De novo Assembly and Annotation of the Blood Transcriptome of the Southern Elephant Seal Mirounga leonina from the South Shetland Islands, Antarctica

Abstract

The southern elephant seal Mirounga leonina is the largest member of the phocid seals and is a highly sexually dimorphic predator at circumpolar regions. In this study, we generated the first high-quality de novo assembly of the blood transcriptome of M. leonina using the Illumina MiSeq platform. A total of 40.5 million raw reads were obtained and assembled using the Oases assembly platform, resulting in 46,445 contigs with an N50 value of 634 bp. We performed functional gene annotations through pathway analyses of the Gene Ontology (GO), Eukaryotic Orthologous Groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Ortholog comparison revealed that a high proportion of the M. leonina blood transcriptome had significant sequence homology within pinnipeds. M. leonine, a deep and long-diving seal, is routinely exposed to progressive hypoxia during dives. KEGG analyses detected an intact hypoxia-inducible factor-1 (HIF-1) signaling pathway, which is a key metabolism pathway in the adaptive homeostatic responses to hypoxia. In the blood transcriptome, many essential genes involved in innate immunity were detected, which suggests that the blood serves as a host defense modulator against exogenous infections in M. leonina. This genomic resource will be useful for understanding adaptive metabolism upon repeated breath-hold dives and determining the health status of southern elephant seals without the need to sacrifice them in experiments.