Phylogeography of Y chromosome haplogroups A & B in Africa

A dissertation submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Medicine Johannesburg, 2014Evolution and historical events over the past 300 000 years have contributed in shaping the gene pool of sub-Saharan African populations. By examining patterns of Y chromosome variation, through the screening of single nucleotide polymorphisms (SNPs) and short tandem repeats (STRs), the present study aimed to characterise the phylogeography of ancient African Y chromosome haplogroups found in populations across sub-Saharan Africa, as well as understand the genetic affinities of these populations. In order to screen the large number of the markers required, seven multiplex single base extension assays were developed. These were used to refine the resolution of Y chromosomes commonly found in Africa, but also included a few markers to delineate the common non-African Y chromosome haplogroups, following a hierarchical screening process. In total, 1667 males were screened, and these data were compiled together with comparative published data. The resultant SNP and STR dataset was used in illustrating, more specifically, the phylogeographies of haplogroups A and B. The wide geographic distribution of haplogroup A, together with its position at the root of the phylogeny and high diversity, support an early diversification of the haplogroup into its subclades, which subsequently spread across Africa. The distribution of major haplogroup B subclades, however, are possibly due to post-glacial migrations in the case of haplogroup B-M112, and recent population expansions, leading to the common presence of haplogroup B-M152 across sub-Saharan Africa. The spread of haplogroup E, however, created the biggest impact on African populations; with its expansion likely resulting in the diminished presence of many of the subclades of haplogroups A and B. The Y chromosome compositions of present sub-Saharan African populations are, thus, the result of several diversification events, followed by migration, and mixing of population groups, over the course of modern human existence

Development of a single base extension method to resolve Y chromosome haplogroups in sub-Saharan African populations

<p>Abstract</p> <p>Background</p> <p>The ability of the Y chromosome to retain a record of its evolution has seen it become an essential tool of molecular anthropology. In the last few years, however, it has also found use in forensic genetics, providing information on the geographic origin of individuals. This has been aided by the development of efficient screening methods and an increased knowledge of geographic distribution. In this study, we describe the development of single base extension assays used to resolve 61 Y chromosome haplogroups, mainly within haplogroups A, B and E, found in Africa.</p> <p>Results</p> <p>Seven multiplex assays, which incorporated 60 Y chromosome markers, were developed. These resolved Y chromosomes to 61 terminal branches of the major African haplogroups A, B and E, while also including a few Eurasian haplogroups found occasionally in African males. Following its validation, the assays were used to screen 683 individuals from Southern Africa, including south eastern Bantu speakers (BAN), Khoe-San (KS) and South African Whites (SAW). Of the 61 haplogroups that the assays collectively resolved, 26 were found in the 683 samples. While haplogroup sharing was common between the BAN and KS, the frequencies of these haplogroups varied appreciably. Both groups showed low levels of assimilation of Eurasian haplogroups and only two individuals in the SAW clearly had Y chromosomes of African ancestry.</p> <p>Conclusions</p> <p>The use of these single base extension assays in screening increased haplogroup resolution and sampling throughput, while saving time and DNA. Their use, together with the screening of short tandem repeat markers would considerably improve resolution, thus refining the geographic ancestry of individuals.</p

Y-Chromosome variation in Southern African Khoe-San populations based on whole-genome sequences

Abstract: Although the human Y chromosome has effectively shown utility in uncovering facets of human evolution and population histories, the ascertainment bias present in early Y-chromosome variant data sets limited the accuracy of diversity and TMRCA estimates obtained from them. The advent of next-generation sequencing, however, has removed this bias and allowed for the discovery of thousands of new variants for use in improving the Y-chromosome phylogeny and computing estimates that are more accurate. Here, we describe the high-coverage sequencing of the whole Y chromosome in a data set of 19 male Khoe-San individuals in comparisonwith existingwholeY-chromosome sequence data.Due to the increased resolution,we potentially resolve the source of haplogroup B-P70 in the Khoe-San, and reconcile recently published haplogroup A-M51 data with the most recent version of the ISOGG Y-chromosome phylogeny. Our results also improve the positioning of tentatively placed new branches of the ISOGG Ychromosome phylogeny. The distribution ofmajor Y-chromosome haplogroups in the Khoe-San and other African groups coincide with the emerging picture of African demographic history;with E-M2 linked to the agriculturalist Bantu expansion, E-M35 linked to pastoralist eastern Africanmigrations, B-M112 linked to earlier east-south gene flow, A-M14 linked to shared ancestrywith central African rainforest hunter-gatherers, and A-M51 potentially unique to the Khoe-San

Integrating multi-taxon palaeogenomes and sedimentary ancient DNA to study past ecosystem dynamics

Ancient DNA (aDNA) has played a major role in our understanding of the past. Important advances in the sequencing and analysis of aDNA from a range of organisms have enabled a detailed understanding of processes such as past demography, introgression, domestication, adaptation and speciation. However, to date and with the notable exception of microbiomes and sediments, most aDNA studies have focused on single taxa or taxonomic groups, making the study of changes at the community level challenging. This is rather surprising because current sequencing and analytical approaches allow us to obtain and analyse aDNA from multiple source materials. When combined, these data can enable the simultaneous study of multiple taxa through space and time, and could thus provide a more comprehensive understanding of ecosystem-wide changes. It is therefore timely to develop an integrative approach to aDNA studies by combining data from multiple taxa and substrates. In this review, we discuss the various applications, associated challenges and future prospects of such an approach

Khoe-San genomes reveal unique variation and confirm the deepest population divergence in Homo sapiens

Abstract: The southern African indigenous Khoe-San populations harbor the most divergent lineages of all living peoples. Exploring their genomes is key to understanding deep human history. We sequenced 25 full genomes from five Khoe-San populations, revealing many novel variants, that 25% of variants are unique to the Khoe-San, and that the Khoe-San group harbors the greatest level of diversity across the globe. In line with previous studies, we found several gene regions with extreme values in genome-wide scans for selection, potentially caused by natural selection in the lineage leading to Homo sapiens and more recent in time. These gene regions included immunity-, sperm-, brain-, diet-, and muscle-related genes. When accounting for recent admixture, all Khoe-San groups display genetic diversity approaching the levels in other African groups and a reduction in effective population size starting around 100,000 years ago. Hence, all human groups show a reduction in effective population size commencing around the time of the Out-of- Africa migrations, which coincides with changes in the paleoclimate records, changes that potentially impacted all humans at the time

Population Genetics of Human Genomic Elements

The genomes of living organisms are composed of a multitude of functional units, which interact with each other and their environment in a highly regulated fashion, to facilitate the expression of an enduring (and evolving) phenotype. Several approaches have emerged in the effort to identify these functional units and explore their activities. In this thesis, I have taken a population genetics approach; evaluating how the distribution of genetic variation in the human genome has been shaped through the actions of natural selection on functional genomic elements. In the first paper, I interrogate a catalogue of elements derived from biochemical signatures for signals of selection; finding significant signals of purifying selection on regulatory elements, independent of linked-purifying selection. In the second paper, I explore the pseudogene class of genomic elements, and find that a large proportion of a particular subclass, transcribed duplicated pseudogenes, has experienced significant amounts of positive selection. In the third paper, I focus on protein coding genes and variants that disrupt their open reading frames. Specifically, I examine the distribution of loss-of-function variants in the Khoe-San population; gauging their functional significance and exploring the biological roles of affected genes. In the final paper, instead of using population genetics to uncover and explore genomic elements, I use a major genomic element – the Y chromosome – as an effective tool to study the evolutionary history of a human population

Population Genetics of Human Genomic Elements

The genomes of living organisms are composed of a multitude of functional units, which interact with each other and their environment in a highly regulated fashion, to facilitate the expression of an enduring (and evolving) phenotype. Several approaches have emerged in the effort to identify these functional units and explore their activities. In this thesis, I have taken a population genetics approach; evaluating how the distribution of genetic variation in the human genome has been shaped through the actions of natural selection on functional genomic elements. In the first paper, I interrogate a catalogue of elements derived from biochemical signatures for signals of selection; finding significant signals of purifying selection on regulatory elements, independent of linked-purifying selection. In the second paper, I explore the pseudogene class of genomic elements, and find that a large proportion of a particular subclass, transcribed duplicated pseudogenes, has experienced significant amounts of positive selection. In the third paper, I focus on protein coding genes and variants that disrupt their open reading frames. Specifically, I examine the distribution of loss-of-function variants in the Khoe-San population; gauging their functional significance and exploring the biological roles of affected genes. In the final paper, instead of using population genetics to uncover and explore genomic elements, I use a major genomic element – the Y chromosome – as an effective tool to study the evolutionary history of a human population

Population Genetics of Human Genomic Elements

The genomes of living organisms are composed of a multitude of functional units, which interact with each other and their environment in a highly regulated fashion, to facilitate the expression of an enduring (and evolving) phenotype. Several approaches have emerged in the effort to identify these functional units and explore their activities. In this thesis, I have taken a population genetics approach; evaluating how the distribution of genetic variation in the human genome has been shaped through the actions of natural selection on functional genomic elements. In the first paper, I interrogate a catalogue of elements derived from biochemical signatures for signals of selection; finding significant signals of purifying selection on regulatory elements, independent of linked-purifying selection. In the second paper, I explore the pseudogene class of genomic elements, and find that a large proportion of a particular subclass, transcribed duplicated pseudogenes, has experienced significant amounts of positive selection. In the third paper, I focus on protein coding genes and variants that disrupt their open reading frames. Specifically, I examine the distribution of loss-of-function variants in the Khoe-San population; gauging their functional significance and exploring the biological roles of affected genes. In the final paper, instead of using population genetics to uncover and explore genomic elements, I use a major genomic element – the Y chromosome – as an effective tool to study the evolutionary history of a human population

Patterns of variation in cis-regulatory regions : examining evidence of purifying selection

Background: With only 2 % of the human genome consisting of protein coding genes, functionality across the rest of the genome has been the subject of much debate. This has gained further impetus in recent years due to a rapidly growing catalogue of genomic elements, based primarily on biochemical signatures (e.g. the ENCODE project). While the assessment of functionality is a complex task, the presence of selection acting on a genomic region is a strong indicator of importance. In this study, we apply population genetic methods to investigate signals overlaying several classes of regulatory elements. Results: We disentangle signals of purifying selection acting directly on regulatory elements from the confounding factors of demography and purifying selection linked to e.g. nearby protein coding regions. We confirm the importance of regulatory regions proximal to coding sequence, while also finding differential levels of selection at distal regions. We note differences in purifying selection among transcription factor families. Signals of constraint at some genomic classes were also strongly dependent on their physical location relative to coding sequence. In addition, levels of selection efficacy across genomic classes differed between African and non-African populations. Conclusions: In order to assign a valid signal of selection to a particular class of genomic sequence, we show that it is crucial to isolate the signal by accounting for the effects of demography and linked-purifying selection. Our study highlights the intricate interplay of factors affecting signals of selection on functional elements

Additional file 1: of Patterns of variation in cis-regulatory regions: examining evidence of purifying selection

Supplementary Methods, Tables and Figures. (PDF 8572 kb