DUAL FUNCTION OF THE REGION-SPECIFIC HOMEOTIC GENE SPALT DURING DROSOPHILA TRACHEAL SYSTEM-DEVELOPMENT

We report that the region-specific homeotic gene spalt affects the Dro sophila tracheal system at two different stages of embryonic developme nt. Both lack-of-function and gain-of-function experiments show that b lastodermal spalt activity restricts tracheal development to 10 bilate rally positioned pairs of tracheal placodes in the trunk region by rep ressing placode formation in parasegments 2, 3 and 14. The results sug gest that the activity of the zink-finger type transcription factor en coded by spalt suppresses the molecular pathway that establishes trach eal development. spalt function is also necessary for the directed mig ration of the dorsal trunk cells, a distinct subset of tracheal cells. This process is a prerequisite for the formation of the dorsal trunk generated by fusion of adjacent tracheal metameres into a common tubul ar structure. The directed cell migration, in which spalt gene functio n participates, seems to be independent of branch fusion and general t racheal cell migration processes.