfair comparison metrics, and unrealistic simulated data. Using STAR (Dobin et al., Bioinformatics, 2013. 29(1): p. 15-21) as an exemplar, we demonstrate that correcting these deficiencies makes its accuracy equal or better than that of TopHat2. Furthermore, this exercise highlighted some serious issues with the TopHat2 algorithms, such as poor recall of alignments with a moderate (>3) number of mismatches, low sensitivity and high false discovery rate for splice junction detection, loss of precision for the realignment algorithm, and large number of false chimeric alignments.
Population genetic consequences of the Allee effect and the role of offspring-number variation
Meike J. Wittmann, Wilfried Gabriel, Dirk Metzler
(Submitted on 21 Nov 2013)
A strong demographic Allee effect in which the expected population growth rate is negative below a certain critical population size can cause high extinction probabilities in small introduced populations. However, many species are repeatedly introduced to the same location and eventually one population may overcome the Allee effect by chance. With the help of stochastic models, we investigate how much genetic diversity such successful populations harbour on average and how this depends on offspring-number variation, an important source of stochastic variability in population size. We find that with increasing variability, the Allee effect increasingly promotes genetic diversity in successful populations. Successful Allee-effect populations with highly variable population dynamics escape rapidly from the region of small population sizes and do not linger around the critical population size. Therefore, they are exposed to relatively little genetic drift. We show that here—unlike in classical population genetics models—the role of offspring-number variation cannot be accounted for by an effective-population-size correction. Thus, our results highlight the importance of detailed biological knowledge, in this case on the probability distribution of family sizes, when predicting the evolutionary potential of newly founded populations or when using genetic data to reconstruct their demographic history.
Calibrated birth-death phylogenetic time-tree priors for Bayesian inference
Joseph Heled, Alexei J.Drummond
(Submitted on 19 Nov 2013)
Here we introduce a general class of multiple calibration birth-death tree priors for use in Bayesian phylogenetic inference. All tree priors in this class separate ancestral node heights into a set of “calibrated nodes” and “uncalibrated nodes” such that the marginal distribution of the calibrated nodes is user-specified whereas the density ratio of the birth-death prior is retained for trees with equal values for the calibrated nodes. We describe two formulations, one in which the calibration information informs the prior on ranked tree topologies, through the (conditional) prior, and the other which factorizes the prior on divergence times and ranked topologies, thus allowing uniform, or any arbitrary prior distribution on ranked topologies. While the first of these formulations has some attractive properties the algorithm we present for computing its prior density is computationally intensive. On the other hand, the second formulation is always computationally efficient. We demonstrate the utility of the new class of multiple-calibration tree priors using both small simulations and a real-world analysis and compare the results to existing schemes. The two new calibrated tree priors described in this paper offer greater flexibility and control of prior specification in calibrated time-tree inference and divergence time dating, and will remove the need for indirect approaches to the assessment of the combined effect of calibration densities and tree process priors in Bayesian phylogenetic inference.
Natural Allelic Variations of Xenobiotic Enzymes Pleiotropically Affect Sexual Dimorphism in Oryzias latipes
Takafumi Katsumura, Shoji Oda, Shigeki Nakagome, Tsunehiko Hanihara, Hiroshi Kataoka, Hiroshi Mitani, Shoji Kawamura, Hiroki Oota
Sexual dimorphisms, which are phenotypic differences between males and females, are driven by sexual selection [1, 2]. Interestingly, sexually selected traits show geographic variations within species despite strong directional selective pressures [3, 4]. However, genetic factors that regulate varied sexual differences remain unknown. In this study, we show that polymorphisms in cytochrome P450 (CYP) 1B1, which encodes a xenobiotic-metabolising enzyme, are associated with local differences of sexual dimorphisms in the anal fin morphology of medaka fish (Oryzias latipes). High and low activity CYP1B1 alleles increased and decreased differences in anal fin sizes, respectively. Behavioural and phylogenetic analyses suggest maintenance of the high activity allele by sexual selection, whereas the low activity allele may have evolved by positive selection due to by-product effects of CYP1B1. The present data can elucidate evolutionary mechanisms behind genetic variations in sexual dimorphism and indicate pleiotropic effects of xenobiotic enzymes.