Fig. 4

Phylogenetic tree of gnathostome taxa (based on [8]) and the distribution of the five cardiac MyHC isoforms according to the two hypothesis raised in the present study. The colored boxes indicate the five MyHC isoforms found in the vertebrate heart. The colors in the diagrams indicate predominant isoforms in each cardiac segment of the nine taxa in which the trait is known, either from the present results or from previous studies. In the evolutionary scenario (a), the expression of three cardiac isomyosins (MYH6, MYH7 and MYH7B) is referred as the ancestral condition A. MYH2 expression in the inflow segments would be an acquisition of Chondrichthyes. In the alternative evolutionary scenario (b), the ancestral condition B is characterized by the expression of four cardiac isomyosin (MYH2, MYH6, MYH7 and MYH7B). In both evolutionary scenarios, MYH6 and MYH7 became the only cardiac isomyosins in osteichthyes, except for polypteriforms and amphibians. The former reacquired MYH7B expression in the ventricle, whereas the latter changed MYH7 by MYH15 in this segment. To elaborate this phylogenetic tree, data for sharks come from proteomic analysis (ESI-Quadrupole-Orbitrap, present results) whereas data for batoids, polypteriforms and osteoglossiforms come from immunohistochemistry (present results), for cypriniforms, amphibians and birds using in situ hybridization [4, 6, 17, 49, 59, 60] and for mammals come from of electrophoretic separation [34, 39]. See the text for details. A, atrium; B, bulbus arteriosus; C, conus arteriosus; SV, sinus venosus; V, ventricle