Highly repetitive satellite DNA (satDNA) repeats are found in most eukaryotic genomes. SatDNAs are rapidly evolving and have roles in genome stability and chromosome segregation. Their repetitive nature poses a challenge for genome assembly and makes progress on the detailed study of satDNA structure difficult. Here, we use single-molecule sequencing long reads from Pacific Biosciences (PacBio) to determine the detailed structure of all major autosomal complex satDNA loci in Drosophila melanogaster, with a particular focus on the 260-bp and Responder satellites. We determine the optimal de novo assembly methods and parameter combinations required to produce a high-quality assembly of these previously unassembled satDNA loci and validate this assembly using molecular and computational approaches. We determined that the computationally intensive PBcR-BLASR assembly pipeline yielded better assemblies than the faster and more efficient pipelines based on the MHAP hashing algorithm, and it is essential to validate assemblies of repetitive loci. The assemblies reveal that satDNA repeats are organized into large arrays interrupted by transposable elements. The repeats in the center of the array tend to be homogenized in sequence, suggesting that gene conversion and unequal crossovers lead to repeat homogenization through concerted evolution, although the degree of unequal crossing over may differ among complex satellite loci. We find evidence for higher-order structure within satDNA arrays that suggest recent structural rearrangements. These assemblies provide a platform for the evolutionary and functional genomics of satDNAs in pericentric heterochromatin.

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Satellite DNAs (satDNAs) are among the most dynamically evolving components of eukaryotic genomes and play important roles in genome regulation, genome evolution, and speciation. Despite their abundance and functional impact, we know little about the evolutionary dynamics and molecular mechanisms that shape satDNA distributions in genomes. Here, we use high-quality genome assemblies to study the evolutionary dynamics of two complex satDNAs, Rsp-like and 1.688 g/cm3, in Drosophila melanogaster and its three nearest relatives in the simulans clade. We show that large blocks of these repeats are highly dynamic in the heterochromatin, where their genomic location varies across species. We discovered that small blocks of satDNA that are abundant in X chromosome euchromatin are similarly dynamic, with repeats changing in abundance, location, and composition among species. We detail the proliferation of a rare satellite (Rsp-like) across the X chromosome in D. simulans and D. mauritiana. Rsp-like spread by inserting into existing clusters of the older, more abundant 1.688 satellite, in events likely facilitated by microhomology-mediated repair pathways. We show that Rsp-like is abundant on extrachromosomal circular DNA in D. simulans, which may have contributed to its dynamic evolution. Intralocus satDNA expansions via unequal exchange and the movement of higher order repeats also contribute to the fluidity of the repeat landscape. We find evidence that euchromatic satDNA repeats experience cycles of proliferation and diversification somewhat analogous to bursts of transposable element proliferation. Our study lays a foundation for mechanistic studies of satDNA proliferation and the functional and evolutionary consequences of satDNA movement.

The rapid evolution of repetitive DNA sequences, including heterochromatic regions, satellite DNA, tandem duplications, and transposable elements, can underlie phenotypic evolution and contribute to hybrid incompatibilities between species. However, repetitive genomic regions are fragmented in most contemporary genome assemblies. We generated highly contiguous de novo assemblies for the Drosophila simulans species complex ( D. simulans, D. mauritiana, and D. sechellia ), which speciated~250,000 years ago. These species diverged from their common ancestor with D. melanogaster 3 million years ago. Our assemblies are comparable in contiguity and accuracy to the current D. melanogaster genome, allowing us to directly compare repetitive regions in genomes across different evolutionary times. We find a rapid turnover of satellite DNA and extensive structural variation in heterochromatic regions, while the euchromatic gene content is mostly conserved. Despite the overall preservation of synteny, euchromatin of each species has been sculpted by clade and species-specific inversions, transposable elements (TE), satellite and tRNA tandem arrays, and gene duplications. We also find Y-linked genes rapidly diverging, in terms of copy number and recent duplications from the autosomes. Our assemblies provide a valuable resource for studying genome evolution and its consequences for phenotypic evolution in these genetic model species.The group of four fruit fly species composed of D. melanogaster, D. simulans, D.sechellia and D. mauritiana is collectively known as the D. melanogaster species complex (or mel-complex for short) (Hey and Kliman 1993) and serves as a model system for studying speciation (Tao et al.

ABSTRACTSatellite DNAs (satDNAs) are among the most dynamically evolving components of eukaryotic genomes and play important roles in genome regulation, genome evolution, and speciation. Despite their abundance and functional impact, we know little about the evolutionary dynamics and molecular mechanisms that shape satDNA distributions in genomes. Here we use high-quality genome assemblies to study evolutionary dynamics of two complex satDNAs, Rsp-like and 1.688 gm/cm3, in Drosophila melanogaster and its three nearest relatives in the simulans clade. We show that large blocks of these repeats are highly dynamic in the heterochromatin, where their genomic location varies across species. We discovered that small blocks of satDNA that are abundant in X chromosome euchromatin are similarly dynamic, with repeats changing in abundance, location, and composition among species. We detail the proliferation of a rare satellite (Rsp-like) across the X chromosome in D. simulans and D. mauritiana. Rsp-like spreads by inserting into existing clusters of the older, more abundant 1.688 satellite, in events that were likely facilitated by microhomology-mediated repair pathways. We show that Rsp-like is abundant on extrachromosomal circular DNA in D. simulans, which may have contributed to its dynamic evolution. Intralocus satDNA expansions via unequal exchange and the movement of higher-order repeats also contribute to the fluidity of the repeat landscape. We find evidence that euchromatic satDNA repeats experience cycles of proliferation and diversification somewhat analogous to bursts of transposable element proliferation. Our study lays a foundation for mechanistic studies of satDNA proliferation and the functional and evolutionary consequences of satDNA movement.

Genomic data for priapulans are limited to a single species, restricting broad comparative analyses, and thorough interrogation of questions spanning phylogenomics, ecdysozoan physiology, and development. To help fill this void, we present here a high-quality priapulan genome for the meiofaunal species Tubiluchus corallicola. Our assembly combines Nanopore and Illumina sequencing technologies, and makes use of a whole genome amplification, to generate enough DNA to sequence this small meiofaunal species. We generated a moderately contiguous assembly (2547 scaffolds), with a high level of completeness (Metazoan BUSCOs n = 954, single copy complete = 89.6%, duplicated = 3.9%, fragmented = 3.5%, missing = 3.0%). We then screened the genome for homologs of the Halloween genes, key genes implicated in the ecdysis (moulting) pathway of arthropods, recovering a putative homolog of shadow. The presence of a shadow orthologue in two priapulan genomes suggests that the Halloween genes may not have evolved in a stepwise manner in Panarthropoda, as previously thought, but may have a deeper origin at the base of Ecdysozoa.

Genome assemblies are growing at an exponential rate and have proved indispensable for studying evolution but effort has been biased toward vertebrates and arthropods with particular focus on insects. Onychophora or velvet worms are an ancient group of cryptic, soil dwelling worms noted for their unique mode of prey capture, biogeographic patterns, and diversity of reproductive strategies. They constitute a poorly understood phylum of exclusively terrestrial animals that is sister group to arthropods. Due to this phylogenetic position, they are crucial in understanding the origin of the largest phylum of animals. Despite their significance, there is a paucity of genomic resources for the phylum with only one highly fragmented and incomplete genome publicly available. Initial attempts at sequencing an onychophoran genome proved difficult due to their large genome size and high repeat content. However, leveraging recent advances in long read sequencing technology, we present here the first annotated draft genome for the phylum. With a total size of 5.6Gb, the gigantism of the Epiperipatus broadwayi genome arises from having high repeat content, intron size inflation, and extensive gene family expansion. Additionally, we report a previously unknown diversity of onychophoran hemocyanins that suggests the diversification of copper-mediated oxygen carriers occurred independently in Onychophora­­ after its split from Arthropoda, parallel to the independent diversification of hemocyanins in each of the main arthropod lineages.