Masu salmon species complex relationships and sex chromosomes revealed from analyses of the masu salmon (Oncorhynchus masou masou) genome assembly.

Masu salmon (Oncorhynchus masou) are the only Pacific salmon endemic to Asia. Some researchers prefer to categorize these salmon into 4 subspecies (masu-Oncorhynchus masou masou, amago-Oncorhynchus masou ishikawae, Biwa-Oncorhynchus masou subsp., and Formosan-Oncorhynchus masou formosanus), while others prefer individual species designations. Even though the masu salmon fishery is thousands of years old, classification of the diversity within the masu salmon species complex remains elusive. In this study, a genetic map and reference genome assembly were generated for 1 species/subspecies (masu) to provide resources for understanding the species complex. In O. m. masou, the sex chromosome was determined to be chromosome 7. Resequenced genomes from 2 other putative subspecies (amago and Biwa) provided evidence that they do not share the same sex chromosome. Principal component and admixture analyses clustered the amago and Biwa salmon close together. This supported previous findings of a close relationship between amago and Biwa salmon and a more distant relationship to masu salmon for both. Additional analyses of the masu salmon species complex will benefit from using the new reference genome assembly.