The Furrow

In Drosophila retinal pattern formation and cell-type determination begins in a monolayer epithelium: the eye field of the larval eye-antennal imaginal disc (1-3). These processes are progressive: they are associated with a moving indentation in the surface of the epithelium, called the morphogenic furrow (4-9). In the predecessor project to this proposal we set out to identify genes that are involved in photoreceptor cell-type determination by screening for genes which interact with weak mutations in the glass (gl) gene. gl encodes a DNA binding Zinc-finger protein (10) that can act to regulate gene expression specifically in photoreceptor cells (11-13) and gl expression begins in the morphogenic furrow (11, 12). The most significant result of this study was our discovery that hedgehog (hh), a gene which was previously known for its function in embryonic segmentation, encodes the primary signal for furrow movement, and thus for gl expression and photoreceptor cell determination (9, 14). We exploited this result to extend our understanding of the workings of the morphogenic furrow by testing the function of two other segmentation genes, first wingless (wg) and then patched (ptc) (15). All of these genes function in other aspects of nervous system development in Drosophila (16-20) and in vertebrates (21-27). Recently it has been shown that a vertebrate homolog of hh functions in the developing retina (25).

In recent years the molecular mechanisms that drive the morphogenic furrow have begun to emerge (9), and we have been in the forefront of these advances (see progress report below). This is now seen to be a complex process involving many genes and many of the genes that act in furrow movement were previously known for their functions in embryonic development, as genes in the "segment-polarity" class (28-30). The hh gene encodes a protein that is secreted and cleaved (31-34), and is expressed behind the morphogenic furrow in the developing eye. When hh is removed, the furrow stops (14, 35) and ectopic expression of hh anterior to the furrow can be sufficient to induce an ectopic furrow (36). The ptc gene encodes a probable twelve-pass trans-membrane protein (37, 38), which has an opposite mutant phenotype to that of hh. ptc has phenotypic effects in the cells that receive the Hh signal and it has been suggested that Ptc is a receptor for the Hh protein (39-42) . The Ptc protein is expressed ahead of the morphogenic furrow in the eye, and acts as an inhibitor of the furrow (15, 43), and this inhibition is relieved by the receipt of the Hh signal. A third protein has been shown to act in this pathway, the cAMP-dependent protein kinase (PKA) (44-48). Mutations that remove PKA have phenotypes in the embryo and eye, that are similar to those of ptc (19, 49). Another segment polarity gene, wg, acts as an inhibitor of furrow initiation on the dorsal margin of the developing eye (15, 50) and ectopic expression of the Wg protein can block furrow expression (50). Recently we have shown that the ecdysone pathway also interacts in this process (51).

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