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

Abstract

Blastomeres of the 16-cell stage embryos of the sea urchin, Hemicentrotus pulcherrimus, were separated by an elutriator. By differential display, several 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 rRNA 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 asymmetrically along a vegetal-animal axis of the 16-cell embryos and are enriched in micromeres, and this is also confirmed by whole mount in situ hybridization as well as electron microscopic in situ hybridization. As development proceeds, 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 speculated considering roles of extramitochondrial mt 16S rRNA in Drosophila development that extramitochondrial mt rRNA may be implicated in development of sea urchin embryos.