Variation of nonsynonymous/synonymous rate ratios at HLA genes over time and phylogenetic context

Variation of nonsynonymous/synonymous rate ratios at HLA genes over time and phylogenetic context

B&aacuterbara D Bitarello, Rodrigo dos Santos Francisco, Diogo Meyer

Many HLA loci show an excess of nonsynonymous (dN) with respect to synonymous (dS) substitutions at codons of the antigen recognition site (ARS), a hallmark of adaptive evolution. However, it remains unclear how these changes are distributed over time and across branches of the HLA phylogeny. In particular, although HLA alleles can be assigned to functionally and phylogenetically defined groups (“lineages”), a test for differences in ω (ω = dN/dS) within and between lineages is lacking. We analysed variation of ω across divergence times and phylogenetic contexts (placement of branches in the phylogeny). We found a significant positive correlation between ω at ARS codons and divergence time, and that branches between lineages have higher ω than those within lineages. The excess of nonsynonymous hanges between lineages attained significance when we used non-ARS codons to account for the fact that, even under purifying selection, ω is inflated for recently diverged alleles. Although less intensely selected, within-lineage variation at ARS codons bears evidence of selection, in the form of higher ω than those of non-ARS codons. Our results show that ω ratios of class I HLA genes vary over time, and are higher in branches connecting alleles from distinct lineages. These results suggest that although within-lineage variation bears evidence of balancing selection, the between-lineage changes have been more intensely selected. Our findings indicate the importance of considering the effect of timescale when analysing ω values over a wide spectrum of divergences, and the value of using additional markers (in our case the tightly linked non-ARS codons) to account for the temporal dynamics of ω.

Dead or just asleep? Variance of microsatellite allele distributions in the human Y-chromosome.

Dead or just asleep? Variance of microsatellite allele distributions in the human Y-chromosome.

Joe Flood

Several different methods confirm that a number of micro-satellites on the human Y-chromosome have allele distributions with different variances in different haplogroups, after adjusting for coalescent times. This can be demonstrated through both heteroscedasticity tests and by poor correlation of the variance vectors in different subclades. The most convincing demonstration however is the complete inactivity of some markers in certain subclades – “microsatellite death”, while they are still active in companion subclades. Many microsatellites have declined in activity as they proceed down through descendant subclades. This appears to confirm the theory of microsatellite life cycles, in which point mutations cause a steady decay in activity. However, the changes are too fast to be caused by point mutations alone, and slippage events may be implicated. The rich microsatellite terrain exposed in our large single-haplotype samples provides new opportunities for genotyping and analysis.

The impact of macroscopic epistasis on long-term evolutionary dynamics

The impact of macroscopic epistasis on long-term evolutionary dynamics

Benjamin H. Good, Michael M. Desai
(Submitted on 18 Aug 2014)

Genetic interactions can strongly influence the fitness effects of individual mutations, yet the impact of these epistatic interactions on evolutionary dynamics remains poorly understood. Here we investigate the evolutionary role of epistasis over 50,000 generations in a well-studied laboratory evolution experiment in E. coli. The extensive duration of this experiment provides a unique window into the effects of epistasis during long-term adaptation to a constant environment. Guided by analytical results in the weak-mutation limit, we develop a computational framework to assess the compatibility of a given epistatic model with the observed patterns of fitness gain and mutation accumulation through time. We find that the average fitness trajectory alone provides little power to distinguish between competing models, including those that lack any direct epistatic interactions between mutations. However, when combined with the mutation trajectory, these observables place strong constraints on the set of possible models of epistasis, ruling out most existing explanations of the data. Instead, we find the strongest support for a “two-epoch” model of adaptation, in which an initial burst of diminishing returns epistasis is followed by a steady accumulation of mutations under a constant distribution of fitness effects. Our results highlight the need for additional DNA sequencing of these populations, as well as for more sophisticated models of epistasis that are compatible with all of the experimental data.

The Genetic Legacy of the Expansion of Turkic-Speaking Nomads Across Eurasia

The Genetic Legacy of the Expansion of Turkic-Speaking Nomads Across Eurasia

Bayazit Yunusbayev, Mait Metspalu, Ene Metspalu, Albert Valeev, Sergei Litvinov, Ruslan Valiev, Vita Akhmetova, Elena Balanovska, Oleg Balanovsky, Shahlo Turdikulova, Dilbar Dalimova, Pagbajabyn Nymadawa, Ardeshir Bahmanimehr, Hovhannes Sahakyan, Kristiina Tambets, Sardana Fedorova, Nikolay Barashkov, Irina Khidiatova, Evelin Mihailov, Rita Khusainova, Larisa Damba, Miroslava Derenko, Boris Malyarchuk, Ludmila Osipova, Mikhail Voevoda, Levon Yepiskoposyan, Toomas Kivisild, Elza Khusnutdinova, Richard Villems

The Turkic peoples represent a diverse collection of ethnic groups defined by the Turkic languages. These groups have dispersed across a vast area, including Siberia, Northwest China, Central Asia, East Europe, the Caucasus, Anatolia, the Middle East, and Afghanistan. The origin and early dispersal history of the Turkic peoples is disputed, with candidates for their ancient homeland ranging from the Transcaspian steppe to Manchuria in Northeast Asia. Previous genetic studies have not identified a clear-cut unifying genetic signal for the Turkic peoples, which lends support for language replacement rather than demic diffusion as the model for the Turkic language?s expansion. We addressed the genetic origin of 373 individuals from 22 Turkic-speaking populations, representing their current geographic range, by analyzing genome-wide high-density genotype data. Most of the Turkic peoples studied, except those in Central Asia, genetically resembled their geographic neighbors, in agreement with the elite dominance model of language expansion. However, western Turkic peoples sampled across West Eurasia shared an excess of long chromosomal tracts that are identical by descent (IBD) with populations from present-day South Siberia and Mongolia (SSM), an area where historians center a series of early Turkic and non-Turkic steppe polities. The observed excess of long chromosomal tracts IBD (> 1cM) between populations from SSM and Turkic peoples across West Eurasia was statistically significant. Finally, we used the ALDER method and inferred admixture dates (~9th?17th centuries) that overlap with the Turkic migrations of the 5th?16th centuries. Thus, our results indicate historical admixture among Turkic peoples, and the recent shared ancestry with modern populations in SSM supports one of the hypothesized homelands for their nomadic Turkic and related Mongolic ancestors.

MIPSTR: a method for multiplex genotyping of germ-line and somatic STR variation across many individuals

MIPSTR: a method for multiplex genotyping of germ-line and somatic STR variation across many individuals
Keisha Dawn Carlson, Peter H Sudmant, Maximilian Oliver Press, Evan E Eichler, Jay Shendure, Christine Queitsch

Abstract Short tandem repeats (STRs) are highly mutable genetic elements that often reside in functional genomic regions. The cumulative evidence of genetic studies on individual STRs suggests that STR variation profoundly affects phenotype and contributes to trait heritability. Despite recent advances in sequencing technology, STR variation has remained largely inaccessible across many individuals compared to single nucleotide variation or copy number variation. STR genotyping with short-read sequence data is confounded by (1) the difficulty of uniquely mapping short, low-complexity reads and (2) the high rate of STR amplification stutter. Here, we present MIPSTR, a robust, scalable, and affordable method that addresses these challenges. MIPSTR uses targeted capture of STR loci by single-molecule Molecular Inversion Probes (smMIPs) and a unique mapping strategy. Targeted capture and mapping strategy resolve the first challenge; the use of single molecule information resolves the second challenge. Unlike previous methods, MIPSTR is capable of distinguishing technical error due to amplification stutter from somatic STR mutations. In proof-of-principle experiments, we use MIPSTR to determine germ-line STR genotypes for 102 STR loci with high accuracy across diverse populations of the plant A. thaliana. We show that putatively functional STRs may be identified by deviation from predicted STR variation and by association with quantitative phenotypes. Employing DNA mixing experiments and a mutant deficient in DNA repair, we demonstrate that MIPSTR can detect low-frequency somatic STR variants. MIPSTR is applicable to any organism with a high-quality reference genome and is scalable to genotyping many thousands of STR loci in thousands of individuals.

An estimate of the average number of recessive lethal mutations carried by humans

An estimate of the average number of recessive lethal mutations carried by humans
Ziyue Gao, Darrel Waggoner, Matthew Stephens, Carole Ober, Molly Przeworski
(Submitted on 28 Jul 2014)

The effects of inbreeding on human health depend critically on the number and severity of recessive, deleterious mutations carried by individuals. In humans, existing estimates of these quantities are based on comparisons between consanguineous and non-consanguineous couples, an approach that confounds socioeconomic and genetic effects of inbreeding. To circumvent this limitation, we focused on a founder population with almost complete Mendelian disease ascertainment and a known pedigree. By considering all recessive lethal diseases reported in the pedigree and simulating allele transmissions, we estimated that each haploid set of human autosomes carries on average 0.29 (95% credible interval [0.10, 0.83]) autosomal, recessive alleles that lead to complete sterility or severe disorders at birth or before reproductive age when homozygous. Comparison to existing estimates of the deleterious effects of all recessive alleles suggests that a substantial fraction of the burden of autosomal, recessive variants is due to single mutations that lead to death between birth and reproductive age. In turn, the comparison to estimates from other eukaryotes points to a surprising constancy of the average number of recessive lethal mutations across organisms with markedly different genome sizes.

Comparative Performance of Two Whole Genome Capture Methodologies on Ancient DNA Illumina Libraries

Comparative Performance of Two Whole Genome Capture Methodologies on Ancient DNA Illumina Libraries
Maria Avila-Arcos, Marcela Sandoval-Velasco, Hannes Schroeder, Meredith L Carpenter, Anna-Sapfo Malaspinas, Nathan Wales, Fernando Peñaloza, Carlos D Bustamante, M. Thomas P Gilbert

1. The application of whole genome capture (WGC) methods to ancient DNA (aDNA) promises to increase the efficiency of ancient genome sequencing. 2. We compared the performance of two recently developed WGC methods in enriching human aDNA within Illumina libraries built using both double-stranded (DSL) and single-stranded (SSL) build protocols. Although both methods effectively enriched aDNA, one consistently produced marginally better results, giving us the opportunity to further explore the parameters influencing WGC experiments. 3. Our results suggest that bait length has an important influence on library enrichment. Moreover, we show that WGC biases against the shorter molecules that are enriched in SSL preparation protocols. Therefore application of WGC to such samples is not recommended without future optimization. Lastly, we document the effect of WGC on other features including clonality, GC composition and repetitive DNA content of captured libraries. 4. Our findings provide insights for researchers planning to perform WGC on aDNA, and suggest future tests and optimization to improve WGC efficiency.

Author post: Hybrid origins and the earliest stages of diploidization in the highly successful recent polyploid Capsella bursa-pastoris

This guest post is by Gavin Douglas (@gmdougla), Stephen Wright (@stepheniwright), and Tanja Slotte (@tanjaslotte) on their paper Douglas et al. Hybrid origins and the earliest stages of diploidization in the highly successful recent polyploid Capsella bursa-pastoris. bioRxived here.

photo credit: Tanja Slotte

photo credit: Tanja Slotte

In this preprint we investigate the mode of origin and evolutionary consequences of polyploidy in the highly successful tetraploid plant Capsella bursa-pastoris. We analyze high-coverage massively parallel genomic sequence data and first show that C. bursa-pastoris is a recent hybrid of two Capsella lineages leading to C. grandiflora and C. orientalis. This settles a long-standing uncertainty regarding the origins of C. bursa-pastoris. Second, we investigate patterns of nonfunctionalization and gene loss, and while we find little evidence for rapid, massive genome-wide fractionation, our analyses suggest that there is a decrease in the efficacy of selection in this recently formed tetraploid.

Allopolyploid origins of Capsella bursa-pastoris

Determining the evolutionary origin of C. bursa-pastoris has proven to be difficult and many contradictory hypotheses have been suggested, including that the tetraploid is an autopolyploid of a single Capsella species. Part of the complication has been the relatively low levels of sequence divergence between homeologous gene copies, and across the diploid Capsella lineages. Given population genomic sequences from all three Capsella species mentioned, we were able to address this question again with several different approaches.
C. bursa-pastoris undergoes disomic inheritance, meaning that genes duplicated as a result of polyploidy (homeologs) are independently inherited. Thus, one of the major tasks with our genomic data was to partition out the sequences from the two homeologous subgenomes. Because of the low levels of sequence divergence between homeologs (3% on average), this can be a challenging task. We took two approaches to generate phased genome sequence for inferring species origins; de novo assembly of short reads and phasing of SNPs from mapping reads to the reference genome of the diploid Capsella rubella. Phylogenetic trees generated from de novo assemblies of these species overwhelmingly support one C. bursa-pastoris homeolog forming a clade with C. grandiflora and the other with C. orientalis. The distribution of SNPs and transposable elements shared between these species also strongly support this hybridization model, which we estimate occurred within the last 100-300,000 years.
One reason the hybrid origins of C. bursa-pastoris is exciting is due to the divergent evolution of its progenitor lineages. C. orientalis and C. grandiflora differ both in their mating system and geographical distribution. Given that C. bursa-pastoris is a highly successful weed found worldwide, it will be interesting in future work to assess whether this divergence between the C. orientalis and C. grandiflora lineages contributed to the tetraploid’s adaptability.

Decreased efficacy of selection in the recently arisen polyploid
Following genome duplications the majority of redundant loci are expected to become lost over time through the process of diploidization. This model has been supported by several ancient polyploid events, including in Arabidopsis. Capsella bursa-pastoris presents an interesting model for studying the early phases of diploidization, and allows for an investigation of the rate of gene loss as well as the relative importance of relaxed selection vs. positive selection during early stages of gene inactivation. We searched for large deletions spanning genes using several approaches both based on determination of exact breakpoints and by cross-referencing low-coverage regions in C. bursa-pastoris with other Capsella species. Although we identified proportionately more large deletions segregating in C. bursa-pastoris than in the diploids, we did not find evidence for massive genomic changes in the tetraploid.
We were able to demonstrate relaxation of selection by analyzing the site frequency spectrum of SNPs segregating at 0-fold nonsynonymous sites in the three Capsella species. We also investigated SNPs causing putatively deleterious effects, such as premature stop codons, segregating in the three Capsella. Many of these SNPs are shared between the three species, although segregating at low frequencies in C. grandiflora. Since this shared deleterious variation inherited from progenitors seems to be responsible for a large proportion of the earliest stages of gene degeneration, this data supports a model of genome fractionation that is given a “head start” from standing variation. A key message following from this result is that we should be giving more weight to purely historical explanations of gene loss when studying biased fractionation.

Extraordinarily wide genomic impact of a selective sweep associated with the evolution of sex ratio distorter suppression

Extraordinarily wide genomic impact of a selective sweep associated with the evolution of sex ratio distorter suppression
Emily A Hornett, Bruce Moran, Louise A Reynolds, Sylvain Charlat, Samuel Tazzyman, Nina Wedell, Chris D Jiggins, Gregory Hurst

Symbionts that distort their host?s sex ratio by favouring the production and survival of females are common in arthropods. Their presence produces intense Fisherian selection to return the sex ratio to parity, typified by the rapid spread of host ?suppressor? loci that restore male survival/development. In this study, we investigated the genomic impact of a selective event of this kind in the butterfly Hypolimnas bolina. Through linkage mapping we first identified a genomic region that was necessary for males to survive Wolbachia-induced killing. We then investigated the genomic impact of the rapid spread of suppression that converted the Samoan population of this butterfly from a 100:1 female-biased sex ratio in 2001, to a 1:1 sex ratio by 2006. Models of this process revealed the potential for a chromosome-wide selective sweep. To measure the impact directly, the pattern of genetic variation before and after the episode of selection was compared. Significant changes in allele frequencies were observed over a 25cM region surrounding the suppressor locus, alongside generation of linkage disequilibrium. The presence of novel allelic variants in 2006 suggests that the suppressor was introduced via immigration rather than through de novo mutation. In addition, further sampling in 2010 indicated that many of the introduced variants were lost or had reduced in frequency since 2006. We hypothesise that this loss may have resulted from a period of purifying selection – removing deleterious material that introgressed during the initial sweep. Our observations of the impact of suppression of sex ratio distorting activity reveal an extraordinarily wide genomic imprint, reflecting its status as one of the strongest selective forces in nature.

Sequencing of the human IG light chain loci from a hydatidiform mole BAC library reveals locus-specific signatures of genetic diversity

Sequencing of the human IG light chain loci from a hydatidiform mole BAC library reveals locus-specific signatures of genetic diversity

Corey T Watson, Karyn Meltz Steinberg, Tina A Graves-Lindsay, Rene L Warren, Maika Malig, Jacqueline E Schein, Richard K Wilson, Rob Holt, Evan Eichler, Felix Breden

Germline variation at immunoglobulin gene (IG) loci is critical for pathogen-mediated immunity, but establishing complete reference sequences in these regions is problematic because of segmental duplications and somatically rearranged source DNA. We sequenced BAC clones from the essentially haploid hydatidiform mole, CHM1, across the light chain IG loci, kappa (IGK) and lambda (IGL), creating single haplotype representations of these regions. The IGL haplotype is 1.25Mb of contiguous sequence with four novel V gene and one novel C gene alleles and an 11.9kbp insertion. The IGK haplotype consists of two 644kbp proximal and 466kbp distal contigs separated by a gap also present in the reference genome sequence. Our effort added an additional 49kbp of unique sequence extending into this gap. The IGK haplotype contains six novel V gene and one novel J gene alleles and a 16.7kbp region with increased sequence identity between the two IGK contigs, exhibiting signatures of interlocus gene conversion. Our data facilitated the first comparison of nucleotide diversity between the light and IG heavy (IGH) chain haplotypes within a single genome, revealing a three to six fold enrichment in the IGH locus, supporting the theory that the heavy chain may be more important in determining antigenic specificity.