Dynamic protein assembly and architecture of the large solitary membraneless organelle during germline development in the wasp Nasonia vitripennis

Abstract

Germ cells in different animals assemble characteristic membraneless organelles referred to as germ granules, which contain RNA and proteins required for germline development. Typically, the germ granules are small spherical or amorphous cytoplasmic granules and often, they assemble around membrane-bound organelles such as nuclei, mitochondria and endoplasmic reticulum. In particular, in egg chambers of the fruit fly Drosophila, nurse cells assemble perinuclear granules, referred to as nuage, along with multiple small germ granules formed at the posterior pole of the oocyte (polar granules). Nuage is assembled in a very similar way in the wasp Nasonia vitripennis, despite the long evolutionary distance from Drosophila. In contrast, Nasonia forms a very different single germ granule, called the oosome, at the posterior, which is about 40 times larger than a homologous Drosophila polar granule. Here, using molecular and super-resolution imaging approaches, we provide insights into protein assembly and architecture of the oosome during germline development. Interestingly, unlike the fly, the wasp utilizes alternatively spliced RNA-helicase Vasa isoforms during germline development and oosome formation. The isoforms differ by an unstructured region, containing repeats of phenylalanine and glycine, that is similar to functional domains characteristic of nucleoporins. In addition, while other conserved components of germ granules, such as Oskar, Aubergine and Tudor proteins are recruited to the oosome, these polypeptides show a distinct and specific localization within the oosome. Of particular note, Tudor protein forms a shell encapsulating the oosome, while small Oskar/Vasa/Aubergine granules occur inside the oosome core. Also, in surprising contrast to Drosophila egg chambers, we found that a subset of the wasp nurse cells located in anterior show dramatic DNA damage and assemble higher levels of nuage than their posterior counterparts. The characteristics of two distinct nurse cell populations suggest a mechanistic link between the higher amounts of nuage assembled in anterior nurse cells and their need to silence transposable elements in the presence of double-strand DNA breaks. Our results point to the high degree of plasticity in the assembly of membraneless organelles, which adapt to specific developmental needs of different organisms, and suggest that novel molecular features of conserved proteins result in the unique architecture of the oosome in the wasp.