Academic journal article Genetics

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

Academic journal article Genetics

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

Article excerpt

THE proper formation of a tissue or organ requires the precise specification of cell fates and the subsequent maintenance of these differentiated fates. Often, these processes take place during embryonic development. In many instances, disruption of tissue morphogenesis results in organismal lethality, complicating mechanistic studies. In this regard, the Drosophila melanogaster egg chamber is a useful tool for study. The ovary is a nonessential organ. Thus, genes with essential roles in formation of the mature Drosophila egg can be studied using adult animals. In addition, abundant genetic and molecular tools are available in Drosophila, facilitating mechanistic analysis of tissue morphogenesis (Sullivan et al. 2000).

Each Drosophila ovary is composed of 16-20 ovarioles (Spradling 1993). The germline stem cells and their associated somatic niche are found at the anterior tip of the ovariole in a region known as the germarium (Spradling 1993) (Figure 1, A and B). Division of a germline stem cell produces a daughter cell known as a cystoblast (Spradling et al. 2001; Gonzalez-Reyes 2003). The cystoblast undergoes four rounds of cell division to produce a cyst containing 16 germ cells. The cyst is eventually surrounded by a layer of somatic cells known as follicle cells. As the cyst progresses through the germarium, oocyte fate is specified (Deng and Lin 2001; Riechmann and Ephrussi 2001; Huynh and St Johnston 2004). Thus, once the cyst emerges from the germarium as an egg chamber, it contains 15 nurse cells and one oocyte (Spradling 1993) (Figure 1, A and B). The egg chamber progresses through 14 stages of morphogenesis before it is competent for fertilization. During these stages, the cyst grows in size and distinct fates are specified in the follicle cells (Spradling 1993). Although 14 different stages of egg chamber maturation can be identified, based on morphological features and the cell fates that are specified, egg chamber maturation represents a developmental continuum. Thus, not every stage of egg chamber development is observed in all ovarioles.

Between stages 2 and 6 there is a buildup of gurken (grk) messenger RNA (mRNA) and protein within the oocyte (Neuman-Silberberg and Schupbach 1993, 1996). This pool of Grk signals to the overlying follicle cells to adopt posterior cell fates (Gonzalez-Reyes et al. 1995; Roth et al. 1995). Once posterior fate has been specified, the follicle cells signal back to the oocyte. Although the nature of this signal is still unknown, the Notch and Hippo pathways are thought to contribute to the process (Ruohola et al. 1991; Meignin et al. 2007; Polesello and Tapon 2007; Yu et al. 2008; Yan et al. 2011). In addition, proteins within the extracellular matrix and factors that link the oocyte to the follicle cells are also involved (Deng and Ruohola-Baker 2000; Frydman and Spradling 2001; MacDougall et al. 2001). This signaling cascade initiates a reorganization of oocyte microtubules. Prior to this, oocyte microtubules are arranged with minus ends enriched at the posterior pole (Theurkauf et al. 1992). However, the redistribution of microtubules that takes place in response to Grk signaling results in minus ends enriched along the anterior and lateral cortex and plus ends that display a slight enrichment at the posterior pole of the oocyte (Theurkauf et al. 1992; Clark et al. 1994, 1997; Parton et al. 2011; Sanghavi et al. 2012). This reorganization is required for migration of the oocyte nucleus from the posterior to the anterior of the oocyte (Gonzalez-Reyes et al. 1995; Roth et al. 1995) (Figure 1A). In addition, this microtubule organization is required for the posterior localization of oskar (osk) mRNA, the anterior localization of bicoid (bcd)mRNA,and the localization of grk mRNA at the dorsal-anterior margin (St Johnston 2005). The precise localization of these mRNAs and their resulting protein products is essential for polarizing the oocyte and the resulting embryo (Berleth et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.