Myosin isoforms and their subunits in the lungfish Protopterus annectens: Changes during development and the annual cycle

Myosin isoforms and their light and heavy subunits were analyzed, in a comparison with the profile of myosin ATPase during the development and annual cycle of the African lungfish Protopterus annectens. Axial muscle of the larvae contained three isoforms of larval myosin. In adult animals, larval isomyosins were shown to coexist with small amounts of fast isomyosins in axial muscle. By contrast, fin muscles displayed essentially the fast isomyosins and a slow myosin isoform. No significant differences were noted in myosin content between swimming, cocooning, and water‐readapted animals. Larval and fast myosin isoforms were composed of two different heavy chains, HCI and HCf, respectively, in association with fast light chains. The slow myosin isoform was characterized by a distinct heavy chain, HCs, which was associated with two slow light chains. Analysis of the profile of myosin ATPase revealed only type IIC fibers in the axial muscles of larvae, while the adult muscles were composed of type IIC fibers in the central part and adult type IIA, IIB, and I fibers in the peripheral part. This study shows that P. annectens resembles the neotenic urodelan amphibian Ambystoma mexicanum at the level of the distribution of muscle fiber types and the expression of myosin isoforms.