Localization of mitochondrial large rRNA in germinal granules and the consequent segregation of germ line.

This article reviews recent studies on cytoplasmic factors for germ-line establishment in Drosophila and Xenopus. In a variety of animal groups, the cytoplasmic factors for germ-line differentiation have been postulated to be localized in the germ plasm. We have found that mitochondrial large rRNA (mtlrRNA) is present in germinal granules, the distinctive morphological markers of germ plasm in the fruit-fly and the frog. MtlrRNA has been identified as a cytoplasmic factor that induce pole cells, the progenitor of the germ line, in uv-sterilized Drosophila embryos. The developmental stages of Xenopus at which mtlrRNA is present in germinal granules correspond to the stages of germ line segregation. Based on our findings and available data on posterior class gene products as well as on classical developmental biology of germ cells, we discuss the probable role of mtlrRNA in germ-line segregation from the somatic line in these animals.