The genome sequence of the fin whale, Balaenoptera physalus (Linnaeus, 1758) (Artiodactyla: Balaenopteridae)
Nicholas J. Davison, Phillip A. Morin, L Filipe C Castro, Mariana F Nery
TL;DR
The paper presents the genome sequence of the fin whale, including two haplotypes and the mitochondrial genome, as part of a project to sequence species in Britain and Ireland.
Contribution
The novel contribution is the high-quality genome assembly of the fin whale, including chromosomal pseudomolecules and sex chromosomes.
Findings
The genome assembly includes two haplotypes with lengths of 3,442.54 and 2,850.21 megabases.
Haplotype 1 is scaffolded into 23 chromosomal pseudomolecules, including X and Y sex chromosomes.
The mitochondrial genome is 16.4 kilobases long.
Abstract
We present a genome assembly from an individual male Balaenoptera physalus (fin whale; Chordata; Mammalia; Artiodactyla; Balaenopteridae). The assembly contains two haplotypes with total lengths of 3 442.54 megabases and 2 850.21 megabases. Most of haplotype 1 (79.11%) is scaffolded into 23 chromosomal pseudomolecules, including the X and Y sex chromosomes. Haplotype 2 was assembled to scaffold level. The mitochondrial genome has also been assembled, with a length of 16.4 kilobases. This assembly was generated as part of the Darwin Tree of Life project, which produces reference genomes for eukaryotic species found in Britain and Ireland.
Genes, proteins, chemicals, diseases, species, mutations and cell lines named across the full text — each resolved to its canonical identifier and authoritative record.
Click any figure to enlarge with its caption.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6| Platform | PacBio HiFi | Hi-C |
|---|---|---|
|
| mBalPhy2 | mBalPhy2 |
|
| SAN00003288 | SAN00003288 |
|
| SAMEA114493136 | SAMEA114493136 |
|
| SAMEA114493144 | SAMEA114493144 |
|
| muscle | muscle |
|
| Revio | Illumina NovaSeq X |
|
| ERR13245265 | ERR13248928 |
|
| 8.00 million | 2 692.03 million |
|
| 82.14 Gb | 406.50 Gb |
|
| mBalPhy2.hap1.1 | mBalPhy2.hap2.1 |
|
| GCA_965194825.1 | GCA_965194765.1 |
|
| chromosome | scaffold |
|
| 3 442.54 | 2 850.21 |
|
| 23 | scaffold-level |
|
| 4 127 | 3 706 |
|
| 2.08 Mb | 2.31 Mb |
|
| 2 611 | 2 448 |
|
| 110.87 Mb | 106.92 Mb |
|
| 197.29 | - |
|
| X and Y | - |
|
| Mitochondrion: 16.4 kb | - |
| INSDC accession | Molecule | Length (Mb) | GC% |
|---|---|---|---|
| 1 | 215.55 | 41.50 | |
| 2 | 200.59 | 41 | |
| 3 | 185.94 | 40.50 | |
| 4 | 155.67 | 39 | |
| 5 | 151.80 | 38.50 | |
| 6 | 142.26 | 40.50 | |
| 7 | 128.10 | 41.50 | |
| 8 | 128.02 | 40 | |
| 9 | 117.78 | 39.50 | |
| 10 | 114.40 | 42 | |
| 11 | 114.32 | 43 | |
| 12 | 113.45 | 43 | |
| 13 | 109.76 | 41.50 | |
| 14 | 108.97 | 43 | |
| 15 | 100.29 | 41.50 | |
| 16 | 93.56 | 45.50 | |
| 17 | 90.56 | 40.50 | |
| 18 | 85.40 | 39 | |
| 19 | 75.79 | 45 | |
| 20 | 66.03 | 46 | |
| 21 | 47.46 | 41.50 | |
| X | 152.38 | 40.50 | |
| Y | 25.40 | 41.50 |
| Measure | Value | Benchmark |
|---|---|---|
| EBP summary (haplotype 1) | 6.8.Q65 | 6.C.Q40 |
| Contig N50 length | 2.08 Mb | ≥ 1 Mb |
| Scaffold N50 length | 110.87 Mb | = chromosome N50 |
| Consensus quality (QV) | Haplotype 1: 65.6; haplotype 2: 65.5; combined: 65.6 | ≥ 40 |
|
| Haplotype 1: 94.59%; Haplotype 2: 89.62%; combined:
| ≥ 95% |
| BUSCO | C:95.6% [S:92.7%; D:2.9%]; F:1.1%; M:3.3%; n:13 335 | S > 90%; D < 5% |
| Percentage of assembly assigned to
| 79.11% | ≥ 90% |
- —Wellcome Trust
Peer Reviews
No public reviews on file for this paper yet. If you reviewed it on a platform where reviews are public (OpenReview, ICLR, NeurIPS, ICML), you can paste yours below so the community can read it here.
Videos
No videos yet. Explain this paper in a talk, walkthrough, or lecture? Add one.
Taxonomy
TopicsMarine animal studies overview · Ichthyology and Marine Biology · Genomics and Phylogenetic Studies
Species taxonomy
Eukaryota; Opisthokonta; Metazoa; Eumetazoa; Bilateria; Deuterostomia; Chordata; Craniata; Vertebrata; Gnathostomata; Teleostomi; Euteleostomi; Sarcopterygii; Dipnotetrapodomorpha; Tetrapoda; Amniota; Mammalia; Theria; Eutheria; Boreoeutheria; Laurasiatheria; Artiodactyla; Whippomorpha; Cetacea; Mysticeti; Balaenopteridae; Balaenoptera; Balaenoptera physalus (Linnaeus, 1758) (NCBI:txid9770)
Background
The fin whale ( Balaenoptera physalus) is the second largest animal on earth (up to 23–26 m in length), only slightly smaller than its close relative, the blue whale ( Balaenoptera musculus) (up to 33 m in length). Fin whales are antitropically distributed in temperate to subpolar waters, following seasonal migrations that isolate populations in each hemisphere, and are subdivided into three oceanic subspecies in the North Pacific ( B. p. velifera), North Atlantic ( B. p. physalus), and the Southern Ocean ( B. p. quoyi) ( Archer et al., 2019).
Fin whales are baleen whales that lunge- and filter-feed on a variety of small schooling fish, squid, and krill, varying by season and locality. They swim fast (5 to 8 knots, up to 15 knots) and cover large distances in search of food and during seasonal migrations, typically traveling singly or in small groups. Maturation occurs at around 25 years, and individuals have been known to live for 80 to 90 years ( Aguilar & García-Vernet, 2018).
More fin whales were killed than any other species during industrial whaling, with approximately 874 000 fin whales killed in the 20th century ( Rocha et al., 2014), reducing global abundance by ~70% ( Edwards et al., 2015), and some populations by ~99% ( Nigenda-Morales et al., 2023). Fin whales are listed as Vulnerable globally, and Endangered in the Mediterranean Sea ( Cooke, 2018). Although commercial whaling effectively stopped in the 1980s due to the global moratorium, some aboriginal (Greenland) and commercial whaling (Japan, Iceland) of fin whales has continued. Other threats include ship strikes, entanglement in commercial fishing gear, pollution, and climate change.
Two scaffold-level genome assemblies have previously been generated for Balaenoptera physalus (GCA_023338255.1, GCA_008795845.1; submitted by Senckenberg Bik-F and the Korea Ocean Research & Development Institute) (data obtained via NCBI datasets, O’Leary et al., 2024). We present a chromosome-level genome sequence for the species, produced using the Tree of Life pipeline from a specimen collected from Orkney, Scotland, UK ( Figure 1).
Members of the Scottish Marine Animal Stranding Scheme with the Balaenoptera physalus (mBalPhy2) carcass from which samples were taken for genome sequencing (photo credit Nick Davison).
Methods
Sample acquisition
The specimen used for genome sequencing was an adult male Balaenoptera physalus (specimen ID SAN00003288, ToLID mBalPhy2; Figure 1), collected from Echnaloch Bay, Burray, Orkney, Scotland, United Kingdom (latitude 58.8568, longitude –2.9242) on 2021-07-21. The specimen was collected and identified by Nick Davison (Scottish Marine Animal Stranding Scheme, University of Glasgow). Sample metadata were collected in line with the Darwin Tree of Life project standards described by Lawniczak et al. (2022).
Nucleic acid extraction
Protocols for high molecular weight (HMW) DNA extraction developed at the Wellcome Sanger Institute (WSI) Tree of Life Core Laboratory are available on protocols.io ( Howard et al., 2025). The mBalPhy2 sample was weighed and triaged to determine the appropriate extraction protocol. Tissue from the muscle was homogenised by cryogenic disruption using the Covaris cryoPREP ^®^ Automated Dry Pulverizer. HMW DNA was extracted using the Automated MagAttract v2 protocol. DNA was sheared into an average fragment size of 12–20 kb following the Megaruptor®3 for LI PacBio protocol. Sheared DNA was purified by manual SPRI (solid-phase reversible immobilisation). The concentration of the sheared and purified DNA was assessed using a Nanodrop spectrophotometer and Qubit Fluorometer using the Qubit dsDNA High Sensitivity Assay kit. Fragment size distribution was evaluated by running the sample on the FemtoPulse system.
PacBio HiFi library preparation and sequencing
Library preparation and sequencing were performed at the WSI Scientific Operations core. Libraries were prepared using the SMRTbell Prep Kit 3.0 (Pacific Biosciences, California, USA), following the manufacturer’s instructions. The kit includes reagents for end repair/A-tailing, adapter ligation, post-ligation SMRTbell bead clean-up, and nuclease treatment. Size selection and clean-up were performed using diluted AMPure PB beads (Pacific Biosciences). DNA concentration was quantified using a Qubit Fluorometer v4.0 (ThermoFisher Scientific) and the Qubit 1X dsDNA HS assay kit. Final library fragment size was assessed with the Agilent Femto Pulse Automated Pulsed Field CE Instrument (Agilent Technologies) using the gDNA 55 kb BAC analysis kit.
The sample was sequenced on a Revio instrument (Pacific Biosciences). The prepared library was normalised to 2 nM, and 15 μL was used for making complexes. Primers were annealed and polymerases bound to generate circularised complexes, following the manufacturer’s instructions. Complexes were purified using 1.2X SMRTbell beads, then diluted to the Revio loading concentration (200–300 pM) and spiked with a Revio sequencing internal control. The sample was sequenced on a Revio 25M SMRT cell. The SMRT Link software (Pacific Biosciences), a web-based workflow manager, was used to configure and monitor the run and to carry out primary and secondary data analysis.
Hi-C
** Sample preparation and crosslinking **
The Hi-C sample was prepared from 20–50 mg of frozen muscle tissue from the mBalPhy2 sample using the Arima-HiC v2 kit (Arima Genomics). Following the manufacturer’s instructions, tissue was fixed and DNA crosslinked using TC buffer to a final formaldehyde concentration of 2%. The tissue was homogenised using the Diagnocine Power Masher-II. Crosslinked DNA was digested with a restriction enzyme master mix, biotinylated, and ligated. Clean-up was performed with SPRISelect beads before library preparation. DNA concentration was measured with the Qubit Fluorometer (Thermo Fisher Scientific) and Qubit HS Assay Kit. The biotinylation percentage was estimated using the Arima-HiC v2 QC beads.
** Hi-C library preparation and sequencing **
Biotinylated DNA constructs were fragmented using a Covaris E220 sonicator and size selected to 400–600 bp using SPRISelect beads. DNA was enriched with Arima-HiC v2 kit Enrichment beads. End repair, A-tailing, and adapter ligation were carried out with the NEBNext Ultra II DNA Library Prep Kit (New England Biolabs), following a modified protocol where library preparation occurs while DNA remains bound to the Enrichment beads. Library amplification was performed using KAPA HiFi HotStart mix and a custom Unique Dual Index (UDI) barcode set (Integrated DNA Technologies). Depending on sample concentration and biotinylation percentage determined at the crosslinking stage, libraries were amplified with 10–16 PCR cycles. Post-PCR clean-up was performed with SPRISelect beads. Libraries were quantified using the AccuClear Ultra High Sensitivity dsDNA Standards Assay Kit (Biotium) and a FLUOstar Omega plate reader (BMG Labtech).
Prior to sequencing, libraries were normalised to 10 ng/μL. Normalised libraries were quantified again to create equimolar and/or weighted 2.8 nM pools. Pool concentrations were checked using the Agilent 4200 TapeStation (Agilent) with High Sensitivity D500 reagents before sequencing. Sequencing was performed using paired-end 150 bp reads on the Illumina NovaSeq X.
Genome assembly
Prior to assembly of the PacBio HiFi reads, a database of k-mer counts ( k = 31) was generated from the filtered reads using FastK. GenomeScope2 ( Ranallo-Benavidez et al., 2020) was used to analyse the k-mer frequency distributions, providing estimates of genome size, heterozygosity, and repeat content.
The HiFi reads were assembled using Hifiasm in Hi-C phasing mode ( Cheng et al., 2021; Cheng et al., 2022), producing two haplotypes. Hi-C reads ( Rao et al., 2014) were mapped to the primary contigs using bwa-mem2 ( Vasimuddin et al., 2019). Contigs were further scaffolded with Hi-C data in YaHS ( Zhou et al., 2023), using the --break option for handling potential misassemblies. The scaffolded assemblies were evaluated using Gfastats ( Formenti et al., 2022), BUSCO ( Manni et al., 2021) and MERQURY.FK ( Rhie et al., 2020).
The mitochondrial genome was assembled using MitoHiFi ( Uliano-Silva et al., 2023), which runs MitoFinder ( Allio et al., 2020) and uses these annotations to select the final mitochondrial contig and to ensure the general quality of the sequence.
Assembly curation
The assembly was decontaminated using the Assembly Screen for Cobionts and Contaminants ( ASCC) pipeline. TreeVal was used to generate the flat files and maps for use in curation. Manual curation was conducted primarily in PretextView and HiGlass ( Kerpedjiev et al., 2018). Scaffolds were visually inspected and corrected as described by Howe et al. (2021). Manual corrections included 33 breaks and 171 joins. The curation process is documented at https://gitlab.com/wtsi-grit/rapid-curation. PretextSnapshot was used to generate a Hi-C contact map of the final assembly.
Assembly quality assessment
The Merqury.FK tool ( Rhie et al., 2020) was run in a Singularity container ( Kurtzer et al., 2017) to evaluate k-mer completeness and assembly quality for both haplotypes using the k-mer databases ( k = 31) computed prior to genome assembly. The analysis outputs included assembly QV scores and completeness statistics.
The genome was analysed using the BlobToolKit pipeline, a Nextflow implementation of the earlier Snakemake version ( Challis et al., 2020). The pipeline aligns PacBio reads using minimap2 ( Li, 2018) and SAMtools ( Danecek et al., 2021) to generate coverage tracks. It runs BUSCO ( Manni et al., 2021) using lineages identified from the NCBI Taxonomy ( Schoch et al., 2020). For the three domain-level lineages, BUSCO genes are aligned to the UniProt Reference Proteomes database ( Bateman et al., 2023) using DIAMOND blastp ( Buchfink et al., 2021). The genome is divided into chunks based on the density of BUSCO genes from the closest taxonomic lineage, and each chunk is aligned to the UniProt Reference Proteomes database with DIAMOND blastx. Sequences without hits are chunked using seqtk and aligned to the NT database with blastn ( Altschul et al., 1990). The BlobToolKit suite consolidates all outputs into a blobdir for visualisation. The BlobToolKit pipeline was developed using nf-core tooling ( Ewels et al., 2020) and MultiQC ( Ewels et al., 2016), with containerisation through Docker ( Merkel, 2014) and Singularity ( Kurtzer et al., 2017).
Genome sequence report
Sequence data
PacBio sequencing of the Balaenoptera physalus specimen generated 82.14 Gb (gigabases) from 8.00 million reads, which were used to assemble the genome. GenomeScope2.0 analysis estimated the haploid genome size at 3 160.70 Mb, with a heterozygosity of 0.26% and repeat content of 36.13% ( Figure 2). These estimates guided expectations for the assembly. Based on the estimated genome size, the sequencing data provided approximately 25× coverage. Hi-C sequencing produced 406.50 Gb from 2 692.03 million reads, which were used to scaffold the assembly. Table 1 summarises the specimen and sequencing details.
Frequency distribution of k-mers generated using GenomeScope2.The plot shows observed and modelled k-mer spectra, providing estimates of genome size, heterozygosity, and repeat content based on unassembled sequencing reads.
Assembly statistics
The genome was assembled into two haplotypes using Hi-C phasing. Haplotype 1 was curated to chromosome level, while haplotype 2 was assembled to scaffold level. The final assembly has a total length of 3 442.54 Mb in 2 611 scaffolds, with 1 516 gaps, and a scaffold N50 of 110.87 Mb ( Table 2).
Most of the assembly sequence (79.11%) was assigned to 23 chromosomal-level scaffolds, representing 21 autosomes and the X and Y sex chromosomes. These chromosome-level scaffolds, confirmed by Hi-C data, are named according to size ( Figure 3; Table 3). Scaffolds in the following regions have highly repetitive sequence, and have uncertain order and orientation: 20 to 22.5 Mbp on Chromosome 4, 22.4 to 32.3 and 108.1 to 119.4 Mbp on Chromosome 6, 26 to 42.3 Mbp on Chromosome 7, 73 to 88.4 Mbp on Chromosome 10, 88.1 to 97.2 Mbp on Chromosome 13, 34.5 to 39.1 Mbp on Chromosome 17, 23.7 to 29.6 Mbp on Chromosome 19, 1.2 to 7.7 Mbp on Chromosome 21 and 18 to 24.3 Mbp on Chromosome Y. The sex chromosomes were identified by the depth of PacBio read coverage.
Hi-C contact map of the Balaenoptera physalus genome assembly.Assembled chromosomes are shown in order of size and labelled along the axes, with a megabase scale shown below. The plot was generated using PretextSnapshot.
Table 3.: Chromosomal pseudomolecules in the haplotype 1 genome assembly of Balaenoptera physalus mBalPhy2.
The mitochondrial genome was also assembled. This sequence is included as a contig in the multifasta file of the genome submission and as a standalone record.
For haplotype 1, the estimated QV is 65.6, and for haplotype 2, 65.5. When the two haplotypes are combined, the assembly achieves an estimated QV of 65.6. The k-mer completeness is 94.59% for haplotype 1, 89.62% for haplotype 2, and 99.16% for the combined haplotypes ( Figure 4).
Evaluation of k-mer completeness using MerquryFK.This plot illustrates the recovery of k-mers from the original read data in the final assemblies. The horizontal axis represents k-mer multiplicity, and the vertical axis shows the number of k-mers. The black curve represents k-mers that appear in the reads but are not assembled. The green curve corresponds to k-mers shared by both haplotypes, and the red and blue curves show k-mers found only in one of the haplotypes.
BUSCO analysis using the cetartiodactyla_odb10 reference set ( n = 13 335) identified 95.6% of the expected gene set (single = 92.7%, duplicated = 2.9%) for haplotype 1. The snail plot in Figure 5 summarises the scaffold length distribution and other assembly statistics for haplotype 1. The blob plot in Figure 6 shows the distribution of scaffolds by GC proportion and coverage for haplotype 1.
Assembly metrics for mBalPhy2.hap1.1.The BlobToolKit snail plot provides an overview of assembly metrics and BUSCO gene completeness. The circumference represents the length of the whole genome sequence, and the main plot is divided into 1 000 bins around the circumference. The outermost blue tracks display the distribution of GC, AT, and N percentages across the bins. Scaffolds are arranged clockwise from longest to shortest and are depicted in dark grey. The longest scaffold is indicated by the red arc, and the deeper orange and pale orange arcs represent the N50 and N90 lengths. A light grey spiral at the centre shows the cumulative scaffold count on a logarithmic scale. A summary of complete, fragmented, duplicated, and missing BUSCO genes in the set is presented at the top right. An interactive version of this figure can be accessed on the BlobToolKit viewer.
BlobToolKit GC-coverage plot for mBalPhy2.hap1.1.Blob plot showing sequence coverage (vertical axis) and GC content (horizontal axis). The circles represent scaffolds, with the size proportional to scaffold length and the colour representing phylum membership. The histograms along the axes display the total length of sequences distributed across different levels of coverage and GC content. An interactive version of this figure is available on the BlobToolKit viewer.
Table 4 lists the assembly metric benchmarks adapted from Rhie et al. (2021) and the Earth BioGenome Project Report on Assembly Standards September 2024. The EBP metric, calculated for the haplotype 1, is 6.8.Q65.
Table 4.: Earth Biogenome Project summary metrics for the Balaenoptera physalus assembly.
Wellcome Sanger Institute – Legal and Governance
The materials that have contributed to this genome note have been supplied by a Darwin Tree of Life Partner. The submission of materials by a Darwin Tree of Life Partner is subject to the ‘Darwin Tree of Life Project Sampling Code of Practice’, which can be found in full on the Darwin Tree of Life website. By agreeing with and signing up to the Sampling Code of Practice, the Darwin Tree of Life Partner agrees they will meet the legal and ethical requirements and standards set out within this document in respect of all samples acquired for, and supplied to, the Darwin Tree of Life Project. Further, the Wellcome Sanger Institute employs a process whereby due diligence is carried out proportionate to the nature of the materials themselves, and the circumstances under which they have been/are to be collected and provided for use. The purpose of this is to address and mitigate any potential legal and/or ethical implications of receipt and use of the materials as part of the research project, and to ensure that in doing so we align with best practice wherever possible. The overarching areas of consideration are:
Ethical review of provenance and sourcing of the materialLegality of collection, transfer and use (national and international)
Each transfer of samples is further undertaken according to a Research Collaboration Agreement or Material Transfer Agreement entered into by the Darwin Tree of Life Partner, Genome Research Limited (operating as the Wellcome Sanger Institute), and in some circumstances, other Darwin Tree of Life collaborators.
The reference list from the paper itself. Each links out to its DOI / PubMed record.
- 1Aguilar A García-Vernet R : Fin whale: Balaenoptera physalus (Linnaeus, 1758).In: B. Würsig, J. G. M. Thewissen, and K. M. Kovacs (eds), Encyclopedia of Marine Mammals.San Diego: Academic Press,2018;368–71.
- 2Allio R Schomaker-Bastos A Romiguier J : Mito Finder: efficient automated large-scale extraction of mitogenomic data in target enrichment phylogenomics. Mol Ecol Resour. 2020;20(4):892–905. 10.1111/1755-0998.13160 32243090 PMC 7497042 · doi ↗ · pubmed ↗
- 3Altschul SF Gish W Miller W : Basic Local Alignment Search Tool. J Mol Biol. 1990;215(3):403–410. 10.1016/S 0022-2836(05)80360-2 2231712 · doi ↗ · pubmed ↗
- 4Archer FI Brownell RL Jr Hancock-Hanser BL : Revision of fin whale Balaenoptera physalus (Linnaeus, 1758) subspecies using genetics. J Mammal. 2019;100(5):1653–70. 10.1093/jmammal/gyz 121 · doi ↗
- 5Bateman A Martin MJ Orchard S : Uni Prot: the Universal Protein Knowledgebase in 2023. Nucleic Acids Res. 2023;51(D 1):D 523–D 531. 10.1093/nar/gkac 1052 36408920 PMC 9825514 · doi ↗ · pubmed ↗
- 6Buchfink B Reuter K Drost HG : Sensitive protein alignments at Tree-of-Life scale using DIAMOND. Nat Methods. 2021;18(4):366–368. 10.1038/s 41592-021-01101-x 33828273 PMC 8026399 · doi ↗ · pubmed ↗
- 7Challis R Richards E Rajan J : Blob Tool Kit – interactive quality assessment of genome assemblies. G 3 (Bethesda). 2020;10(4):1361–1374. 10.1534/g 3.119.400908 32071071 PMC 7144090 · doi ↗ · pubmed ↗
- 8Cheng H Concepcion GT Feng X : Haplotype-resolved de novo assembly using phased assembly graphs with hifiasm. Nat Methods. 2021;18(2):170–175. 10.1038/s 41592-020-01056-5 33526886 PMC 7961889 · doi ↗ · pubmed ↗