On the association between chromosomal rearrangements and genic evolution in humans and chimpanzees

Inversions on human chromosomes

Human chromosome inversions are types of balanced structural variations, making them difficult to analyze. Thanks to PEM (paired-end sequencing and mapping), there has been tremendous progress in studying inversions. Inversions play an important role as an evolutionary factor, contributing to the formation of gonosomes, speciation of chimpanzees and humans, and inv17q21.3 or inv8p23.1 exhibit the features of natural selection. Both inversions have been related to pathogenic phenotype by directly affecting a gene structure (e.g., inv5p15.1q14.1), regulating gene expression (e.g., inv7q21.3q35) and by predisposing to other secondary arrangements (e.g., inv7q11.23). A polymorphism of human inversions is documented by the InvFEST database (a database that stores information about clinical predictions, validations, frequency of inversions, etc.), but only a small fraction of these inversions is validated, and a detailed analysis is complicated by the frequent location of breakpoints within regions of repetitive sequences.

Wheat heat tolerance is impaired by heightened deletions in the distal end of 4AL chromosomal arm

Heat stress (HS) causes substantial damages to worldwide crop production. As a cool season crop, wheat (Triticum aestivum) is sensitive to HS-induced damages. To support the genetic improvement of wheat HS tolerance (HST), we conducted fine mapping of TaHST1, a locus required for maintaining wheat vegetative and reproductive growth under elevated temperatures. TaHST1 was mapped to the distal terminus of 4AL chromosome arm using genetic populations derived from two BC₆ F₆ breeding lines showing tolerance (E6015-4T) or sensitivity (E6015-3S) to HS. The 4AL region carrying TaHST1 locus was approximately 0.949 Mbp and contained the last 19 high confidence genes of 4AL according to wheat reference genome sequence. Resequencing of E6015-3S and E6015-4T and haplotype analysis of 3087 worldwide wheat accessions revealed heightened deletion polymorphisms in the distal 0.949 Mbp region of 4AL, which was confirmed by the finding of frequent gene losses in this region in eight genome-sequenced hexaploid wheat cultivars. The great majority (86.36%) of the 3087 lines displayed different degrees of nucleotide sequence deletions, with only 13.64% of them resembling E6015-4T in this region. These deletions can impair the presence and/or function of TaHST1 and surrounding genes, thus rendering global wheat germplasm vulnerable to HS or other environmental adversities. Therefore, conscientious and urgent efforts are needed in global wheat breeding programmes to optimize the structure and function of 4AL distal terminus by ensuring the presence of TaHST1 and surrounding genes. The new information reported here will help to accelerate the ongoing global efforts in improving wheat HST.

Ancestral and neo-sex chromosomes contribute to population divergence in a dioecious plant

Empirical evidence from several animal groups suggests sex chromosomes disproportionately contribute to reproductive isolation. This effect may be enhanced when sex chromosomes are associated with turnover of sex determination systems resulting from structural rearrangements to the chromosomes. We investigated these predictions in the dioecious plant Rumex hastatulus, which is composed of populations of two different sex chromosome cytotypes caused by an X-autosome fusion. Using population genomic analyses, we investigated the demographic history of R. hastatulus and explored the contributions of ancestral and neo-sex chromosomes to population genetic divergence. Our study revealed that the cytotypes represent genetically divergent populations with evidence for historical but not contemporary gene flow between them. In agreement with classical predictions, we found that the ancestral X chromosome was disproportionately divergent compared with the rest of the genome. Excess differentiation was also observed on the Y chromosome, even when we used measures of differentiation that control for differences in effective population size. Our estimates of the timing of the origin of neo-sex chromosomes in R. hastatulus are coincident with cessation of gene flow, suggesting that the chromosomal fusion event that gave rise to the origin of the XYY cytotype may have also contributed to reproductive isolation.

Human inversions and their functional consequences

Polymorphic inversions are a type of structural variants that are difficult to analyze owing to their balanced nature and the location of breakpoints within complex repeated regions. So far, only a handful of inversions have been studied in detail in humans and current knowledge about their possible functional effects is still limited. However, inversions have been related to phenotypic changes and adaptation in multiple species. In this review, we summarize the evidences of the functional impact of inversions in the human genome. First, given that inversions have been shown to inhibit recombination in heterokaryotes, chromosomes displaying different orientation are expected to evolve independently and this may lead to distinct gene-expression patterns. Second, inversions have a role as disease-causing mutations both by directly affecting gene structure or regulation in different ways, and by predisposing to other secondary arrangements in the offspring of inversion carriers. Finally, several inversions show signals of being selected during human evolution. These findings illustrate the potential of inversions to have phenotypic consequences also in humans and emphasize the importance of their inclusion in genome-wide association studies.

Unraveling the effect of genomic structural changes in the rhesus macaque - implications for the adaptive role of inversions

BACKGROUND

By reshuffling genomes, structural genomic reorganizations provide genetic variation on which natural selection can work. Understanding the mechanisms underlying this process has been a long-standing question in evolutionary biology. In this context, our purpose in this study is to characterize the genomic regions involved in structural rearrangements between human and macaque genomes and determine their influence on meiotic recombination as a way to explore the adaptive role of genome shuffling in mammalian evolution.

RESULTS

We first constructed a highly refined map of the structural rearrangements and evolutionary breakpoint regions in the human and rhesus macaque genomes based on orthologous genes and whole-genome sequence alignments. Using two different algorithms, we refined the genomic position of known rearrangements previously reported by cytogenetic approaches and described new putative micro-rearrangements (inversions and indels) in both genomes. A detailed analysis of the rhesus macaque genome showed that evolutionary breakpoints are in gene-rich regions, being enriched in GO terms related to immune system. We also identified defense-response genes within a chromosome inversion fixed in the macaque lineage, underlying the relevance of structural genomic changes in evolutionary and/or adaptation processes. Moreover, by combining in silico and experimental approaches, we studied the recombination pattern of specific chromosomes that have suffered rearrangements between human and macaque lineages.

CONCLUSIONS

Our data suggest that adaptive alleles - in this case, genes involved in the immune response - might have been favored by genome rearrangements in the macaque lineage.

Novel H3K4me3 marks are enriched at human- and chimpanzee-specific cytogenetic structures

Human and chimpanzee genomes are 98.8% identical within comparable sequences. However, they differ structurally in nine pericentric inversions, one fusion that originated human chromosome 2, and content and localization of heterochromatin and lineage-specific segmental duplications. The possible functional consequences of these cytogenetic and structural differences are not fully understood and their possible involvement in speciation remains unclear. We show that subtelomeric regions—regions that have a species-specific organization, are more divergent in sequence, and are enriched in genes and recombination hotspots—are significantly enriched for species-specific histone modifications that decorate transcription start sites in different tissues in both human and chimpanzee. The human lineage-specific chromosome 2 fusion point and ancestral centromere locus as well as chromosome 1 and 18 pericentric inversion breakpoints showed enrichment of human-specific H3K4me3 peaks in the prefrontal cortex. Our results reveal an association between plastic regions and potential novel regulatory elements.

Recombination rates and genomic shuffling in human and chimpanzee--a new twist in the chromosomal speciation theory

A long-standing question in evolutionary biology concerns the effect of recombination in shaping the genomic architecture of organisms and, in particular, how this impacts the speciation process. Despite efforts employed in the last decade, the role of chromosomal reorganizations in the human-chimpanzee speciation process remains unresolved. Through whole-genome comparisons, we have analyzed the genome-wide impact of genomic shuffling in the distribution of human recombination rates during the human-chimpanzee speciation process. We have constructed a highly refined map of the reorganizations and evolutionary breakpoint regions in the human and chimpanzee genomes based on orthologous genes and genome sequence alignments. The analysis of the most recent human and chimpanzee recombination maps inferred from genome-wide single-nucleotide polymorphism data revealed that the standardized recombination rate was significantly lower in rearranged than in collinear chromosomes. In fact, rearranged chromosomes presented significantly lower recombination rates than chromosomes that have been maintained since the ancestor of great apes, and this was related with the lineage in which they become fixed. Importantly, inverted regions had lower recombination rates than collinear and noninverted regions, independently of the effect of centromeres. Our observations have implications for the chromosomal speciation theory, providing new evidences for the contribution of inversions in suppressing recombination in mammals.

Refinement of Bos taurus sequence assembly based on BAC-FISH experiments

Background

The sequencing of the cow genome was recently published (Btau_4.0 assembly). A second, alternate cow genome assembly (UMD2), based on the same raw sequence data, was also published. The two assemblies have been subsequently updated to Btau_4.2 and UMD3.1, respectively.

Results

We compared the Btau_4.2 and UMD3.1 alternate assemblies. Inconsistencies were grouped into three main categories: (i) DNA segments showing almost coincidental chromosomal mapping but discordant orientation (inversions); (ii) DNA segments showing a discordant map position along the same chromosome; and (iii) sequences present in one chromosomal assembly but absent in the corresponding chromosome of the other assembly. The latter category mainly consisted of large amounts of scaffolds that were unassigned in Btau_4.2 but successfully mapped in UMD3.1. We sampled 70 inconsistencies and identified appropriate cow BACs for each of them. These clones were then utilized in FISH experiments on cow metaphase or interphase nuclei in order to disambiguate the discrepancies. In almost all instances the FISH results agreed with the UMD3.1 assembly. Occasionally, however, the mapping data of both assemblies were discordant with the FISH results.

Conclusions

Our work demonstrates how FISH, which is assembly independent, can be efficiently used to solve assembly problems frequently encountered using the shotgun approach.

Inferring the history of interchromosomal gene transposition in Drosophila using n-dimensional parsimony

Gene transposition puts a new gene copy in a novel genomic environment. Moreover, genes moving between the autosomes and the X chromosome experience change in several evolutionary parameters. Previous studies of gene transposition have not utilized the phylogenetic framework that becomes possible with the availability of whole genomes from multiple species. Here we used parsimonious reconstruction on the genomic distribution of gene families to analyze interchromosomal gene transposition in Drosophila. We identified 782 genes that have moved chromosomes within the phylogeny of 10 Drosophila species, including 87 gene families with multiple independent movements on different branches of the phylogeny. Using this large catalog of transposed genes, we detected accelerated sequence evolution in duplicated genes that transposed when compared to the parental copy at the original locus. We also observed a more refined picture of the biased movement of genes from the X chromosome to the autosomes. The bias of X-to-autosome movement was significantly stronger for RNA-based movements than for DNA-based movements, and among DNA-based movements there was an excess of genes moving onto the X chromosome as well. Genes involved in female-specific functions moved onto the X chromosome while genes with male-specific functions moved off the X. There was a significant overrepresentation of proteins involving chromosomal function among transposed genes, suggesting that genetic conflict between sexes and among chromosomes may be a driving force behind gene transposition in Drosophila.

On the sequence-directed nature of human gene mutation: the role of genomic architecture and the local DNA sequence environment in mediating gene mutations underlying human inherited disease

Different types of human gene mutation may vary in size, from structural variants (SVs) to single base-pair substitutions, but what they all have in common is that their nature, size and location are often determined either by specific characteristics of the local DNA sequence environment or by higher order features of the genomic architecture. The human genome is now recognized to contain "pervasive architectural flaws" in that certain DNA sequences are inherently mutation prone by virtue of their base composition, sequence repetitivity and/or epigenetic modification. Here, we explore how the nature, location and frequency of different types of mutation causing inherited disease are shaped in large part, and often in remarkably predictable ways, by the local DNA sequence environment. The mutability of a given gene or genomic region may also be influenced indirectly by a variety of noncanonical (non-B) secondary structures whose formation is facilitated by the underlying DNA sequence. Since these non-B DNA structures can interfere with subsequent DNA replication and repair and may serve to increase mutation frequencies in generalized fashion (i.e., both in the context of subtle mutations and SVs), they have the potential to serve as a unifying concept in studies of mutational mechanisms underlying human inherited disease.

A time-invariant principle of genome evolution

Uncovering general principles of genome evolution that are time-invariant and that operate in germ and somatic cells has implications for genome-wide association studies (GWAS), gene therapy, and disease genomics. Here we investigate the relationship between structural alterations (e.g., insertions and deletions) and single-nucleotide substitutions by comparing the following genomes that diverged at different times across germ- and somatic-cell lineages: (i) the reference human and chimpanzee genome (in million years), (ii) the reference human and personal genomes (in tens of thousands of years), and (iii) structurally altered regions in cancer and genetically engineered cells (in days). At the species level, genes with structural alteration in nearby regions show increased single-nucleotide changes and tend to evolve faster. In personal genomes, the single-nucleotide substitution rate is higher near sites of structural alteration and decreases with increasing distance. In human cancer cell populations and in cells genetically engineered using zinc-finger nucleases, single-nucleotide changes occur frequently near sites of structural alterations. We present evidence that structural alteration induces single-nucleotide changes in nearby regions and discuss possible molecular mechanisms that contribute to this phenomenon. We propose that the low fidelity of nonreplicative error-prone repair polymerases, which are used during insertion or deletion, result in break-repair-induced single-nucleotide mutations in the vicinity of structural alteration. Thus, in the mutational landscape, structural alterations are linked to single-nucleotide changes across different time scales in both somatic- and germ-cell lineages. We discuss implications for genome evolution, GWAS, disease genomics, and gene therapy and emphasize the need to investigate both types of mutations within a single framework.

Molecular cloning of the baboon UDP-glucuronosyltransferase 2B gene family and their activity in conjugating morphine

Glucuronidation by UDP-glucuronyltransferase 2B enzymes (UGT2Bs) is a major pathway for the elimination of endobiotics and xenobiotics, including therapeutic drugs. Morphine, a probe drug for UGT2B7, is metabolized to morphine-3-beta-glucuronide (M3G) and morphine-6-beta-glucuronide (M6G) in humans. Morphine has been used in a series of experiments in the baboon to characterize developmental changes in fetal glucuronidation. This study identifies the baboon UGT2B family of enzymes, compares them with that of the human and the monkey (Macaca fascicularis), and measures the activity of the individual baboon UGT2Bs toward morphine. UGT2B cDNAs were cloned from the liver of adult and newborn baboons and expressed in human embryonic kidney 293 cells. The UGT activity toward morphine was assessed by the rate of formation of M3G and M6G by high-performance liquid chromatography. Eight baboon UGT2Bs were cloned and identified: UGT2B41 and UGT2B42, which are 90% homologous to human UGT2B4; UGT2B43, which is 93% homologous to human UGT2B15; and UGT2B39, UGT2B40, UGT2B44, UGT2B45, and UGT2B46, which are 89 to 91% homologous to human UGT2B7. Homology between baboon and monkey UGT2B ranged from 92.6 to 99.1%, with the primary protein structure of UGT2B43 being 99.1% identical to monkey UGT2B20, including a unique R96I substitution. Gene conversion interfered with the phylogenetic signal in the baboon UGT2B7-like and the monkey UGT2B4-like groups and led to concerted evolution of these enzymes. All of the baboon UGT2Bs metabolized morphine to both M3G and M6G. This study lays the foundation for investigating the regulation of UGT2B enzymes during fetal and neonatal development in the baboon.

An exon-based comparative variant analysis pipeline to study the scale and role of frameshift and nonsense mutation in the human-chimpanzee divergence

Chimpanzees and humans are closely related but differ in many deadly human diseases and other characteristics in physiology, anatomy, and pathology. In spite of decades of extensive research, crucial questions about the molecular mechanisms behind the differences are yet to be understood. Here I report ExonVar, a novel computational pipeline for Exon-based human-chimpanzee comparative Variant analysis. The objective is to comparatively analyze mutations specifically those that caused the frameshift and nonsense mutations and to assess their scale and potential impacts on human-chimpanzee divergence. Genomewide analysis of human and chimpanzee exons with ExonVar identified a number of species-specific, exon-disrupting mutations in chimpanzees but much fewer in humans. Many were found on genes involved in important biological processes such as T cell lineage development, the pathogenesis of inflammatory diseases, and antigen induced cell death. A “less-is-more” model was previously established to illustrate the role of the gene inactivation and disruptions during human evolution. Here this analysis suggested a different model where the chimpanzee-specific exon-disrupting mutations may act as additional evolutionary force that drove the human-chimpanzee divergence. Finally, the analysis revealed a number of sequencing errors in the chimpanzee and human genome sequences and further illustrated that they could be corrected without resequencing.

Sequencing primate genomes: what have we learned?

We summarize the progress in whole-genome sequencing and analyses of primate genomes. These emerging genome datasets have broadened our understanding of primate genome evolution revealing unexpected and complex patterns of evolutionary change. This includes the characterization of genome structural variation, episodic changes in the repeat landscape, differences in gene expression, new models regarding speciation, and the ephemeral nature of the recombination landscape. The functional characterization of genomic differences important in primate speciation and adaptation remains a significant challenge. Limited access to biological materials, the lack of detailed phenotypic data and the endangered status of many critical primate species have significantly attenuated research into the genetic basis of primate evolution. Next-generation sequencing technologies promise to greatly expand the number of available primate genome sequences; however, such draft genome sequences will likely miss critical genetic differences within complex genomic regions unless dedicated efforts are put forward to understand the full spectrum of genetic variation.

Patterns of autosomal divergence between the human and chimpanzee genomes support an allopatric model of speciation

There is a large variation in divergence times across genomic regions between human and chimpanzee. It has been suggested that this could partly result from selection against ancestral gene flow between incipient species in regions of the genome containing genetic incompatibilities. It is possible that such barriers to gene flow could arise in specific genes or in chromosomal inversions. I analysed patterns of lineage sorting that occur between human, chimpanzee and gorilla genomic sequences by examining divergent site patterns in >18 Mb genomic alignments. I develop a method to normalise site patterns by the mutational spectrum to minimise errors caused by misinference caused by recurrent mutation. Here I show that divergence times appear to be uniform between coding and noncoding sequences and between inverted and non-rearranged portions of chromosomes. I therefore find no evidence to support the large-scale accumulation of genetic incompatibilities at speciation genes or chromosomal inversions in the ancestral population of humans and chimpanzees. In addition, site patterns that are discordant with the species tree occur more frequently in regions with high human recombination rates. This could indicate the action of selective sweeps in the ancestral population, but could also be indicative of increased rates of homoplasy in these regions. I argue that these observations are compatible with a neutral allopatric model of speciation.

Gene regulation in primates evolves under tissue-specific selection pressures

Regulatory changes have long been hypothesized to play an important role in primate evolution. To identify adaptive regulatory changes in humans, we performed a genome-wide survey for genes in which regulation has likely evolved under natural selection. To do so, we used a multi-species microarray to measure gene expression levels in livers, kidneys, and hearts from six humans, chimpanzees, and rhesus macaques. This comparative gene expression data allowed us to identify a large number of genes, as well as specific pathways, whose inter-species expression profiles are consistent with the action of stabilizing or directional selection on gene regulation. Among the latter set, we found an enrichment of genes involved in metabolic pathways, consistent with the hypothesis that shifts in diet underlie many regulatory adaptations in humans. In addition, we found evidence for tissue-specific selection pressures, as well as lower rates of protein evolution for genes in which regulation evolves under natural selection. These observations are consistent with the notion that adaptive circumscribed changes in gene regulation have fewer deleterious pleiotropic effects compared with changes at the protein sequence level.

The genomic distribution of intraspecific and interspecific sequence divergence of human segmental duplications relative to human/chimpanzee chromosomal rearrangements

BACKGROUND

It has been suggested that chromosomal rearrangements harbor the molecular footprint of the biological phenomena which they induce, in the form, for instance, of changes in the sequence divergence rates of linked genes. So far, all the studies of these potential associations have focused on the relationship between structural changes and the rates of evolution of single-copy DNA and have tried to exclude segmental duplications (SDs). This is paradoxical, since SDs are one of the primary forces driving the evolution of structure and function in our genomes and have been linked not only with novel genes acquiring new functions, but also with overall higher DNA sequence divergence and major chromosomal rearrangements.

RESULTS

Here we take the opposite view and focus on SDs. We analyze several of the features of SDs, including the rates of intraspecific divergence between paralogous copies of human SDs and of interspecific divergence between human SDs and chimpanzee DNA. We study how divergence measures relate to chromosomal rearrangements, while considering other factors that affect evolutionary rates in single copy DNA.

CONCLUSION

We find that interspecific SD divergence behaves similarly to divergence of single-copy DNA. In contrast, old and recent paralogous copies of SDs do present different patterns of intraspecific divergence. Also, we show that some relatively recent SDs accumulate in regions that carry inversions in sister lineages.

Trends in the evolution of reptilian chromosomes

Reptiles are a karyologically heterogeneous group, where some orders and suborders exhibit characteristics similar to those of anamniotes and others share similarities with homeotherms. The class also shows different evolutionary trends, for instance in genome and chromosome size and composition. The turtle DNA base composition is similar to that of mammals, whereas that of lizards and snakes is more similar to that of anamniotes. The major karyological differences between turtles and squamates are the size and composition of the genome and the rate at which chromosomes change. Turtles have larger and more variable genome sizes, and a greater amount of middle repetitive DNA that differs even among related species. In lizards and snakes size of the genome are smaller, single-copy DNA is constant within each suborder, and differences in repetitive DNA involve fractions that become increasingly heterogeneous with widening phylogenetic distance. With regard to variation in karyotype morphology, turtles and crocodiles show low variability in chromosome number, morphology, and G-banding pattern. Greater variability is found among squamates, which have a similar degree of karyotypic change-as do some mammals, such as carnivores and bats-and in which there are also differences among congeneric species. An interesting relationship has been highlighted in the entire class Reptilia between rates of change in chromosomes, number of living species, and rate of extinction. However, different situations obtain in turtles and crocodiles on the one hand, and squamates on the other. In the former, the rate of change in chromosomes is lower and the various evolutionary steps do not seem to have entailed marked chromosomal variation, whereas squamates have a higher rate of change in chromosomes clearly related to the number of living species, and chromosomal variation seems to have played an important role in the evolution of several taxa. The different evolutionary trends in chromosomes observed between turtles and crocodiles on the one hand and squamates on the other might depend on their different patterns of G-banding.

Molecular mechanisms of chromosomal rearrangement during primate evolution

Breakpoint analysis of the large chromosomal rearrangements which have occurred during primate evolution promises to yield new insights into the underlying mechanisms of mutagenesis. Comparison of these evolutionary breakpoints with those that are disease-associated in humans, and which occur during either meiotic or mitotic cell division, should help to identify basic mechanistic similarities as well as differences. It has recently become clear that segmental duplications (SDs) have had a very significant impact on genome plasticity during primate evolution. In comparisons of the human and chimpanzee genomes, SDs have been found in flanking regions of 70-80% of inversions and approximately 40% of deletions/duplications. A strong spatial association between primate-specific breakpoints and SDs has also become evident from comparisons of human with other mammalian genomes. The lineage-specific hyperexpansion of certain SDs observed in the genomes of human, chimpanzee, gorilla and gibbon is indicative of the intrinsic instability of some SDs in primates. However, since many primate-specific breakpoints map to regions lacking SDs, but containing interspersed high-copy repetitive sequence elements such as SINEs, LINEs, LTRs, alpha-satellites and (AT)( n ) repeats, we may infer that a range of different molecular mechanisms have probably been involved in promoting chromosomal breakage during the evolution of primate genomes.