Although usually recognized as a kind of ‘fish’ like actinopterygian fishes, cartilaginous fishes (chondrichthyans) form a distinct class of vertebrates with more than 1,200 species, known mostly as sharks and rays ( Figure 1; Nelson et al., 2016). This taxonomic class has the longest evolutionary history among vertebrates of about 400 million years, in terms of the divergence of extant members ( Naylor et al., 2012). Whereas its diversity might not be widely recognized, species in this taxon are characterized by several unique traits including electromagnetic sensing (all cartilaginous fishes), electricity generation (electric rays), diverse morphology sometimes with a flattened body (angelsharks and most rays) and/or a toothed rostrum (sawsharks and sawfishes). The highlight of their biological enigmas is in their reproductive modes with high plasticity between oviparity and viviparity, and occasionally parthenogenesis and intersexuality ( Penfold and Wyffels, 2019). Mainly because of overfishing, many cartilaginous fish populations are declining ( Pacoureau et al., 2021), and evidence-based resource management would greatly benefit from the establishment of genomic platforms.

Despite these outstanding evolutionary and biological importance, modern genomic approaches have only recently been applied to cartilaginous fishes (reviewed in Kuraku, 2021). The only exception is the effort commenced before 2010 on the elephant fish Callorhinchus milii ( Venkatesh et al., 2014), a member of the Holocephali (chimaeras and ratfishes), the more species-poor chondrichthyan lineage, with a relatively small genome size of about 1.9 giga basepairs (Gbp). In contrast, most elasmobranchs have genomes of more than 3 Gbp plagued with abundant repetitive elements.

The Squalomix Consortium ( Figure 2A) was launched in 2020 aiming to provide the genome sequence and other genome-wide data for chondrichthyan species including transcriptomes and epigenomes. Sample processing and data production is conducted by the Molecular Life History Laboratory at the National Institute of Genetics, Mishima, Japan, and the Laboratory for Phyloinformatics in RIKEN Kobe, Japan, which harbors a DNA Analysis Facility. The consortium is funded by academic agencies as of May 2022 and is seeking additional funding sources, especially from industrial groups oriented toward the conservation of biodiversity and marine environments. In November 2020, the Squalomix Consortium became affiliated with Earth BioGenome Project (EBP), the global initiative to promote biodiversity genomics ( Lewin et al., 2022). The collaborative network at the Squalomix Consortium includes an extensive range of expertise and worldwide distribution.

In Squalomix, sample collection is performed cautiously to minimize the sacrifice of wildlife—especially those with an endangered status. The collection focuses mainly on the rich marine fauna in Japan’s neighboring temperate waters, with occasional sources from death stranding for elusive species. The project collaborates closely with local aquariums oriented toward academic science. Their contributions play indispensable roles in relaying offshore sampling and enable sustainable sampling of embryos and blood from live individuals, although the latter approach is limited to species that can be bred in captivity and are amenable to husbandry.

Another strength of the Squalomix Consortium is its expertise in laboratory solutions that are not confined to DNA sequencing, but additionally explore post-genome approaches to decipher the molecular basis of chondrichthyan phenotypic evolution. Access to fresh tissues from local aquaria facilitates embryological analysis, genome size quantification with flow cytometry, and karyotyping from cell cultures ( Figure 3). Remarkably, cell culture in cartilaginous fishes, which was long thought difficult because of their high body fluid osmolarity, was enabled by modifying the culture medium with balancing osmolytes ( Uno et al., 2020). Our cytological expertise also allowed various epigenomic analyses that benefit from whole genome sequencing, on transcription factor binding with ChIP-seq ( Hara et al., 2018) and chromatin openness with ATAC-seq, in addition to long-range DNA interactions with Hi-C ( Kadota et al., 2020; Onimaru et al., 2021). These techniques contributed to biological analyses based on the draft genome sequences of three shark species ( Hara et al., 2018), which launched the Squalomix Consortium.

The sequencing strategy in the Squalomix Consortium is designed to accommodate genomic characteristics of cartilaginous fishes, mostly with large, repetitive genomes. In the standard protocol formulated in January 2021 ( Figure 3), we start by estimating genome size using flow cytometry and karyotyping as well as by ‘survey’ sequencing of transcriptomes, which serves for species identity verification with an assembled mitochondrial DNA sequence. These initial steps ensure sample authenticity and quality. We then proceed to genome sequencing, which employs both short-read and long-read high-fidelity (‘HiFi’) sequencing platforms, together with Hi-C data production for chromosome-scale scaffolding based on three-dimensional DNA interactions. The long-read data are obtained using the Sequel II or IIe platforms (Pacific Biosciences, Inc.) with a minimum sequencing depth of 20x. The assembly outputs are evaluated with reference to their coverage of protein-coding gene space, as well as transcriptome data, genome size, and karyotypic organization obtained separately. These validations allow us to scrutinize the inclusion of those genomic regions that are difficult to sequence and assemble, such as the Hox C genes that were previously thought to be missing in elasmobranchs but were retrieved by elaborate annotation ( Hara et al., 2018; reviewed in Kuraku, 2021). Complete genome assemblies are critical to validate gene loss and variations in gene repertoires via synteny/phylogeny comparisons, previously suggested for visual opsins and conventional olfactory receptors ( Hara et al., 2018). The standard procedure outlined above ( Figure 3) has been applied to several study species, including the red stingray Hemitrygon akajei ( Figure 2B) for which a draft genome assembly has been made available for BLAST searches at the Squalomix sequence archive ( Figure 4A; https://transcriptome.riken.jp/squalomix/).

The Squalomix Consortium aims not only to sequence and analyze the genomes but also to tightly interact with other research groups whose target species list contains cartilaginous fishes including other EBP-affiliated projects (see below). To maximize mutual benefit among those projects, some animal samples from our collection could be provided for genome sequencing at other sites. The Squalomix Consortium offers laboratory experiments for genome size quantification or karyotype analysis for species listed by other consortia, provided that fresh cells are available. The sample transfer will be processed in accordance with the Nagoya Protocol and other relevant regulations. Inclusive cooperation respecting complementary expertise is expected to overcome the long-standing difficulty in studying elasmobranchs sustainably and contribute to disentangling the marine ecosystems for effective conservation.

Once produced, genome assemblies pass rigid quality controls and are deposited in the NCBI Genome under the NCBI BioProject ID PRJNA707598 and made available as database for BLAST searches at our Squalomix sequence archive ( https://transcriptome.riken.jp/squalomix/). This archive also has a link to the up-to-date listing of the species for which genome sequences are available, filed by the GenomeSync database ( http://genomesync.org/). The archive website also hosts a gateway to genome browsers powered by JBrowse2 that allow users to visualize specific genomic regions and load additional tracks including base composition, gene models, repetitive elements, and aligned RNA-seq reads ( Figure 4C). We also provide comprehensive matrices of expression profiles for predicted genes of the brownbanded bamboo shark Chiloscyllium punctatum and the cloudy catshark Scyliorhinus torazame that were already quantified and normalized based on RNA-seq data of various tissues for our past publication ( Hara et al., 2018).

Some elasmobranch genomes have already been sequenced by other pioneering working groups ( https://www.ncbi.nlm.nih.gov/data-hub/genome/?taxon=7777&reference_only=true). This includes the Vertebrate Genomes Project (VGP), whose data production format employs a suite of modern promising solutions including optical mapping and Hi-C scaffolding as well as long-read and short-read sequencing, to cover all vertebrate species ( Rhie et al., 2021). The initial VGP progress report released the genome sequences of the thorny skate Amblyraja radiata (NCBI Genome ID, GCA_010909765.2). The Darwin Tree of Life (DToL) Project partly links with VGP and aims to sequence all eukaryotic species in Britain and Ireland. DToL’s first chondrichthyan genome is that of the small-spotted catshark Scyliorhinus canicula, the egg-laying species most widely studied in developmental biology and endocrinology (NCBI Genome ID, GCA_902713615.1). The recently launched European Reference Genome Atlas (ERGA) also plans to produce reference chromosome anchored genomes of multiple species from this geography including cartilaginous fish aiming to empower conservation efforts ( Formenti et al., 2022). Researchers in China launched the Fish10K project that partially targets cartilaginous fishes ( Fan, et al., 2020). In addition, the DNA Zoo project puts special emphasis on Hi-C scaffolding ( Rao et al., 2014), often using available genome assemblies already released by other groups as input and performing chromosome-scale genome scaffolding using Hi-C data even in the presence of intra-specific genomic variations. So far, the DNA Zoo effort produced the chromosome-scale genome assemblies of the brownbanded bamboo shark C. punctatum and the whale shark Rhincodon typus, each of which was produced using samples from multiple individuals ( Hoencamp et al., 2021). All the above efforts are expected to be coordinated under the overarching EBP initiative, in order to play complementary roles towards the global aim of generating high-quality genomic resources.

Products from this consortium are deposited in NCBI under the BioProject ID PRJNA707598 and are available at our Squalomix sequence archive ( https://transcriptome.riken.jp/squalomix/).