As the deepest vertebrate in the ocean, the hadal snailfish (Pseudoliparis swirei), which lives at a depth of 6,000–8,000 m, is a representative case for studying adaptation to extreme environments. Despite some preliminary studies on this species in recent years, including their loss of pigmentation, visual and skeletal calcification genes, and the role of trimethylamine N-oxide in adaptation to high-hydrostatic pressure, it is still unknown how they evolved and why they are among the few vertebrate species that have successfully adapted to the deep-sea environment. Using genomic data from different trenches, we found that the hadal snailfish may have entered and fully adapted to such extreme environments only in the last few million years. Meanwhile, phylogenetic relationships show that they spread into different trenches in the Pacific Ocean within a million years. Comparative genomic analysis has also revealed that the genes associated with perception, circadian rhythms, and metabolism have been extensively modified in the hadal snailfish to adapt to its unique environment. More importantly, the tandem duplication of a gene encoding ferritin significantly increased their tolerance to reactive oxygen species, which may be one of the important factors in their adaptation to high-hydrostatic pressure.

Pelagic cephalopods have evolved a series of fascinating traits, such as excellent visual acuity, high-speed agility, and photophores for adaptation to open pelagic oceans. However, the genetic mechanisms underpinning these traits are not well understood. Thus, in this study, we obtained high-quality genomes of two purpleback flying squid species (Sthenoteuthis oualaniensis and Sthenoteuthis sp.), with sizes of 5450 Mb and 5651 Mb, respectively. Comparative genomic analyses revealed that the S-crystallin subfamily SL20-1 associated with visual acuity in the purpleback flying squid lineage was significantly expanded, and the evolution of high-speed agility for the species was accompanied by significant positive selection pressure on genes related to energy metabolism. These molecular signals might have contributed to the evolution of their adaptative predatory and anti-predatory traits. In addition, the transcriptomic analysis provided clear indications of the evolution of the photophores of purpleback flying squids, especially the recruitment of new genes and energy metabolism-related genes which may have played key functional roles in the process.

The gold barb (Barbodes semifasciolatus), a member of the Cyprinidae family, exhibits remarkable adaptability to highly acidic environments, making it an ideal model for studying extreme environmental adaptation. However, its genome has not been previously characterized. To address this, we assembled a high-quality chromosome-scale genome for B. semifasciolatus using High-Fidelity (HiFi) sequencing and Hi-C technology. The resulting haplotype-resolved assemblies, spanning 776 Mb and 779 Mb across 25 chromosomes, achieved genome coverages of 99.5% and 99.7%, respectively, and included four gap-free chromosomes. Genome quality assessment using BUSCO indicated a high completeness score of 98.2% for haplotype1 and 98.3% for haplotype2, further validated by strong synteny with the zebrafish (Danio rerio), confirming the assembly’s integrity and continuity. Through integration of full-length transcriptome data, RNA sequencing, and homology-based annotation, we identified 26,057 protein-coding genes with 2,087 pseudogenes in haplotype 2, and 25,622 protein-coding genes with 2,101 pseudogenes in haplotype 1. This high-resolution genome assembly is a crucial resource for advancing research in the Cyprinidae, particularly for understanding adaptive evolution in extreme environments.

Whole-genome duplication is a major evolutionary force, with salmonids providing a key model system. In this study, we generated a high-quality masu salmon genome using PacBio HiFi, ONT ultra-long and Hi-C sequencing technologies. The final phased assembly includes two haplotypes: HAP1 (2.5 Gb) and HAP2 (2.4 Gb). The contiguity of the assembly is excellent, with scaffold N50 values of 99.28 Mb (HAP1) and 98.82 Mb (HAP2). Notably, the 33 largest scaffolds in each haplotype cover over 99.6% of the genome, approaching chromosome-level scale. The genomic completeness was assessed at 98.3% (HAP1) and 97.7% (HAP2) via BUSCO analysis. Finally, we identified 41,858 and 40,970 protein-coding genes for HAP1 and HAP2, respectively. Furthermore, to enable research into reproductive strategies, we generated 90 transcriptome datasets from the semelparous masu salmon across key post-spawning time points, alongside a comparative dataset of 60 transcriptomes from the iteroparous rainbow trout. Together, these high-quality genomic and transcriptomic resources offer a powerful platform for investigating both post-WGD genome evolution and the molecular basis of semelparity in salmonids.