Faculty Bibliography

The Drosophila ommatidia contain two classes of photoreceptor cells (PR's), the outer and the inner PR's. We performed an enhancer trap screen in order to target genes specifically expressed in PR's. Using the UAS/GAL4 method with enhanced green fluorescent protein (eGFP) as a vital marker, we screened 180000 flies. Out of 2730 lines exhibiting new eGFP patterns, we focused on 16 lines expressing eGFP in particular subsets of PR's. In particular, we describe three lines inserted near the spalt major, m-spondin and furrowed genes, whose respective expression patterns resemble those genes. These genes had not been reported to be expressed in the adult eye. These examples clearly show the ability of our screen to target genes expressed in the adult Drosophila eye.

The maternal determinant Bicoid (Bcd) represents the paradigm of a morphogen that provides positional information for pattern formation. However, as bicoid seems to be a recently acquired gene in flies, the question was raised as to how embryonic patterning is achieved in organisms with more ancestral modes of development. Because the phylogenetically conserved Hunchback (Hb) protein had previously been shown to act as a morphogen in abdominal patterning, we asked which functions of Bcd could be performed by Hb. By reestablishing a proposed ancient regulatory circuitry in which maternal Hb controls zygotic hunchback expression, we show that Hb is able to form thoracic segments in the absence of Bcd.

The Torso signal transduction pathway exhibits two opposite effects on the activity of the Bicoid (Bcd) morphogen: (i) Bcd function is repressed by Torso (Tor) at the anterior pole of the embryo leading to a retraction of the expression of many Bcd targets from the most anterior region of the embryo, where the Tor tyrosine kinase receptor is activated, and (ii) Bcd function is strengthened by Tor in a broader anterior region, as indicated by a shift of the posterior border of Bcd targets towards the anterior pole in embryos deprived from Tor activity. Anterior repression of Bcd targets was not observed in embryos lacking maternal contribution of D-sor, which acts downstream of Tor and encodes a MAP-kinase kinase. This indicates that the Ras signalling cascade is directly involved in this process, although the known transcriptional effectors of the Tor pathway, tll and hkb, are not (Ronchi, E., Treisman, J., Dostatni, N., Struhl, G. and Desplan, C. (1993) Cell 74, 347-355). Bcd is a good in vitro substrate for phosphorylation by MAP-kinase and phosphorylation of the protein occur in vivo on MAP-kinase sites. In the presence of a Bcd mutant that could no longer be phosphorylated by MAP-kinase, expression of Bcd targets remained repressed by Tor at the pole while strengthening of Bcd activity was reduced. These experiments indicate that phosphorylation of Bcd by MAP-kinase is likely to be required for the Tor pathway to induce its full positive effect on Bcd. This suggests that Tor signalling acts at a distance from the anterior pole by direct modification of the diffusing Bcd morphogen.

Color vision is achieved by comparing the inputs from retinal photoreceptor neurons that differ in their wavelength sensitivity. Recent studies have elucidated the distribution and phylogeny of opsins, the family of light-sensitive molecules involved in this process. Interesting new findings suggest that animals have evolved a strategy to achieve specific sensitivity through the mutually exclusive expression of different opsin genes in photoreceptors.

Munster (Mu) is a homeobox-containing gene of the Paired-class which is specifically expressed in the developing Bolwig organs, the Drosophila larval eyes. This expression is first detected during early germ band retraction stage (stage 12 from 7 h 20 at 25 degrees C) and persists until the end of embryogenesis. Mu homeodomain is most similar to that of Aristaless and D-Goosecoid. Strikingly, the Munster gene maps within 6 kb of D-goosecoid, in the same genomic region as aristaless, suggesting that these genes are part of a homeobox gene cluster.

Four maternal systems are known to pattern the early Drosophila embryo. The key component of the anterior system is the homeodomain protein Bicoid (Bcd). Bcd needs the contribution of another anterior morphogen, Hunchback (Hb), to function properly: Bcd and Hb synergize to organize anterior development. A molecular mechanism for this synergy has been proposed to involve specific interactions of Bcd and Hb with TATA-binding protein-associated factors (TAFIIs) that are components of the general transcription machinery. Bcd contains three putative activation domains: a glutamine-rich region, which interacts in vitro with TAFII110; an alanine-rich domain, which targets TAFII60; and a C-terminal acidic region, which has an unknown role. We have generated flies carrying bcd transgenes lacking one or several of these domains to test their function in vivo. Surprisingly, a bcd transgene that lacks all three putative activation domains is able to rescue the bcdE1 null phenotype to viability. Moreover, the development of these embryos is not affected by the presence of dominant negative mutations in TAFII110 or TAFII60. This means that the interactions observed in vitro between Bcd and TAFII60 or TAFII110 aid transcriptional activation but are dispensable for normal development.

In the Drosophila retina, the R7 and R8 inner photoreceptors integrate light information to detect polarized or color light. In fart, there are three types of ommatidia with a stochastic distribution. One set expresses rhodopsin3 (rh3) in R7 and rh5 in R8, another the rh4-rh6 pair, while the last one expresses rh3 in both R7 and R8. We are studying the mechanisms underlying the exclusive pxpression of rh genes as well as the signaling events between the R7 and R8 cells that allow the coordinate expression of rh. Using genetic situations where there are supernumerary R7 cells, or where there are no R7 cells, we concluded that there is a reciprocal communication between R7 and R8, In fact, we can propose that there is a 'ground state' (rh4/rh6) that does not require interaction between R7 and R8, and an acquired state frh3/rh5) that requires exchange of information between the cells. As control of rh expression is purely transrriptional, an analysis of the rh promoters has allowed us to understand their modes of regulation. Consistent with the model of ground/acquired states, rh4 contains promoter elements that allow its expression in all R7 cells, and an dement that represses its transcription in rh3-expressing cells. To the contrary, rh3 (and rho) is only activated in a subset of ommatidia in response to the activity of the orthodenticle (old) gene, a gene that has been shown to be critical in vortpbrate photoreceptor development. Using genetir approaches, we hope to understand how the diversification of rhodopsin genes and modification of iliir f'xprfssion have evolved to achieve color vision and phototactic behavior.