A second genetic screen for gurken mRNA mislocalisation uncovers novel phenotypes of piRNA pathway mutants in Drosophila.

Transposon silencing is essential for germline development. In Drosophila oogenesis, DNA damage caused by transposon activation affects microtubule-dependent mRNA localisation in the oocyte and impairs embryonic axes formation. Our previous EMS mutagenesis screen for gurken mRNA mislocalisation on chromosome 3L of Drosophila melanogaster identified several piRNA pathway mutants ( Hayashi et al., 2014). Here, we report the screen for chromosome 3R. We identified ten mutation groups disrupting gurken mRNA localisation and other mutations affecting different aspects of oogenesis. We found that mutations in karyopherin-β3 affect localisation and translation of gurken mRNA in a transposon silencing-independent manner. Characterisation of the new mutation in vreteno revealed that the piRNA pathway is essential for the basal stalk development, the process of holding the ovariole and encapsulating the first egg chambers. Females transheterozygous for vreteno and armitage mutations both showed abnormal basal stalks, defective egg chamber formation and loss of germline cells. We also found that the mutation in the Zinc Finger motif of Spindle-E shows a hypomorphic transposon activation phenotype, consistent with the previous study ( Ott et al., 2014). Further characterisation showed that the Zinc Finger is required for robust ping-pong piRNA biogenesis and the nuage localisation of AGO3, but not of Aubergine, suggesting that it is involved in a specific step of ping-pong biogenesis.