Converting a morphogenetic field into a defined tissue or organ involves the precise interaction of a complex network of genes. The defining event in such a process is the discrete expression of so-called 'field-selector genes', which, in the case of the fruitfly wing, ensure that the cluster of cells in which they are expressed develop into a wing rather than into another organ. But how do selector genes orchestrate the development of complex structures and how do they impart specificity to globally deployed signalling pathways such as Decapentaplegic (Dpp) and Notch, which pattern the wing as well as many other organs? Investigating how selector genes direct the expression of field-specific responses has been the focus of a recent study by Guss et al. Their results show that the expression of wing-specific responses requires both wing-specific selector genes and commonly deployed signalling pathways.

The selector gene complex that specifies wing identity consists of two proteins, Vestigial (Vg) and Scalloped (Sd). The sd gene encodes a DNA-binding protein that is required cell-autonomously for the expression of wing patterning genes such as cut , spalt and vg. Sd binds directly to the cis-regions of the target genes it regulates. In addition, mutating the Sd-binding sites in the promoters of cut, spalt and vg eliminates their expression. It is therefore possible that, in the wing, field-selector genes control directly the expression of all target genes, rather than controlling only a few members of the gene-regulatory network. However, cut, spalt and vg are also known targets of the Notch and Dpp signalling pathways. This observation led Guss et al. to test whether field-specific genes and commonly deployed signalling pathways pattern the wing field by converging on the control regions of the same genes. Indeed, only when binding sites for both selector and signalling genes are present on a synthetic promoter construct can wing-specific gene expression and patterning occur.

This study has made a step towards clarifying the relationship between two genetic systems that are required to specify and pattern a structure, by showing that both systems depend on each other. Although these results are specific to wing development, an obligatory combinatorial input on target promoters might be a general mechanism by which selector genes work with signalling pathways to control the extensive genetic network that specifies the development of complex structures.