Biology subjects

Balacco, J.

Publications and source records attributed to Balacco, J..

18 recordsLinked to original sources

bioRxiv2026

Manta and devil ray genome assemblies uncover signatures of pelagic adaptation

Manta and devil rays (Mobulidae) are iconic marine megafauna highly adapted to life in the open ocean, with forward-facing mouths, enlarged pectoral fins and specialised filter-feeding apparatus. These innovations have enabled mobulids to occupy vast pelagic habitats, yet the genetic changes underlying them remain poorly understood. Here we use a combination of chromatin conformation capture, PacBio HiFi and Nanopore long-reads to generate highly contiguous annotated chromosome-level reference genomes for two mobulid species: the Atlantic pygmy devil ray (Mobula hypostoma) and the Atlantic manta ray (Mobula yarae). The assemblies span 3.5 and 4.0 Gb, with contig N50s of 23.3 and 7.3 Mb, respectively. We combine these into a Progressive Cactus whole genome alignment with benthic batoid outgroups to characterise the landscape of structural variation and identify candidate molecular regions associated with pelagic adaptation. This approach revealed that over 1.5 million structural rearrangements occurred along the ancestral mobulid lineage, with deletions encompassing three times more sequence compared to the benthic sister lineage. Genes affected by mobulid specific deletions were significantly enriched for neurogenesis pathways linked to craniofacial patterning, with several deletions overlapping the putative promoter sequences of craniofacial and roof of mouth development genes. Our findings highlight candidate developmental pathways through which ancestral deletions may have driven mobulid adaptations, while the high-quality genome resources provide a foundation for comparative and functional studies in Chondrichthyes.

evolutionary biology↗

bioRxiv2026

Chromosome-level genome assembly of the European leaf-toed gecko, Euleptes europaea

The European leaf-toed gecko (Euleptes europaea) is a small, nocturnal gecko endemic to the western Mediterranean. As a phylogenetically distinctive member of the Gondwanan family Sphaerodactylidae, it represents an important species for studying Mediterranean island biogeography, adaptation, and reptile genome evolution. The species also occupies a key position for investigating the evolution of sex chromosomes, as geckos exhibit remarkable diversity and frequent transitions in sex-determination systems. We present a chromosome-level genome assembly of Euleptes europaea generated as part of the Vertebrate Genomes Project. The 1.8 Gb assembly has a scaffold N50 of 102.3 Mb (contig N50 27 Mb), with 21 chromosome-scale scaffolds corresponding to the known karyotype (2n = 42). The primary assembly has a BUSCO completeness of 97.80% (95.60% as single-copy), a k-mer completeness of 96.00%, and a k-mer quality value (QV) of 61.20. Repetitive elements account for 53.20% of the genome and genome annotation identified 18,633 protein-coding genes. This high-quality reference genome will facilitate studies of genome evolution, island adaptation, and sex chromosome evolution across geckos and other reptiles.

genomics↗

bioRxiv2026

Widespread introgression and a potential role for a neo-sex chromosome in booby diversification and speciation

Speciation in marine taxa is dynamic and complex, often occurring in the absence of absolute geographic barriers to gene flow. Boobies comprise a clade of seven highly mobile seabird species with high diversity in the eastern Pacific Ocean, where no land barriers separate extant sister species and hybridization is widespread. They exhibit striking diversity in bare-part coloration, an important signal of mate quality, and possess a novel multiple sex chromosome system (Z1Z2Z1Z2 / Z1Z2W), likely resulting from a fusion between an ancestral autosomal microchromosome and the W chromosome. To understand speciation processes in the context of gene flow, we sequenced and analyzed 29 short-read booby genomes and assembled a reference northern gannet genome to (1) test for introgression, (2) characterize genomic patterns of divergence across species, and (3) investigate the gene content and evolution of the neo-sex chromosome. We found that divergence among eastern Pacific sister taxa is temporally associated with periods of glacial maxima. Between periods of glacial maxima, two pairs of sister species (blue-footed and Peruvian boobies; masked and Nazca boobies) exhibit strong signatures of episodic introgression, indicating that speciation has occurred over multiple periods of divergence followed by gene flow. However, in the third pair of sister species, genomic signatures showed higher divergence within brown booby populations than between brown and Cocos boobies. We also identified the epidermal differentiation complex - a gene cassette involved in skin, feather, beak, and claw development - on the neo-sex region of the W chromosome, where it spans the boundary between the putatively recombining and non-recombining regions, with interspecific variation in the exact recombination suppression boundary. We inferred that this complex may contribute to diversity in feather and bare part coloration across boobies. Together, these results reveal that diversification and speciation in a clade of highly mobile seabirds emerged from a complex evolutionary history involving historical climate dynamics, introgression, and sex chromosome evolution.

evolutionary biology↗

bioRxiv2026

The diploid reference genome of a human embryonic stem cell line

Advances in DNA sequencing and assembly technologies are spurring a shift from haploid reference genomes to sample-specific diploid assemblies. Here, we generated the first telomere-to-telomere (T2T) diploid reference for the widely used human embryonic stem cell (hESC) line, H9 (WAe009-A). This haplotype-resolved assembly is highly accurate with comprehensive annotation of genes, segmental duplications, methylation, and chromatin conformation. Pangenomic and phased-locus inference point to H9s mixed ancestry with a predominant European component. H9-specific genomic features include near-perfect telomeres [~]1.65-fold longer than other T2T assemblies, consistent with telomerase activity during pluripotency; chromosome 17 inversions that can predispose offspring to neurological syndromes; and expansions of ncRNA clusters, with overall genomic stability maintained despite extensive culturing. Mapping multi-omic datasets to the genome, we demonstrate the power of this resource for allele-specific, high-precision transcriptomic, genetic, and epigenetic analyses, with far-reaching implications for human development and disease.

genomics↗

bioRxiv2026

Elevated recessive lethal frequencies drive hatching failure following near extinction in 'Alala, the Hawaiian crow

Near-extinction events impose severe genomic bottlenecks that can have lasting fitness consequences, yet the specific mechanisms involved remain poorly understood. Alal[a], the last remaining Hawaiian crow, narrowly avoided extinction when a conservation breeding program was initiated from just eight genetic founders. Although the conservation breeding program has since recovered to [~]120 birds, it remains plagued by egg hatching failure rates >50%. To investigate the impacts of this bottleneck on hatching failure and other fitness components, we generated a chromosome-level reference genome and resequenced 175 individuals, including 78 deceased embryos. Although long runs of homozygosity (ROH) >1Mb are abundant in Alal[a] (mean FROH=0.32), associations between FROH and measures of survival and reproduction, including egg failure, were weak or nonexistent. Instead, we identify two recessive lethal haplotypes that together account for nearly all of the excess hatching failure and have persisted in the population at high frequency (15-25%), hinting at impaired purifying selection. Eco-evolutionary simulation models demonstrate that these findings are expected for a species that has endured a severe population bottleneck and exhibits modest levels of pre-bottleneck heterozygosity. Our findings suggest that elevated recessive allele frequencies may be a broadly important consequence of population bottlenecks.

evolutionary biology↗

bioRxiv2025

CAD-C: An engineered nuclease enables repair-free in situ proximity ligation and nucleosome-resolution chromosome walks in human cells

Chromosome conformation capture (3C)-derived methods have become an indispensable tool in the study of gene regulation. The three-dimensional contacts probed by 3C methods depend strongly on the properties of the enzyme used to fragment chromatin prior to proximity-driven ligation. Micrococcal nuclease (MNase), used in Micro-C, increases resolution at the expense of low ligation efficiency and the need for extensive enzyme titration. To overcome these limitations, we engineered a TEV protease-activatable caspase-activated DNase (CAD) to enable an efficient, low-sequence-bias, and high-resolution proximity ligation assay we call CAD-C. CAD-C was successful on the first attempt for each human cell line tested and the resulting datasets capture loops, TADs, compartments, and stripes similarly to Micro-C. However, compared to Micro-C and Hi-C, CAD-C shows enhanced sensitivity for promoter-enhancer loops. Leveraging the ligation-competent DNA ends produced by CAD cleavage, we show that CAD-C is compatible with a highly streamlined, repair-free protocol and produces multi-step CADwalks, consecutive ligations between nucleosomal or sub-nucleosomal fragments. With these walks, we probe local chromatin fiber folding contacts, nucleosomal and sub-nucleosomal footprints, and long-range nuclear organization regimes in human cell lines. CAD-C is an efficient, robust chromatin structure assay that can span sub-nucleosomal to chromosomal length scales in a single experiment.

genomics↗

bioRxiv2025

The complete genome of a songbird

Bird genomes are the smallest among amniotes, but remain challenging to assemble due to their structural complexity. This study presents the first fully phased, diploid, telomere-to-telomere (T2T) reference genome for the zebra finch (Taeniopygia guttata), a model organism for neuroscience and evolutionary genomics. Combining multiple sequencing strategies resulted in closing nearly all gaps, adding [~]90 Mbp of previously missing sequence (7.8%). This includes T2T assemblies for all microchromosomes, including dot chromosomes, and the previously almost entirely missing chr16. The T2T genome is comprehensively annotated for genes, repeats, structural variants, and long-read methylation calls. Complete centromeric structures were assembled and annotated along with kinetochore binding sites. Relative to the previous high-quality reference of the Vertebrate Genomes Project, 2,778 (8.51%) previously unassembled or unannotated genes were identified, of which 9% overlap with segmental duplications. This first complete genome of a songbird, now the new public reference, illuminates avian genome architecture and function.

genomics↗

bioRxiv2025

Chromosome-level genome assembly of the Brazilian merganser (Mergus octosetaceus), a rare and elusive waterfowl species

BackgroundThe Brazilian merganser (Mergus octosetaceus) is a critically endangered bird, with fewer than 250 mature individuals remaining in fragmented populations of the Cerrado biome in Brazil. It is an ecologically demanding species that requires clear water of fast-flowing rivers for diving and foraging, and riverside cavities for nesting. Technical and logistical challenges have restricted sampling and research to few genetic studies so far. The genetic results consistently revealed a low genetic diversity and high inbreeding in the species. Besides, the lack of a reference genome has precluded deeper evolutionary and conservation genetic analyses for the species. ResultsWe generated the first high-quality, chromosome-level genome assembly for Mergus octosetaceus using PacBio HiFi long reads and Arima Hi-C data from a female specimen. The assembly is 1.25 Gb in length, is highly complete, scoring 98.9% of BUSCO completeness, and includes both sex chromosomes (Z and W). Annotated repetitive elements comprise 18.94% of the genome, with evidence of recent activity, particularly among LINEs and LTRs retrotransposons. We also produced and annotated the complete mitochondrial genome, identifying 37 genes and the control region. Comparative synteny analysis with other Anatidae species revealed strong chromosomal conservation, with several inversion events in macrochromosomes. Demographic history reconstruction indicated fluctuations in effective population size, with significant reductions overlapped by major temperature changes, highlighting potential climate sensitivity of this lineage. ConclusionThis reference genome provides a fundamental resource for M. octosetaceus, enabling insights into genome evolution, chromosomal dynamics within Anatidae, and past population history. It is a remarkable foundation for future research and conservation strategies for managing and restoring this critically endangered species.

evolutionary biology↗

bioRxiv2025

Seasonal brain regeneration and chromosome instability are linked to selection on DNA repair in Sorex araneus

Sorex araneus, the Eurasian common shrew, has seasonal brain size plasticity (Dehnels phenomenon) and abundant intraspecific chromosomal rearrangements, but genomic contributions to these traits remain unknown. We couple a chromosome-scale genome assembly with seasonal brain transcriptomes to discover relationships between molecular changes and both traits. Positively selected genes enriched the Fanconi anemia DNA repair pathway, which prevents the accumulation of chromosomal aberrations, and is likely involved in chromosomal rearrangements (FANCI, FAAP100). Genes involved in neurogenesis show either signatures of positive selection (PCDHA6), seasonal differential expression in the cortex and hippocampus (Notch signaling), or both (SOX9), suggesting a role for cellular proliferation in seasonal brain shrinkage and regrowth. Both positive selection and evolutionary upregulation in the shrew hypothalamus of VEGFA and SPHK2 indicate adaptations in hypothalamic metabolic homeostasis have evolved together with Dehnels phenomenon. These findings reveal genomic changes central to the evolution of both chromosomal instability and cyclical patterns in brain gene expression that characterizes mammalian brain size plasticity. TeaserGenomic and expression variations are key to chromosomal instability and seasonal brain plasticity in the common shrew.

evolutionary biology↗

bioRxiv2025

Haplotype-resolved reference genomes of the sea turtle clade unveil ultra-syntenic genomes with hotspots of divergence

BackgroundReference genomes for the entire sea turtle clade have the potential to reveal the genetic basis of traits driving the ecological and phenotypic diversity in these ancient and iconic marine species. Furthermore, these genomic resources can support conservation efforts and deepen our understanding of their unique evolution. ResultsWe present haplotype-resolved, chromosome-level reference genomes and high-quality gene annotations for five sea turtle species. This completes the catalog of reference genomes of the entire sea turtle clade when combined with our previously published reference genomes. Our analysis reveals remarkable genome synteny and collinearity across all species, despite the clades origin dating back more than 60 million years. Regions of high interspecific genetic distance and intraspecific genetic diversity are consistently clustered in genomic hotspots, which are enriched with genes coding for immune response proteins, olfactory receptors, zinc fingers, and G-protein-coupled receptors. These hotspot regions may offer insights into the genetic mechanisms driving phenotypic divergence among species, and represent areas of significant adaptive potential. Ancient demographic analysis revealed a synchronous population expansion among sea turtle species during the Pleistocene, with varying magnitudes of demographic change, likely shaped by their diverse ecological adaptations, and biogeographic contexts. ConclusionsOur work provides genomic resources for exploring genetic diversity, evolutionary adaptations, and demographic histories of sea turtles. We outline genomic regions with increased diversity, linked to immune response, sensory evolution, and adaptation to varying environments that have historically been subject to strong diversifying selection, and likely will underpin sea turtles responses to future environmental change. These reference genomes can assist conservation by providing insights into the demographic and evolutionary processes that sustain and threaten these iconic species.

genomics↗

bioRxiv2025

Insights into the Evolution of Ancient Shark and Ray Sex Chromosomes

While sex-determining mechanisms have been extensively characterized in many vertebrates, they have not been explored in chondrichthyan fishes until relatively recently. In the present study, we used high-quality whole genome reference assemblies to examine the putative sex chromosomes of 14 elasmobranch species spanning nine orders. We describe four newly assembled reference genomes belonging to the white shark Carcharodon carcharias, the Atlantic stingray Hypanus sabinus, the smalltooth sawfish Pristis pectinata, and the zebra shark Stegostoma tigrinum. We conducted sex chromosome identification and verification using short-read sequence data collected for multiple individuals for three of the species. This revealed putative pseudoautosomal regions (PARs) and, in one instance, a candidate sex chromosome reassignment. A synteny analysis revealed an ancient and shared origin of the chromosomes within elasmobranchs considerably older than any previously proposed scenario, and a potential candidate gene involved in sex determination shared across all examined species. The synteny analysis also revealed a historical fusion and the formation of neo-Y chromosomes between two myliobatiform species. Our results show that there has been strong conservation and homology of the X chromosomes among elasmobranchs in spite of their varied features and different evolutionary histories.

evolutionary biology↗

bioRxiv2025

Evaluation of sequencing reads at scale using rdeval

MotivationLarge sequencing data sets are produced and deposited into public archives at unprecedented rates. The availability of tools that can reliably and efficiently generate and store sequencing read summary statistics has become critical. ResultsAs part of the effort by the Vertebrate Genomes Project (VGP) to generate high-quality reference genomes at scale, we sought to address the community need for efficient sequencing data evaluation by developing rdeval, a standalone tool to quickly compute and dynamically display sequencing read metrics. Rdeval can either run on the fly or store key sequence data metrics in read sketches, with dramatic compression gains. Statistics can then be efficiently recalled from sketches for additional processing. Rdeval can convert fa*[.gz] files to and from other popular formats including BAM and CRAM for better compression. Overall, while CRAM achieves the best compression, the gain is marginal, and BAM achieves the best compromise between data compression and accessing speed. Rdeval also generates a detailed visual report with multiple data analytics that can be exported in various formats. We showcase rdevals functionalities using human and VGP read data from different sequencing platforms and species. For PacBio long-read sequencing, our analysis shows dramatic improvements both in read length and quality over time, and a benefit of additional coverage for genome assembly. Availability and implementationRdeval is implemented in C++ for data processivity and in R for data visualization. Precompiled releases (Linux, MacOS, Windows) and commented source code for rdeval are available under MIT license at https://github.com/vgl-hub/rdeval. Documentation is available using ReadTheDocs (https://rdeval-documentation.readthedocs.io). Rdeval is also available in Bioconda and in Galaxy (https://usegalaxy.org). An automated test workflow ensures the consistency of software updates. Supplementary informationSupplementary data are available at Bioinformatics online.

bioinformatics↗

bioRxiv2024

Persistent genomic erosion in whooping cranes despite demographic recovery

Integrating in-situ (wild) and ex-situ (captive) conservation efforts can mitigate genetic diversity loss and help prevent extinction of endangered wild populations. The whooping crane (Grus americana) experienced severe population declines in the 18th century, culminating into a collapse to 16 individuals in 1941. Legal protections and conservation actions have since increased the population to approximately 840 individuals, yet the impact on genomic diversity remains unclear. We analysed the temporal dynamics of genomic erosion by sequencing a high-quality genome reference, and re-sequencing 16 historical and 37 modern genomes, including wild individuals and four generations of captive-bred individuals. Genomic demographic reconstructions reveal a steady decline, accelerating over the past 300 years with the European settlement of North America. Temporal genomic analyses show that despite demographic recovery, the species has lost 70% of its genetic diversity and has increased their inbreeding. Although the modern population bottleneck reduced the ancestral genetic load, modern populations possess more realized load than masked load, possibly resulting in a chronic loss of fitness. Integrating pedigree and genomic data, we underscore the role of breeding management in reducing recent inbreeding. Yet ongoing heterozygosity loss, load accumulation, and background inbreeding argues against the species downlisting from their current Endangered status on the IUCN Red List and the Endangered Species Act. The presence of private genetic variation in wild and captive populations suggests that wild-captive crosses could enhance genetic diversity and reduce the realized load. Our findings emphasize the role of genomics in informing conservation management and policy.

evolutionary biology↗

bioRxiv2023

Single-cell long-read mRNA isoform regulation is pervasive across mammalian brain regions, cell types, and development

RNA isoforms influence cell identity and function. Until recently, technological limitations prevented a genome-wide appraisal of isoform influence on cell identity in various parts of the brain. Using enhanced long-read single-cell isoform sequencing, we comprehensively analyze RNA isoforms in multiple mouse brain regions, cell subtypes, and developmental timepoints from postnatal day 14 (P14) to adult (P56). For 75% of genes, full-length isoform expression varies along one or more axes of phenotypic origin, underscoring the pervasiveness of isoform regulation across multiple scales. As expected, splicing varies strongly between cell types. However, certain gene classes including neurotransmitter release and reuptake as well as synapse turnover, harbor significant variability in the same cell type across anatomical regions, suggesting differences in network activity may influence cell-type identity. Glial brain-region specificity in isoform expression includes strong poly(A)-site regulation, whereas neurons have stronger TSS regulation. Furthermore, developmental patterns of cell-type specific splicing are especially pronounced in the murine adolescent transition from P21 to P28. The same cell type traced across development shows more isoform variability than across adult anatomical regions, indicating a coordinated modulation of functional programs dictating neural development. As most cell-type specific exons in P56 mouse hippocampus behave similarly in newly generated data from human hippocampi, these principles may be extrapolated to human brain. However, human brains have evolved additional cell-type specificity in splicing, suggesting gain-of-function isoforms. Taken together, we present a detailed single-cell atlas of full-length brain isoform regulation across development and anatomical regions, providing a previously unappreciated degree of isoform variability across multiple scales of the brain.

genetics↗

bioRxiv2022

Pangenomics provides insights into the role of synanthropy in barn swallow evolution

Insights into the evolution of non-model organisms are often limited by the lack of reference genomes. As part of the Vertebrate Genomes Project, we present a new reference genome and a pangenome produced with High-Fidelity long reads for the barn swallow Hirundo rustica. We then generated a reference-free multialignment with other bird genomes to identify genes under selection. Conservation analyses pointed at genes enriched for transcriptional regulation and neurodevelopment. The most conserved gene is CAMK2N2, with a potential role in fear memory formation. In addition, using all publicly available data, we generated a comprehensive catalogue of genetic markers. Genome-wide linkage disequilibrium scans identified potential selection signatures at multiple loci. The top candidate region comprises several genes and includes BDNF, a gene involved in stress response, fear memory formation, and tameness. We propose that the strict association with humans in this species is linked with the evolution of pathways typically under selection in domesticated taxa.

genomics↗

bioRxiv2021

Single-nuclei isoform RNA sequencing reveals combination patterns of transcript elements across human brain cell types

Single-nuclei RNA-Seq is being widely employed to investigate cell types, especially of human brain and other frozen samples. In contrast to single-cell approaches, however, the majority of single-nuclei RNA counts originate from partially processed RNA leading to intronic cDNAs, thus hindering the investigation of complete isoforms. Here, using microfluidics, PCR-based artifact removal, target enrichment, and long-read sequencing, we developed single-nuclei isoform RNA-sequencing ( SnISOr-Seq), and applied it to the analysis of human adult frontal cortex samples. We found that exons associated with autism exhibit coordinated and more cell-type specific inclusion than exons associated with schizophrenia or ALS. We discovered two distinct modes of combination patterns: first, those distinguishing cell types in the human brain. These are enriched in combinations of TSS-exon, exon-polyA site, and distant (non-adjacent) exon pairs. Second, those with all isoform combinations found within one neural cell type, which are enriched in adjacent exon pairs. Furthermore, adjacent exon pairs are predominantly mutually associated, while distant pairs are frequently mutually exclusive. Finally, we observed that human-specific exons are as tightly coordinated as conserved exons, pointing to an efficient evolutionary mechanism underpinning coordination. SnISOr-Seq opens the door to single-nuclei long-read isoform analysis in the human brain, and in any frozen, archived or hard-to-dissociate sample.

genomics↗

bioRxiv2021

Benchmarking ultra-high molecular weight DNA preservation methods for long-read and long-range sequencing

Studies in vertebrate genomics require sampling from a broad range of tissue types, taxa, and localities. Recent advancements in long-read and long-range genome sequencing have made it possible to produce high-quality chromosome-level genome assemblies for almost any organism. However, adequate tissue preservation for the requisite ultra-high molecular weight DNA (uHMW DNA) remains a major challenge. Here we present a comparative study of preservation methods for field and laboratory tissue sampling, across vertebrate classes and different tissue types. We find that no single method is best for all cases. Instead, the optimal storage and extraction methods vary by taxa, by tissue, and by down-stream application. Therefore, we provide sample preservation guidelines that ensure sufficient DNA integrity and amount required for use with long-read and long-range sequencing technologies across vertebrates. Our best practices generated the uHMW DNA needed for the high-quality reference genomes for Phase 1 of the Vertebrate Genomes Project (VGP), whose ultimate mission is to generate chromosome-level reference genome assemblies of all [~]70,000 extant vertebrate species.

genomics↗

bioRxiv2020

Complete vertebrate mitogenomes reveal widespread gene duplications and repeats

Modern sequencing technologies should make the assembly of the relatively small mitochondrial genomes an easy undertaking. However, few tools exist that address mitochondrial assembly directly. As part of the Vertebrate Genomes Project (VGP) we have developed mitoVGP, a fully automated pipeline for similarity-based identification of mitochondrial reads and de novo assembly of mitochondrial genomes that incorporates both long (>10 kbp, PacBio or Nanopore) and short (100-300 bp, Illumina) reads. Our pipeline led to successful complete mitogenome assemblies of 100 vertebrate species of the VGP. We have observed that tissue type and library size selection have considerable impact on mitogenome sequencing and assembly. Comparing our assemblies to purportedly complete reference mitogenomes based on short-read sequencing, we have identified errors, missing sequences, and incomplete genes in those references, particularly in repeat regions. Our assemblies have also identified novel gene region duplications, shedding new light on mitochondrial genome evolution and organization.

genomics↗