FAILURE OF MUSCLE MYOSIN HEAVY-CHAIN GENE-EXPRESSION IN QUARTER ASCIDIAN EMBRYOS DEVELOPED FROM THE SECONDARY MUSCLE LINEAGE CELLS

Muscle cells of the ascidian tadpole larva originate from two different lineages, the primary (B4.1 line) and secondary (A4.1 and b4.2 lines) lineages. Experiments with 8-cell embryos have indicated that isolated blastomeres of the primary lineage show autonomous muscle development, whereas blastomeres of the secondary lineage rarely develop the differentiation markers (muscle-specific antigens and specific enzyme activity) in isolation. However, there is the possibility that A4.1 and b4.2 quarter embryos might express a muscle-specific gene but the transcripts might not be translated into proteins, thus we would not be able to detect the muscle differentiation. In order to examine the possibility, four blastomere-pairs (a4.2, b4.2, A4.1, and B4.1 pairs), isolated from the 8-cell embryo of Halocynthia roretzi, were allowed to develop into quarter embryos, and the occurrence of transcripts of myosin heavy-chain gene was determined by in situ hybridization of whole-mount specimens. The transcripts were evident only in B4.1 quarter embryos and not in A4.1, b4.2 and a4.2 quarter embryos. Thus, the proportion of A4.1 and b4.2 quarter embryos that develop muscle cells does not increase even when examined at the transcriptional level.