Centromere emergence in evolution

Mechanisms of Rapid Karyotype Evolution in Mammals

Chromosome reshuffling events are often a foundational mechanism by which speciation can occur, giving rise to highly derivative karyotypes even amongst closely related species. Yet, the features that distinguish lineages prone to such rapid chromosome evolution from those that maintain stable karyotypes across evolutionary time are still to be defined. In this review, we summarize lineages prone to rapid karyotypic evolution in the context of Simpson's rates of evolution-tachytelic, horotelic, and bradytelic-and outline the mechanisms proposed to contribute to chromosome rearrangements, their fixation, and their potential impact on speciation events. Furthermore, we discuss relevant genomic features that underpin chromosome variation, including patterns of fusions/fissions, centromere positioning, and epigenetic marks such as DNA methylation. Finally, in the era of telomere-to-telomere genomics, we discuss the value of gapless genome resources to the future of research focused on the plasticity of highly rearranged karyotypes.

Repositioning of centromere-associated repeats during karyotype evolution in Oryzias fishes

The karyotype, which is the number and shape of chromosomes, is a fundamental characteristic of all eukaryotes. Karyotypic changes play an important role in many aspects of evolutionary processes, including speciation. In organisms with monocentric chromosomes, it was previously thought that chromosome number changes were mainly caused by centric fusions and fissions, whereas chromosome shape changes, that is, changes in arm numbers, were mainly due to pericentric inversions. However, recent genomic and cytogenetic studies have revealed examples of alternative cases, such as tandem fusions and centromere repositioning, found in the karyotypic changes within and between species. Here, we employed comparative genomic approaches to investigate whether centromere repositioning occurred during karyotype evolution in medaka fishes. In the medaka family (Adrianichthyidae), the three phylogenetic groups differed substantially in their karyotypes. The Oryzias latipes species group has larger numbers of chromosome arms than the other groups, with most chromosomes being metacentric. The O. javanicus species group has similar numbers of chromosomes to the O. latipes species group, but smaller arm numbers, with most chromosomes being acrocentric. The O. celebensis species group has fewer chromosomes than the other two groups and several large metacentric chromosomes that were likely formed by chromosomal fusions. By comparing the genome assemblies of O. latipes, O. javanicus, and O. celebensis, we found that repositioning of centromere-associated repeats might be more common than simple pericentric inversion. Our results demonstrated that centromere repositioning may play a more important role in karyotype evolution than previously appreciated.

Centromere innovations within a mouse species

Mammalian centromeres direct faithful genetic inheritance and are typically characterized by regions of highly repetitive and rapidly evolving DNA. We focused on a mouse species, Mus pahari, that we found has evolved to house centromere-specifying centromere protein-A (CENP-A) nucleosomes at the nexus of a satellite repeat that we identified and termed π-satellite (π-sat), a small number of recruitment sites for CENP-B, and short stretches of perfect telomere repeats. One M. pahari chromosome, however, houses a radically divergent centromere harboring ~6 mega-base pairs of a homogenized π-sat-related repeat, π-satB, that contains >20,000 functional CENP-B boxes. There, CENP-B abundance promotes accumulation of microtubule-binding components of the kinetochore and a microtubule-destabilizing kinesin of the inner centromere. We propose that the balance of pro- and anti-microtubule binding by the new centromere is what permits it to segregate during cell division with high fidelity alongside the older ones whose sequence creates a markedly different molecular composition.

Centromere Innovations Within a Mouse Species

Mammalian centromeres direct faithful genetic inheritance and are typically characterized by regions of highly repetitive and rapidly evolving DNA. We focused on a mouse species, Mus pahari, that we found has evolved to house centromere-specifying CENP-A nucleosomes at the nexus of a satellite repeat that we identified and term π-satellite (π-sat), a small number of recruitment sites for CENP-B, and short stretches of perfect telomere repeats. One M. pahari chromosome, however, houses a radically divergent centromere harboring ~6 Mbp of a homogenized π-sat-related repeat, π-satB, that contains >20,000 functional CENP-B boxes. There, CENP-B abundance drives accumulation of microtubule-binding components of the kinetochore, as well as a microtubule-destabilizing kinesin of the inner centromere. The balance of pro- and anti-microtubule-binding by the new centromere permits it to segregate during cell division with high fidelity alongside the older ones whose sequence creates a markedly different molecular composition.

Characterization of centromeric DNA of Gossypium anomalum reveals sequence-independent enrichment dynamics of centromeric repeats

Centromeres in eukaryotes are composed of highly repetitive DNAs, which evolve rapidly and are thought to achieve a favorable structure in mature centromeres. However, how the centromeric repeat evolves into an adaptive structure is largely unknown. We characterized the centromeric sequences of Gossypium anomalum through chromatin immunoprecipitation against CENH3 antibodies. We revealed that the G. anomalum centromeres contained only retrotransposon-like repeats but were depleted in long arrays of satellites. These retrotransposon-like centromeric repeats were present in the African-Asian and Australian lineage species, suggesting that they might have arisen in the common ancestor of these diploid species. Intriguingly, we observed a substantial increase and decrease in copy numbers among African-Asian and Australian lineages, respectively, for the retrotransposon-derived centromeric repeats without apparent structure or sequence variation in cotton. This result indicates that the sequence content is not a decisive aspect of the adaptive evolution of centromeric repeats or at least retrotransposon-like centromeric repeats. In addition, two active genes with potential roles in gametogenesis or flowering were identified in CENH3 nucleosome-binding regions. Our results provide new insights into the constitution of centromeric repetitive DNA and the adaptive evolution of centromeric repeats in plants.

Centromere repositioning and shifts in wheat evolution

The centromere is the region of a chromosome that directs its separation and plays an important role in cell division and reproduction of organisms. Elucidating the dynamics of centromeres is an alternative strategy for exploring the evolution of wheat. Here, we comprehensively analyzed centromeres from the de novo-assembled common wheat cultivar Aikang58 (AK58), Chinese Spring (CS), and all sequenced diploid and tetraploid ancestors by chromatin immunoprecipitation sequencing, whole-genome bisulfite sequencing, RNA sequencing, assay for transposase-accessible chromatin using sequencing, and comparative genomics. We found that centromere-associated sequences were concentrated during tetraploidization and hexaploidization. Centromeric repeats of wheat (CRWs) have undergone expansion during wheat evolution, with strong interweaving between the A and B subgenomes post tetraploidization. We found that CENH3 prefers to bind with younger CRWs, as directly supported by immunocolocalization on two chromosomes (1A and 2A) of wild emmer wheat with dicentromeric regions, only one of which bound with CENH3. In a comparison of AK58 with CS, obvious centromere repositioning was detected on chromosomes 1B, 3D, and 4D. The active centromeres showed a unique combination of lower CG but higher CHH and CHG methylation levels. We also found that centromeric chromatin was more open than pericentromeric chromatin, with higher levels of gene expression but lower gene density. Frequent introgression between tetraploid and hexaploid wheat also had a strong influence on centromere position on the same chromosome. This study also showed that active wheat centromeres were genetically and epigenetically determined.

CENP-A: A Histone H3 Variant with Key Roles in Centromere Architecture in Healthy and Diseased States

Centromeres are key architectural components of chromosomes. Here, we examine their construction, maintenance, and functionality. Focusing on the mammalian centromere- specific histone H3 variant, CENP-A, we highlight its coevolution with both centromeric DNA and its chaperone, HJURP. We then consider CENP-A de novo deposition and the importance of centromeric DNA recently uncovered with the added value from new ultra-long-read sequencing. We next review how to ensure the maintenance of CENP-A at the centromere throughout the cell cycle. Finally, we discuss the impact of disrupting CENP-A regulation on cancer and cell fate.

Evolution of tissue and developmental specificity of transcription start sites in Bos taurus indicus

To further the understanding of the evolution of transcriptional regulation, we profiled genome-wide transcriptional start sites (TSSs) in two sub-species, Bos taurus taurus and Bos taurus indicus, that diverged approximately 500,000 years ago. Evolutionary and developmental-stage differences in TSSs were detected across the sub-species, including translocation of dominant TSS and changes in TSS distribution. The 16% of all SNPs located in significant differentially used TSS clusters across sub-species had significant shifts in allele frequency (472 SNPs), indicating they may have been subject to selection. In spleen and muscle, a higher relative TSS expression was observed in Bos indicus than Bos taurus for all heat shock protein genes, which may be responsible for the tropical adaptation of Bos indicus.

Forutan et al. measure RNA expression and map transcription start sites (TSSs) between two sub-species of cattle in adult and fetal tissues and demonstrate translocation and changes in TSS distribution between the sub-species. This study provides insight into cattle adaptation and provides support of rapidly evolving TSS between two recently diverged subspecies.

Heterochromatin and microsatellites detection in karyotypes of four sea turtle species: Interspecific chromosomal differences

The wide variation in size and content of eukaryotic genomes is mainly attributed to the accumulation of repetitive DNA sequences, like microsatellites, which are tandemly repeated DNA sequences. Sea turtles share a diploid number (2n) of 56, however recent molecular cytogenetic data have shown that karyotype conservatism is not a rule in the group. In this study, the heterochromatin distribution and the chromosomal location of microsatellites (CA)n, (GA)n, (CAG)n, (GATA)n, (GAA)n, (CGC)n and (GACA)n in Chelonia mydas, Caretta caretta, Eretmochelys imbricata and Lepidochelys olivacea were comparatively investigated. The obtained data showed that just the (CA)n, (GA)n, (CAG)n and (GATA)n microsatellites were located on sea turtle chromosomes, preferentially in heterochromatic regions of the microchromosomes (mc). Variations in the location of heterochromatin and microsatellites sites, especially in some pericentromeric regions of macrochromosomes, corroborate to proposal of centromere repositioning occurrence in Cheloniidae species. Furthermore, the results obtained with the location of microsatellites corroborate with the temperature sex determination mechanism proposal and the absence of heteromorphic sex chromosomes in sea turtles. The findings are useful for understanding part of the karyotypic diversification observed in sea turtles, especially those that explain the diversification of Carettini from Chelonini species.

Single-cell strand sequencing of a macaque genome reveals multiple nested inversions and breakpoint reuse during primate evolution

Rhesus macaque is an Old World monkey that shared a common ancestor with human ∼25 Myr ago and is an important animal model for human disease studies. A deep understanding of its genetics is therefore required for both biomedical and evolutionary studies. Among structural variants, inversions represent a driving force in speciation and play an important role in disease predisposition. Here we generated a genome-wide map of inversions between human and macaque, combining single-cell strand sequencing with cytogenetics. We identified 375 total inversions between 859 bp and 92 Mbp, increasing by eightfold the number of previously reported inversions. Among these, 19 inversions flanked by segmental duplications overlap with recurrent copy number variants associated with neurocognitive disorders. Evolutionary analyses show that in 17 out of 19 cases, the Hominidae orientation of these disease-associated regions is always derived. This suggests that duplicated sequences likely played a fundamental role in generating inversions in humans and great apes, creating architectures that nowadays predispose these regions to disease-associated genetic instability. Finally, we identified 861 genes mapping at 156 inversions breakpoints, with some showing evidence of differential expression in human and macaque cell lines, thus highlighting candidates that might have contributed to the evolution of species-specific features. This study depicts the most accurate fine-scale map of inversions between human and macaque using a two-pronged integrative approach, such as single-cell strand sequencing and cytogenetics, and represents a valuable resource toward understanding of the biology and evolution of primate species.

Centromeres under Pressure: Evolutionary Innovation in Conflict with Conserved Function

Centromeres are essential genetic elements that enable spindle microtubule attachment for chromosome segregation during mitosis and meiosis. While this function is preserved across species, centromeres display an array of dynamic features, including: (1) rapidly evolving DNA; (2) wide evolutionary diversity in size, shape and organization; (3) evidence of mutational processes to generate homogenized repetitive arrays that characterize centromeres in several species; (4) tolerance to changes in position, as in the case of neocentromeres; and (5) intrinsic fragility derived by sequence composition and secondary DNA structures. Centromere drive underlies rapid centromere DNA evolution due to the "selfish" pursuit to bias meiotic transmission and promote the propagation of stronger centromeres. Yet, the origins of other dynamic features of centromeres remain unclear. Here, we review our current understanding of centromere evolution and plasticity. We also detail the mutagenic processes proposed to shape the divergent genetic nature of centromeres. Changes to centromeres are not simply evolutionary relics, but ongoing shifts that on one side promote centromere flexibility, but on the other can undermine centromere integrity and function with potential pathological implications such as genome instability.

Epigenetics as an Evolutionary Tool for Centromere Flexibility

Centromeres are the complex structures responsible for the proper segregation of chromosomes during cell division. Structural or functional alterations of the centromere cause aneuploidies and other chromosomal aberrations that can induce cell death with consequences on health and survival of the organism as a whole. Because of their essential function in the cell, centromeres have evolved high flexibility and mechanisms of tolerance to preserve their function following stress, whether it is originating from within or outside the cell. Here, we review the main epigenetic mechanisms of centromeres’ adaptability to preserve their functional stability, with particular reference to neocentromeres and holocentromeres. The centromere position can shift in response to altered chromosome structures, but how and why neocentromeres appear in a given chromosome region are still open questions. Models of neocentromere formation developed during the last few years will be hereby discussed. Moreover, we will discuss the evolutionary significance of diffuse centromeres (holocentromeres) in organisms such as nematodes. Despite the differences in DNA sequences, protein composition and centromere size, all of these diverse centromere structures promote efficient chromosome segregation, balancing genome stability and adaptability, and ensuring faithful genome inheritance at each cellular generation.

What makes a centromere?

Centromeres are the eukaryotic chromosomal sites at which the kinetochore forms and attaches to spindle microtubules to orchestrate chromosomal segregation in mitosis and meiosis. Although centromeres are essential for cell division, their sequences are not conserved and evolve rapidly. Centromeres vary dramatically in size and organization. Here we categorize their diversity and explore the evolutionary forces shaping them. Nearly all centromeres favor AT-rich DNA that is gene-free and transcribed at a very low level. Repair of frequent centromere-proximal breaks probably contributes to their rapid sequence evolution. Point centromeres are only ~125 bp and are specified by common protein-binding motifs, whereas short regional centromeres are 1-5 kb, typically have unique sequences, and may have pericentromeric repeats adapted to facilitate centromere clustering. Transposon-rich centromeres are often ~100-300 kb and are favored by RNAi machinery that silences transposons, by suppression of meiotic crossovers at centromeres, and by the ability of some transposons to target centromeres. Megabase-length satellite centromeres arise in plants and animals with asymmetric female meiosis that creates centromere competition, and favors satellite monomers one or two nucleosomes in length that position and stabilize centromeric nucleosomes. Holocentromeres encompass the length of a chromosome and may differ dramatically between mitosis and meiosis. We propose a model in which low level transcription of centromeres facilitates the formation of non-B DNA that specifies centromeres and promotes loading of centromeric nucleosomes.

Evolution of Marsupial Genomes

Marsupial genomes, which are packaged into large chromosomes, provide a powerful resource for studying the mechanisms of genome evolution. The extensive and valuable body of work on marsupial cytogenetics, combined more recently with genome sequence data, has enabled prediction of the 2n = 14 karyotype ancestral to all marsupial families. The application of both chromosome biology and genome sequencing, or chromosomics, has been a necessary approach for various aspects of mammalian genome evolution, such as understanding sex chromosome evolution and the origin and evolution of transmissible tumors in Tasmanian devils. The next phase of marsupial genome evolution research will employ chromosomics approaches to begin addressing fundamental questions in marsupial genome evolution and chromosome evolution more generally. The answers to these complex questions will impact our understanding across a broad range of fields, including the genetics of speciation, genome adaptation to environmental stressors, and species management.

Centromere Repeats: Hidden Gems of the Genome

Satellite DNAs are now regarded as powerful and active contributors to genomic and chromosomal evolution. Paired with mobile transposable elements, these repetitive sequences provide a dynamic mechanism through which novel karyotypic modifications and chromosomal rearrangements may occur. In this review, we discuss the regulatory activity of satellite DNA and their neighboring transposable elements in a chromosomal context with a particular emphasis on the integral role of both in centromere function. In addition, we discuss the varied mechanisms by which centromeric repeats have endured evolutionary processes, producing a novel, species-specific centromeric landscape despite sharing a ubiquitously conserved function. Finally, we highlight the role these repetitive elements play in the establishment and functionality of de novo centromeres and chromosomal breakpoints that underpin karyotypic variation. By emphasizing these unique activities of satellite DNAs and transposable elements, we hope to disparage the conventional exemplification of repetitive DNA in the historically-associated context of ‘junk’.

Comparison of Oryza sativa and Oryza brachyantha Genomes Reveals Selection-Driven Gene Escape from the Centromeric Regions

Centromeres are dynamic chromosomal regions, and the genetic and epigenetic environment of the centromere is often regarded as oppressive to protein-coding genes. Here, we used comparative genomic and phylogenomic approaches to study the evolution of centromeres and centromere-linked genes in the genus Oryza We report a 12.4-Mb high-quality BAC-based pericentromeric assembly for Oryza brachyantha, which diverged from cultivated rice (Oryza sativa) ∼15 million years ago. The synteny analyses reveal seven medium (>50 kb) pericentric inversions in O. sativa and 10 in O. brachyantha Of these inversions, three resulted in centromere movement (Chr1, Chr7, and Chr9). Additionally, we identified a potential centromere-repositioning event, in which the ancestral centromere on chromosome 12 in O. brachyantha jumped ∼400 kb away, possibly mediated by a duplicated transposition event (>28 kb). More strikingly, we observed an excess of syntenic gene loss at and near the centromeric regions (P < 2.2 × 10^-16). Most (33/47) of the missing genes moved to other genomic regions; therefore such excess could be explained by the selective loss of the copy in or near centromeric regions after gene duplication. The pattern of gene loss immediately adjacent to centromeric regions suggests centromere chromatin dynamics (e.g., spreading or microrepositioning) may drive such gene loss.

Inversion variants in human and primate genomes

For many years, inversions have been proposed to be a direct driving force in speciation since they suppress recombination when heterozygous. Inversions are the most common large-scale differences among humans and great apes. Nevertheless, they represent large events easily distinguishable by classical cytogenetics, whose resolution, however, is limited. Here, we performed a genome-wide comparison between human, great ape, and macaque genomes using the net alignments for the most recent releases of genome assemblies. We identified a total of 156 putative inversions, between 103 kb and 91 Mb, corresponding to 136 human loci. Combining literature, sequence, and experimental analyses, we analyzed 109 of these loci and found 67 regions inverted in one or multiple primates, including 28 newly identified inversions. These events overlap with 81 human genes at their breakpoints, and seven correspond to sites of recurrent rearrangements associated with human disease. This work doubles the number of validated primate inversions larger than 100 kb, beyond what was previously documented. We identified 74 sites of errors, where the sequence has been assembled in the wrong orientation, in the reference genomes analyzed. Our data serve two purposes: First, we generated a map of evolutionary inversions in these genomes representing a resource for interrogating differences among these species at a functional level; second, we provide a list of misassembled regions in these primate genomes, involving over 300 Mb of DNA and 1978 human genes. Accurately annotating these regions in the genome references has immediate applications for evolutionary and biomedical studies on primates.

Genomic properties of chromosomal bands are linked to evolutionary rearrangements and new centromere formation in primates

Chromosomal rearrangements in humans are largely related to pathological conditions, and phenotypic effects are also linked to alterations in the expression profile following nuclear relocation of genes between functionally different compartments, generally occupying the periphery or the inner part of the cell nuclei. On the other hand, during evolution, chromosomal rearrangements may occur apparently without damaging phenotypic effects and are visible in currently phylogenetically related species. To increase our insight into chromosomal reorganisation in the cell nucleus, we analysed 18 chromosomal regions endowed with different genomic properties in cell lines derived from eight primate species covering the entire evolutionary tree. We show that homologous loci, in spite of their evolutionary relocation along the chromosomes, generally remain localised to the same functional compartment of the cell nuclei. We conclude that evolutionarily successful chromosomal rearrangements are those that leave the nuclear position of the regions involved unchanged. On the contrary, in pathological situations, the effect typically observed is on gene structure alteration or gene nuclear reposition. Moreover, our data indicate that new centromere formation could potentially occur everywhere in the chromosomes, but only those emerging in very GC-poor/gene-poor regions, generally located in the nuclear periphery, have a high probability of being retained through evolution. This suggests that, in the cell nucleus of related species, evolutionary chromosomal reshufflings or new centromere formation does not alter the functionality of the regions involved or the interactions between different loci, thus preserving the expression pattern of orthologous genes.

Cytogenetic comparison of heteromorphic and homomorphic sex chromosomes in Coccinia (Cucurbitaceae) points to sex chromosome turnover

Our understanding of the evolution of plant sex chromosomes is increasing rapidly due to high-throughput sequencing data and phylogenetic and molecular-cytogenetic approaches that make it possible to infer the evolutionary direction and steps leading from homomorphic to heteromorphic sex chromosomes. Here, we focus on four species of Coccinia, a genus of 25 dioecious species, including Coccinia grandis, the species with the largest known plant Y chromosome. Based on a phylogeny for the genus, we selected three species close to C. grandis to test the distribution of eight repetitive elements including two satellites, and several plastid and mitochondrial probes, that we had previously found to have distinct accumulation patterns in the C. grandis genome. Additionally, we determined C-values and performed immunostaining experiments with (peri-)centromere-specific antibodies on two species (for comparison with C. grandis). In spite of no microscopic chromosomal heteromorphism, single pairs of chromosomes in male cells of all three species accumulate some of the very same repeats that are enriched on the C. grandis Y chromosome, pointing to either old (previous) sex chromosomes or incipient (newly arising) ones, that is, to sex chromosome turnover. A 144-bp centromeric satellite repeat (CgCent) that characterizes all C. grandis chromosomes except the Y is highly abundant in all centromeric regions of the other species, indicating that the centromeric sequence of the Y chromosome diverged very recently.

The Promises and Challenges of Genomic Studies of Human Centromeres

Human centromeres are genomic regions that act as sites of kinetochore assembly to ensure proper chromosome segregation during mitosis and meiosis. Although the biological importance of centromeres in genome stability, and ultimately, cell viability are well understood, the complete sequence content and organization in these multi-megabase-sized regions remains unknown. The lack of a high-resolution reference assembly inhibits standard bioinformatics protocols, and as a result, sequence-based studies involving human centromeres lag far behind the advances made for the non-repetitive sequences in the human genome. In this chapter, I introduce what is known about the genomic organization in the highly repetitive regions spanning human centromeres, and discuss the challenges these sequences pose for assembly, alignment, and data interpretation. Overcoming these obstacles is expected to issue a new era for centromere genomics, which will offer new discoveries in basic cell biology and human biomedical research.

Rapid proliferation and nucleolar organizer targeting centromeric retrotransposons in cotton

Centromeric chromatin in most eukaryotes is composed of highly repetitive centromeric retrotransposons and satellite repeats that are highly variable even among closely related species. The evolutionary mechanisms that underlie the rapid evolution of centromeric repeats remain unknown. To obtain insight into the evolution of centromeric repeats following polyploidy, we studied a model diploid progenitor (Gossypium raimondii, D-genome) of the allopolyploid (AD-genome) cottons, G. hirsutum and G. barbadense. Sequence analysis of chromatin-immunoprecipitated DNA showed that the G. raimondii centromeric repeats originated from retrotransposon-related sequences. Comparative analysis showed that nine of the 10 analyzed centromeric repeats were absent from the centromeres in the A-genome and related diploid species (B-, F- and G-genomes), indicating that they colonized the centromeres of D-genome lineage after the divergence of the A- and D- ancestral species or that they were ancestrally retained prior to the origin of Gossypium. Notably, six of the nine repeats were present in both the A- and D-subgenomes in tetraploid G. hirsutum, and increased in abundance in both subgenomes. This finding suggests that centromeric repeats may spread and proliferate between genomes subsequent to polyploidization. Two repeats, Gr334 and Gr359 occurred in both the centromeres and nucleolar organizer regions (NORs) in D- and AD-genome species, yet localized to just the NORs in A-, B-, F-, and G-genome species. Contained within is a story of an established centromeric repeat that is eliminated and allopolyploidization provides an opportunity for reinvasion and reestablishment, which broadens our evolutionary understanding behind the cycles of centromeric repeat establishment and targeting.

Cytogenetic Analysis of the South American Fruit Fly Anastrepha fraterculus (Diptera:Tephritidae) Species Complex: Construction of Detailed Photographic Polytene Chromosome Maps of the Argentinian Af. sp.1 Member

Genetic and cytogenetic studies constitute a significant basis for understanding the biology of insect pests and the design and the construction of genetic tools for biological control strategies. Anastrepha fraterculus is an important pest of the Tephritidae family. It is distributed from southern Texas through eastern Mexico, Central America and South America causing significant crop damage and economic losses. Currently it is considered as a species complex; until now seven members have been described based on multidisciplinary approaches. Here we report the cytogenetic analysis of an Argentinian population characterized as Af. sp.1 member of the Anastrepha fraterculus species complex. The mitotic karyotype and the first detailed photographic maps of the salivary gland polytene chromosomes are presented. The mitotic metaphase complement consists of six (6) pairs of chromosomes, including one pair of heteromorphic sex chromosomes, with the male being the heterogametic sex. The analysis of the salivary gland polytene complement shows a total number of five long chromosomes that correspond to the five autosomes of the mitotic karyotype and a heterochromatic network corresponding to the sex chromosomes. Comparison of the polytene chromosome maps between this species and Anastrepha ludens shows significant similarity. The polytene maps presented here are suitable for cytogenetic studies that could shed light on the species limits within this species complex and support the development of genetic tools for sterile insect technique (SIT) applications.

Three potato centromeres are associated with distinct haplotypes with or without megabase-sized satellite repeat arrays

We report discoveries of different haplotypes associated with the centromeres of three potato chromosomes, including haplotypes composed of long arrays of satellite repeats and haplotypes lacking the same repeats. These results are in favor of the hypothesis that satellite repeat-based centromeres may originate from neocentromeres that lack repeats.

The CentO satellite confers translational and rotational phasing on cenH3 nucleosomes in rice centromeres

Plant and animal centromeres comprise megabases of highly repeated satellite sequences, yet centromere function can be specified epigenetically on single-copy DNA by the presence of nucleosomes containing a centromere-specific variant of histone H3 (cenH3). We determined the positions of cenH3 nucleosomes in rice (Oryza sativa), which has centromeres composed of both the 155-bp CentO satellite repeat and single-copy non-CentO sequences. We find that cenH3 nucleosomes protect 90-100 bp of DNA from micrococcal nuclease digestion, sufficient for only a single wrap of DNA around the cenH3 nucleosome core. cenH3 nucleosomes are translationally phased with 155-bp periodicity on CentO repeats, but not on non-CentO sequences. CentO repeats have an ∼10-bp periodicity in WW dinucleotides and in micrococcal nuclease cleavage, providing evidence for rotational phasing of cenH3 nucleosomes on CentO and suggesting that satellites evolve for translational and rotational stabilization of centromeric nucleosomes.

Maize centromeres expand and adopt a uniform size in the genetic background of oat

Most existing centromeres may have originated as neocentromeres that activated de novo from noncentromeric regions. However, the evolutionary path from a neocentromere to a mature centromere has been elusive. Here we analyzed the centromeres of nine chromosomes that were transferred from maize into oat as the result of an inter-species cross. Centromere size and location were assayed by chromatin immunoprecipitation for the histone variant CENH3, which is a defining feature of functional centromeres. Two isolates of maize chromosome 3 proved to contain neocentromeres in the sense that they had moved from the original site, whereas the remaining seven centromeres (1, 2, 5, 6, 8, 9, and 10) were retained in the same area in both species. In all cases, the CENH3-binding domains were dramatically expanded to encompass a larger area in the oat background (∼3.6 Mb) than the average centromere size in maize (∼1.8 Mb). The expansion of maize centromeres appeared to be restricted by the transcription of genes located in regions flanking the original centromeres. These results provide evidence that (1) centromere size is regulated; (2) centromere sizes tend to be uniform within a species regardless of chromosome size or origin of the centromere; and (3) neocentromeres emerge and expand preferentially in gene-poor regions. Our results suggest that centromere size expansion may be a key factor in the survival of neocentric chromosomes in natural populations.

The interplay between genome organization and nuclear architecture of primate evolutionary neo-centromeres

An Evolutionary Neo-Centromere (ENC) is a centromere that emerged in an ectopic region of a chromosome during evolution. It is thought that the old centromere must be inactivated because dicentric chromosomes are not viable. The aim of the present study was to investigate whether 3D arrangement in the interphase nucleus of the novel and old centromeric domains was affected by the repositioning event. The data we present here strongly indicate that the ENC phenomenon does not affect the 3D location of either novel or old centromeres. Very likely, other features, such as gene density, rather than the newly acquired or lost functions, define positioning in the nucleus.

Repeatless and repeat-based centromeres in potato: implications for centromere evolution

Centromeres in most higher eukaryotes are composed of long arrays of satellite repeats. By contrast, most newly formed centromeres (neocentromeres) do not contain satellite repeats and instead include DNA sequences representative of the genome. An unknown question in centromere evolution is how satellite repeat-based centromeres evolve from neocentromeres. We conducted a genome-wide characterization of sequences associated with CENH3 nucleosomes in potato (Solanum tuberosum). Five potato centromeres (Cen4, Cen6, Cen10, Cen11, and Cen12) consisted primarily of single- or low-copy DNA sequences. No satellite repeats were identified in these five centromeres. At least one transcribed gene was associated with CENH3 nucleosomes. Thus, these five centromeres structurally resemble neocentromeres. By contrast, six potato centromeres (Cen1, Cen2, Cen3, Cen5, Cen7, and Cen8) contained megabase-sized satellite repeat arrays that are unique to individual centromeres. The satellite repeat arrays likely span the entire functional cores of these six centromeres. At least four of the centromeric repeats were amplified from retrotransposon-related sequences and were not detected in Solanum species closely related to potato. The presence of two distinct types of centromeres, coupled with the boom-and-bust cycles of centromeric satellite repeats in Solanum species, suggests that repeat-based centromeres can rapidly evolve from neocentromeres by de novo amplification and insertion of satellite repeats in the CENH3 domains.

The evolution of African great ape subtelomeric heterochromatin and the fusion of human chromosome 2

Chimpanzee and gorilla chromosomes differ from human chromosomes by the presence of large blocks of subterminal heterochromatin thought to be composed primarily of arrays of tandem satellite sequence. We explore their sequence composition and organization and show a complex organization composed of specific sets of segmental duplications that have hyperexpanded in concert with the formation of subterminal satellites. These regions are highly copy number polymorphic between and within species, and copy number differences involving hundreds of copies can be accurately estimated by assaying read-depth of next-generation sequencing data sets. Phylogenetic and comparative genomic analyses suggest that the structures have arisen largely independently in the two lineages with the exception of a few seed sequences present in the common ancestor of humans and African apes. We propose a model where an ancestral human-chimpanzee pericentric inversion and the ancestral chromosome 2 fusion both predisposed and protected the chimpanzee and human genomes, respectively, to the formation of subtelomeric heterochromatin. Our findings highlight the complex interplay between duplicated sequences and chromosomal rearrangements that rapidly alter the cytogenetic landscape in a short period of evolutionary time.

Distribution of repetitive DNAs and the hybrid origin of the red vizcacha rat (Octodontidae)

Great genome size (GS) variations described in desert-specialist octodontid rodents include diploid species ( Octomys mimax and Octodontomys gliroides ) and putative tetraploid species ( Tympanoctomys barrerae and Pipanacoctomys aureus ). Because of its high DNA content, elevated chromosome number, and gigas effect, the genome of T. barrerae is claimed to have resulted from tetraploidy. Alternatively, the origin of its GS has been attributed to the accumulation of repetitive sequences. To better characterize the extent and origin of these repetitive DNA, self-genomic in situ hybridization (self-GISH), whole-comparative genomic hybridization (W-CGH), and conventional GISH were conducted in mitotic and meiotic chromosomes. Self-GISH on T. barrerae mitotic plates together with comparative self-GISH (using its closest relatives) discriminate a pericentromeric and a telomeric DNA fraction. As most of the repetitive sequences are pericentromeric, it seems that the large GS of T. barrerae is not due to highly repeated sequences accumulated along chromosomes arms. W-CGH using red-labeled P. aureus DNA and green-labeled O. mimax DNA simultaneously on chromosomes of T. barrerae revealed a yellow-orange fluorescence over a repetitive fraction of the karyotype. However, distinctive red-only fluorescent signals were also detected at some centromeres and telomeres, indicating closer homology with the DNA sequences of P. aureus. Conventional GISH using an excess of blocking DNA from either P. aureus or O. mimax labeled only a fraction of the T. barrerae genome, indicating its double genome composition. These data point to a hybrid nature of the T. barrerae karyotype, suggesting a hybridization event in the origin of this species.

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.

Centromere repositioning in mammals

The evolutionary history of chromosomes can be tracked by the comparative hybridization of large panels of bacterial artificial chromosome clones. This approach has disclosed an unprecedented phenomenon: 'centromere repositioning', that is, the movement of the centromere along the chromosome without marker order variation. The occurrence of evolutionary new centromeres (ENCs) is relatively frequent. In macaque, for instance, 9 out of 20 autosomal centromeres are evolutionarily new; in donkey at least 5 such neocentromeres originated after divergence from the zebra, in less than 1 million years. Recently, orangutan chromosome 9, considered to be heterozygous for a complex rearrangement, was discovered to be an ENC. In humans, in addition to neocentromeres that arise in acentric fragments and result in clinical phenotypes, 8 centromere-repositioning events have been reported. These 'real-time' repositioned centromere-seeding events provide clues to ENC birth and progression. In the present paper, we provide a review of the centromere repositioning. We add new data on the population genetics of the ENC of the orangutan, and describe for the first time an ENC on the X chromosome of squirrel monkeys. Next-generation sequencing technologies have started an unprecedented, flourishing period of rapid whole-genome sequencing. In this context, it is worth noting that these technologies, uncoupled from cytogenetics, would miss all the biological data on evolutionary centromere repositioning. Therefore, we can anticipate that classical and molecular cytogenetics will continue to have a crucial role in the identification of centromere movements. Indeed, all ENCs and human neocentromeres were found following classical and molecular cytogenetic investigations.

Centromere identity, function, and epigenetic propagation across cell divisions

The key to understanding centromere identity is likely to lie in the chromatin containing the histone H3 variant CENP-A. CENP-A is the prime candidate to carry the epigenetic information that specifies the chromosomal location of the centromere in nearly all eukaryotic species, raising questions fundamental to understanding chromosome inheritance: How is the epigenetic centromere mark propagated? What physical properties of CENP-A-containing complexes are important for epigenetically marking centromeres? What are the molecules that recognize centromeric chromatin and serve as the foundation for the mitotic kinetochore? We discuss recent advances from our research groups that have yielded substantial insight into these questions and present our current understanding of the centromere. Future work promises an understanding of the molecular processes that confer fidelity to genome transmission at cell division.

Genome-wide characterization of centromeric satellites from multiple mammalian genomes

Despite its importance in cell biology and evolution, the centromere has remained the final frontier in genome assembly and annotation due to its complex repeat structure. However, isolation and characterization of the centromeric repeats from newly sequenced species are necessary for a complete understanding of genome evolution and function. In recent years, various genomes have been sequenced, but the characterization of the corresponding centromeric DNA has lagged behind. Here, we present a computational method (RepeatNet) to systematically identify higher-order repeat structures from unassembled whole-genome shotgun sequence and test whether these sequence elements correspond to functional centromeric sequences. We analyzed genome datasets from six species of mammals representing the diversity of the mammalian lineage, namely, horse, dog, elephant, armadillo, opossum, and platypus. We define candidate monomer satellite repeats and demonstrate centromeric localization for five of the six genomes. Our analysis revealed the greatest diversity of centromeric sequences in horse and dog in contrast to elephant and armadillo, which showed high-centromeric sequence homogeneity. We could not isolate centromeric sequences within the platypus genome, suggesting that centromeres in platypus are not enriched in satellite DNA. Our method can be applied to the characterization of thousands of other vertebrate genomes anticipated for sequencing in the near future, providing an important tool for annotation of centromeres.

Genome-wide mapping of cytosine methylation revealed dynamic DNA methylation patterns associated with genes and centromeres in rice

We conducted genome-wide mapping of cytosine methylation using methylcytosine immunoprecipitation combined with Illumina sequencing. The chromosomal distribution pattern of methylated DNA is similar to the heterochromatin distribution pattern on rice chromosomes. The DNA methylation patterns of rice genes are similar to those in Arabidopsis thaliana, including distinct methylation patterns asssociated with gene bodies and promoters. The DNA sequences in the core domains of rice Cen4, Cen5 and Cen8 showed elevated methylation levels compared with sequences in the pericentromeric regions. In addition, elevated methylation levels were associated with the DNA sequences in the CENH3-binding subdomains, compared with the sequences in the flanking H3 subdomains. In contrast, the centromeric domain of Cen11, which is composed exclusively of centromeric satellite DNA, is hypomethylated compared with the pericentromeric domains. Thus, the DNA sequences associated with functional centromeres can be either hypomethylated or hypermethylated. The methylation patterns of centromeric DNA appear to be correlated with the composition of the associated DNA sequences. We propose that both hypomethylation and hypermethylation of CENH3-associated DNA sequences can serve as epigenetic marks to distinguish where CENH3 deposition will occur within the surrounding H3 chromatin.

Centromere inactivation and epigenetic modifications of a plant chromosome with three functional centromeres

A chromosome with two functional centromeres is cytologically unstable and can only be stabilized when one of the two centromeres becomes inactivated via poorly understood mechanisms. Here, we report a transmissible chromosome with multiple centromeres in wheat. This chromosome encompassed one large and two small domains containing the centromeric histone CENH3. The two small centromeres are in a close vicinity and often fused as a single centromere on metaphase chromosomes. This fused centromere contained approximately 30% of the CENH3 compared to the large centromere. An intact tricentric chromosome was transmitted to about 70% of the progenies, which was likely a consequence of the dominating pulling capacity of the large centromere during anaphases of meiosis. The tricentric chromosome showed characteristics typical to dicentric chromosomes, including chromosome breaks and centromere inactivation. Remarkably, inactivation was always associated with the small centromeres, indicating that small centromeres are less likely to survive than large ones in dicentric chromosomes. The inactivation of the small centromeres also coincided with changes of specific histone modifications, including H3K27me2 and H3K27me3, of the pericentromeric chromatin.

Uncoupling of satellite DNA and centromeric function in the genus Equus

In a previous study, we showed that centromere repositioning, that is the shift along the chromosome of the centromeric function without DNA sequence rearrangement, has occurred frequently during the evolution of the genus Equus. In this work, the analysis of the chromosomal distribution of satellite tandem repeats in Equus caballus, E. asinus, E. grevyi, and E. burchelli highlighted two atypical features: 1) several centromeres, including the previously described evolutionary new centromeres (ENCs), seem to be devoid of satellite DNA, and 2) satellite repeats are often present at non-centromeric termini, probably corresponding to relics of ancestral now inactive centromeres. Immuno-FISH experiments using satellite DNA and antibodies against the kinetochore protein CENP-A demonstrated that satellite-less primary constrictions are actually endowed with centromeric function. The phylogenetic reconstruction of centromere repositioning events demonstrates that the acquisition of satellite DNA occurs after the formation of the centromere during evolution and that centromeres can function over millions of years and many generations without detectable satellite DNA. The rapidly evolving Equus species gave us the opportunity to identify different intermediate steps along the full maturation of ENCs.

Centromeres are the functional elements controlling chromosome segregation during cell division. Vertebrate centromeres, which typically contain large amounts of tandem repeats (satellite DNA), are highly conserved for function but not for DNA sequence, suggesting that centromeric function is mainly determined by epigenetic factors. Evolutionary centromere repositioning is the shift of a centromere to a new position in the absence of structural chromosome rearrangements. In previous work, we demonstrated that centromere repositioning was exceptionally frequent during the evolution of the genus Equus (horses, asses, and zebras). In the present paper, we show that several Equus centromeres, including all the previously described evolutionary new centromeres, are apparently satellite-free, supporting the idea that large blocks of repeats are not necessarily required for the stability of centromeres. Our results suggest that centromere repositioning might be a two-step event: first, a neocentromere arises in a satellite-less region; satellite repeats may then colonize this repositioned centromere at a later stage, giving rise to a “mature” centromere. The rapidly evolving Equus species gave us the opportunity to catch snapshots of several evolutionary novel centromeres in different stages during their maturation.

Shared Y chromosome repetitive DNA sequences in stallion and donkey as visualized using whole-genomic comparative hybridization

The genome of stallion (Spanish breed) and donkey (Spanish endemic Zamorano-Leonés) were compared using whole comparative genomic in situ hybridization (W-CGH) technique, with special reference to the variability observed in the Y chromosome. Results show that these diverging genomes still share some highly repetitive DNA families localized in pericentromeric regions and, in the particular case of the Y chromosome, a sub-family of highly repeated DNA sequences, greatly expanded in the donkey genome, accounts for a large part of the chromatin in the stallion Y chromosome.

Centromere repositioning in cucurbit species: implication of the genomic impact from centromere activation and inactivation

The centromere of an eukaryotic chromosome can move to a new position during evolution, which may result in a major alteration of the chromosome morphology and karyotype. This centromere repositioning phenomenon has been extensively documented in mammalian species and was implicated to play an important role in mammalian genome evolution. Here we report a centromere repositioning event in plant species. Comparative fluorescence in situ hybridization mapping using common sets of fosmid clones between two pairs of cucumber (Cucumis sativus L.) and melon (Cucumis melo L.) chromosomes revealed changes in centromere positions during evolution. Pachytene chromosome analysis revealed that the current centromeres of all four cucumber and melon chromosomes are associated with distinct pericentromeric heterochromatin. Interestingly, inactivation of a centromere in the original centromeric region was associated with a loss or erosion of its affixed pericentromeric heterochromatin. Thus, both centromere activation and inactivation in cucurbit species were associated with a gain/loss of a large amount of pericentromeric heterochromatin.

Identification of a maize neocentromere in an oat-maize addition line

We report a neocentromere event on maize chromosome 3 that occurred due to chromosome breakage. The neocentromere lies on a fragment of the short arm that lacks the primary centromere DNA elements, CentC and CRM. It is transmitted in the genomic background of oat via a new centromere (and kinetochore), as shown by immunolocalization of the oat CENH3 protein. Despite normal transmission of the maize fragment in most progeny, neocentromeres appear to vary in size within the same tissue, as shown by fluorescent measurements. A secondary truncation in one line lowered mitotic transmission to 3% and precipitously reduced the size of the chromosome. The results support the view that neocentromere formation is generally associated with major genomic disturbances such as wide species crosses or deletion of an existing centromere. The data further suggest that new centromeres may undergo a period of instability that is corrected over a period of several generations.

The rest repression of the neurosecretory phenotype is negatively modulated by BHC80, a protein of the BRAF/HDAC complex

Expression of neurosecretion by nerve cells requires the levels of the transcription repressor element-1 silencing transcription factor (REST) to be very low. However, when high-REST clones of PC12 cells, defective of neurosecretion, were fused to other high-REST, non-neurosecretory cells, some neurosecretion was recovered. To clarify the mechanism of this recovery, we fused defective PC12 cells with human lymphocytes. A cytogenetic analysis revealed all hybrid clones that recovered neurosecretion to contain a fragment of chromosome 11 including the gene encoding BHC80, a protein of one of the complexes that mediate REST repression. In these clones, REST levels were as high as in defective PC12, whereas BHC80, localized in the nucleus, was 4- to 5-fold higher. Transient transfection of defective PC12 with various amounts of BHC80 cDNA induced (1) in defective PC12, the reexpression of only neurosecretion mRNAs; (2) in defective PC12 cotransfected with the REST negative construct DNA-binding domain (to attenuate gene repression), the recovery of a weak, but complete neurosecretory phenotype, including dense-core granules and their regulated exocytosis. Chromatin immunoprecipitation and immunodepletion analyses revealed the extensive BHC80 association with REST at the genes of two neurosecretion proteins, chromograninB and SNAP25, however only in the low-REST PC12, whereas in high-REST defective PC12 no association was appreciable. In defective PC12 transfected with BHC80 some association was reestablished. Therefore, the recovery of neurosecretion observed after fusion/transfection of defective PC12 depends on the reciprocal level of BHC80 and REST, with BHC80 working as a negative modulator of REST repression. This role appears of possible cell physiological and pathological importance.

New insights into centromere organization and evolution from the white-cheeked gibbon and marmoset

The evolutionary history of alpha-satellite DNA, the major component of primate centromeres, is hardly defined because of the difficulty in its sequence assembly and its rapid evolution when compared with most genomic sequences. By using several approaches, we have cloned, sequenced, and characterized alpha-satellite sequences from two species representing critical nodes in the primate phylogeny: the white-cheeked gibbon, a lesser ape, and marmoset, a New World monkey. Sequence analyses demonstrate that white-cheeked gibbon and marmoset alpha-satellite sequences are formed by units of approximately 171 and approximately 342 bp, respectively, and they both lack the high-order structure found in humans and great apes. Fluorescent in situ hybridization characterization shows a broad dispersal of alpha-satellite in the white-cheeked gibbon genome including centromeric, telomeric, and chromosomal interstitial localizations. On the other hand, centromeres in marmoset appear organized in highly divergent dimers roughly of 342 bp that show a similarity between monomers much lower than previously reported dimers, thus representing an ancient dimeric structure. All these data shed light on the evolution of the centromeric sequences in Primates. Our results suggest radical differences in the structure, organization, and evolution of alpha-satellite DNA among different primate species, supporting the notion that 1) all the centromeric sequence in Primates evolved by genomic amplification, unequal crossover, and sequence homogenization using a 171 bp monomer as the basic seeding unit and 2) centromeric function is linked to relatively short repeated elements, more than higher-order structure. Moreover, our data indicate that complex higher-order repeat structures are a peculiarity of the hominid lineage, showing the more complex organization in humans.

Intergenic locations of rice centromeric chromatin

Centromeres are sites for assembly of the chromosomal structures that mediate faithful segregation at mitosis and meiosis. Plant and animal centromeres are typically located in megabase-sized arrays of tandem satellite repeats, making their precise mapping difficult. However, some rice centromeres are largely embedded in nonsatellite DNA, providing an excellent model to study centromere structure and evolution. We used chromatin immunoprecipitation and 454 sequencing to define the boundaries of nine of the 12 centromeres of rice. Centromere regions from chromosomes 8 and 9 were found to share synteny, most likely reflecting an ancient genome duplication. For four centromeres, we mapped discrete subdomains of binding by the centromeric histone variant CENH3. These subdomains were depleted in both intact and nonfunctional genes relative to interspersed subdomains lacking CENH3. The intergenic location of rice centromeric chromatin resembles the situation for human neocentromeres and supports a model of the evolution of centromeres from gene-poor regions.

Before a cell divides, its chromosomes must be duplicated and then separated to provide each daughter cell with an identical genome copy. To accomplish this separation, the cell-division apparatus attaches to structures on the chromosomes called centromeres. Most plant and animal centromeres contain highly repetitive DNA sequences and specific proteins such as CENH3; however, it is not known which of the many repeats bind CENH3. Some rice centromeres, however, consist largely of single-copy DNA, providing a tractable model for investigating CENH3-binding patterns. Using modern DNA sequencing technology and an antibody to CENH3, we were able to find which sequences in the rice genome are bound by CENH3. We uncovered evidence that one centromere, Cen8, which has lost much of its repetitive content through a rearrangement within the last approximately 5 million years, is derived from a highly repetitive centromeric region that was duplicated along with the rest of the genome 50–70 million years ago. We also found that CENH3 is bound discontinuously in centromeric subdomains that have fewer genes than subdomains lacking CENH3. These results suggest, not only that centromeres evolve in gene-poor regions, but also how centromeres might evolve from single-copy to repetitive sequences.

A key centromere protein is found to bind discontinuously to subdomains of centromeres that are depleted in genes, suggesting that centromeres evolve in gene-poor regions.

Evolutionary new centromeres in primates

The centromere has a pivotal role in structuring chromosomal architecture, but remains a poorly understood and seemingly paradoxical "black hole." Centromeres are a very rapidly evolving segment of the genome and it is now known that centromere shifts in evolution are not rare and must be considered on a par with other chromosome rearrangements. Recently, unprecedented findings on neocentromeres and evolutionary new centromeres (ENC) have helped clarify the relationship of the centromere within the genome and shown that these two phenomena are two faces of the same coin. No prominent sequence features are known that promote centromere formation and both types of new centromeres are formed epigenetically, both clinical neocentromeres and ENC cluster at chromosomal "hotspots." The clustering of neocentromeres in 8p is probably the result of the relatively high frequency of noncanonical pairing. Studies on the evolution of the chromosomes 3, 13, and 15 help explain why there are clusters of neocentromeres. These domains often correspond to ancestral inactivated centromeres and some regions can preserve features that trigger neocentromere emergence over tens of millions of years. Neocentromeres may be correlated with the distribution of segmental duplications (SDs) in regions of extreme plasticity that often can be characterized as gene deserts. Further, because centromeres and associated pericentric regions are dynamically complex, centromere shifts may turbocharge genome reorganization by influencing the distribution of heterochromatin. The "reuse" of regions as centromere seeding-points in evolution and in human clinical cases further extends the concept of "reuse" of specific domains for "chromosomal events."

Evolutionary-new centromeres preferentially emerge within gene deserts

BACKGROUND

Evolutionary-new centromeres (ENCs) result from the seeding of a centromere at an ectopic location along the chromosome during evolution. The novel centromere rapidly acquires the complex structure typical of eukaryote centromeres. This phenomenon has played an important role in shaping primate karyotypes. A recent study on the evolutionary-new centromere of macaque chromosome 4 (human 6) showed that the evolutionary-new centromere domain was deeply restructured, following the seeding, with respect to the corresponding human region assumed as ancestral. It was also demonstrated that the region was devoid of genes. We hypothesized that these two observations were not merely coincidental and that the absence of genes in the seeding area constituted a crucial condition for the evolutionary-new centromere fixation in the population.

RESULTS

To test our hypothesis, we characterized 14 evolutionary-new centromeres selected according to conservative criteria. Using different experimental approaches, we assessed the extent of genomic restructuring. We then determined the gene density in the ancestral domain where each evolutionary-new centromere was seeded.

CONCLUSIONS

Our study suggests that restructuring of the seeding regions is an intrinsic property of novel evolutionary centromeres that could be regarded as potentially detrimental to the normal functioning of genes embedded in the region. The absence of genes, which was found to be of high statistical significance, appeared as a unique favorable scenario permissive of evolutionary-new centromere fixation in the population.

A satellite-like sequence, representing a "clone gap" in the human genome, was likely involved in the seeding of a novel centromere in macaque

Although the human genome sequence is generally considered "finished", the latest assembly (NCBI Build 36.1) still presents a number of gaps. Some of them are defined as "clone gaps" because they separate neighboring contigs. Evolutionary new centromeres are centromeres that repositioned along the chromosome, without marker order variation, during evolution. We have found that one human "clone gap" at 18q21.2 corresponds to an evolutionary new centromere in Old World Monkeys (OWM). The partially sequenced gap revealed a satellite-like structure. DNA stretches of the same satellite were found in the macaque (flanking the chromosome 18 centromere) and in the marmoset (New World Monkey), which was used as an outgroup. These findings strongly suggested that the repeat was present at the time of novel centromere seeding in OWM ancestor. We have provided, therefore, the first instance of a specific sequence hypothesized to have played a role in triggering the emergence of an evolutionary new centromere.

Refinement of macaque synteny arrangement with respect to the official rheMac2 macaque sequence assembly

We have compared the synteny block organization of the official macaque genome sequence assembly (Jan. 2006; rheMac2) with an independent assembly that used a molecular cytogenetic approach. The mapping of four synteny segments, ranging in size from 4 Mb to 24 Mb, was found to be inconsistent between the two datasets. We specifically investigated these discrepancies by appropriate co-hybridization FISH experiments with validated reference probes located outside the area under study. We found that in the macaque rheMac2 release three synteny segments were wrongly mapped and one segment was incorrectly oriented.

Phylogenomics of African guenons

The karyotypes of 28 specimens belonging to 26 species of Cercopithecinae have been compared with each other and with human karyotype by chromosome banding and, for some of them, by Zoo-FISH (human painting probes) techniques. The study includes the first description of the karyotypes of four species and a synonym of Cercopithecus nictitans. The chromosomal homologies obtained provide us with new data on a large number of rearrangements. This allows us to code chromosomal characters to draw Cercopithecini phylogenetic trees, which are compared to phylogenetic data based on DNA sequences. Our findings show that some of the superspecies proposed by Kingdon (1997 The Kingdon Field Guide to African Mammals, Academic Press.) and Groves (2001 Primates Taxonomy, Smithsonian Institution Press) do not form homogeneous groups and that the genus Cercopithecus is paraphyletic, in agreement with previous molecular analyses. The evolution of Cercopithecini karyotypes is mainly due to non-centromeric chromosome fissions and centromeric shifts or inversions. Non-Robertsonian translocations occurred in C. hamlyni and C. neglectus. The position of chromosomal rearrangements in the phylogenetic tree leads us to propose that the Cercopithecini evolution proceeded by either repeated fission events facilitated by peculiar genomic structures or successive reticulate phases, in which heterozygous populations for few rearranged chromosomes were present, allowing the spreading of chromosomal forms in various combinations, before the speciation process.

Primate chromosome evolution: ancestral karyotypes, marker order and neocentromeres

In 1992 the Japanese macaque was the first species for which the homology of the entire karyotype was established by cross-species chromosome painting. Today, there are chromosome painting data on more than 50 species of primates. Although chromosome painting is a rapid and economical method for tracking translocations, it has limited utility for revealing intrachromosomal rearrangements. Fortunately, the use of BAC-FISH in the last few years has allowed remarkable progress in determining marker order along primate chromosomes and there are now marker order data on an array of primate species for a good number of chromosomes. These data reveal inversions, but also show that centromeres of many orthologous chromosomes are embedded in different genomic contexts. Even if the mechanisms of neocentromere formation and progression are just beginning to be understood, it is clear that these phenomena had a significant impact on shaping the primate genome and are fundamental to our understanding of genome evolution. In this report we complete and integrate the dataset of BAC-FISH marker order for human syntenies 1, 2, 4, 5, 8, 12, 17, 18, 19, 21, 22 and the X. These results allowed us to develop hypotheses about the content, marker order and centromere position in ancestral karyotypes at five major branching points on the primate evolutionary tree: ancestral primate, ancestral anthropoid, ancestral platyrrhine, ancestral catarrhine and ancestral hominoid. Current models suggest that between-species structural rearrangements are often intimately related to speciation. Comparative primate cytogenetics has become an important tool for elucidating the phylogeny and the taxonomy of primates. It has become increasingly apparent that molecular cytogenetic data in the future can be fruitfully combined with whole-genome assemblies to advance our understanding of primate genome evolution as well as the mechanisms and processes that have led to the origin of the human genome.