Phylogeny of myosin heavy chain isoforms. Tree constructed using the minimum evolution method and bootstrapped with 1,000 repetitions. Using well-characterized myosin isoforms and routing the tree using Drosophila myosin heavy chain, it is clear that both the slow and fast isoforms identified (Danio smyhc1 and Danio myhc4) cluster with previously described slow and fast isoforms, respectively. It is also clear that fish and amniote isoforms cluster together, demonstrating that the origin of slow and fast isoforms occurred before the divergence of Sarcopterygians and Tetrapods.

The expression of slow and fast myosin heavy chains in zebrafish. a,b,d-f: Expression of smyhc1. g-l: Expression of myhc4. All views are dorsal. a: Expression is clearly visible in the adaxial cells of the formed somites and also of the presomitic mesoderm at both seven-somite stages. b-d: Single confocal sections at the 10-somite stage showing smyhc1 expression (b) and expression of F59 (c) demonstrating colocalization in the adaxial cells (d). e: Adaxial expression is maintained at the 13-somite stage. f: At the 28-somite stage expression is seen in the lateral slow muscle postmigration and in the most posterior somites in the adaxial and migrating slow muscle. g: Expression of myhc4 in the adaxial cells and presomitic mesoderm at the seven-somite stage. h-j: Single confocal images of myhc4 expression (h) together with F59 (i) demonstrating adaxial cell localization (j). k: Expression is maintained in the adaxial cells at the 13-somite stage. l: At 17-somites just after migration of the slow muscle initiates, the expression domain expands to include the fast muscle.

Cross-sections of slow and fast myosin heavy chain expression. a,f: Expression of smyhc1(a) and myhc4 (f) is seen in the adaxial cells at the 10-somite stage. b-d,g-i: Single confocal images of fast red heavy chain in situs and F59 antibody labeling at the 26-somite stage. b-d: smyhc1 expression is restricted to the slow muscle as demonstrated by smyhc1 in situ (b) and F59 antibody labeling (c) overlapping (d). e: A 28-somite embryo at the level of the yolk extension showing expression of smyhc1 in the single layer of superficial slow muscle (arrowheads) and the medial muscle pioneer cells (arrows). g-i: The restriction of myhc4 to the fast muscle after slow muscle migration is demonstrated by the nonoverlapping patterns of myhc4 (g) with F59 (h) seen in i. j: Expression of myhc4 is visible throughout the fast muscle but not in the single layer of superficial slow muscle (arrowheads) or the muscle pioneer cells (arrows).