Analysis of whole mitochondrial genome sequences increases phylogenetic resolution of istiophorid billfishes

The billfishes are a group of highly migratory fishes comprising two extant families. The family Xiphiidae is monotypic and contains only the broadbill swordfish (Xiphias gladius Linnaeus, 1758), while the nine-other species of billfishes constitute the family Istiophoridae. The outcomes of past phylogenetic investigations using partial nuclear and mitochondrial (mtDNA) sequences recommended a restructuring of Istiophoridae, by expanding the three existing genera into five: Makaira, Istiophorus, Kajikia, Istiompax, and Tetrapturus. However, several aspects of the Istiophoridae phylogeny remain unresolved, including the position of the genus Istiompax. In this investigation, we sequenced the whole mitogenomes for all istiophorid billfish species. Phylogenetic analyses supported the presence of five genera within Istiophoridae and resolved the placement of Istiompax as the sister taxon to Kajikia + Tetrapturus. Genetic identity between the striped marlin [Kajikia audax (Philippi, 1887)] and the white marlin [Kajikia albida (Poey, 1860)] was found to be higher than between Indo-Pacific and Atlantic clades of both the blue marlin (Makaira nigricans Lacepede, 1802) and sailfish [Istiophorus platypterus (Shaw, 1792)], suggesting recent isolation or ongoing genetic connectivity between K. audax and K. albida. An assessment of mtDNA gene regions identified NADH dehydrogenase subunit 4 (ND4) as best reflecting the phylogenetic reconstruction using the whole mitogenomes, while cytochrome b (Cytb) was the only gene region able to separate all species of billfishes in the present study. Our investigation increased the phylogenetic resolution of istiophorid billfishes and underscores the need for further investigations to fully resolve the complex genetic relationships within Tetrapturus and Kajikia.