The DExH box helicase region of Spindle-E is necessary for retrotransposon silencing and germline development. Caryn Navarro, Kristen Ott, Tram Nyguyen. Boston University School of Medicine, Boston, MA.

   A large portion of the genome in many organisms contains transposable selfish genetic elements (TEs). TEs can self-replicate and insert into new locations, thereby causing genome instability. The tight regulation of TE transposition is critical in the germline because mutations that occur in these cells are inherited by offspring and may cause disease. A class of small RNAs, the Piwi associated RNAs (piRNAs) are responsible for suppressing the expression of TEs in the germline. Experiments in mice and Drosophila have shown that mutations that disrupt piRNA biogenesis cause elevated retrotransposon levels, defects in germline development, and sterility. To date, little is known about the molecular function of many piRNA pathway proteins. We have chosen to focus our studies on the functional characterization of the piRNA pathway protein, Spindle-E (SpnE) because it is necessary for the generation of most germline piRNAs and therefore likely plays a central role in piRNA biogenesis. SpnE contains a DExH box and a Tudor domain as well as a Zn finger motif. Through the analysis of 12 new mutant spnE alleles, we have found that the highly conserved DExH box helicase region is required for piRNA pathway function, whereas the Zn finger motif is dispensable. Similar to most piRNA mutants, in spnE DExH box mutant ovaries, retrotransposon RNA levels are elevated, Dynein aggregates form, and Aubergine levels are reduced and the protein does not localize to the nuage properly. Additionally, eggs laid by these mutant mothers have severe dorsal/ventral patterning defects. Our mutant analysis also uncovered a new role for SpnE in ovary development. In 3-5 day old adult ovaries single spectrosome containing cells overproliferate and accumulate within the germarium. These cells fail to differentiate and continue to divide leading to a germ cell tumor. This phenotype is germline autonomous and strengthens over time. Our results indicate that SpnE functions at several times during Drosophila ovary development and the DExH box helicase region is important for its function in each of these processes.