Maternally transcribed genes|
Female meiosis in Drosophila
The reproductive system of the female fly engages in a complex process resulting in the highly organized patterning of the mature egg. Four maternal genetic systems are responsible for the polarity of the egg and ultimately, the embryo. Each of these are established in the egg prior to fertilization, in preparation for later function during embryonic development.
Anterior group genes are involved in the localization of Bicoid mRNA to the anterior tip of the egg. After fertilization Bicoid protein acts as a transcription factor to activate and repress genes along the anterio-posterior axis of the embryo.
A second system involves the formation of the pole plasm, a specialized cytoplasm that forms in the posterior of the egg. This process is carried out by posterior group genes, ensuring the posterior localization of Oskar protein and Nanos mRNA. It is this specialized pole plasm that becomes the cytoplasm of the germ cells early in development. Germ cells are fated to give rise to the eggs and sperm produced by adult flies.
The third system established during egg development is the terminal system. These so-called terminal class genes ensure the activation of the receptor Torso at both the anterior and posterior poles of the zygote, shortly after fertilization. As with the dorsal system (see below) the terminal system involves feedback between follicle cells and the egg.
The fourth maternal genetic system, the dorsal system, sets up the dorsoventral polarity of the egg. Dorsal group genes ensure the activation of the receptor Toll in the ventral portion of the embryo. The dorsal system is especially difficult to follow because it involves two receptors and two ligands (the proteins that trigger receptors) as well as both the egg and the follicle cells that surround it.
The dorsal system program is initiated in the middle of the oogenesis process, half way through the making of the egg. A signal is sent from the egg to follicle cells. This signal, the Gurken ligand, is released in the anterior dorsal region of the egg and triggers signaling through the Epidermal growth factor receptor (EGF-R), also known as Torpedo or EGR. This signaling leads to a different developmental fate for dorsal and anterior follicle cells as opposed to that of ventral and posterior follicle cells. It poises ventral follicle cells to undertake their role in dorso-ventral polarity, the activation of Spätzle in the ventral portion of the perivitelline fluid. Gurken signals also result in an adjustment of oocyte polarity during the middle of oogenesis by an unknown mechanism resulting in changes in the cytoskeleton of the egg.
Gurken's effects begin to be felt right after fertilization. Ventral cells, acting through the protease Easter, activate the ligand Spätzle but only in the ventral part of the oocyte. Spätzle signals through the receptor Toll result in nuclear transport of the transcription factor Dorsal, leading to the establishment of dorso-ventral polarity in the developing egg.
More details about oogenesis and the establishment of embryonic polarity will be found at individual gene sites.
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