Specific expression of myosin heavy chain gene in muscle lineage cells of the ascidian embryo.

In ascidians the lineage of embryonic precursor cells is well documented. In this study, we investigated temporal and spatial expression of myosin heavy chain (MHC) gene during embryogenesis of the ascidianHalocynthia roretzi. When the occurrence of MHC transcripts was examined by Northern blot hybridization and in situ hybridization with specific cDNA and antisense RNA probes, transcripts were undetectable in fertilized eggs and cleavage-stage embryos. This suggests that the maternal message for MHC is not directly associated with the synthesis of MHC protein in ascidian embryos. MHC transcripts were initially observed at the gastrula stage. In situ hybridization of whole-mount preparations demonstrated that the transcripts first appeared in nuclei of primary-lineage muscle cells of the early-to-middle gastrula and accumulated rapidly as development progressed. The occurrence of MHC transcripts was restricted to differentiating muscle cells. No hybridization signal was detected in mesenchyme cells which are thought to form adult body-wall muscle. After metamorphosis the amount of MHC transcripts decreased; they became undetectable in juveniles about 10 days after metamorphosis, suggesting an intermission of MHC gene activity prior to the formation of adult bodywall muscle. Thus the expression of MHC gene in ascidian embryos was strictly regulated in both spatial and temporal orders and occurred only in muscle lineage cells.