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• How female germline stem cells are maintained within stem cell niche
• How germline cysts exist germarium (molecular mechanism of encapsulation)
• Maintenance of osk complex onto posterior cortex during late oogenesis

Research Projects

Drosophila oogenesis provides an excellent model system for studying cell fate specification (oocyte selection and maintenance), cytoskeleton dynamics (oocyte cytoskeleton rearrangement), cell polarity (oocyte polarity set-up), inter-cellular transport (nurse cell to oocyte transport), cell-cell communication (oocyte and follicle cells communication), and pattern formation (anterior-posterior and dorsal-ventral patterning within the oocyte). The research interests of my lab revolve around two problems: 1) how germline stem cells maintain their identities during the early stages of oogenesis and 2) how oocyte polarity is maintained during the late stages of oogenesis.

Each Drosophila ovary contains about 15 ovarioles. Oogenesis is initiated at the tip of the ovariole where germline stem cells (GSCs) divide asymmetrically to generate one stem cell and one cystoblast which undergoes 4 synchronous divisions with incomplete cytokinesis to generate one cyst of 16 cells interconnected by ring canals. Among these 16 cells, only one will adopt oocyte fate while the others develop into nurse cells. GSCs reside in a special stem cell niche and can undergo unlimited self-renewal divisions throughout much of female life. It has been shown that both somatic signals from the niche as well as germline signals are required for maintenance of GSCs at the tip of the germarium. The molecules involved in generating somatic signals include fs(1)Yb, Piwi and bone morphogenetic proteins Decapentaplegic (Dpp) and Glass bottom boat (Gbb), while the molecules involve in generating germline signals include at least two translational repressors Nanos (Nos) and Pumillo (Pum). Bag-of-marbles (bam), a novel protein, is required for cystoblast development. Down-regulation of Dpp in somatic tissues and removal of nos, pum function from germline cells result in loss of GSCs. In contrast, over-expression of Dpp in somatic tissues as well as disruption of bam function in germline cells cause expansion of stem cells at the expense of other cells within the germarium. Recently, we identified a novel gene required for the normal development of GSCs whose loss of function yields phenotypes similar to those seen in bam mutants. How this gene functions during oogenesis is currently under investigation.

After the oocyte has been specified, the cyst buds from the germarium and undergoes maturation which is accompanied with dynamic cytoskeleton rearrangements. By stage 9, morphogenetic molecules required for oocyte patterning are strictly localized to specialised compartments and this localization will be maintained for several hours until fertilization. For example, bicoid (bcd) mRNA localizes to the anterior margin of the oocyte, while posterior patterning molecules such as oskar (osk) mRNA is localized to the posterior cortex. Recently, we have shown that there are two redundant pathways, an actin-dependent pathway mediated by Bifocal (Bif) and an actin-independent pathway mediated by Homer, are required specifically for the maintenance of the posterior group molecules in late oogenesis. To further understand how these pathways function during oogenesis, we are trying to identify molecules involves in these two pathways and have identified one Homer interacting protein. Currently, we are elucidating the potential function of this molecule during Drosophila oogenesis.

Fig.1 anti-α-spectrin staining (red) of wt germarium, labeling GSCs and dividing cysts.

Fig.2 osk RNA in-situ in wt (A and C) and bif;hom double mutant (B and D) oocyte. In both wt (A) and bif;hom mutant (B) stage 9 oocyte, osk is localized to the posterior cortex, but in stage 10 mutant oocyte (D), osk is diffusely localized at posterior cortex, compared to wt stage 10 oocyte (C).