Revisiting Bivalve Mollusk Myogenesis: New Immunocytochemical Evidence from Magallana gigas

This study investigated the formation of the contractile apparatus in bivalve mollusk larvae. Elucidation of the molecular aspects of larval development in bivalves is significantly complicated by the lack of antibodies that can selectively label proteins and their structures. To address this issue, polyclonal antibodies were produced against main proteins of bivalve muscles. Immunohistochemistry was used to obtain high-quality confocal images of muscles in bivalve larvae to describe the inclusion of main muscle proteins in the contractile apparatus during the development of the oyster Magallana gigas. The appearance of these proteins in muscle cells was not uniform and, surprisingly, the leading protein in the formation of muscle structures, including striations, was twitchin. The findings obtained support the hypothesis that this protein, similar to vertebrate titin (a protein from the same family), may play an essential role in the assembly of the contractile apparatus. Like another protein detected during this study, myorod, twitchin is believed to be involved in the formation of the catch state (an energy-efficient state of molluskan smooth muscles). The strong evidence for the presence of these proteins within the striated muscles in the larval stage of bivalves raises questions about the origin of this important evolutionary adaptation.