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

Assembly of the threespine stickleback Y chromosome reveals convergent signatures of sex chromosome evolution

BACKGROUND

Heteromorphic sex chromosomes have evolved repeatedly across diverse species. Suppression of recombination between X and Y chromosomes leads to degeneration of the Y chromosome. The progression of degeneration is not well understood, as complete sequence assemblies of heteromorphic Y chromosomes have only been generated across a handful of taxa with highly degenerate sex chromosomes. Here, we describe the assembly of the threespine stickleback (Gasterosteus aculeatus) Y chromosome, which is less than 26 million years old and at an intermediate stage of degeneration. Our previous work identified that the non-recombining region between the X and the Y spans approximately 17.5 Mb on the X chromosome.

RESULTS

We combine long-read sequencing with a Hi-C-based proximity guided assembly to generate a 15.87 Mb assembly of the Y chromosome. Our assembly is concordant with cytogenetic maps and Sanger sequences of over 90 Y chromosome BAC clones. We find three evolutionary strata on the Y chromosome, consistent with the three inversions identified by our previous cytogenetic analyses. The threespine stickleback Y shows convergence with more degenerate sex chromosomes in the retention of haploinsufficient genes and the accumulation of genes with testis-biased expression, many of which are recent duplicates. However, we find no evidence for large amplicons identified in other sex chromosome systems. We also report an excellent candidate for the master sex-determination gene: a translocated copy of Amh (Amhy).

CONCLUSIONS

Together, our work shows that the evolutionary forces shaping sex chromosomes can cause relatively rapid changes in the overall genetic architecture of Y chromosomes.

Effects of Mutations in the Drosophila melanogaster Rif1 Gene on the Replication and Underreplication of Pericentromeric Heterochromatin in Salivary Gland Polytene Chromosomes

In Drosophila salivary gland polytene chromosomes, a substantial portion of heterochromatin is underreplicated. The combination of mutations SuUR^ES and Su(var)3-9⁰⁶ results in the polytenization of a substantial fraction of unique and moderately repeated sequences but has almost no effect on satellite DNA replication. The Rap1 interacting factor 1 (Rif) protein is a conserved regulator of replication timing, and in Drosophila, it affects underreplication in polytene chromosomes. We compared the morphology of pericentromeric regions and labeling patterns of in situ hybridization of heterochromatin-specific DNA probes between wild-type salivary gland polytene chromosomes and the chromosomes of Rif1 mutants and SuUR Su(var)3-9⁰⁶ double mutants. We show that, despite general similarities, heterochromatin zones exist that are polytenized only in the Rif1 mutants, and that there are zones that are under specific control of Su(var)3-9. In the Rif1 mutants, we found additional polytenization of the largest blocks of satellite DNA (in particular, satellite 1.688 of chromosome X and simple satellites in chromosomes X and 4) as well as partial polytenization of chromosome Y. Data on pulsed incorporation of 5-ethynyl-2′-deoxyuridine (EdU) into polytene chromosomes indicated that in the Rif1 mutants, just as in the wild type, most of the heterochromatin becomes replicated during the late S phase. Nevertheless, a significantly increased number of heterochromatin replicons was noted. These results suggest that Rif1 regulates the activation probability of heterochromatic origins in the satellite DNA region.

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.

RepeatModeler2 for automated genomic discovery of transposable element families

The accelerating pace of genome sequencing throughout the tree of life is driving the need for improved unsupervised annotation of genome components such as transposable elements (TEs). Because the types and sequences of TEs are highly variable across species, automated TE discovery and annotation are challenging and time-consuming tasks. A critical first step is the de novo identification and accurate compilation of sequence models representing all of the unique TE families dispersed in the genome. Here we introduce RepeatModeler2, a pipeline that greatly facilitates this process. This program brings substantial improvements over the original version of RepeatModeler, one of the most widely used tools for TE discovery. In particular, this version incorporates a module for structural discovery of complete long terminal repeat (LTR) retroelements, which are widespread in eukaryotic genomes but recalcitrant to automated identification because of their size and sequence complexity. We benchmarked RepeatModeler2 on three model species with diverse TE landscapes and high-quality, manually curated TE libraries: Drosophila melanogaster (fruit fly), Danio rerio (zebrafish), and Oryza sativa (rice). In these three species, RepeatModeler2 identified approximately 3 times more consensus sequences matching with >95% sequence identity and sequence coverage to the manually curated sequences than the original RepeatModeler. As expected, the greatest improvement is for LTR retroelements. Thus, RepeatModeler2 represents a valuable addition to the genome annotation toolkit that will enhance the identification and study of TEs in eukaryotic genome sequences. RepeatModeler2 is available as source code or a containerized package under an open license (https://github.com/Dfam-consortium/RepeatModeler, http://www.repeatmasker.org/RepeatModeler/).

The Y chromosome may contribute to sex-specific ageing in Drosophila

Heterochromatin suppresses repetitive DNA, and a loss of heterochromatin has been observed in aged cells of several species, including humans and Drosophila. Males often contain substantially more heterochromatic DNA than females, due to the presence of a large, repeat-rich Y chromosome, and male flies generally have shorter average life spans than females. Here we show that repetitive DNA becomes de-repressed more rapidly in old male flies relative to females, and repeats on the Y chromosome are disproportionally mis-expressed during aging. This is associated with a loss of heterochromatin at repetitive elements during aging in male flies, and a general loss of repressive chromatin in aged males away from pericentromeric regions and the Y. By generating flies with different sex chromosome karyotypes (XXY females; X0 and XYY males), we show that repeat de-repression and average lifespan is correlated with the number of Y chromosomes. This suggests that sex-specific chromatin differences may contribute to sex-specific aging in flies.

De novo assembly of the olive fruit fly (Bactrocera oleae) genome with linked-reads and long-read technologies minimizes gaps and provides exceptional Y chromosome assembly

BACKGROUND

The olive fruit fly, Bactrocera oleae, is the most important pest in the olive fruit agribusiness industry. This is because female flies lay their eggs in the unripe fruits and upon hatching the larvae feed on the fruits thus destroying them. The lack of a high-quality genome and other genomic and transcriptomic data has hindered progress in understanding the fly's biology and proposing alternative control methods to pesticide use.

RESULTS

Genomic DNA was sequenced from male and female Demokritos strain flies, maintained in the laboratory for over 45 years. We used short-, mate-pair-, and long-read sequencing technologies to generate a combined male-female genome assembly (GenBank accession GCA_001188975.2). Genomic DNA sequencing from male insects using 10x Genomics linked-reads technology followed by mate-pair and long-read scaffolding and gap-closing generated a highly contiguous 489 Mb genome with a scaffold N50 of 4.69 Mb and L50 of 30 scaffolds (GenBank accession GCA_001188975.4). RNA-seq data generated from 12 tissues and/or developmental stages allowed for genome annotation. Short reads from both males and females and the chromosome quotient method enabled identification of Y-chromosome scaffolds which were extensively validated by PCR.

CONCLUSIONS

The high-quality genome generated represents a critical tool in olive fruit fly research. We provide an extensive RNA-seq data set, and genome annotation, critical towards gaining an insight into the biology of the olive fruit fly. In addition, elucidation of Y-chromosome sequences will advance our understanding of the Y-chromosome's organization, function and evolution and is poised to provide avenues for sterile insect technique approaches.

Structural Variation of the X Chromosome Heterochromatin in the Anopheles gambiae Complex

Heterochromatin is identified as a potential factor driving diversification of species. To understand the magnitude of heterochromatin variation within the Anopheles gambiae complex of malaria mosquitoes, we analyzed metaphase chromosomes in An. arabiensis, An. coluzzii, An. gambiae, An. merus, and An. quadriannulatus. Using fluorescence in situ hybridization (FISH) with ribosomal DNA (rDNA), a highly repetitive fraction of DNA, and heterochromatic Bacterial Artificial Chromosome (BAC) clones, we established the correspondence of pericentric heterochromatin between the metaphase and polytene X chromosomes of An. gambiae. We then developed chromosome idiograms and demonstrated that the X chromosomes exhibit qualitative differences in their pattern of heterochromatic bands and position of satellite DNA (satDNA) repeats among the sibling species with postzygotic isolation, An. arabiensis, An. merus, An. quadriannulatus, and An. coluzzii or An. gambiae. The identified differences in the size and structure of the X chromosome heterochromatin point to a possible role of repetitive DNA in speciation of mosquitoes. We found that An. coluzzii and An. gambiae, incipient species with prezygotic isolation, share variations in the relative positions of the satDNA repeats and the proximal heterochromatin band on the X chromosomes. This previously unknown genetic polymorphism in malaria mosquitoes may be caused by a differential amplification of DNA repeats or an inversion in the sex chromosome heterochromatin.

A fly model establishes distinct mechanisms for synthetic CRISPR/Cas9 sex distorters

Synthetic sex distorters have recently been developed in the malaria mosquito, relying on endonucleases that target the X-chromosome during spermatogenesis. Although inspired by naturally-occurring traits, it has remained unclear how they function and, given their potential for genetic control, how portable this strategy is across species. We established Drosophila models for two distinct mechanisms for CRISPR/Cas9 sex-ratio distortion—“X-shredding” and “X-poisoning”—and dissected their target-site requirements and repair dynamics. X-shredding resulted in sex distortion when Cas9 endonuclease activity occurred during the meiotic stages of spermatogenesis but not when Cas9 was expressed from the stem cell stages onwards. Our results suggest that X-shredding is counteracted by the NHEJ DNA repair pathway and can operate on a single repeat cluster of non-essential sequences, although the targeting of a number of such repeats had no effect on the sex ratio. X-poisoning by contrast, i.e. targeting putative haplolethal genes on the X chromosome, induced a high bias towards males (>92%) when we directed Cas9 cleavage to the X-linked ribosomal target gene RpS6. In the case of X-poisoning sex distortion was coupled to a loss in reproductive output, although a dominant-negative effect appeared to drive the mechanism of female lethality. These model systems will guide the study and the application of sex distorters to medically or agriculturally important insect target species.

Harmful insect populations can be eliminated for a lack of females if they are made to produce mostly male offspring. There are genes that occur naturally that make males produce mostly sons and, although we don’t know exactly how they work, this appears to coincide with damage to the X-chromosome during the production of sperm. Recently, we showed in a mosquito species that such sex-biasing genes could also be constructed artificially from first principles. To better understand if this works in other species too, we designed and built male-biasing genes of two types in the fruit fly and determined what is needed to for a shift towards males. We show how different ways of cutting the X-chromosome DNA at different times with CRISPR, results in distinct outcomes and started to ask what cellular processes are involved in this. These models will help us to design such genes for the control of insect species that transmit disease or threaten crops.

Dissecting Fertility Functions of Drosophila Y Chromosome Genes with CRISPR

Gene-poor, repeat-rich regions of the genome are poorly understood and have been understudied due to technical challenges and the misconception that they are degenerating "junk." Yet multiple lines of evidence indicate these regions may be an important source of variation that could drive adaptation and species divergence, particularly through regulation of fertility. The ∼40 Mb Y chromosome of Drosophila melanoga st er contains only 16 known protein-coding genes, and is highly repetitive and entirely heterochromatic. Most of the genes originated from duplication of autosomal genes and have reduced nonsynonymous substitution rates, suggesting functional constraint. We devised a genetic strategy for recovering and retaining stocks with sterile Y-linked mutations and combined it with CRISPR to create mutants with deletions that disrupt three Y-linked genes. Two genes, PRY and FDY, had no previously identified functions. We found that PRY mutant males are subfertile, but FDY mutant males had no detectable fertility defects. FDY, the newest known gene on the Y chromosome, may have fertility effects that are conditional or too subtle to detect. The third gene, CCY, had been predicted but never formally shown to be required for male fertility. CRISPR targeting and RNA interference of CCY caused male sterility. Surprisingly, however, our CCY mutants were sterile even in the presence of an extra wild-type Y chromosome, suggesting that perturbation of the Y chromosome can lead to dominant sterility. Our approach provides an important step toward understanding the complex functions of the Y chromosome and parsing which functions are accomplished by genes vs. repeat elements.

Identification of Y-chromosome scaffolds of the Queensland fruit fly reveals a duplicated gyf gene paralogue common to many Bactrocera pest species

Bactrocera tryoni (Queensland fruit fly) are polyphagous horticultural pests of eastern Australia. Heterogametic males contain a sex-determining Y-chromosome thought to be gene poor and repetitive. Here, we report 39 Y-chromosome scaffolds (~700 kb) from B. tryoni identified using genotype-by-sequencing data and whole-genome resequencing. Male diagnostic PCR assays validated eight Y-scaffolds, and one (Btry4096) contained a novel gene with five exons that encode a predicted 575 amino acid protein. The Y-gene, referred to as typo-gyf, is a truncated Y-chromosome paralogue of X-chromosome gene gyf (1773 aa). The Y-chromosome contained ~41 copies of typo-gyf, and expression occurred in male flies and embryos. Analysis of 13 tephritid transcriptomes confirmed typo-gyf expression in six additional Bactrocera species, including Bactrocera latifrons, Bactrocera dorsalis and Bactrocera zonata. Molecular dating estimated typo-gyf evolved within the past 8.02 million years (95% highest posterior density 10.56-5.52 million years), after the split with Bactrocera oleae. Phylogenetic analysis also highlighted complex evolutionary histories among several Bactrocera species, as discordant nuclear (116 genes) and mitochondrial (13 genes) topologies were observed. B. tryoni Y-sequences may provide useful sites for future transgene insertions, and typo-gyf could act as a Y-chromosome diagnostic marker for many Bactrocera species, although its function is unknown.

Ancestral male recombination in Drosophila albomicans produced geographically restricted neo-Y chromosome haplotypes varying in age and onset of decay

Male Drosophila typically have achiasmatic meiosis, and fusions between autosomes and the Y chromosome have repeatedly created non-recombining neo-Y chromosomes that degenerate. Intriguingly, Drosophila nasuta males recombine, but their close relative D. albomicans reverted back to achiasmy after evolving neo-sex chromosomes. Here we use genome-wide polymorphism data to reconstruct the complex evolutionary history of neo-sex chromosomes in D. albomicans and examine the effect of recombination and its cessation on the initiation of neo-Y decay. Population and phylogenomic analyses reveal three distinct neo-Y types that are geographically restricted. Due to ancestral recombination with the neo-X, overall nucleotide diversity on the neo-Y is similar to the neo-X but severely reduced within neo-Y types. Consistently, the neo-Y chromosomes fail to form a monophyletic clade in sliding window trees outside of the region proximal to the fusion. Based on tree topology changes, we inferred the recombination breakpoints that produced haplotypes specific to each neo-Y type. We show that recombination became suppressed at different time points for the different neo-Y haplotypes. Haplotype age correlates with onset of neo-Y decay, and older neo-Y haplotypes show more fixed gene disruption via frameshift indels and down-regulation of neo-Y alleles. Genes are downregulated independently on the different neo-Ys, but are depleted of testes-expressed genes across all haplotypes. This indicates that genes important for male function are initially shielded from degeneration. Our results offer a time course of the early progression of Y chromosome evolution, showing how the suppression of recombination, through the reversal to achiasmy in D. albomicans males, initiates the process of degeneration.

Unpredictable Effects of the Genetic Background of Transgenic Lines in Physiological Quantitative Traits

Physiology, fitness and disease phenotypes are complex traits exhibiting continuous variation in natural populations. To understand complex trait gene functions transgenic lines of undefined genetic background are often combined to assess quantitative phenotypes ignoring the impact of genetic polymorphisms. Here, we used inbred wild-type strains of the Drosophila Genetics Reference Panel to assess the phenotypic variation of six physiological and fitness traits, namely, female fecundity, survival and intestinal mitosis upon oral infection, defecation rate and fecal pH upon oral infection, and terminal tracheal cell branching in hypoxia. We found continuous variation in the approximately 150 strains tested for each trait, with extreme values differing by more than four standard deviations for all traits. In addition, we assessed the effects of commonly used Drosophila UAS-RNAi transgenic strains and their backcrossed isogenized counterparts, in the same traits plus baseline intestinal mitosis and tracheal branching in normoxia, in heterozygous conditions, when only half of the genetic background was different among strains. We tested 20 non-isogenic strains (10 KK and 10 GD) from the Vienna Drosophila Resource Center and their isogenized counterparts without Gal4 induction. Survival upon infection and female fecundity exhibited differences in 50% and 40% of the tested isogenic vs. non-isogenic pairs, respectively, whereas all other traits were affected in only 10–25% of the cases. When 11 isogenic and their corresponding non-isogenic UAS-RNAi lines were expressed ubiquitously with Gal4, 4 isogenic vs. non-isogenic pairs exhibited differences in survival to infection. Furthermore, when a single UAS-RNAi line was crossed with the same Gal4 transgene inserted in different genetic backgrounds, the quantitative variations observed were unpredictable on the basis of pure line performance. Thus, irrespective of the trait of interest, the genetic background of commonly used transgenic strains needs to be considered carefully during experimentation.

Structural variants exhibit widespread allelic heterogeneity and shape variation in complex traits

It has been hypothesized that individually-rare hidden structural variants (SVs) could account for a significant fraction of variation in complex traits. Here we identified more than 20,000 euchromatic SVs from 14 Drosophila melanogaster genome assemblies, of which ~40% are invisible to high specificity short-read genotyping approaches. SVs are common, with 31.5% of diploid individuals harboring a SV in genes larger than 5kb, and 24% harboring multiple SVs in genes larger than 10kb. SV minor allele frequencies are rarer than amino acid polymorphisms, suggesting that SVs are more deleterious. We show that a number of functionally important genes harbor previously hidden structural variants likely to affect complex phenotypes. Furthermore, SVs are overrepresented in candidate genes associated with quantitative trait loci mapped using the Drosophila Synthetic Population Resource. We conclude that SVs are ubiquitous, frequently constitute a heterogeneous allelic series, and can act as rare alleles of large effect.

A high-quality genome assembly from a single, field-collected spotted lanternfly (Lycorma delicatula) using the PacBio Sequel II system

BACKGROUND

A high-quality reference genome is an essential tool for applied and basic research on arthropods. Long-read sequencing technologies may be used to generate more complete and contiguous genome assemblies than alternate technologies; however, long-read methods have historically had greater input DNA requirements and higher costs than next-generation sequencing, which are barriers to their use on many samples. Here, we present a 2.3 Gb de novo genome assembly of a field-collected adult female spotted lanternfly (Lycorma delicatula) using a single Pacific Biosciences SMRT Cell. The spotted lanternfly is an invasive species recently discovered in the northeastern United States that threatens to damage economically important crop plants in the region.

RESULTS

The DNA from 1 individual was used to make 1 standard, size-selected library with an average DNA fragment size of ∼20 kb. The library was run on 1 Sequel II SMRT Cell 8M, generating a total of 132 Gb of long-read sequences, of which 82 Gb were from unique library molecules, representing ∼36× coverage of the genome. The assembly had high contiguity (contig N50 length = 1.5 Mb), completeness, and sequence level accuracy as estimated by conserved gene set analysis (96.8% of conserved genes both complete and without frame shift errors). Furthermore, it was possible to segregate more than half of the diploid genome into the 2 separate haplotypes. The assembly also recovered 2 microbial symbiont genomes known to be associated with L. delicatula, each microbial genome being assembled into a single contig.

CONCLUSIONS

We demonstrate that field-collected arthropods can be used for the rapid generation of high-quality genome assemblies, an attractive approach for projects on emerging invasive species, disease vectors, or conservation efforts of endangered species.

Dynamic turnover of centromeres drives karyotype evolution in Drosophila

Centromeres are the basic unit for chromosome inheritance, but their evolutionary dynamics is poorly understood. We generate high-quality reference genomes for multiple Drosophila obscura group species to reconstruct karyotype evolution. All chromosomes in this lineage were ancestrally telocentric and the creation of metacentric chromosomes in some species was driven by de novo seeding of new centromeres at ancestrally gene-rich regions, independently of chromosomal rearrangements. The emergence of centromeres resulted in a drastic size increase due to repeat accumulation, and dozens of genes previously located in euchromatin are now embedded in pericentromeric heterochromatin. Metacentric chromosomes secondarily became telocentric in the pseudoobscura subgroup through centromere repositioning and a pericentric inversion. The former (peri)centric sequences left behind shrunk dramatically in size after their inactivation, yet contain remnants of their evolutionary past, including increased repeat-content and heterochromatic environment. Centromere movements are accompanied by rapid turnover of the major satellite DNA detected in (peri)centromeric regions.

A New Portrait of Constitutive Heterochromatin: Lessons from Drosophila melanogaster

Constitutive heterochromatin represents a significant portion of eukaryotic genomes, but its functions still need to be elucidated. Even in the most updated genetics and molecular biology textbooks, constitutive heterochromatin is portrayed mainly as the 'silent' component of eukaryotic genomes. However, there may be more complexity to the relationship between heterochromatin and gene expression. In the fruit fly Drosophila melanogaster, a model for heterochromatin studies, about one-third of the genome is heterochromatic and is concentrated in the centric, pericentric, and telomeric regions of the chromosomes. Recent findings indicate that hundreds of D. melanogaster genes can 'live and work' properly within constitutive heterochromatin. The genomic size of these genes is generally larger than that of euchromatic genes and together they account for a significant fraction of the entire constitutive heterochromatin. Thus, this peculiar genome component in spite its ability to induce silencing, has in fact the means for being quite dynamic. A major scope of this review is to revisit the 'dogma of silent heterochromatin'.

Diversification and collapse of a telomere elongation mechanism

In most eukaryotes, telomerase counteracts chromosome erosion by adding repetitive sequence to terminal ends. Drosophila melanogaster instead relies on specialized retrotransposons that insert exclusively at telomeres. This exchange of goods between host and mobile element-wherein the mobile element provides an essential genome service and the host provides a hospitable niche for mobile element propagation-has been called a "genomic symbiosis." However, these telomere-specialized, jockey family retrotransposons may actually evolve to "selfishly" overreplicate in the genomes that they ostensibly serve. Under this model, we expect rapid diversification of telomere-specialized retrotransposon lineages and, possibly, the breakdown of this ostensibly symbiotic relationship. Here we report data consistent with both predictions. Searching the raw reads of the 15-Myr-old melanogaster species group, we generated de novo jockey retrotransposon consensus sequences and used phylogenetic tree-building to delineate four distinct telomere-associated lineages. Recurrent gains, losses, and replacements account for this retrotransposon lineage diversity. In Drosophila biarmipes, telomere-specialized elements have disappeared completely. De novo assembly of long reads and cytogenetics confirmed this species-specific collapse of retrotransposon-dependent telomere elongation. Instead, telomere-restricted satellite DNA and DNA transposon fragments occupy its terminal ends. We infer that D. biarmipes relies instead on a recombination-based mechanism conserved from yeast to flies to humans. Telomeric retrotransposon diversification and disappearance suggest that persistently "selfish" machinery shapes telomere elongation across Drosophila rather than completely domesticated, symbiotic mobile elements.

Recompleting the Caenorhabditis elegans genome

Caenorhabditis elegans was the first multicellular eukaryotic genome sequenced to apparent completion. Although this assembly employed a standard C. elegans strain (N2), it used sequence data from several laboratories, with DNA propagated in bacteria and yeast. Thus, the N2 assembly has many differences from any C. elegans available today. To provide a more accurate C. elegans genome, we performed long-read assembly of VC2010, a modern strain derived from N2. Our VC2010 assembly has 99.98% identity to N2 but with an additional 1.8 Mb including tandem repeat expansions and genome duplications. For 116 structural discrepancies between N2 and VC2010, 97 structures matching VC2010 (84%) were also found in two outgroup strains, implying deficiencies in N2. Over 98% of N2 genes encoded unchanged products in VC2010; moreover, we predicted ≥53 new genes in VC2010. The recompleted genome of C. elegans should be a valuable resource for genetics, genomics, and systems biology.

Genetic and Molecular Analysis of Essential Genes in Centromeric Heterochromatin of the Left Arm of Chromosome 3 in Drosophila melanogaster

A large portion of the Drosophila melanogaster genome is contained within heterochromatic regions of chromosomes, predominantly at centromeres and telomeres. The remaining euchromatic portions of the genome have been extensively characterized with respect to gene organization, function and regulation. However, it has been difficult to derive similar data for sequences within centromeric (centric) heterochromatin because these regions have not been as amenable to analysis by standard genetic and molecular tools. Here we present an updated genetic and molecular analysis of chromosome 3L centric heterochromatin (3L Het). We have generated and characterized a number of new, overlapping deficiencies (Dfs) which remove regions of 3L Het. These Dfs were critically important reagents in our subsequent genetic analysis for the isolation and characterization of lethal point mutations in the region. The assignment of these mutations to genetically-defined essential loci was followed by matching them to gene models derived from genome sequence data: this was done by using molecular mapping plus sequence analysis of mutant alleles, thereby aligning genetic and physical maps of the region. We also identified putative essential gene sequences in 3L Het by using RNA interference to target candidate gene sequences. We report that at least 25, or just under 2/3 of loci in 3L Het, are essential for viability and/or fertility. This work contributes to the functional annotation of centric heterochromatin in Drosophila, and the genetic and molecular tools generated should help to provide important insights into the organization and functions of gene sequences in 3L Het.

Islands of retroelements are major components of Drosophila centromeres

Centromeres are essential chromosomal regions that mediate kinetochore assembly and spindle attachments during cell division. Despite their functional conservation, centromeres are among the most rapidly evolving genomic regions and can shape karyotype evolution and speciation across taxa. Although significant progress has been made in identifying centromere-associated proteins, the highly repetitive centromeres of metazoans have been refractory to DNA sequencing and assembly, leaving large gaps in our understanding of their functional organization and evolution. Here, we identify the sequence composition and organization of the centromeres of Drosophila melanogaster by combining long-read sequencing, chromatin immunoprecipitation for the centromeric histone CENP-A, and high-resolution chromatin fiber imaging. Contrary to previous models that heralded satellite repeats as the major functional components, we demonstrate that functional centromeres form on islands of complex DNA sequences enriched in retroelements that are flanked by large arrays of satellite repeats. Each centromere displays distinct size and arrangement of its DNA elements but is similar in composition overall. We discover that a specific retroelement, G2/Jockey-3, is the most highly enriched sequence in CENP-A chromatin and is the only element shared among all centromeres. G2/Jockey-3 is also associated with CENP-A in the sister species D. simulans, revealing an unexpected conservation despite the reported turnover of centromeric satellite DNA. Our work reveals the DNA sequence identity of the active centromeres of a premier model organism and implicates retroelements as conserved features of centromeric DNA.