P granules in the germ cells of Caenorhabditis elegans adults are associated with clusters of nuclear pores and contain RNA

The germ cells, and germ cell precursors, in the nematode Caenorhabditis el egans contain distinctive granules called P granules. During early embryoge nesis, P granules are segregated asymmetrically into those blastomeres that eventually produce the germ line. Because of the correlation between P gra nule distribution and the development of the germ line, P granules are wide ly thought to function in some aspect of germ line specification or differe ntiation. Most of the analysis of P granule structure and localization has focused on the early embryo, when P granules are located in the cytoplasm. However, during most of development P granules are associated with germ cel l nuclei. We report here an ultrastructural analysis of the nuclear-associa ted P granules in the germ cells of the adult hermaphrodite gonad. We show that P granules are tightly associated with nuclear pores and that the posi tions of certain structures within the P granules correspond to the positio ns of pores on the nuclear envelope. We present immunocytochemical and ultr astructural data suggesting that P granules can associate, or remain associ ated, with pore-like structures even after they detach from the nuclear env elope during oogenesis. Finally, we show that nuclear-associated P granules in the gonad contain RNA, complementing previous studies showing that cyto plasmic P granules in embryos contain RNA. (C) 2000 Academic Press.