RESEARCH FOCUS: GENETIC AND MOLECULAR ANALYSIS OF OOGENESIS IN DROSOPHILA MELANOGASTER
The mature egg of Drosophila melanogaster contains information that determines the major axis of the embryo. This information is built into the egg during oogenesis and functions in the early embryo to create a regionally distinct pattern of zygotic gene expression. The work in the laboratory focuses on understanding the nature and distribution of this maternal information. In particular, we are analyzing how this spatial information is built into the egg during oogenesis.
As a first step in the analysis, we carried out screens for female sterile and maternal-effect mutations. Several classes of mutations that affect the pattern of the egg or the body pattern of the embryo were identified. We have subsequently concentrated on mutations in our collection that affect not only the spatial pattern of the embryo but also the pattern of the egg shell. These genes act early during oogenesis to establish the spatial pattern of the egg chamber as a whole, including both the pattern of the egg shell which is produced by the follicle cells situated on the outside of the egg chamber as well as the spatial distribution, or local activation of the maternal cytoplasmic determinants that specify the pattern of the embryo. Mosaic analysis has shown that some of the genes in this class act in the germline itself, while a second group of genes functions in the follicle cells.
The analysis of these genes has demonstrated that cell-cell interactions between germline and follicle cells take place during oogenesis and are important for the establishment of the normal pattern of egg shell and embryo. One of the genes required for the dorsoventral pattern of egg and embryo (torpedo) is homologous to the vertebrate Epidermal Growth Factor receptor (EFGr). This vertebrate protein is known to function as a transmembrane receptor with tyrosine kinase activity. A detailed genetic analysis of this gene has revealed that this receptor tyrosine kinase is used at several different stages in the Drosophila life cycle and promotes different cell communication processes. During oogenesis it is expressed in the follicle cells, and enables the follicle cells to receive dorsoventral patterning signals from the germline. The genes that act in the germline are involved in the production of this germline signal. In particular we found that the gene gurken encodes a protein that has homology to the vertebrate signaling molecule TGF-alpha. Therefore, gurken encodes a ligand for torpedo/Egfr. Interestingly, during oogenesis, the gurken RNA becomes localized to the future dorsal side of the oocyte. The localized gurken RNA produces a high concentration of Gurken protein on the dorsal side of the egg. Localization of gurken RNA to one side of the oocyte therefore represents a first step in a cascade of events that determines dorsoventral pattern of the egg. We are conducting further experiments to analyze the mechanisms that lead to the asymmetric localization of the gurken RNA.
Once activated, the torpedo receptor tyrosine kinase must interact with other gene products in the follicle cells to determine the dorsoventral fate of the follicle cells. We are currently analyzing genes that may act as downstream targets of the kinase in follicle cells. Eventually this should allow us to define in precise genetic and molecular terms how a dorsoventral signal is first produced in the germline during oogenesis, how this signal is transmitted from the germline to the overlying follicle cells, how it determines the fate of the follicle cells, and how the follicle cells, in turn, send a new signal to the germ cell that ultimately defines the dorsoventral pattern of the embryo.