The adult musculature in Botryllus schlosseri: differentiation and gene expression

Typically, ascidians have three types of muscles: skeletal in the larva, cardiac and smooth in the post-metamorphic sessile organism. The larval and cardiac muscles are striated, with vertebrate-like arrangement of myofilaments. Instead, the smooth body-wall musculature has intermediate characters between smooth and striated muscle of vertebrates. We studied the musculature in the blastozooids of Botryllus schlosseri analysing its organisation, differentiation and gene expression. We isolated and characterised two transctipts resulted homologous to muscle genes of other adult ascidians: a muscle-type actin (BsMA2) and troponin T (BsTnT-a); moreover, we obtained also the genomic sequence coding for BsMA2. Phylogenetic analyses showed a close relationship between urochordates and vertebrates muscle genes. The BsMA2 genomic sequence was compared in the exon-intron organization with other muscle and cytoplasmic–type actin genes of both invertebrates and vertebrates. Our analysis showed that intron positions are conserved in ascidian and in the other deuterostomes. We detected the expression of the two genes by in situ hybridization (ISH), in order to follow the muscle development throughout the blastogenetic cycle of B. schlosseri. The ISH, in parallel with phalloidin staining experiments, showed that the first diffuse signal of muscle differentiation appears in the intersiphonal epidermis of young buds. Then, the muscle fibers differentiate in the body-wall, while an intense expression of BsMA2 marks the heart myocardium just when it begins contractions. Moreover, the ultrastructure of smooth muscle cells was investigated during differentiation from mesenchymal cells in the bud, in the adult, and in the fibers contraction during regression of zooids.