Expression of larval-type muscle actin-encoding genes in the ascidian Halocynthia roretzi

Temporal and spatial expression of muscle actin (MA)-encoding genes in the Halocynthia roretzi MA (HrMA) cluster was examined by whole-mount in situ hybridization and Northern blot analyses. Expression of these MA was restricted to larval muscle cells. None of the MA from the cluster was expressed in any adult tissues examined, including two different types of adult muscle tissue, body-wall muscle and heart muscle. When Northern hybridization was performed using an HrMA coding region probe under low-stringency conditions, transcripts were detected in body-wall muscle and heart muscle, but not in other adult tissues. In addition, transcripts of different lengths were detected in body-wall muscle and heart muscle. The comparison of amino-acid sequences among ascidian MA indicates that they possess at least two distinct MA isoforms, found in larval muscle and adult body-wall. These suggest that different MA are expressed in each type of ascidian muscle.