Cross-regulatory interactions among tracheal genes support a co-operative model for the induction of tracheal fates in the Drosophila embryo

The Drosophila tracheal system arises from clusters of ectodermal cells tha t invaginate and migrate to originate a network of epithelial tubes. Geneti c analyses have identified several genes that are specifically expressed in the tracheal cells and are required for tracheal development. Among them, trachealess (trh) is able to induce ectopic tracheal pits and therefore it has been suggested that it would act as an inducer of tracheal cell fates; however, this capacity appears to be spatially restricted. Here we analyze the expression of the tracheal specific genes in the early steps of trachea l development and their cross interactions We find that there is a set of p rimary genes including trh and ventral veinless (vvl) whose expression does not depend on any other tracheal gene and a set of downstream genes whose expression requires different combinations of the primary genes. We also fi nd that the combined expression of primary genes is sufficient to induce so me downstream genes but not others. These results indicate that there is no t a single master gene responsible for the appropriate expression of the tr acheal genes and support a model where tracheal cell fates are induced by t he co-operation of several factors rather than by the activity of a single tracheal inducer. (C) 2000 Elsevier Science Ireland Ltd. All rights reserve d.