Silver chimaera genome assembly and identification of the holocephalan sex chromosome sequence

Cartilaginous fishes are divided into holocephalans and elasmobranchs, and comparative studies involving them are expected to elucidate how variable phenotypes and distinctive genomic properties were established in those ancient vertebrate lineages. To date, molecular-level studies on holocephalans have concentrated on the family Callorhinchidae, with a chromosome-scale genome assembly of Callorhinchus milii available. In this study, we focused on the most species-rich holocephalan family Chimaeridae and sequenced the genome of its member, silver chimaera (Chimaera phantasma). We report the first chromosome-scale genome assembly of the Chimaeridae, with high continuity and completeness, which exhibited a large intragenomic variation of chromosome lengths, which is correlated with intron size. This pattern is observed more widely in vertebrates and at least partly accounts for cross-species genome size variation. A male-female comparison identified a silver chimaera genomic scaffold with a double sequence depth for females, which we identify as an X chromosome fragment. This is the first DNA sequence-based evidence of a holocephalan sex chromosome, suggesting a male heterogametic sex determination system. This study, allowing the first chromosome-level comparison among holocephalan genomes, will trigger in-depth understanding of the genomic diversity among vertebrates as well as species population genetic structures based on the genome assembly of high completeness.