Development 107, 723-731 (1989) Printed in Great Britain © The Company of Biologists Limited 1989 723 Dynamics of Drosophila eye development and temporal requirements of sevenless expression KONRAD BASLER and ERNST HAFEN Zoologisches Institut, Universittit Zurich, CH-8057 Zurich, Switzerland Summary The development of the compound eye of Drosophila consists of a linear, stereotyped program starting at the posterior end of the eye imaginal disc and progressing towards the anterior border. The determination of the R7 photoreceptor cells is part of this process and is dependent on the sevenless gene. In this study, we used a heat-shock-inducible sevenless gene as a conditional allele to determine the exact temporal requirements of sevenless gene expression and to reveal the stages of ommatidial development during which the presumptive R7 cell can respond to the presence of sevenless protein. Our results indicate that sevenless gene function is only required during a brief, defined period for the initiation of R7 development; subsequently sevenless is dispensable for both differentiation and function of the R7 photo- receptors. Furthermore, using rescue of R7 cells as an internal marker to monitor the progression of eye development we could examine when and at what rate ommatidial columns form. Key words: sevenless, eye development, cell fate determination, conditional allele, phototaxis. Introduction Generation of cell type diversity during the develop- ment of multicellular organisms involves the successive restriction in the developmental potential of initially totipotent precursor cells. Genetic analysis in Dros- ophila and other organisms have indicated that the individual steps in this process may be controlled by single selector genes. It is of interest to know whether these genes control pathway choices only at a single level in the developmental hierarchy or whether they are used at multiple levels during the development of a specific cell type. To address this question we have studied the temporal and functional requirements for sevenless, a gene controlling the development of a particular cell type in the eye of Drosophila. The compound eye of Drosophila is an ideal system in which to study the hierarchy of developmental events, since their temporal order is spatially displayed during the differentiation of the imaginal disc that gives rise to the eye. Furthermore, the repetitive, highly ordered array of the ommatidial clusters permits the examination of developmental decisions at the single cell level (for recent review see Ready, 1989). The eye develops from the eye imaginal disc during the third larval instar and pupation. Before this period, the eye disc consists of a single layer epithelium of dividing unpatterned cells. Ommatidial assembly does not occur synchronously throughout the disc, but starts at the posterior margin and progresses anteriorly (Ready etal. 1976). Closely associated with the anterior boundary of ommatidial assembly is a morphological depression, called the morphogenetic furrow. Cells anterior to the furrow are unpatterned and randomly organized whereas cells posterior to the furrow become incorpor- ated into the ommatidial clusters in a fixed sequence (Tomlinson and Ready, 1987). This process can be visualized with antibodies against the sevenless protein which stain a subpopulation of ommatidial precursor cells in the developing ommatidia (Fig. 1). Flies homozygous or hemizygous for the sevenless mutation lack all R7 photoreceptor cells (Harris et al. 1976). The mutant R7 precursor is unable to respond to the inductive signal and develops into a non-neuronal cone cell (Harris et al. 1976; Tomlinson and Ready, 1986). The sevenless gene has been cloned (Hafen et al. 1987; Banerjee etal. 1987). It encodes a transmembrane protein with a large extracellular domain and two cytoplasmic domains (Basler and Hafen, 1988; Bowtell et al. 1988), one of which is a tyrosine kinase domain. The sevenless protein most likely acts on the surface of ommatidial precursors as a receptor for an R7-inducing signal. The mutant alleles of sevenless that have been ident- ified so far are all loss-of-function mutations. The lack of temperature-sensitive alleles for sevenless prevented the analysis of the temporal requirements for sevenless function. Transient expression of sevenless under the control of a heat-shock promoter (hsp-sev construct) results in the formation of a narrow stripe of rescued ommatidia in an otherwise mutant background (Basler and Hafen, 1989; Bowtell et al. 1989). The position of