Fig. 5

Proposed model showing a modular nature of muscle development. a Proposed model for Botryllus schlosseri muscle development. Three myogenic regions are depicted: the body wall muscles are derived from mesenchymal cells that evaginated from the dorsal tube, the intersiphonal muscles from the posterior branchial chamber, and the heart formed by either evagination or clustering of mesenchymal cells in the ventrolateral region. The myocardium is formed by invagination; hence, the two-layered tubular heart of the adult is put in place. b Comparison of the cardiopharyngeal muscle network in chordate development. The expression of genes has been assayed in multiple vertebrates [3, 64,65,66,67], Ciona intestinalis [31, 40] and Botryllus schlosseri. A number of similarities characterize the inductive signals specifying the cardiopharyngeal field in vertebrate and ascidian embryogenesis, while in blastogenesis a common origin of body wall muscles and heart muscles is unlikely. The cardiac lineage expresses Nk2.5 and Gata4 in the forming vertebrate hearts as well as their orthologs in both ascidian species. For what concern Mef2 expression no information is currently available in ascidians [68]. An ortholog to vertebrate Mef2 is present in the transcriptomic dataset of blastogenesis. Pharyngeal muscle starts differentiation by activating the paralogues Myf5 and MyoD; only one ortholog, Mrf, is found in ascidians. To activate such in ascidians, Ebf is expressed in the body muscle lineage. Two myosin heavy chain forms characterize the vertebrate heart. Pharyngeal muscles do not express the same isoforms in all vertebrates: ruminants and rodents express the same myosins as in trunk and limb, namely, Myh1 and Myh2; in other animals Myh6 and Myh16 can be expressed in addition. In ascidians, Myh2 is expressed in the heart, and Myh3 in the body wall. Bold text indicates conserved expression within at least two species, gray only transcriptomic data, asterisk only in some species