Localization of mitochondrial large ribosomal RNA in the myoplasm of the early ascidian embryo

Mitochondrial large ribosomal RNA (mtlrRNA) is transferred out of mitochond ria and associates with germinal granules in Drosophila and Xenopus embryos . It has been revealed that mtlrRNA outside of mitochondria is required for formation of the germ-line progenitor, or pole cells in Drosophila. In the present study, the distribution of mtlrRNA was examined in embryos of the ascidian, Halocynthia roretzi, during cleavage stages by wholemount in situ hybridization. Until the 4-cell stage, the distribution of mtlrRNA coincid ed with that of mitochondria, which are localized to the cortical cytoplasm in the posterior region of the embryos. Both mitochondria and mtlrRNA were preferentially partitioned into muscle-lineage blastomeres during cleavage stages; After the 8-cell stage, a discrepancy in intracellular localizatio n of mitochondria and mtlrRNA became evident. Mitochondria translocated int o central yolkless cytoplasm, while mtlrRNA remained in the posterior corte x in the posterior muscle-lineage blastomeres. The significance of the cort ical localization of mtlrRNA in muscle precursor cells in ascidian embryos is obscure. However, the results suggest that mtlrRNA is also transferred o ut of mitochondria in early ascidian embryos and may play some roles in dev elopmental processes.