Risso's dolphin (Grampus griseus Cuvier, 1812) is the only species of genus Grampus and a cosmopolitan marine inhabitant. Here, we report a polymorphic complete mitochondrial genome of G. griseus. The size of the total mitochondrial genome was 16,386 bp in length and contains 13 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and a control region. 37 single nucleotide polymorphic sites (SNPs) were identified compared to the references. Based on the available total mitochondrial dolphin genomes' phylogenetics, G. griseus has formed a clade with 0.1415 distance, sister to the following species of the subfamily Globicephalinae and the taxonomy of Orcinus orca still needs further investigations.

Aquatic habitats are home to large animals such as marine mammals. Toothed whales have special fat deposits in the forehead region (called the melon) of their heads that are associated with echolocation underwater. This fat is also important industrially for human use. Due to the lack of gene expression information on the melon fat of toothed whales, we investigated the melon morphology via the transcriptomic approach. Four parts of the melons of three individual Risso’s dolphins were used for total RNA extraction, cDNA library preparation, and sequencing via next-generation sequencing (NGS) technologies. After the downstream analysis of raw sequence data, we determined that the outer layer of the melon’s ML4 region played multifunctional roles. The 36 differentially expressed genes of outer melon included ASB5, MYH13, MYOM2, and MYOM3. These genes are associated with muscle function and energy metabolism. Gene clustering and functional enrichment analyses also represented enrichments, such as the pentose phosphate pathway and morphogenesis related to lipid metabolism and muscle functions. This study will be crucial for muscle and fat functional-related molecular studies on aquatic mammals. Additionally, the study presents potential pathways, such as melon fat biosynthesis, for sustainable future developments.

The Grampus griseus (Risso's dolphin) is a toothed whale species in the order Cetacea that exhibits cosmopolitanism (Gaspari et al., 2007;Gaspari & Natoli, 2012). Grampus griseu is the only species in the genus Grampus, the fifth-largest member of the family Delphinidae (Baird, 2009), and part of the subfamily Globicephalinae. Species in this subfamily could be cryptic species by evolutionary imperative (Thompson et al., 2013). Even within the subfamily Globicephalinae, G. griseus display a 0.022 distance from other species in the

The Risso's dolphin (Grampus griseus) is one of the migratory marine mammals and they have commonly dispersed in tropical and temperate seas. It is a least concerned species in the IUCN red list of threatened species. However, their population size and factors affecting their population structure are unknown. Due to the wide distribution of this species, their populations might be genetically stable and less structured. To support genetic studies on dolphins and other marine mammals, we assembled the draft genome of Risso's dolphin that was found in Japan. The tissue samples were used to extract high molecular DNA and subjected to sequencing by Illumina HiSeq X, Oxford Nanopore MinION, and Bionano Saphyr. The assembled hybrid genome was 75.9% of complete eukaryotic BUSCOs and the genome size was 2.256 Gb with 2.042 Mb of scaffold N50. De novo assembly of this genome by Bionano Saphyr recovered 2.036 Gb total genome map length and structural variations. The gene structures of this draft genome were identified by BRAKER2, and 9947 genes were recovered. The data will be useful for future studies of cetaceans.

Jaw fats play a key role in echolocation in toothed whales. These fats are located along the outer and inner segments of the lower jawbone. A ribose nucleic acid (RNA) sequencing technique was employed to investigate transcriptomes of these two types of jaw fat tissues in Risso’s dolphins. We identified 1,899 upregulated common genes in both fat tissues. The differentially-expressed genes (DEGs) analysis showed that 34 and nine known genes were significantly upregulated in outer and inner jaw fats, respectively. A functional enrichment analysis was conducted by Enricher; lipid metabolism-related gene ontologies (GO) and pathways were identified (p<0.05). Based on these analyses, APOH, HNF4A, MYF6, SLC1A2, SLC2A2 and ALDOB were key genes for lipid metabolism in the outer jaw fat which are mainly involved with lipoprotein lipase activities. However, APP, DHX9, PXMP4 and THBS4 genes were highly expressed in the inner jaw fat, and their main functional enrichments were amyloid-beta formation and the activation of ECM-receptor interaction. These recent findings provide evidence for de novo lipid synthesis and as a new concept, the APP may be involved with transferring sound wave signals from the inner jaw fat to the brain via neurons, and further studies are necessary for revealing the puzzle of echolocation in toothed whales.