Transverse section of the bare regions in fish (left) and frog (right) striated muscles. Myosin filaments have triangular profiles. In fish, the triangles point the same way, but in frog they have two orientations that give rise to a superlattice arrangment (described below). What is the origin of this transition in the evolution of vertebrate muscle?
A schematic view of the myosin filament lattice in fish (left, simple lattice arrangment) and frog (right, superlattice arrangement)
Distribution of superlattice and simple-lattice muscles in the vertebrates (Luther et al, 1996). Surprisingly, the superlattice form dominates in the vertebrates, from the most primitive hagfish (agnatha), through the lungfish and to the tetrapods. The simple lattice is found in only a few classes, including the bony fish, teleosts. However, the most successful vertebrate in terms of actual numbers are the teleosts.
The biggest surprise of this work was our discovery that sharks have simple lattice in their red (slow) muscle, but superlattice in their white (fast) muscle. The filament lattice has obviously important significance in muscle contractility, but as yet, we do not know what it is.