A large palindrome with interchromosomal gene duplications in the pericentromeric region of the D. melanogaster Y chromosome

Disclosing complex mutational dynamics at a Y chromosome palindrome evolving through intra- and inter-chromosomal gene conversion

The human MSY ampliconic region is mainly composed of large duplicated sequences that are organized in eight palindromes (termed P1-P8), and may undergo arm-to-arm gene conversion. Although the importance of these elements is widely recognized, their evolutionary dynamics are still nuanced. Here, we focused on the P8 palindrome, which shows a complex evolutionary history, being involved in intra- and inter-chromosomal gene conversion. To disclose its evolutionary complexity, we performed a high-depth (50×) targeted next-generation sequencing of this element in 157 subjects belonging to the most divergent lineages of the Y chromosome tree. We found a total of 72 polymorphic paralogous sequence variants that have been exploited to identify 41 Y-Y gene conversion events that occurred during recent human history. Through our analysis, we were able to categorize P8 arms into three portions, whose molecular diversity was modelled by different evolutionary forces. Notably, the outer region of the palindrome is not involved in any gene conversion event and evolves exclusively through the action of mutational pressure. The inner region is affected by Y-Y gene conversion occurring at a rate of 1.52 × 10-5 conversions/base/year, with no bias towards the retention of the ancestral state of the sequence. In this portion, GC-biased gene conversion is counterbalanced by a mutational bias towards AT bases. Finally, the middle region of the arms, in addition to intra-chromosomal gene conversion, is involved in X-to-Y gene conversion (at a rate of 6.013 × 10-8 conversions/base/year) thus being a major force in the evolution of the VCY/VCX gene family.

New insights into the evolution of human Y chromosome palindromes through mutation and gene conversion

About one-quarter of the euchromatic portion of the male-specific region of the human Y chromosome consists of large duplicated sequences that are organized in eight palindromes (termed P1-P8), which undergo arm-to arm gene conversion, a proposed mechanism for maintaining their sequence integrity. Although the relevance of gene conversion in the evolution of palindromic sequences has been profoundly recognized, the dynamic of this mechanism is still nuanced. To shed light into the evolution of these genomic elements, we performed a high-depth (50×) targeted next-generation sequencing of the palindrome P6 in 157 subjects belonging to the most divergent evolutionary lineages of the Y chromosome. We found 118 new paralogous sequence variants, which were placed into the context of a robust Y chromosome phylogeny based on 7240 SNPs of the X-degenerate region. We mapped along the phylogeny 80 gene conversion events that shaped the diversity of P6 arms during recent human history. In contrast to previous studies, we demonstrated that arm-to-arm gene conversion, which occurs at a rate of 6.01 × 10 -6 conversions/base/year, is not biased toward the retention of the ancestral state of sequences. We also found a significantly lower mutation rate of the arms (6.18 × 10-10 mutations/base/year) compared with the spacer (9.16 × 10-10 mutations/base/year), a finding that may explain the observed higher inter-species conservation of arms, without invoking any bias of conversion. Finally, by formally testing the mutation/conversion balance in P6, we found that the arms of this palindrome reached a steady-state equilibrium between mutation and gene conversion.

piRNA-mediated gene regulation and adaptation to sex-specific transposon expression in D. melanogaster male germline

Small noncoding piRNAs act as sequence-specific guides to repress complementary targets in Metazoa. Prior studies in Drosophila ovaries have demonstrated the function of the piRNA pathway in transposon silencing and therefore genome defense. However, the ability of the piRNA program to respond to different transposon landscapes and the role of piRNAs in regulating host gene expression remain poorly understood. Here, we comprehensively analyzed piRNA expression and defined the repertoire of their targets in Drosophila melanogaster testes. Comparison of piRNA programs between sexes revealed sexual dimorphism in piRNA programs that parallel sex-specific transposon expression. Using a novel bioinformatic pipeline, we identified new piRNA clusters and established complex satellites as dual-strand piRNA clusters. While sharing most piRNA clusters, the two sexes employ them differentially to combat the sex-specific transposon landscape. We found two piRNA clusters that produce piRNAs antisense to four host genes in testis, including CG12717/pirate, a SUMO protease gene. piRNAs encoded on the Y chromosome silence pirate, but not its paralog, to exert sex- and paralog-specific gene regulation. Interestingly, pirate is targeted by endogenous siRNAs in a sibling species, Drosophila mauritiana, suggesting distinct but related silencing strategies invented in recent evolution to regulate a conserved protein-coding gene.

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.

Heterochromatin-Enriched Assemblies Reveal the Sequence and Organization of the Drosophila melanogaster Y Chromosome

Heterochromatic regions of the genome are repeat-rich and poor in protein coding genes, and are therefore underrepresented in even the best genome assemblies. One of the most difficult regions of the genome to assemble are sex-limited chromosomes. The Drosophila melanogaster Y chromosome is entirely heterochromatic, yet has wide-ranging effects on male fertility, fitness, and genome-wide gene expression. The genetic basis of this phenotypic variation is difficult to study, in part because we do not know the detailed organization of the Y chromosome. To study Y chromosome organization in D. melanogaster, we develop an assembly strategy involving the in silico enrichment of heterochromatic long single-molecule reads and use these reads to create targeted de novo assemblies of heterochromatic sequences. We assigned contigs to the Y chromosome using Illumina reads to identify male-specific sequences. Our pipeline extends the D. melanogaster reference genome by 11.9 Mb, closes 43.8% of the gaps, and improves overall contiguity. The addition of 10.6 MB of Y-linked sequence permitted us to study the organization of repeats and genes along the Y chromosome. We detected a high rate of duplication to the pericentric regions of the Y chromosome from other regions in the genome. Most of these duplicated genes exist in multiple copies. We detail the evolutionary history of one sex-linked gene family, crystal-Stellate While the Y chromosome does not undergo crossing over, we observed high gene conversion rates within and between members of the crystal-Stellate gene family, Su(Ste), and PCKR, compared to genome-wide estimates. Our results suggest that gene conversion and gene duplication play an important role in the evolution of Y-linked genes.

The Drosophila chromosomal protein Mst77F is processed to generate an essential component of mature sperm chromatin

In most animals, the bulk of sperm DNA is packaged with sperm nuclear basic proteins (SNBPs), a diverse group of highly basic chromosomal proteins notably comprising mammalian protamines. The replacement of histones with SNBPs during spermiogenesis allows sperm DNA to reach an extreme level of compaction, but little is known about how SNBPs actually function in vivo. Mst77F is a Drosophila SNBP with unique DNA condensation properties in vitro, but its role during spermiogenesis remains unclear. Here, we show that Mst77F is required for the compaction of sperm DNA and the production of mature sperm, through its cooperation with protamine-like proteins Mst35Ba/b. We demonstrate that Mst77F is incorporated in spermatid chromatin as a precursor protein, which is subsequently processed through the proteolysis of its N-terminus. The cleavage of Mst77F is very similar to the processing of protamine P2 during human spermiogenesis and notably leaves the cysteine residues in the mature protein intact, suggesting that they participate in the formation of disulfide cross-links. Despite the rapid evolution of SNBPs, sperm chromatin condensation thus involves remarkably convergent mechanisms in distantly related animals.

Y chromosome palindromes and gene conversion

The presence of large and near-identical inverted repeat sequences (called palindromes) is a common feature of the constitutively haploid sex chromosomes of different species. Despite the fact palindromes originated in a non-recombining context, they have evolved a strong recombinational activity in the form of abundant arm-to-arm gene conversion. Their independent appearance in different species suggests they can have a profound biological significance that has yet to be fully clarified. It has been theorized that natural selection may have favored palindromic organization of male-specific genes and that the establishment of intra-palindrome gene conversion has strong adaptive significance. Arm-to-arm gene conversion allows the efficient removal of deleterious mutations, increases the fixation rate of beneficial mutations and has played an important role in modulating the equilibrium between gene loss and acquisition during Y chromosome evolution. Additionally, a palindromic organization of duplicates could favor the formation of unusual chromatin structures and could optimize the use of gene conversion as a mechanism to maintain the structural integrity of male-specific genes. In this review, we describe the structural features of palindromes on mammalian sex chromosomes and summarize different hypotheses regarding palindrome evolution and the functional benefits of arm-to-arm gene conversion on the unique haploid portion of the nuclear genome.

The Y chromosomes of the great apes

The great apes (orangutans, gorillas, chimpanzees, bonobos and humans) descended from a common ancestor around 13 million years ago, and since then their sex chromosomes have followed very different evolutionary paths. While great-ape X chromosomes are highly conserved, their Y chromosomes, reflecting the general lability and degeneration of this male-specific part of the genome since its early mammalian origin, have evolved rapidly both between and within species. Understanding great-ape Y chromosome structure, gene content and diversity would provide a valuable evolutionary context for the human Y, and would also illuminate sex-biased behaviours, and the effects of the evolutionary pressures exerted by different mating strategies on this male-specific part of the genome. High-quality Y-chromosome sequences are available for human and chimpanzee (and low-quality for gorilla). The chromosomes differ in size, sequence organisation and content, and while retaining a relatively stable set of ancestral single-copy genes, show considerable variation in content and copy number of ampliconic multi-copy genes. Studies of Y-chromosome diversity in other great apes are relatively undeveloped compared to those in humans, but have nevertheless provided insights into speciation, dispersal, and mating patterns. Future studies, including data from larger sample sizes of wild-born and geographically well-defined individuals, and full Y-chromosome sequences from bonobos, gorillas and orangutans, promise to further our understanding of population histories, male-biased behaviours, mutation processes, and the functions of Y-chromosomal genes.

The intimate genetics of Drosophila fertilization

The union of haploid gametes at fertilization initiates the formation of the diploid zygote in sexually reproducing animals. This founding event of embryogenesis includes several fascinating cellular and nuclear processes, such as sperm-egg cellular interactions, sperm chromatin remodelling, centrosome formation or pronuclear migration. In comparison with other aspects of development, the exploration of animal fertilization at the functional level has remained so far relatively limited, even in classical model organisms. Here, we have reviewed our current knowledge of fertilization in Drosophila melanogaster, with a special emphasis on the genes involved in the complex transformation of the fertilizing sperm nucleus into a replicated set of paternal chromosomes.

Radical remodeling of the Y chromosome in a recent radiation of malaria mosquitoes

Y chromosomes control essential male functions in many species, including sex determination and fertility. However, because of obstacles posed by repeat-rich heterochromatin, knowledge of Y chromosome sequences is limited to a handful of model organisms, constraining our understanding of Y biology across the tree of life. Here, we leverage long single-molecule sequencing to determine the content and structure of the nonrecombining Y chromosome of the primary African malaria mosquito, Anopheles gambiae We find that the An. gambiae Y consists almost entirely of a few massively amplified, tandemly arrayed repeats, some of which can recombine with similar repeats on the X chromosome. Sex-specific genome resequencing in a recent species radiation, the An. gambiae complex, revealed rapid sequence turnover within An. gambiae and among species. Exploiting 52 sex-specific An. gambiae RNA-Seq datasets representing all developmental stages, we identified a small repertoire of Y-linked genes that lack X gametologs and are not Y-linked in any other species except An. gambiae, with the notable exception of YG2, a candidate male-determining gene. YG2 is the only gene conserved and exclusive to the Y in all species examined, yet sequence similarity to YG2 is not detectable in the genome of a more distant mosquito relative, suggesting rapid evolution of Y chromosome genes in this highly dynamic genus of malaria vectors. The extensive characterization of the An. gambiae Y provides a long-awaited foundation for studying male mosquito biology, and will inform novel mosquito control strategies based on the manipulation of Y chromosomes.

The Release 6 reference sequence of the Drosophila melanogaster genome

Drosophila melanogaster plays an important role in molecular, genetic, and genomic studies of heredity, development, metabolism, behavior, and human disease. The initial reference genome sequence reported more than a decade ago had a profound impact on progress in Drosophila research, and improving the accuracy and completeness of this sequence continues to be important to further progress. We previously described improvement of the 117-Mb sequence in the euchromatic portion of the genome and 21 Mb in the heterochromatic portion, using a whole-genome shotgun assembly, BAC physical mapping, and clone-based finishing. Here, we report an improved reference sequence of the single-copy and middle-repetitive regions of the genome, produced using cytogenetic mapping to mitotic and polytene chromosomes, clone-based finishing and BAC fingerprint verification, ordering of scaffolds by alignment to cDNA sequences, incorporation of other map and sequence data, and validation by whole-genome optical restriction mapping. These data substantially improve the accuracy and completeness of the reference sequence and the order and orientation of sequence scaffolds into chromosome arm assemblies. Representation of the Y chromosome and other heterochromatic regions is particularly improved. The new 143.9-Mb reference sequence, designated Release 6, effectively exhausts clone-based technologies for mapping and sequencing. Highly repeat-rich regions, including large satellite blocks and functional elements such as the ribosomal RNA genes and the centromeres, are largely inaccessible to current sequencing and assembly methods and remain poorly represented. Further significant improvements will require sequencing technologies that do not depend on molecular cloning and that produce very long reads.

Recombination dynamics of a human Y-chromosomal palindrome: rapid GC-biased gene conversion, multi-kilobase conversion tracts, and rare inversions

The male-specific region of the human Y chromosome (MSY) includes eight large inverted repeats (palindromes) in which arm-to-arm similarity exceeds 99.9%, due to gene conversion activity. Here, we studied one of these palindromes, P6, in order to illuminate the dynamics of the gene conversion process. We genotyped ten paralogous sequence variants (PSVs) within the arms of P6 in 378 Y chromosomes whose evolutionary relationships within the SNP-defined Y phylogeny are known. This allowed the identification of 146 historical gene conversion events involving individual PSVs, occurring at a rate of 2.9–8.4×10⁻⁴ events per generation. A consideration of the nature of nucleotide change and the ancestral state of each PSV showed that the conversion process was significantly biased towards the fixation of G or C nucleotides (GC-biased), and also towards the ancestral state. Determination of haplotypes by long-PCR allowed likely co-conversion of PSVs to be identified, and suggested that conversion tract lengths are large, with a mean of 2068 bp, and a maximum in excess of 9 kb. Despite the frequent formation of recombination intermediates implied by the rapid observed gene conversion activity, resolution via crossover is rare: only three inversions within P6 were detected in the sample. An analysis of chimpanzee and gorilla P6 orthologs showed that the ancestral state bias has existed in all three species, and comparison of human and chimpanzee sequences with the gorilla outgroup confirmed that GC bias of the conversion process has apparently been active in both the human and chimpanzee lineages.

The sex-determining role of the human Y chromosome makes it male-specific, and always present in only a single copy. This solo lifestyle has endowed it with some bizarre features, among which are eight large DNA units constituting about a quarter of the chromosome's length, and containing many genes important for sperm production. These units are called palindromes, since, taking into account the polarity of the DNA strands, the sequence is the same read from either end of the unit. We investigated the details of a process (gene conversion) that transfers sequence variants in one half of a palindrome into the other, thereby maintaining >99.9% similarity between the halves. We analysed patterns of sequence variants within one palindrome in a set of Y chromosomes whose evolutionary relationships are known. This allowed us to identify past gene conversion events, and to demonstrate a bias towards events that eliminate new variants, and retain old ones. Gene conversion has therefore acted during human evolution to retard sequence change in these regions. Analysis of the chimpanzee and gorilla versions of the palindrome shows that the dynamic processes we see in human Y chromosomes have a deep evolutionary history.

Transgenerational propagation and quantitative maintenance of paternal centromeres depends on Cid/Cenp-A presence in Drosophila sperm

In Drosophila melanogaster, as in many animal and plant species, centromere identity is specified epigenetically. In proliferating cells, a centromere-specific histone H3 variant (CenH3), named Cid in Drosophila and Cenp-A in humans, is a crucial component of the epigenetic centromere mark. Hence, maintenance of the amount and chromosomal location of CenH3 during mitotic proliferation is important. Interestingly, CenH3 may have different roles during meiosis and the onset of embryogenesis. In gametes of Caenorhabditis elegans, and possibly in plants, centromere marking is independent of CenH3. Moreover, male gamete differentiation in animals often includes global nucleosome for protamine exchange that potentially could remove CenH3 nucleosomes. Here we demonstrate that the control of Cid loading during male meiosis is distinct from the regulation observed during the mitotic cycles of early embryogenesis. But Cid is present in mature sperm. After strong Cid depletion in sperm, paternal centromeres fail to integrate into the gonomeric spindle of the first mitosis, resulting in gynogenetic haploid embryos. Furthermore, after moderate depletion, paternal centromeres are unable to re-acquire normal Cid levels in the next generation. We conclude that Cid in sperm is an essential component of the epigenetic centromere mark on paternal chromosomes and it exerts quantitative control over centromeric Cid levels throughout development. Hence, the amount of Cid that is loaded during each cell cycle appears to be determined primarily by the preexisting centromeric Cid, with little flexibility for compensation of accidental losses.

Compositional bias is a major determinant of the distribution pattern and abundance of palindromes in Drosophila melanogaster

Palindromic sequences are important DNA motifs related to gene regulation, DNA replication and recombination, and thus, investigating the evolutionary forces shaping the distribution pattern and abundance of palindromes in the genome is substantially important. In this article, we analyzed the abundance of palindromes in the genome, and then explored the possible effects of several genomic factors on the palindrome distribution and abundance in Drosophila melanogaster. Our results show that the palindrome abundance in D. melanogaster deviates from random expectation and the uneven distribution of palindromes across the genome is associated with local GC content, recombination rate, and coding exon density. Our data suggest that base composition is the major determinant of the distribution pattern and abundance of palindromes and the correlation between palindrome density and recombination is a side-product of the effect of compositional bias on the palindrome abundance.

Sex, rebellion and decadence: the scandalous evolutionary history of the human Y chromosome

It can be argued that the Y chromosome brings some of the spirit of rock&roll to our genome. Equal parts degenerate and sex-driven, the Y has boldly rebelled against sexual recombination, one of the sacred pillars of evolution. In evolutionary terms this chromosome also seems to have adopted another of rock&roll's mottos: living fast. Yet, it appears to have refused to die young. In this manuscript the Y chromosome will be analyzed from the intersection between structural, evolutionary and functional biology. Such integrative approach will present the Y as a highly specialized product of a series of remarkable evolutionary processes. These led to the establishment of a sex-specific genomic niche that is maintained by a complex balance between selective pressure and the genetic diversity introduced by intrachromosomal recombination. Central to this equilibrium is the "polish or perish" dilemma faced by the male-specific Y genes: either they are polished by the acquisition of male-related functions or they perish via the accumulation of inactivating mutations. Thus, understanding to what extent the idiosyncrasies of Y recombination may impact this chromosome's role in sex determination and male germline functions should be regarded as essential for added clinical insight into several male infertility phenotypes. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.