Remarkably high repeat content in the genomes of sparrows: the importance of genome assembly completeness for transposable element discovery.

Benham, P.; Cicero, C.; Escalona, M.; Beraut, E.; Fairbairn, C.; Marimuthu, M. P. A.; Nguyen, O.; Sahasrabudhe, R.; King, B. L.; Thomas, W. K.; Kovach, A. I.; Nachman, M. W.; Bowie, R. C. K.

doi:10.1101/2023.10.26.564301

bioRxiv · 10.1101/2023.10.26.564301

Remarkably high repeat content in the genomes of sparrows: the importance of genome assembly completeness for transposable element discovery.

Benham, P.·Cicero, C.·Escalona, M.·Beraut, E.·Fairbairn, C.·Marimuthu, M. P. A.·Nguyen, O.·Sahasrabudhe, R.·King, B. L.·Thomas, W. K.·Kovach, A. I.·Nachman, M. W.·Bowie, R. C. K.

View original source ↗Export metadata ↓

Abstract

Transposable elements (TE) play critical roles in shaping genome evolution. However, the highly repetitive sequence content of TEs is a major source of assembly gaps. This makes it difficult to decipher the impact of these elements on the dynamics of genome evolution. The increased capacity of long-read sequencing technologies to span highly repetitive regions of the genome should provide novel insights into patterns of TE diversity. Here we report the generation of highly contiguous reference genomes using PacBio long read and Omni-C technologies for three species of sparrows in the family Passerellidae. To assess the influence of sequencing technology on TE annotation, we compared these assemblies to three chromosome-level sparrow assemblies recently generated by the Vertebrate Genomes Project and nine other sparrow species generated using a variety of short- and long-read technologies. All long-read based assemblies were longer in length (range: 1.12-1.41 Gb) than short-read assemblies (0.91-1.08 Gb). Assembly length was strongly correlated with the amount of repeat content, with longer genomes showing much higher levels of repeat content than typically reported for the avian order Passeriformes. Repeat content for the Bells sparrow (31.2% of genome) was the highest level reported to date for a songbird genome assembly and was more in line with woodpecker (order Piciformes) genomes. CR1 LINE elements retained from an expansion that occurred 25-30 million years ago were the most abundant TEs in the song sparrow genome. Although the other five sparrow species also exhibit evidence for a spike in CR1 LINE activity at 25-30 million years ago, LTR elements stemming from more recent expansions were the most abundant elements in these species. LTRs were uniquely abundant in the Bells sparrow genome deriving from two recent peaks of activity. Higher levels of repeat content (79.2-93.7%) were found on the W chromosome relative to the Z (20.7-26.5) or autosomes (16.1-30.9%). These patterns support a dynamic model of transposable element expansion and contraction underpinning the seemingly constrained and small sized genomes of birds. Our work highlights how the resolution of difficult-to-assemble regions of the genome with new sequencing technologies promises to transform our understanding of avian genome evolution.

Source connections

Published article ↗

Explore related subjects

genomics Preprint

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Benham, P., Cicero, C., Escalona, M., Beraut, E., Fairbairn, C., Marimuthu, M. P. A., Nguyen, O., Sahasrabudhe, R., King, B. L., Thomas, W. K., Kovach, A. I., Nachman, M. W., Bowie, R. C. K.. 2023-10-31. Remarkably high repeat content in the genomes of sparrows: the importance of genome assembly completeness for transposable element discovery.. https://doi.org/10.1101/2023.10.26.564301

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related preprints

bioRxiv2026

Structural variation in repeat elements is widespread in normal human tissues and in tumorigenesis

Somatic mosaicism contributes to genomic variation, yet postzygotic structural variants remain under-characterized. We performed long- and short-read WGS from multiple individuals (n=47 normal tissues; n=168 samples) and identified mosaic structural variants in all individuals and germ layers, impacting a median 285.2 kb/genome. Nearly half of breakpoints were independently validated, with tissue distributions reflecting both early and late developmental origins. Most mosaic variants were repeat-mediated and 8.3% overlapped functional elements, an enrichment compared to germline variants. To extend these analyses in samples where long-read sequencing is infeasible, we measured repeat alterations from short-read sequencing, recapitulating mosaic tissue-specific differences. We characterized tumor- and tissue- specific variation in repeats across 15 cancer types and found tumor-related repeat variation to be similar in scale to that of normal mosaic variation. Tracking repeat changes in cell-free DNA provided a noninvasive approach for tumor monitoring. Our analyses revealed widespread repeat-driven structural variation in health and disease.

genomics↗

bioRxiv2026

RNA isoform-resolved multiplexed sequencing with bioorthogonal barcoding

RNA isoform dysregulation drives disease pathogenesis and is the target of FDA-approved splice-switching therapeutics. However, multiplexed sequencing methods discard splice junction information because only 3' termini are barcoded and counted. Here, we repurpose acylation and click chemistries to conjugate bioorthogonal barcodes (bobcodes) directly onto multiple internal positions along cellular RNAs. Bobcoded RNAs from multiple samples are pooled for multiplexed cDNA synthesis, during which reverse transcriptase switches from each RNA template onto its tethered bobcode with greater than 99% accuracy in species mixing experiments. Bobcode attachment intervals set cDNA insert sizes without a library fragmentation step, and priming with poly(dT) or random hexamers selects between 3'-end counting and full-length isoform capture. A bioorthogonal barcode-sequencing (BOB-seq v0.1) drug screen identifies transcriptome-wide on- and off-target RNA splicing effects and outperforms existing multiplexing RNA sequencing methods in workflow simplicity, sample-to-sample variability, and barcoding accuracy. Bobcodes add isoform resolution to scalable multiplexed RNA sequencing.

genomics↗

bioRxiv2026

Structural polymorphism and population-variable coding capacity of HERV-K(HML-2) in human pangenomes

Approximately 8% of the human genome is derived from ancient retroviral infections. The most recently integrated of these endogenous retroviruses is the HERV-K(HML-2) clade, whose expression has been associated with cancer, amyotrophic lateral sclerosis, and embryogenesis. Studies of HERV expression, particularly HML-2, have relied predominantly on short-read sequencing. However, the high similarity among HML-2 proviruses prevents many short reads from being assigned uniquely to individual loci. We therefore compared haplotype-resolved long-read genome assemblies from 292 donors to resolve variation in proviral structure and coding capacity. Several loci previously thought to be fixed were structurally polymorphic. Tandem arrays occurred at 13 loci and contained up to six proviral copies in a single array. At 8q11.23, we identified a previously undescribed full-length provirus in one haplotype. All 583 other haplotypes carried a solo-LTR. We found that standard reference genomes failed to represent the coding capacity retained in many individuals, whose proviruses contained intact open reading frames despite disruptive mutations in the reference sequences. Short-read genotypes left 32.5% of the tested donor-variant pairs unresolved at sites associated with viral reading frames. These findings show why HML-2 expression must be interpreted in the context of the structural and coding alleles each individual carries.

genomics↗