Maternal PIWI proteins are essential for embryonic mitosis and chromatin integrity. Sneha Mani¹, Heather Megosh², Haifan Lin¹, *First and second authors equally contributed to this work. 1) Cell Biology, Yale University, New Haven, CT; 2) Cell Biology, Duke University, Durham, NC.

   PIWI proteins in Drosophila have been implicated in transcriptional and posttranscriptional gene silencing mediated by small non-coding RNAs. Although these proteins are known to be required for germline development, their somatic function remains elusive. Here, we examine the role of maternal Piwi, Aub and Ago3 during early embryogenesisthe first phase of somatic development.
    In syncytial embryos, Piwi has a dynamic localization pattern that is embryonic stage-dependent; most of Piwi is localized in the cytoplasm during mitotic cycles 1-13 after which it is moves into the nucleus. Aub and Ago3 are diffusely cytoplasmic till Stage 9, after which they localize more obviously to the perinuclear region. Embryos depleted of any one of the three maternal PIWI proteins display various severe mitotic defects including abnormal nuclear morphology, cell cycle arrest, asynchronous nuclear divisions and aberrant nuclear migration. A more thorough examination of early embryonic cell divisions reveals roles for all three PIWI proteins in the assembly of mitotic machinery and in the regulation of progression through mitosis. Additionally, embryos depleted of maternal PIWI exhibit various deficiencies in markers of chromatin organization.
    These observations suggest that maternal Piwi, Aub and Ago3 play a critical role in the maintenance of chromatin structure and cell cycle progression during embryogenesis, with compromised chromatin integrity as a possible cause of the observed cell cycle defects. Our study demonstrates the essential function of PIWI proteins in somatic development.