Adaptive evolution of Cid, a centromere-specific histone in Drosophila

Molecular evolution of the mammalian kinetochore complex

Mammalian centromeres are satellite-rich chromatin domains that serve as sites for kinetochore complex assembly. Centromeres are highly variable in sequence and satellite organization across species, but the processes that govern the co-evolutionary dynamics between rapidly evolving centromeres and their associated kinetochore proteins remain poorly understood. Here, we pursue a course of phylogenetic analyses to investigate the molecular evolution of the complete kinetochore complex across primate and rodent species with divergent centromere repeat sequences and features. We show that many protein components of the core centromere associated network (CCAN) harbor signals of adaptive evolution, consistent with their intimate association with centromere satellite DNA and roles in the stability and recruitment of additional kinetochore proteins. Surprisingly, CCAN and outer kinetochore proteins exhibit comparable rates of adaptive divergence, suggesting that changes in centromere DNA can ripple across the kinetochore to drive adaptive protein evolution within distant domains of the complex. Our work further identifies kinetochore proteins subject to lineage-specific adaptive evolution, including rapidly evolving proteins in species with centromere satellites characterized by higher-order repeat structure and lacking CENP-B boxes. Thus, features of centromeric chromatin beyond the linear DNA sequence may drive selection on kinetochore proteins. Overall, our work spotlights adaptively evolving proteins with diverse centromere-associated functions, including centromere chromatin structure, kinetochore protein assembly, kinetochore-microtubule association, cohesion maintenance, and DNA damage response pathways. These adaptively evolving kinetochore protein candidates present compelling opportunities for future functional investigations exploring how their concerted changes with centromere DNA ensure the maintenance of genome stability.

KNL1 and NDC80 represent new universal markers for the detection of functional centromeres in plants

Centromere is the chromosomal site of kinetochore assembly and microtubule attachment for chromosome segregation. Given its importance, markers that allow specific labeling of centromeric chromatin throughout the cell cycle and across all chromosome types are sought for facilitating various centromere studies. Antibodies against the N-terminal region of CENH3 are commonly used for this purpose, since CENH3 is the near-universal marker of functional centromeres. However, because the N-terminal region of CENH3 is highly variable among plant species, antibodies directed against this region usually function only in a small group of closely related species. As a more versatile alternative, we present here antibodies targeted to the conserved domains of two outer kinetochore proteins, KNL1 and NDC80. Sequence comparison of these domains across more than 350 plant species revealed a high degree of conservation, particularly within a six amino acid motif, FFGPVS in KNL1, suggesting that both antibodies would function in a wide range of plant species. This assumption was confirmed by immunolabeling experiments in angiosperm (monocot and dicot) and gymnosperm species, including those with mono-, holo-, and meta-polycentric chromosomes. In addition to centromere labeling on condensed chromosomes during cell division, both antibodies detected the corresponding regions in the interphase nuclei of most species tested. These results demonstrated that KNL1 and NDC80 are better suited for immunolabeling centromeres than CENH3, because antibodies against these proteins offer incomparably greater versatility across different plant species which is particularly convenient for studying the organization and function of the centromere in non-model species.

Plant Artificial Chromosomes: Construction and Transformation

With the decline of cultivated land and increase of the population in recent years, an agricultural revolution is urgently needed to produce more food to improve the living standards of humans. As one of the foundations of synthetic biology, artificial chromosomes hold great potential for advancing crop improvement. They offer opportunities to increase crop yield and quality, while enhancing crop resistance to disease. The progress made in plant artificial chromosome technology enables selective modification of existing chromosomes or the synthesis of new ones to improve crops and study gene function. However, current artificial chromosome technologies still face limitations, particularly in the synthesis of repeat sequences and the transformation of large DNA fragments. In this review, we will introduce the structure of plant centromeres, the construction of plant artificial chromosomes, and possible methods for transforming large fragments into plant cells.

Centromere structure and function: lessons from Drosophila

The fruit fly Drosophila melanogaster serves as a powerful model organism for advancing our understanding of biological processes, not just by studying its similarities with other organisms including ourselves but also by investigating its differences to unravel the underlying strategies that evolved to achieve a common goal. This is particularly true for centromeres, specialized genomic regions present on all eukaryotic chromosomes that function as the platform for the assembly of kinetochores. These multiprotein structures play an essential role during cell division by connecting chromosomes to spindle microtubules in mitosis and meiosis to mediate accurate chromosome segregation. Here, we will take a historical perspective on the study of fly centromeres, aiming to highlight not only the important similarities but also the differences identified that contributed to advancing centromere biology. We will discuss the current knowledge on the sequence and chromatin organization of fly centromeres together with advances for identification of centromeric proteins. Then, we will describe both the factors and processes involved in centromere organization and how they work together to provide an epigenetic identity to the centromeric locus. Lastly, we will take an evolutionary point of view of centromeres and briefly discuss current views on centromere drive.

Variation in the CENP-A sequence association landscape across diverse inbred mouse strains

Centromeres are crucial for chromosome segregation, but their underlying sequences evolve rapidly, imposing strong selection for compensatory changes in centromere-associated kinetochore proteins to assure the stability of genome transmission. While this co-evolution is well documented between species, it remains unknown whether population-level centromere diversity leads to functional differences in kinetochore protein association. Mice (Mus musculus) exhibit remarkable variation in centromere size and sequence, but the amino acid sequence of the kinetochore protein CENP-A is conserved. Here, we apply k-mer-based analyses to CENP-A chromatin profiling data from diverse inbred mouse strains to investigate the interplay between centromere variation and kinetochore protein sequence association. We show that centromere sequence diversity is associated with strain-level differences in both CENP-A positioning and sequence preference along the mouse core centromere satellite. Our findings reveal intraspecies sequence-dependent differences in CENP-A/centromere association and open additional perspectives for understanding centromere-mediated variation in genome stability.

Mutation pressure, drift, and the pace of molecular coevolution

Most aspects of the molecular biology of cells involve tightly coordinated intermolecular interactions requiring specific recognition at the nucleotide and/or amino acid levels. This has led to long-standing interest in the degree to which constraints on interacting molecules result in conserved vs. accelerated rates of sequence evolution, with arguments commonly being made that molecular coevolution can proceed at rates exceeding the neutral expectation. Here, a fairly general model is introduced to evaluate the degree to which the rate of evolution at functionally interacting sites is influenced by effective population sizes (Ne), mutation rates, strength of selection, and the magnitude of recombination between sites. This theory is of particular relevance to matters associated with interactions between organelle- and nuclear-encoded proteins, as the two genomic environments often exhibit dramatic differences in the power of mutation and drift. Although genes within low Ne environments can drive the rate of evolution of partner genes experiencing higher Ne, rates exceeding the neutral expectation require that the former also have an elevated mutation rate. Testable predictions, some counterintuitive, are presented on how patterns of coevolutionary rates should depend on the relative intensities of drift, selection, and mutation.

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.

X-chromosome target specificity diverged between dosage compensation mechanisms of two closely related Caenorhabditis species

An evolutionary perspective enhances our understanding of biological mechanisms. Comparison of sex determination and X-chromosome dosage compensation mechanisms between the closely related nematode species Caenorhabditis briggsae (Cbr) and Caenorhabditis elegans (Cel) revealed that the genetic regulatory hierarchy controlling both processes is conserved, but the X-chromosome target specificity and mode of binding for the specialized condensin dosage compensation complex (DCC) controlling X expression have diverged. We identified two motifs within Cbr DCC recruitment sites that are highly enriched on X: 13 bp MEX and 30 bp MEX II. Mutating either MEX or MEX II in an endogenous recruitment site with multiple copies of one or both motifs reduced binding, but only removing all motifs eliminated binding in vivo. Hence, DCC binding to Cbr recruitment sites appears additive. In contrast, DCC binding to Cel recruitment sites is synergistic: mutating even one motif in vivo eliminated binding. Although all X-chromosome motifs share the sequence CAGGG, they have otherwise diverged so that a motif from one species cannot function in the other. Functional divergence was demonstrated in vivo and in vitro. A single nucleotide position in Cbr MEX can determine whether Cel DCC binds. This rapid divergence of DCC target specificity could have been an important factor in establishing reproductive isolation between nematode species and contrasts dramatically with the conservation of target specificity for X-chromosome dosage compensation across Drosophila species and for transcription factors controlling developmental processes such as body-plan specification from fruit flies to mice.

The Dynamic Structure and Rapid Evolution of Human Centromeric Satellite DNA

The complete sequence of a human genome provided our first comprehensive view of the organization of satellite DNA associated with heterochromatin. We review how our understanding of the genetic architecture and epigenetic properties of human centromeric DNA have advanced as a result. Preliminary studies of human and nonhuman ape centromeres reveal complex, saltatory mutational changes organized around distinct evolutionary layers. Pockets of regional hypomethylation within higher-order α-satellite DNA, termed centromere dip regions, appear to define the site of kinetochore attachment in all human chromosomes, although such epigenetic features can vary even within the same chromosome. Sequence resolution of satellite DNA is providing new insights into centromeric function with potential implications for improving our understanding of human biology and health.

Centromere drive: chromatin conflict in meiosis

Centromeres are essential loci in eukaryotes that are necessary for the faithful segregation of chromosomes in mitosis and meiosis. Centromeres organize the kinetochore, the protein machine that attaches sister chromatids or homologous chromosomes to spindle microtubules and regulates their disjunction. Centromeres have both genetic and epigenetic determinants, which can come into conflict in asymmetric female meiosis in seed plants and animals. The centromere drive model was proposed to describe this conflict and explain how it leads to the rapid evolution of both centromeres and kinetochores. Recent studies confirm key aspects of the centromere drive model, clarify its mechanisms, and implicate rapid centromere/kinetochore evolution in hybrid inviability between species.

Recurrent but Short-Lived Duplications of Centromeric Proteins in Holocentric Caenorhabditis Species

Centromeric histones (CenH3s) are essential for chromosome inheritance during cell division in most eukaryotes. CenH3 genes have rapidly evolved and undergone repeated gene duplications and diversification in many plant and animal species. In Caenorhabditis species, two independent duplications of CenH3 (named hcp-3 for HoloCentric chromosome-binding Protein 3) were previously identified in C. elegans and C. remanei. Using phylogenomic analyses in 32 Caenorhabditis species, we find strict retention of the ancestral hcp-3 gene and 10 independent duplications. Most hcp-3L (hcp-3-like) paralogs are only found in 1-2 species, are expressed in both males and females/hermaphrodites, and encode histone fold domains with 69-100% identity to ancestral hcp-3. We identified novel N-terminal protein motifs, including putative kinetochore protein-interacting motifs and a potential separase cleavage site, which are well conserved across Caenorhabditis HCP-3 proteins. Other N-terminal motifs vary in their retention across paralogs or species, revealing potential subfunctionalization or functional loss following duplication. An N-terminal extension in the hcp-3L gene of C. afra revealed an unprecedented protein fusion, where hcp-3L fused to duplicated segments from hcp-4 (nematode CENP-C). By extending our analyses beyond CenH3, we found gene duplications of six inner and outer kinetochore genes in Caenorhabditis, which appear to have been retained independent of hcp-3 duplications. Our findings suggest that centromeric protein duplications occur frequently in Caenorhabditis nematodes, are selectively retained for short evolutionary periods, then degenerate or are lost entirely. We hypothesize that unique challenges associated with holocentricity in Caenorhabditis may lead to this rapid "revolving door" of kinetochore protein paralogs.

Molecular conflicts disrupting centromere maintenance contribute to Xenopus hybrid inviability

Although central to evolution, the causes of hybrid inviability that drive reproductive isolation are poorly understood. Embryonic lethality occurs when the eggs of the frog X. tropicalis are fertilized with either X. laevis or X. borealis sperm. We observed that distinct subsets of paternal chromosomes failed to assemble functional centromeres, causing their mis-segregation during embryonic cell divisions. Core centromere DNA sequence analysis revealed little conservation among the three species, indicating that epigenetic mechanisms that normally operate to maintain centromere integrity are disrupted on specific paternal chromosomes in hybrids. In vitro reactions combining X. tropicalis egg extract with either X. laevis or X. borealis sperm chromosomes revealed that paternally matched or overexpressed centromeric histone CENP-A and its chaperone HJURP could rescue centromere assembly on affected chromosomes in interphase nuclei. However, although the X. laevis chromosomes maintained centromeric CENP-A in metaphase, X. borealis chromosomes did not and also displayed ultra-thin regions containing ribosomal DNA. Both centromere assembly and morphology of X. borealis mitotic chromosomes could be rescued by inhibiting RNA polymerase I or preventing the collapse of stalled DNA replication forks. These results indicate that specific paternal centromeres are inactivated in hybrids due to the disruption of associated chromatin regions that interfere with CENP-A incorporation, at least in some cases due to conflicts between replication and transcription machineries. Thus, our findings highlight the dynamic nature of centromere maintenance and its susceptibility to disruption in vertebrate interspecies hybrids.

Meiotic drive in house mice: mechanisms, consequences, and insights for human biology

Meiotic drive occurs when one allele at a heterozygous site cheats its way into a disproportionate share of functional gametes, violating Mendel's law of equal segregation. This genetic conflict typically imposes a fitness cost to individuals, often by disrupting the process of gametogenesis. The evolutionary impact of meiotic drive is substantial, and the phenomenon has been associated with infertility and reproductive isolation in a wide range of organisms. However, cases of meiotic drive in humans remain elusive, a finding that likely reflects the inherent challenges of detecting drive in our species rather than unique features of human genome biology. Here, we make the case that house mice (Mus musculus) present a powerful model system to investigate the mechanisms and consequences of meiotic drive and facilitate translational inferences about the scope and potential mechanisms of drive in humans. We first detail how different house mouse resources have been harnessed to identify cases of meiotic drive and the underlying mechanisms utilized to override Mendel's rules of inheritance. We then summarize the current state of knowledge of meiotic drive in the mouse genome. We profile known mechanisms leading to transmission bias at several established drive elements. We discuss how a detailed understanding of meiotic drive in mice can steer the search for drive elements in our own species. Lastly, we conclude with a prospective look into how new technologies and molecular tools can help resolve lingering mysteries about the prevalence and mechanisms of selfish DNA transmission in mammals.

Evolution of eukaryotic centromeres by drive and suppression of selfish genetic elements

Despite the universal requirement for faithful chromosome segregation, eukaryotic centromeres are rapidly evolving. It is hypothesized that rapid centromere evolution represents an evolutionary arms race between selfish genetic elements that drive, or propagate at the expense of organismal fitness, and mechanisms that suppress fitness costs. Selfish centromere DNA achieves preferential inheritance in female meiosis by recruiting more effector proteins that alter spindle microtubule interaction dynamics. Parallel pathways for effector recruitment are adaptively evolved to suppress functional differences between centromeres. Opportunities to drive are not limited to female meiosis, and selfish transposons, plasmids and B chromosomes also benefit by maximizing their inheritance. Rapid evolution of selfish genetic elements can diversify suppressor mechanisms in different species that may cause hybrid incompatibility.

Cross-species incompatibility between a DNA satellite and the Drosophila Spartan homolog poisons germline genome integrity

Satellite DNA spans megabases of eukaryotic sequence and evolves rapidly.1-6 Paradoxically, satellite-rich genomic regions mediate strictly conserved, essential processes such as chromosome segregation and nuclear structure.7-10 A leading resolution to this paradox posits that satellite DNA and satellite-associated chromosomal proteins coevolve to preserve these essential functions.11 We experimentally test this model of intragenomic coevolution by conducting the first evolution-guided manipulation of both chromosomal protein and DNA satellite. The 359bp satellite spans an 11 Mb array in Drosophila melanogaster that is absent from its sister species, Drosophila simulans.12-14 This species-specific DNA satellite colocalizes with the adaptively evolving, ovary-enriched protein, maternal haploid (MH), the Drosophila homolog of Spartan.15 To determine if MH and 359bp coevolve, we swapped the D. simulans version of MH ("MH[sim]") into D. melanogaster. MH[sim] triggers ovarian cell death, reduced ovary size, and loss of mature eggs. Surprisingly, the D. melanogaster mh-null mutant has no such ovary phenotypes,15 suggesting that MH[sim] is toxic in a D. melanogaster background. Using both cell biology and genetics, we discovered that MH[sim] poisons oogenesis through a DNA-damage pathway. Remarkably, deleting the D. melanogaster-specific 359bp satellite array completely restores mh[sim] germline genome integrity and fertility, consistent with a history of coevolution between these two fast-evolving loci. Germline genome integrity and fertility are also restored by overexpressing topoisomerase II (Top2), suggesting that MH[sim] interferes with Top2-mediated processing of 359bp. The observed 359bp-MH[sim] cross-species incompatibility supports a model under which seemingly inert repetitive DNA and essential chromosomal proteins must coevolve to preserve germline genome integrity.

Centromere drive: model systems and experimental progress

Centromeres connect chromosomes and spindle microtubules to ensure faithful chromosome segregation. Paradoxically, despite this conserved function, centromeric DNA evolves rapidly and centromeric proteins show signatures of positive selection. The centromere drive hypothesis proposes that centromeric DNA can act like a selfish genetic element and drive non-Mendelian segregation during asymmetric female meiosis. Resulting fitness costs lead to genetic conflict with the rest of the genome and impose a selective pressure for centromeric proteins to adapt by suppressing the costs. Here, we describe experimental model systems for centromere drive in yellow monkeyflowers and mice, summarize key findings demonstrating centromere drive, and explain molecular mechanisms. We further discuss efforts to test if centromeric proteins are involved in suppressing drive-associated fitness costs, highlight a model for centromere drive and suppression in mice, and put forth outstanding questions for future research.

Population Scale Analysis of Centromeric Satellite DNA Reveals Highly Dynamic Evolutionary Patterns and Genomic Organization in Long-Tailed and Rhesus Macaques

Centromeric satellite DNA (cen-satDNA) consists of highly divergent repeat monomers, each approximately 171 base pairs in length. Here, we investigated the genetic diversity in the centromeric region of two primate species: long-tailed (Macaca fascicularis) and rhesus (Macaca mulatta) macaques. Fluorescence in situ hybridization and bioinformatic analysis showed the chromosome-specific organization and dynamic nature of cen-satDNAsequences, and their substantial diversity, with distinct subfamilies across macaque populations, suggesting increased turnovers. Comparative genomics identified high level polymorphisms spanning a 120 bp deletion region and a remarkable interspecific variability in cen-satDNA size and structure. Population structure analysis detected admixture patterns within populations, indicating their high divergence and rapid evolution. However, differences in cen-satDNA profiles appear to not be involved in hybrid incompatibility between the two species. Our study provides a genomic landscape of centromeric repeats in wild macaques and opens new avenues for exploring their impact on the adaptive evolution and speciation of primates.

Satellitome analysis illuminates the evolution of ZW sex chromosomes of Triportheidae fishes (Teleostei: Characiformes)

Satellites are an abundant source of repetitive DNAs that play an essential role in the chromosomal organization and are tightly linked with the evolution of sex chromosomes. Among fishes, Triportheidae stands out as the only family where almost all species have a homeologous ZZ/ZW sex chromosomes system. While the Z chromosome is typically conserved, the W is always smaller, with variations in size and morphology between species. Here, we report an analysis of the satellitome of Triportheus auritus (TauSat) by integrating genomic and chromosomal data, with a special focus on the highly abundant and female-biased satDNAs. In addition, we investigated the evolutionary trajectories of the ZW sex chromosomes in the Triportheidae family by mapping satDNAs in selected representative species of this family. The satellitome of T. auritus comprised 53 satDNA families of which 24 were also hybridized by FISH. Most satDNAs differed significantly between sexes, with 19 out of 24 being enriched on the W chromosome of T. auritus. The number of satDNAs hybridized into the W chromosomes of T. signatus and T. albus decreased to six and four, respectively, in accordance with the size of their W chromosomes. No TauSat probes produced FISH signals on the chromosomes of Agoniates halecinus. Despite its apparent conservation, our results indicate that each species differs in the satDNA accumulation on the Z chromosome. Minimum spanning trees (MSTs), generated for three satDNA families with different patterns of FISH mapping data, revealed different homogenization rates between the Z and W chromosomes. These results were linked to different levels of recombination between them. The most abundant satDNA family (TauSat01) was exclusively hybridized in the centromeres of all 52 chromosomes of T. auritus, and its putative role in the centromere evolution was also highlighted. Our results identified a high differentiation of both ZW chromosomes regarding satellites composition, highlighting their dynamic role in the sex chromosomes evolution.

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.

Variation and Evolution of Human Centromeres: A Field Guide and Perspective

We are entering a new era in genomics where entire centromeric regions are accurately represented in human reference assemblies. Access to these high-resolution maps will enable new surveys of sequence and epigenetic variation in the population and offer new insight into satellite array genomics and centromere function. Here, we focus on the sequence organization and evolution of alpha satellites, which are credited as the genetic and genomic definition of human centromeres due to their interaction with inner kinetochore proteins and their importance in the development of human artificial chromosome assays. We provide an overview of alpha satellite repeat structure and array organization in the context of these high-quality reference data sets; discuss the emergence of variation-based surveys; and provide perspective on the role of this new source of genetic and epigenetic variation in the context of chromosome biology, genome instability, and human disease.

Functional Diversification of Chromatin on Rapid Evolutionary Timescales

Repeat-enriched genomic regions evolve rapidly and yet support strictly conserved functions like faithful chromosome transmission and the preservation of genome integrity. The leading resolution to this paradox is that DNA repeat-packaging proteins evolve adaptively to mitigate deleterious changes in DNA repeat copy number, sequence, and organization. Exciting new research has tested this model of coevolution by engineering evolutionary mismatches between adaptively evolving chromatin proteins of one species and the DNA repeats of a close relative. Here, we review these innovative evolution-guided functional analyses. The studies demonstrate that vital, chromatin-mediated cellular processes, including transposon suppression, faithful chromosome transmission, and chromosome retention depend on species-specific versions of chromatin proteins that package species-specific DNA repeats. In many cases, the ever-evolving repeats are selfish genetic elements, raising the possibility that chromatin is a battleground of intragenomic conflict.

Parallel pathways for recruiting effector proteins determine centromere drive and suppression

Selfish centromere DNA sequences bias their transmission to the egg in female meiosis. Evolutionary theory suggests that centromere proteins evolve to suppress costs of this "centromere drive." In hybrid mouse models with genetically different maternal and paternal centromeres, selfish centromere DNA exploits a kinetochore pathway to recruit microtubule-destabilizing proteins that act as drive effectors. We show that such functional differences are suppressed by a parallel pathway for effector recruitment by heterochromatin, which is similar between centromeres in this system. Disrupting the kinetochore pathway with a divergent allele of CENP-C reduces functional differences between centromeres, whereas disrupting heterochromatin by CENP-B deletion amplifies the differences. Molecular evolution analyses using Murinae genomes identify adaptive evolution in proteins in both pathways. We propose that centromere proteins have recurrently evolved to minimize the kinetochore pathway, which is exploited by selfish DNA, relative to the heterochromatin pathway that equalizes centromeres, while maintaining essential functions.

Selfish centromeres, selfless heterochromatin

Centromeres are specialized regions on chromosomes recruiting a set of proteins required for faithful chromosome segregation. Differences in centromere strength can potentially bias chromosome segregation toward one of the daughter cells during division. Kumon et al. propose a new model of evolutionary impact on the balance of centromere strength.

Annual and perennial Medicago show signatures of parallel adaptation to climate and soil in highly conserved genes

Human induced environmental change may require rapid adaptation of plant populations and crops, but the genomic basis of environmental adaptation remain poorly understood. We analysed polymorphic loci from the perennial crop Medicago sativa (alfalfa or lucerne) and the annual legume model species M. truncatula to search for a common set of candidate genes that might contribute to adaptation to abiotic stress in both annual and perennial Medicago species. We identified a set of candidate genes of adaptation associated with environmental gradients along the distribution of the two Medicago species. Candidate genes for each species were detected in homologous genomic linkage blocks using genome-environment (GEA) and genome-phenotype association analyses. Hundreds of GEA candidate genes were species-specific, of these, 13.4% (M. sativa) and 24% (M. truncatula) were also significantly associated with phenotypic traits. A set of 168 GEA candidates were shared by both species, which was 25.4% more than expected by chance. When combined, they explained a high proportion of variance for certain phenotypic traits associated with adaptation. Genes with highly conserved functions dominated among the shared candidates and were enriched in gene ontology terms that have shown to play a central role in drought avoidance and tolerance mechanisms by means of cellular shape modifications and other functions associated with cell homeostasis. Our results point to the existence of a molecular basis of adaptation to abiotic stress in Medicago determined by highly conserved genes and gene functions. We discuss these results in light of the recently proposed omnigenic model of complex traits.

Gametic specialization of centromeric histone paralogs in Drosophila virilis

In most eukaryotes, centromeric histone (CenH3) proteins mediate mitosis and meiosis and ensure epigenetic inheritance of centromere identity. We hypothesized that disparate chromatin environments in soma versus germline might impose divergent functional requirements on single CenH3 genes, which could be ameliorated by gene duplications and subsequent specialization. Here, we analyzed the cytological localization of two recently identified CenH3 paralogs, Cid1 and Cid5, in Drosophila virilis using specific antibodies and epitope-tagged transgenic strains. We find that only ancestral Cid1 is present in somatic cells, whereas both Cid1 and Cid5 are expressed in testes and ovaries. However, Cid1 is lost in male meiosis but retained throughout oogenesis, whereas Cid5 is lost during female meiosis but retained in mature sperm. Following fertilization, only Cid1 is detectable in the early embryo, suggesting that maternally deposited Cid1 is rapidly loaded onto paternal centromeres during the protamine-to-histone transition. Our studies reveal mutually exclusive gametic specialization of divergent CenH3 paralogs. Duplication and divergence might allow essential centromeric genes to resolve an intralocus conflict between maternal and paternal centromeric requirements in many animal species.

Molecular Complexes at Euchromatin, Heterochromatin and Centromeric Chromatin

Chromatin consists of a complex of DNA and histone proteins as its core components and plays an important role in both packaging DNA and regulating DNA metabolic pathways such as DNA replication, transcription, recombination, and chromosome segregation. Proper functioning of chromatin further involves a network of interactions among molecular complexes that modify chromatin structure and organization to affect the accessibility of DNA to transcription factors leading to the activation or repression of the transcription of target DNA loci. Based on its structure and compaction state, chromatin is categorized into euchromatin, heterochromatin, and centromeric chromatin. In this review, we discuss distinct chromatin factors and molecular complexes that constitute euchromatin-open chromatin structure associated with active transcription; heterochromatin-less accessible chromatin associated with silencing; centromeric chromatin-the site of spindle binding in chromosome segregation.

The Adenine/Thymine Deleterious Selection Model for GC Content Evolution at the Third Codon Position of the Histone Genes in Drosophila

The evolution of the GC (guanine cytosine) content at the third codon position of the histone genes (H1, H2A, H2B, H3, H4, H2AvD, H3.3A, H3.3B, and H4r) in 12 or more Drosophila species is reviewed. For explaining the evolution of the GC content at the third codon position of the genes, a model assuming selection with a deleterious effect for adenine/thymine and a size effect is presented. The applicability of the model to whole-genome genes is also discussed.

The Centromere Histone Is Conserved and Associated with Tandem Repeats Sharing a Conserved 19-bp Box in the Holocentromere of Meloidogyne Nematodes

Although centromeres have conserved function, centromere-specific histone H3 (CenH3) and centromeric DNA evolve rapidly. The centromere drive model explains this phenomenon as a consequence of the conflict between fast-evolving DNA and CenH3, suggesting asymmetry in female meiosis as a crucial factor. We characterized evolution of the CenH3 protein in three closely related, polyploid mitotic parthenogenetic species of the Meloidogyne incognita group, and in the distantly related meiotic parthenogen Meloidogyne hapla. We identified duplication of the CenH3 gene in a putative sexual ancestral Meloidogyne. We found that one CenH3 (αCenH3) remained conserved in all extant species, including in distant Meloidogyne hapla, whereas the other evolved rapidly and under positive selection into four different CenH3 variants. This pattern of CenH3 evolution in Meloidogyne species suggests the subspecialization of CenH3s in ancestral sexual species. Immunofluorescence performed on mitotic Meloidogyne incognita revealed a dominant role of αCenH3 on its centromere, whereas the other CenH3s have lost their function in mitosis. The observed αCenH3 chromosome distribution disclosed cluster-like centromeric organization. The ChIP-Seq analysis revealed that in M. incognita αCenH3-associated DNA dominantly comprises tandem repeats, composed of divergent monomers which share a completely conserved 19-bp long box. Conserved αCenH3-associated DNA is also confirmed in the related mitotic Meloidogyne incognita group species suggesting preservation of both centromere protein and DNA constituents. We hypothesize that the absence of centromere drive in mitosis might allow for CenH3 and its associated DNA to achieve an equilibrium in which they can persist for long periods of time.

Selfish chromosomal drive shapes recent centromeric histone evolution in monkeyflowers

Centromeres are essential mediators of chromosomal segregation, but both centromeric DNA sequences and associated kinetochore proteins are paradoxically diverse across species. The selfish centromere model explains rapid evolution by both components via an arms-race scenario: centromeric DNA variants drive by distorting chromosomal transmission in female meiosis and attendant fitness costs select on interacting proteins to restore Mendelian inheritance. Although it is clear than centromeres can drive and that drive often carries costs, female meiotic drive has not been directly linked to selection on kinetochore proteins in any natural system. Here, we test the selfish model of centromere evolution in a yellow monkeyflower (Mimulus guttatus) population polymorphic for a costly driving centromere (D). We show that the D haplotype is structurally and genetically distinct and swept to a high stable frequency within the past 1500 years. We use quantitative genetic mapping to demonstrate that context-dependence in the strength of drive (from near-100% D transmission in interspecific hybrids to near-Mendelian in within-population crosses) primarily reflects variable vulnerability of the non-driving competitor chromosomes, but also map an unlinked modifier of drive coincident with kinetochore protein Centromere-specific Histone 3 A (CenH3A). Finally, CenH3A exhibits a recent (<1000 years) selective sweep in our focal population, implicating local interactions with D in ongoing adaptive evolution of this kinetochore protein. Together, our results demonstrate an active co-evolutionary arms race between DNA and protein components of the meiotic machinery in Mimulus, with important consequences for individual fitness and molecular divergence.

Centromeres must mediate faithful chromosomal transmission during cell division and sexual reproduction, but both the DNA and protein components of centromeres diverge rapidly across species. The selfish centromere model argues that this paradoxical diversity results from a genetic conflict between centromeric DNA variants driving through female meiosis to gain over-transmission and kinetochore proteins co-evolving to re-establish Mendelian segregation. We use whole genome sequencing and genetic crossing experiments to demonstrate active evolutionary interactions between a selfish centromere and a key kinetochore protein (CenH3A) in the wildflower Mimulus guttatus. We show that both inter-specific and intra-population differences in CenH3A affect centromeric drive in hybrids, and that adaptive evolution of CenH3A has followed the recent and costly spread of the driver in a wild population. This work provides novel empirical support for the proposed antagonistic co-evolution of the DNA and protein components of centromeres, with important consequences for understanding cellular function, individual fitness, and species divergence.

Sequence, Chromatin and Evolution of Satellite DNA

Satellite DNA consists of abundant tandem repeats that play important roles in cellular processes, including chromosome segregation, genome organization and chromosome end protection. Most satellite DNA repeat units are either of nucleosomal length or 5–10 bp long and occupy centromeric, pericentromeric or telomeric regions. Due to high repetitiveness, satellite DNA sequences have largely been absent from genome assemblies. Although few conserved satellite-specific sequence motifs have been identified, DNA curvature, dyad symmetries and inverted repeats are features of various satellite DNAs in several organisms. Satellite DNA sequences are either embedded in highly compact gene-poor heterochromatin or specialized chromatin that is distinct from euchromatin. Nevertheless, some satellite DNAs are transcribed into non-coding RNAs that may play important roles in satellite DNA function. Intriguingly, satellite DNAs are among the most rapidly evolving genomic elements, such that a large fraction is species-specific in most organisms. Here we describe the different classes of satellite DNA sequences, their satellite-specific chromatin features, and how these features may contribute to satellite DNA biology and evolution. We also discuss how the evolution of functional satellite DNA classes may contribute to speciation in plants and animals.

Population and subspecies diversity at mouse centromere satellites

BACKGROUND

Mammalian centromeres are satellite-rich chromatin domains that execute conserved roles in kinetochore assembly and chromosome segregation. Centromere satellites evolve rapidly between species, but little is known about population-level diversity across these loci.

RESULTS

We developed a k-mer based method to quantify centromere copy number and sequence variation from whole genome sequencing data. We applied this method to diverse inbred and wild house mouse (Mus musculus) genomes to profile diversity across the core centromere (minor) satellite and the pericentromeric (major) satellite repeat. We show that minor satellite copy number varies more than 10-fold among inbred mouse strains, whereas major satellite copy numbers span a 3-fold range. In contrast to widely held assumptions about the homogeneity of mouse centromere repeats, we uncover marked satellite sequence heterogeneity within single genomes, with diversity levels across the minor satellite exceeding those at the major satellite. Analyses in wild-caught mice implicate subspecies and population origin as significant determinants of variation in satellite copy number and satellite heterogeneity. Intriguingly, we also find that wild-caught mice harbor dramatically reduced minor satellite copy number and elevated satellite sequence heterogeneity compared to inbred strains, suggesting that inbreeding may reshape centromere architecture in pronounced ways.

CONCLUSION

Taken together, our results highlight the power of k-mer based approaches for probing variation across repetitive regions, provide an initial portrait of centromere variation across Mus musculus, and lay the groundwork for future functional studies on the consequences of natural genetic variation at these essential chromatin domains.

Adaptive evolution of an essential telomere protein restricts telomeric retrotransposons

Essential, conserved cellular processes depend not only on essential, strictly conserved proteins but also on essential proteins that evolve rapidly. To probe this poorly understood paradox, we exploited the rapidly evolving Drosophila telomere-binding protein, cav/HOAP, which protects chromosomes from lethal end-to-end fusions. We replaced the D. melanogaster HOAP with a highly diverged version from its close relative, D. yakuba. The D. yakuba HOAP ('HOAP[yak]') localizes to D. melanogaster telomeres and protects D. melanogaster chromosomes from fusions. However, HOAP[yak] fails to rescue a previously uncharacterized HOAP function: silencing of the specialized telomeric retrotransposons that, instead of telomerase, maintain chromosome length in Drosophila. Whole genome sequencing and cytogenetics of experimentally evolved populations revealed that HOAP[yak] triggers telomeric retrotransposon proliferation, resulting in aberrantly long telomeres. This evolution-generated, separation-of-function allele resolves the paradoxical observation that a fast-evolving essential gene directs an essential, strictly conserved function: telomeric retrotransposon containment, not end-protection, requires evolutionary innovation at HOAP.

Biochemical evidence for diverse strategies in the inner kinetochore

The kinetochore is a complex structure whose function is absolutely essential. Unlike the centromere, the kinetochore at first appeared remarkably well conserved from yeast to humans, especially the microtubule-binding outer kinetochore. However, recent efforts towards biochemical reconstitution of diverse kinetochores challenge the notion of a similarly conserved architecture for the constitutively centromere-associated network of the inner kinetochore. This review briefly summarizes the evidence from comparative genomics for interspecific variability in inner kinetochore composition and focuses on novel biochemical evidence indicating that even homologous inner kinetochore protein complexes are put to different uses in different organisms.

Innovation of heterochromatin functions drives rapid evolution of essential ZAD-ZNF genes in Drosophila

Contrary to dogma, evolutionarily young and dynamic genes can encode essential functions. We find that evolutionarily dynamic ZAD-ZNF genes, which encode the most abundant class of insect transcription factors, are more likely to encode essential functions in Drosophila melanogaster than ancient, conserved ZAD-ZNF genes. We focus on the Nicknack ZAD-ZNF gene, which is evolutionarily young, poorly retained in Drosophila species, and evolves under strong positive selection. Yet we find that it is necessary for larval development in D. melanogaster. We show that Nicknack encodes a heterochromatin-localizing protein like its paralog Oddjob, also an evolutionarily dynamic yet essential ZAD-ZNF gene. We find that the divergent D. simulans Nicknack protein can still localize to D. melanogaster heterochromatin and rescue viability of female but not male Nicknack-null D. melanogaster. Our findings suggest that innovation for rapidly changing heterochromatin functions might generally explain the essentiality of many evolutionarily dynamic ZAD-ZNF genes in insects.

Many, but not all, lineage-specific genes can be explained by homology detection failure

Genes for which homologs can be detected only in a limited group of evolutionarily related species, called “lineage-specific genes,” are pervasive: Essentially every lineage has them, and they often comprise a sizable fraction of the group’s total genes. Lineage-specific genes are often interpreted as “novel” genes, representing genetic novelty born anew within that lineage. Here, we develop a simple method to test an alternative null hypothesis: that lineage-specific genes do have homologs outside of the lineage that, even while evolving at a constant rate in a novelty-free manner, have merely become undetectable by search algorithms used to infer homology. We show that this null hypothesis is sufficient to explain the lack of detected homologs of a large number of lineage-specific genes in fungi and insects. However, we also find that a minority of lineage-specific genes in both clades are not well explained by this novelty-free model. The method provides a simple way of identifying which lineage-specific genes call for special explanations beyond homology detection failure, highlighting them as interesting candidates for further study.

Lineage-specific gene families may arise from evolutionary innovations such as de novo gene origination, or may simply mean that a similarity search program failed to identify more distant homologs. A new computational method for modeling the expected decay of similarity search scores with evolutionary distance allows distinction between the two explanations.

CenH3 distribution reveals extended centromeres in the model beetle Tribolium castaneum

Centromeres are chromosomal domains essential for kinetochore assembly and correct chromosome segregation. Inconsistent in their underlying DNA sequences, centromeres are defined epigenetically by the presence of the centromere-specific histone H3 variant CenH3. Most of the analyzed eukaryotes have monocentric chromosomes in which CenH3 proteins deposit into a single, primary constriction visible at metaphase chromosomes. Contrary to monocentrics, evolutionary sporadic holocentric chromosomes lack a primary constriction and have kinetochore activity distributed along the entire chromosome length. In this work, we identified cCENH3 protein, the centromeric H3 histone of the coleopteran model beetle Tribolium castaneum. By ChIP-seq analysis we disclosed that cCENH3 chromatin assembles upon a repertoire of repetitive DNAs. cCENH3 in situ mapping revealed unusually elongated T. castaneum centromeres that comprise approximately 40% of the chromosome length. Being the longest insect regional centromeres evidenced so far, T. castaneum centromeres are characterized by metapolycentric structure composed of several individual cCENH3-containing domains. We suggest that the model beetle T. castaneum with its metapolycentromeres could represent an excellent model for further studies of non-canonical centromeres in insects.

Loss of inner kinetochore genes is associated with the transition to an unconventional point centromere in budding yeast

Background

The genomic sequences of centromeres, as well as the set of proteins that recognize and interact with centromeres, are known to quickly diverge between lineages potentially contributing to post-zygotic reproductive isolation. However, the actual sequence of events and processes involved in the divergence of the kinetochore machinery is not known. The patterns of gene loss that occur during evolution concomitant with phenotypic changes have been used to understand the timing and order of molecular changes.

Methods

I screened the high-quality genomes of twenty budding yeast species for the presence of well-studied kinetochore genes. Based on the conserved gene order and complete genome assemblies, I identified gene loss events. Subsequently, I searched the intergenic regions to identify any un-annotated genes or gene remnants to obtain additional evidence of gene loss.

Results

My analysis identified the loss of four genes (NKP1, NKP2, CENPL/IML3 and CENPN/CHL4) of the inner kinetochore constitutive centromere-associated network (CCAN/also known as CTF19 complex in yeast) in both the Naumovozyma species for which genome assemblies are available. Surprisingly, this collective loss of four genes of the CCAN/CTF19 complex coincides with the emergence of unconventional centromeres in N. castellii and N. dairenensis. My study suggests a tentative link between the emergence of unconventional point centromeres and the turnover of kinetochore genes in budding yeast.

Adaptations for centromere function in meiosis

The aim of mitosis is to segregate duplicated chromosomes equally into daughter cells during cell division. Meiosis serves a similar purpose, but additionally separates homologous chromosomes to produce haploid gametes for sexual reproduction. Both mitosis and meiosis rely on centromeres for the segregation of chromosomes. Centromeres are the specialized regions of the chromosomes that are attached to microtubules during their segregation. In this review, we describe the adaptations and layers of regulation that are required for centromere function during meiosis, and their role in meiosis-specific processes such as homolog-pairing and recombination. Since female meiotic divisions are asymmetric, meiotic centromeres are hypothesized to evolve quickly in order to favor their own transmission to the offspring, resulting in the rapid evolution of many centromeric proteins. We discuss this observation using the example of the histone variant CENP-A, which marks the centromere and is essential for centromere function. Changes in both the size and the sequence of the CENP-A N-terminal tail have led to additional functions of the protein, which are likely related to its roles during meiosis. We highlight the importance of CENP-A in the inheritance of centromere identity, which is dependent on the stabilization, recycling, or re-establishment of CENP-A-containing chromatin during meiosis.

Centromeric Transcription: A Conserved Swiss-Army Knife

In most species, the centromere is comprised of repetitive DNA sequences, which rapidly evolve. Paradoxically, centromeres fulfill an essential function during mitosis, as they are the chromosomal sites wherein, through the kinetochore, the mitotic spindles bind. It is now generally accepted that centromeres are transcribed, and that such transcription is associated with a broad range of functions. More than a decade of work on this topic has shown that centromeric transcripts are found across the eukaryotic tree and associate with heterochromatin formation, chromatin structure, kinetochore structure, centromeric protein loading, and inner centromere signaling. In this review, we discuss the conservation of small and long non-coding centromeric RNAs, their associations with various centromeric functions, and their potential roles in disease.

Ancient Coretention of Paralogs of Cid Centromeric Histones and Cal1 Chaperones in Mosquito Species

Despite their essential role in chromosome segregation in most eukaryotes, centromeric histones (CenH3s) evolve rapidly and are subject to gene turnover. We previously identified four instances of gene duplication and specialization of Cid, which encodes for the CenH3 in Drosophila. We hypothesized that retention of specialized Cid paralogs could be selectively advantageous to resolve the intralocus conflict that occurs on essential genes like Cid, which are subject to divergent selective pressures to perform multiple functions. We proposed that intralocus conflict could be a widespread phenomenon that drives evolutionary innovation in centromeric proteins. If this were the case, we might expect to find other instances of coretention and specialization of centromeric proteins during animal evolution. Consistent with this hypothesis, we find that most mosquito species encode two CenH3 (mosqCid) genes, mosqCid1 and mosqCid2, which have been coretained for over 150 My. In addition, Aedes species encode a third mosqCid3 gene, which arose from an independent gene duplication of mosqCid1. Like Drosophila Cid paralogs, mosqCid paralogs evolve under different selective constraints and show tissue-specific expression patterns. Analysis of mosqCid N-terminal protein motifs further supports the model that mosqCid paralogs have functionally diverged. Extending our survey to other centromeric proteins, we find that all Anopheles mosquitoes encode two CAL1 paralogs, which are the chaperones that deposit CenH3 proteins at centromeres in Diptera, but a single CENP-C paralog. The ancient coretention of paralogs of centromeric proteins adds further support to the hypothesis that intralocus conflict can drive their coretention and functional specialization.

Functional Significance of Satellite DNAs: Insights From Drosophila

Since their discovery more than 60 years ago, satellite repeats are still one of the most enigmatic parts of eukaryotic genomes. Being non-coding DNA, satellites were earlier considered to be non-functional “junk,” but recently this concept has been extensively revised. Satellite DNA contributes to the essential processes of formation of crucial chromosome structures, heterochromatin establishment, dosage compensation, reproductive isolation, genome stability and development. Genomic abundance of satellites is under stabilizing selection owing of their role in the maintenance of vital regions of the genome – centromeres, pericentromeric regions, and telomeres. Many satellites are transcribed with the generation of long or small non-coding RNAs. Misregulation of their expression is found to lead to various defects in the maintenance of genomic architecture, chromosome segregation and gametogenesis. This review summarizes our current knowledge concerning satellite functions, the mechanisms of regulation and evolution of satellites, focusing on recent findings in Drosophila. We discuss here experimental and bioinformatics data obtained in Drosophila in recent years, suggesting relevance of our analysis to a wide range of eukaryotic organisms.

The Telomere Paradox: Stable Genome Preservation with Rapidly Evolving Proteins

Telomeres ensure chromosome length homeostasis and protection from catastrophic end-to-end chromosome fusions. All eukaryotes require this essential, strictly conserved telomere-dependent genome preservation. However, recent evolutionary analyses of mammals, plants, and flies report pervasive rapid evolution of telomere proteins. The causes of this paradoxical observation - that unconserved machinery underlies an essential, conserved function - remain enigmatic. Indeed, these fast-evolving telomere proteins bind, extend, and protect telomeric DNA, which itself evolves slowly in most systems. We hypothesize that the universally fast-evolving subtelomere - the telomere-adjacent, repetitive sequence - is a primary driver of the 'telomere paradox'. Under this model, radical sequence changes in the subtelomere perturb subtelomere-dependent, telomere functions. Compromised telomere function then spurs adaptation of telomere proteins to maintain telomere length homeostasis and protection. We propose an experimental framework that leverages both protein divergence and subtelomeric sequence divergence to test the hypothesis that subtelomere sequence evolution shapes recurrent innovation of telomere machinery.

Recurrent Losses and Rapid Evolution of the Condensin II Complex in Insects

Condensins play a crucial role in the organization of genetic material by compacting and disentangling chromosomes. Based on studies in a few model organisms, the condensins I and II complexes are considered to have distinct functions, with the condensin II complex playing a role in meiosis and somatic pairing of homologous chromosomes in Drosophila. Intriguingly, the Cap-G2 subunit of condensin II is absent in Drosophila melanogaster, and this loss may be related to the high levels of chromosome pairing seen in flies. Here, we find that all three non-SMC subunits of condensin II (Cap-G2, Cap-D3, and Cap-H2) have been repeatedly and independently lost in taxa representing multiple insect orders, with some taxa lacking all three. We also find that all non-Dipteran insects display near-uniform low-pairing levels regardless of their condensin II complex composition, suggesting that some key aspects of genome organization are robust to condensin II subunit losses. Finally, we observe consistent signatures of positive selection in condensin subunits across flies and mammals. These findings suggest that these ancient complexes are far more evolutionarily labile than previously suspected, and are at the crossroads of several forms of genomic conflicts. Our results raise fundamental questions about the specific functions of the two condensin complexes in taxa that have experienced subunit losses, and open the door to further investigations to elucidate the diversity of molecular mechanisms that underlie genome organization across various life forms.

Standard Deviations: The Biological Bases of Transmission Ratio Distortion

The rule of Mendelian inheritance is remarkably robust, but deviations from the equal transmission of alternative alleles at a locus [a.k.a. transmission ratio distortion (TRD)] are also commonly observed in genetic mapping populations. Such TRD reveals locus-specific selection acting at some point between the diploid heterozygous parents and progeny genotyping and therefore can provide novel insight into otherwise-hidden genetic and evolutionary processes. Most of the classic selfish genetic elements were discovered through their biasing of transmission, but many unselfish evolutionary and developmental processes can also generate TRD. In this review, we describe methodologies for detecting TRD in mapping populations, detail the arenas and genetic interactions that shape TRD during plant and animal reproduction, and summarize patterns of TRD from across the genetic mapping literature. Finally, we point to new experimental approaches that can accelerate both detection of TRD and characterization of the underlying genetic mechanisms.

Intrinsic elasticity of nucleosomes is encoded by histone variants and calibrated by their binding partners

Histone variants fine-tune transcription, replication, DNA damage repair, and faithful chromosome segregation. Whether and how nucleosome variants encode unique mechanical properties to their cognate chromatin structures remains elusive. Here, using in silico and in vitro nanoindentation methods, extending to in vivo dissections, we report that histone variant nucleosomes are intrinsically more elastic than their canonical counterparts. Furthermore, binding proteins, which discriminate between histone variant nucleosomes, suppress this innate elasticity and also compact chromatin. Interestingly, when we overexpress the binding proteins in vivo, we also observe increased compaction of chromatin enriched for histone variant nucleosomes, correlating with diminished access. Taken together, these data suggest a plausible link between innate mechanical properties possessed by histone variant nucleosomes, the adaptability of chromatin states in vivo, and the epigenetic plasticity of the underlying locus.

Phosphorylation of Drosophila CENP-A on serine 20 regulates protein turn-over and centromere-specific loading

Centromeres are specialized chromosomal regions epigenetically defined by the presence of the histone H3 variant CENP-A. CENP-A is required for kinetochore formation which is essential for chromosome segregation during mitosis. Spatial restriction of CENP-A to the centromere is tightly controlled. Its overexpression results in ectopic incorporation and the formation of potentially deleterious neocentromeres in yeast, flies and in various human cancers. While the contribution of posttranslational modifications of CENP-A to these processes has been studied in yeast and mammals to some extent, very little is known about Drosophila melanogaster. Here, we show that CENP-A is phosphorylated at serine 20 (S20) by casein kinase II and that in mitotic cells, the phosphorylated form is enriched on chromatin. Importantly, our results reveal that S20 phosphorylation regulates the turn-over of prenucleosomal CENP-A by the SCFPpa-proteasome pathway and that phosphorylation promotes removal of CENP-A from ectopic but not from centromeric sites in chromatin. We provide multiple lines of evidence for a crucial role of S20 phosphorylation in controlling restricted incorporation of CENP-A into centromeric chromatin in flies. Modulation of the phosphorylation state of S20 may provide the cells with a means to fine-tune CENP-A levels in order to prevent deleterious loading to extra-centromeric sites.

EvoChromo: towards a synthesis of chromatin biology and evolution

Over the past few years, interest in chromatin and its evolution has grown. To further advance these interests, we organized a workshop with the support of The Company of Biologists to debate the current state of knowledge regarding the origin and evolution of chromatin. This workshop led to prospective views on the development of a new field of research that we term 'EvoChromo'. In this short Spotlight article, we define the breadth and expected impact of this new area of scientific inquiry on our understanding of both chromatin and evolution.

Haplotypes spanning centromeric regions reveal persistence of large blocks of archaic DNA

Despite critical roles in chromosome segregation and disease, the repetitive structure and vast size of centromeres and their surrounding heterochromatic regions impede studies of genomic variation. Here we report the identification of large-scale haplotypes (cenhaps) in humans that span the centromere-proximal regions of all metacentric chromosomes, including the arrays of highly repeated α-satellites on which centromeres form. Cenhaps reveal deep diversity, including entire introgressed Neanderthal centromeres and equally ancient lineages among Africans. These centromere-spanning haplotypes contain variants, including large differences in α-satellite DNA content, which may influence the fidelity and bias of chromosome transmission. The discovery of cenhaps creates new opportunities to investigate their contribution to phenotypic variation, especially in meiosis and mitosis, as well as to more incisively model the unexpectedly rich evolution of these challenging genomic regions.

Centromere Transcription: Means and Motive

The chromosome biology field at large has benefited from studies of the cell cycle components, protein cascades and genomic landscape that are required for centromere identity, assembly and stable transgenerational inheritance. Research over the past 20 years has challenged the classical descriptions of a centromere as a stable, unmutable, and transcriptionally silent chromosome component. Instead, based on studies from a broad range of eukaryotic species, including yeast, fungi, plants, and animals, the centromere has been redefined as one of the more dynamic areas of the eukaryotic genome, requiring coordination of protein complex assembly, chromatin assembly, and transcriptional activity in a cell cycle specific manner. What has emerged from more recent studies is the realization that the transcription of specific types of nucleic acids is a key process in defining centromere integrity and function. To illustrate the transcriptional landscape of centromeres across eukaryotes, we focus this review on how transcripts interact with centromere proteins, when in the cell cycle centromeric transcription occurs, and what types of sequences are being transcribed. Utilizing data from broadly different organisms, a picture emerges that places centromeric transcription as an integral component of centromere function.