Asymmetrical distribution of mitochondrial rRNA into small micromeres of sea urchin embryos

Blastomeres of the 16-cell stage embryos of the sea urchin, Hemicentrotus p ulcherrimus, were separated by an elutriator. By differential display, seve ral RNA species that are enriched in micromeres are detected and their cDNA was cloned. One of the cloned cDNA encodes mt 12S rRNA. cDNA for mt 16S rR NA was also cloned from the cDNA library of unfertilized eggs. Two mt rRNAs contain poly(A) tails in their 3' ends. Both mt rRNAs distribute asymmetri cally along a vegetal-animal axis of the 16-cell embryos and are enriched i n micromeres, and this is also confirmed by whole mount in situ hybridizati on as well as electron microscopic in situ hybridization. As development pr oceeds, these mt rRNAs become more enriched in small micromeres. Results of electron microscopical in situ hybridization reveal both mt rRNAs localize extramitochondrially. Though at present we have no evidence on the role of the extramitochondrial mt rRNAs in sea urchin development, it is speculate d considering roles of extramitochondrial mt 16S rRNA in Drosophila develop ment that extramitochondrial mt rRNA may be implicated in development of se a urchin embryos.