Institut für Entwicklungsbiologie der Universität zu Köln


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Prof. Dr. Siegfried Roth

Establishment of the anterior-posterior and dorsal-ventral axes of the Drosophila egg

The establishment of the anterior-posterior and dorsal-ventral axes of the Drosophila egg relies on processes of cell-cell communication between different populations of cells within the egg chamber. The exchange of information is achieved through intracellular signaling utilizing the well conserved EGF (epidermal growth factor) and the TGF-beta (transforming growth factor-beta) signaling cascades. Misregulation of these pathways causes abnormal development and cancer in mammals. The Drosophila model system offers powerful genetic and molecular approaches to investigate how cells communicate through the modulation and the interaction of these signaling cascades. In the lab we focus on understanding the logic underlying EGF and TGF-beta signaling, in particular, on how the interaction between the two pathways leads to the determination of cell fate in the follicular epithelium surrounding the oocyte. Findings in Drosophila can often be extended to mammalian cells because of the high conservation of the corresponding molecules and mechanisms.

During oogenesis signaling from the follicle cells brings about cytoskeletal rearrangements in the oocyte. The reorganisation of the oocyte cytoskeleton is a prerequisite for the correct localisation of mRNAs (bicoid, oskar and gurken) which act as cytoplasmic determinants of cell fate or as sources for morphogenetic signals that are essential for the proper establishment of the embryonic body axes. A fascinating and still unresolved issue is the nature of the signal from the follicle cell which initiates the changes in cytoskeletal organisation. We are currently conducting extensive genetic screens to identify this signal and other key components in the process. Our goal is to understand the mechanisms by which cytoskeletal dynamics are influenced and how they lead to the correct axial organisation of the egg.

Figure 1:
Follicle cell patterning and the Drosophila egg
Sections through Drosophila egg chambers show that the oocyte is surrounded by a monolayer epithelium of follicle cells. The EGF-like Gurken protein (blue, left section) is produced close to the oocyte nucleus. The protein is secreted and taken up by the overlaying follicle cells. The second section shows the oocyte nucleus localized to the future dorsal site of the egg. The EGF signal from the proximity of the nucleus regulates target expression (blue) in the follicle cells. Different target genes are shown in sections 2, 3 and 4. The chorion pattern of the deposited egg (seen below) results form differential target gene expression.

The evolutionary perspective

A deeper understanding of axis formation in Drosophila requires an evolutionary perspective. Only by comparing Drosophila to more primitive insects can we distinguish the derived aspects of axis formation from those which are basic and therefore likely to be present in all insects. To address the evolution of axis formation we are working with the red flour beetle Tribolium castaneum which shows a more basic form of embryogenesis, the so called short germ development found in most primitive insects. We have already characterized homologues of severval Drosophila dorsoventral pattern formation genes from Tribolium. Surprisingly, the maternal morphogen gradient which establishes the dorsal-ventral axis in Drosophila is also present in Tribolium. It forms globally in the whole egg as in Drosophila even though its positional information is only required in more posterior regions of the Tribolium egg. The formation of a maternal gradient along the whole anterior-posterior axis in a short germ insect might be a preadaptation for the derived long-germ mode of development found in higher Dipterans, like Drosophila. We are now using genetic and molecular techniques including germ line transformation and RNA interference that have recently been developed for Tribolium to enable a better understanding of the extent to which strategies employed in axis determination are conserved.

Figure 2:
Gastrulation in Tribolium
Scanning electronmicroscopic picture (A) and nuclear staining (B) of the same gastrulating Tribolium castaneum embryo. As typical of the short germ type of development the embryo forms from a germ rudiment which occupies only a small portion of the egg surface the remainder giving rise to extraembryonic tissue (serosa). At this stage the embryo has only anlagen for the anteriormost segments. More posterior segments will form later from a growth zone.