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Key Points As a consequence of its key role in male sex determination, the Y chromosome has unique genetic properties that lead to it carrying highly informative haplotypes that evolve largely through the simple accumulation of mutations. Advances in technology have allowed ~10 Mb of Y-chromosome DNA to be sequenced from large population samples, with consequent unbiased ascertainment of their genetic variation. Y-Chromosome sequences can be assembled into a robust phylogeny, which can be calibrated using estimates of the mutation rate from family studies, known archaeological events or ancient DNA samples. The calibrated Y-chromosome phylogeny reveals male expansions corresponding to the migration of modern humans out of Africa ~60,000 years ago, the colonization of the Americas ~15,000 years ago and more recent technology-driven population expansions. The Y chromosome has a particularly important role in forensic genetics, as it allows male-specific DNA profiles to be compared at an increasingly high resolution. In genealogical studies, the male-line inheritance of the Y chromosome makes it a perfect tool for studies of male family history, which has led to a burgeoning area of citizen science. The Y chromosome is central to disorders of sex determination and spermatogenesis. Recently, mosaic somatic loss of the Y chromosome in ageing men has been associated with an increased risk of cancer mortality and Alzheimer disease. Genetic variation of the human Y chromosome plays a key part in studies of human evolution, population history, genealogy, forensics and male medical genetics. This Review outlines how next-generation sequencing has contributed to recent progress in these fields. The properties of the human Y chromosome – namely, male specificity, haploidy and escape from crossing over — make it an unusual component of the genome, and have led to its genetic variation becoming a key part of studies of human evolution, population history, genealogy, forensics and male medical genetics. Next-generation sequencing (NGS) technologies have driven recent progress in these areas. In particular, NGS has yielded direct estimates of mutation rates, and an unbiased and calibrated molecular phylogeny that has unprecedented detail. Moreover, the availability of direct-to-consumer NGS services is fuelling a rise of 'citizen scientists', whose interest in resequencing their own Y chromosomes is generating a wealth of new data.

A fundamental question in the biology of sex differences has eluded direct study in humans: How does sex-chromosome dosage (SCD) shape genome function? To address this, we developed a systematic map of SCD effects on gene function by analyzing genome-wide expression data in humans with diverse sex-chromosome aneuploidies (XO, XXX, XXY, XYY, and XXYY). For sex chromosomes, we demonstrate a pattern of obligate dosage sensitivity among evolutionarily preserved X-Y homologs and update prevailing theoretical models for SCD compensation by detecting X-linked genes that increase expression with decreasing X- and/or Y-chromosome dosage. We further show that SCD-sensitive sex-chromosome genes regulate specific coexpression networks of SCD-sensitive autosomal geneswith critical cellular functions and a demonstrable potential to mediate previously documented SCD effects on disease. These gene coexpression results converge with analysis of transcription factor binding site enrichment and measures of gene expression in murine knockout models to spotlight the dosage-sensitive X-linked transcription factor ZFX as a key mediator of SCD effects on wider genome expression. Our findings characterize the effects of SCD broadly across the genome, with potential implications for human phenotypic variation.

Mosaic loss of chromosome Y (LOY) in circulating white blood cells is the most common form of clonal mosaicism 1 – 5 , yet our knowledge of the causes and consequences of this is limited. Here, using a computational approach, we estimate that 20% of the male population represented in the UK Biobank study ( n  = 205,011) has detectable LOY. We identify 156 autosomal genetic determinants of LOY, which we replicate in 757,114 men of European and Japanese ancestry. These loci highlight genes that are involved in cell-cycle regulation and cancer susceptibility, as well as somatic drivers of tumour growth and targets of cancer therapy. We demonstrate that genetic susceptibility to LOY is associated with non-haematological effects on health in both men and women, which supports the hypothesis that clonal haematopoiesis is a biomarker of genomic instability in other tissues. Single-cell RNA sequencing identifies dysregulated expression of autosomal genes in leukocytes with LOY and provides insights into why clonal expansion of these cells may occur. Collectively, these data highlight the value of studying clonal mosaicism to uncover fundamental mechanisms that underlie cancer and other ageing-related diseases. A genome-wide association study of mosaic loss of chromosome Y (LOY) in UK Biobank participants identifies 156 genetic determinants of LOY, showing that LOY is associated with cancer and non-haematological health outcomes.

The human X and Y chromosomes evolved from an ordinary pair of autosomes, but millions of years ago genetic decay ravaged the Y chromosome, and only three per cent of its ancestral genes survived. We reconstructed the evolution of the Y chromosome across eight mammals to identify biases in gene content and the selective pressures that preserved the surviving ancestral genes. Our findings indicate that survival was nonrandom, and in two cases, convergent across placental and marsupial mammals. We conclude that the gene content of the Y chromosome became specialized through selection to maintain the ancestral dosage of homologous X–Y gene pairs that function as broadly expressed regulators of transcription, translation and protein stability. We propose that beyond its roles in testis determination and spermatogenesis, the Y chromosome is essential for male viability, and has unappreciated roles in Turner’s syndrome and in phenotypic differences between the sexes in health and disease. A study comparing the Y chromosome across mammalian species reveals that selection to maintain the ancestral dosage of homologous X–Y gene pairs preserved a handful of genes on the Y chromosome while the rest were lost; the survival of broadly expressed dosage-sensitive regulators of gene expression suggest that the human Y chromosome is essential for male viability. Evolution and function of the Y chromosome Mammalian Y chromosomes, known for their roles in sex determination and male fertility, often contain repetitive sequences that make them harder to assemble than the rest of the genome. To counter this problem Henrik Kaessmann and colleagues have developed a new transcript assembly approach based on male-specific RNA/genomic sequencing data to explore Y evolution across 15 species representing all major mammalian lineages. They find evidence for two independent sex chromosome originations in mammals and one in birds. Their analysis of the Y/W gene repertoires suggests that although some genes evolved novel functions in sex determination/spermatogenesis as a result of temporal/spatial expression changes, most Y genes probably persisted, at least initially, as a result of dosage constraints. In a parallel study, Daniel Bellott and colleagues reconstructed the evolution of the Y chromosome, using a comprehensive comparative analysis of the genomic sequence of X–Y gene pairs from seven placental mammals and one marsupial. They conclude that evolution streamlined the gene content of the human Y chromosome through selection to maintain the ancestral dosage of homologous X–Y gene pairs that regulate gene expression throughout the body. They propose that these genes make the Y chromosome essential for male viability and contribute to differences between the sexes in health and disease.

The Y chromosome and the mitochondrial genome have been used to estimate when the common patrilineal and matrilineal ancestors of humans lived. We sequenced the genomes of 69 males from nine populations, including two in which we find basal branches of the Y-chromosome tree. We identify ancient phylogenetic structure within African haplogroups and resolve a long-standing ambiguity deep within the tree. Applying equivalent methodologies to the Y chromosome and the mitochondrial genome, we estimate the time to the most recent common ancestor (T MRCA ) of the Y chromosome to be 120 to 156 thousand years and the mitochondrial genome T MRCA to be 99 to 148 thousand years. Our findings suggest that, contrary to previous claims, male lineages do not coalesce significantly more recently than female lineages.

Key Points Recently developed genomic technologies have shed light on the genomic composition of the ancient Y chromosomes of some primates and Drosophila melanogaster and have shown that Y chromosomes in these species largely conform to the previously held view of being degenerate. The presence of evolutionary strata confirmed by genome sequencing of the sex chromosomes supports that Y-chromosome degeneration occurred through successive arrest of recombination over time. In addition, the enrichment of Y chromosomes for genes of male-beneficial functions suggests that sexually antagonistic mutations may have a role in Y-chromosome evolution. Genome sequencing of young Y chromosomes in plants and neo-Y chromosomes in Drosophila spp. have provided insight into the molecular processes that trigger initiation of Y-chromosome degeneration. Empirical evidence suggests that gene silencing occurs before pseudogenization. Empirical observations in Drosophila neo-sex chromosomes, primate Y chromosomes and theoretical models and computer simulations show that degeneration is not a linear process, and so Y chromosomes in these species will probably not completely degenerate in the future. The Y chromosomes of many species, including humans, are gene-poor and degenerate. The recent application of genome-wide technologies to evolutionarily old and young Y chromosomes has provided insight into the processes that have shaped them and their future. The human Y chromosome is intriguing not only because it harbours the master-switch gene that determines gender but also because of its unusual evolutionary history. The Y chromosome evolved from an autosome, and its evolution has been characterized by massive gene decay. Recent whole-genome and transcriptome analyses of Y chromosomes in humans and other primates, in Drosophila species and in plants have shed light on the current gene content of the Y chromosome, its origins and its long-term fate. Furthermore, comparative analysis of young and old Y chromosomes has given further insights into the evolutionary and molecular forces triggering Y-chromosome degeneration and into the evolutionary destiny of the Y chromosome.

Background Y-chromosome DNA (Y-DNA) has been used for tracing paternal lineages and offers a clear path from an individual to a known, or likely, direct paternal ancestor. The advance of next-generation sequencing (NGS) technologies increasingly improves the resolution of the non-recombining region of the Y-chromosome (NRY). However, a lack of suitable computer tools prevents the use of NGS data from the Y-DNA studies. Results We developed Y-LineageTracker, a high-throughput analysis framework that not only utilizes state-of-the-art methodologies to automatically determine NRY haplogroups and identify microsatellite variants of Y-chromosome on a fine scale, but also optimizes comprehensive Y-DNA analysis methods for NGS data. Notably, Y-LineageTracker integrates the NRY haplogroup and Y-STR analysis modules with recognized strategies to robustly suggest an interpretation for paternal genetics and evolution. NRY haplogroup module mainly covers haplogroup classification, clustering analysis, phylogeny construction, and divergence time estimation of NRY haplogroups, and Y-STR module mainly includes Y-STR genotyping, statistical calculation, network analysis, and estimation of time to the most recent common ancestor (TMRCA) based on Y-STR haplotypes. Performance comparison indicated that Y-LineageTracker outperformed existing Y-DNA analysis tools for the high performance and satisfactory visualization effect. Conclusions Y-LineageTracker is an open-source and user-friendly command-line tool that provide multiple functions to efficiently analyze Y-DNA from NGS data at both Y-SNP and Y-STR level. Additionally, Y-LineageTracker supports various formats of input data and produces high-quality figures suitable for publication. Y-LineageTracker is coded with Python3 and supports Windows, Linux, and macOS platforms, and can be installed manually or via the Python Package Index (PyPI). The source code, examples, and manual of Y-LineageTracker are freely available at https://www.picb.ac.cn/PGG/resource.php or CodeOcean ( https://codeocean.com/capsule/7424381/tree ).

Background The origin of sex chromosomes requires the establishment of recombination suppression between the proto-sex chromosomes. In many fish species, the sex chromosome pair is homomorphic with a recent origin, providing species for studying how and why recombination suppression evolved in the initial stages of sex chromosome differentiation, but this requires accurate sequence assembly of the X and Y (or Z and W) chromosomes, which may be difficult if they are recently diverged. Results Here we produce a haplotype-resolved genome assembly of zig-zag eel ( Mastacembelus armatus ), an aquaculture fish, at the chromosomal scale. The diploid assembly is nearly gap-free, and in most chromosomes, we resolve the centromeric and subtelomeric heterochromatic sequences. In particular, the Y chromosome, including its highly repetitive short arm, has zero gaps. Using resequencing data, we identify a ~7 Mb fully sex-linked region (SLR), spanning the sex chromosome centromere and almost entirely embedded in the pericentromeric heterochromatin. The SLRs on the X and Y chromosomes are almost identical in sequence and gene content, but both are repetitive and heterochromatic, consistent with zero or low recombination. We further identify an HMG-domain containing gene HMGN6 in the SLR as a candidate sex-determining gene that is expressed at the onset of testis development. Conclusions Our study supports the idea that preexisting regions of low recombination, such as pericentromeric regions, can give rise to SLR in the absence of structural variations between the proto-sex chromosomes.

Humans expanded out of Africa 50,000-70,000 years ago, but many details of this migration are poorly understood. Here, Haber et al. sequence Y chromosomes belonging to a rare African lineage and analyze... Present-day humans outside Africa descend mainly from a single expansion out ∼50,000–70,000 years ago, but many details of this expansion remain unclear, including the history of the male-specific Y chromosome at this time. Here, we reinvestigate a rare deep-rooting African Y-chromosomal lineage by sequencing the whole genomes of three Nigerian men described in 2003 as carrying haplogroup DE* Y chromosomes, and analyzing them in the context of a calibrated worldwide Y-chromosomal phylogeny. We confirm that these three chromosomes do represent a deep-rooting DE lineage, branching close to the DE bifurcation, but place them on the D branch as an outgroup to all other known D chromosomes, and designate the new lineage D0. We consider three models for the expansion of Y lineages out of Africa ∼50,000–100,000 years ago, incorporating migration back to Africa where necessary to explain present-day Y-lineage distributions. Considering both the Y-chromosomal phylogenetic structure incorporating the D0 lineage, and published evidence for modern humans outside Africa, the most favored model involves an origin of the DE lineage within Africa with D0 and E remaining there, and migration out of the three lineages (C, D, and FT) that now form the vast majority of non-African Y chromosomes. The exit took place 50,300–81,000 years ago (latest date for FT lineage expansion outside Africa – earliest date for the D/D0 lineage split inside Africa), and most likely 50,300–59,400 years ago (considering Neanderthal admixture). This work resolves a long-running debate about Y-chromosomal out-of-Africa/back-to-Africa migrations, and provides insights into the out-of-Africa expansion more generally.

How consistent are the evolutionary trajectories of sex chromosomes shortly after they form? Insights into the evolution of recombination, differentiation, and degeneration can be provided by comparing closely related species with homologous sex chromosomes. The sex chromosomes of the threespine stickleback (Gasterosteus aculeatus) and its sister species, the Japan Sea stickleback (G. nipponicus), have been well characterized. Little is known, however, about the sex chromosomes of their congener, the blackspotted stickleback (G. wheatlandi). We used pedigrees to obtain experimentally phased whole genome sequences from blackspotted stickleback X and Y chromosomes. Using multispecies gene trees and analysis of shared duplications, we demonstrate that Chromosome 19 is the ancestral sex chromosome and that its oldest stratum evolved in the common ancestor of the genus. After the blackspotted lineage diverged, its sex chromosomes experienced independent and more extensive recombination suppression, greater X–Y differentiation, and a much higher rate of Y degeneration than the other two species. These patterns may result from a smaller effective population size in the blackspotted stickleback. A recent fusion between the ancestral blackspotted stickleback Y chromosome and Chromosome 12, which produced a neo-X and neo-Y, may have been favored by the very small size of the recombining region on the ancestral sex chromosome. We identify six strata on the ancestral and neo-sex chromosomes where recombination between the X and Y ceased at different times. These results confirm that sex chromosomes can evolve large differences within and between species over short evolutionary timescales.