Diversity of Lipid Distribution in Fish Skeletal Muscle.

Adipose tissue is a lipid storage organ characterized by the pronounced accumulation of adipocytes. Although adipose tissues are found in various parts of the vertebrate body, it is unclear whether these tissues have a common ancestral origin or have evolved in several phylogenetic lineages by independent adipocyte accumulation events. To gain insight into the evolutionary history of vertebrate adipose tissues, we determined the distribution of adipocytes by oil red O staining in skeletal muscle of 10 teleost species spanning eight orders: Tetraodontiformes, Pleuronectiformes, Spariformes, Salmoniformes, Clupeiformes, Beloniformes, Osmeriformes, and Cypriniformes. Accumulation of adipocytes in the myoseptum was observed in many species, including red sea-bream, rainbow trout, Pacific herring, Pacific saury, zebrafish and giant danio. We also found some order-, species-, and swimming mode-specific distribution patterns of adipocytes: 1) almost complete absence of intramuscular adipocytes in the order Tetraodontiformes (torafugu and spotted green pufferfish), 2) clear adipocyte accumulation in the inclinator muscles of fin in Japanese flounder, 3) a large intramuscular adipose tissue at the root of the dorsal fin in ayu, and 4) thick lipid layers consisting of subcutaneous adipose tissue and red muscle lipids in pelagic migratory fish (Pacific herring and Pacific saury). Of note, Pacific herring and Pacific saury are phylogenetically distinct species sharing a similar niche and swimming mode, suggesting that their analogous adipocyte/lipid distribution patterns are the consequence of convergent evolution. The potentially heterogeneous origin of adipose tissues has significant implications for the interpretation of their functional diversity.