The Role of Drosophila taranis in Polycomb Mediated Transcriptional Silencing and in Maintenance of the Germline Stem Cell Niche

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Biomedical Genetics, 2012.The Polycomb group (PcGs) proteins are implicated in epigenetic transcriptional repression during development, stem cell maintenance and tumorigenesis. The molecular mechanism by which PcGs silence target loci is not fully understood. Here we show that Drosophila taranis (tara) is required for positioning Pc to its target genes. Embryos lacking tara exhibit partial homeotic transformation in the cuticular segments, a phenotype associated with Pc mutants. Consistent with the homeotic transformation, tara loss of function results in misexpression of homeotic gene Ultrabithorax (Ubx) and in reduced Pc recruitment on polytene chromosomes. Moreover, Drosophila embryos lacking taranis misexpress engrailed and lose Pc binding to its Polycomb response element (PRE). These observations suggest that Tara might be involved in transcriptional regulation of Pc target genes. Mass spectrometry revealed that Tara physically interacts with Heat shock cognate70-4 (hsc4), an enhancer of Pc. We further found that taranis genetically interact with hsc4. This interaction affects the expression pattern of homeotic genes, Ubx and Abdominal-B (Abd-B) in the developing embryo. tara and hsc4 double mutant embryos show pronounced homeotic transformation similar to Pc loss of function. Our study shows taranis as an important regulator of PRE activity in the transcriptional silencing mechanism carried out by PcGs. Maintenance of tissue homeostasis requires a functional stem cell environment (a.k.a niche). The Drosophila testis provides for an excellent model for studying the stem cell niche, which contains germ line stem cells along with the somatic cyst cells and the hub cells. Here we show that taranis is required for maintaining testis niche. tara is expressed in the Drosophila testis tip. Knocking down taranis in the niche results in a loss of germline stem cells. We show that the reduction in stem cells might involve Notch signaling in the niche. Interestingly we found that tara is only required in somatic cells for their proliferation. Thus, the results suggest that taranis functions in somatic cells in the niche to regulate germ line proliferation and maintenance