The regulation of empty spiracles by Abdominal-B mediates an abdominal segment identity function

The empty spiracles (ems) homeo box gene is required for the development of the Drosophila larval filzkörper, which are structural specializations of the eighth abdominal segment. Filzkörper development is also dependent on the function of the homeotic selector gene Abdominal-B (Abd-B). Here, we show that ems is a downstream gene that is transcriptionally regulated by Abd-B proteins. This regulation is mediated by an Abd-B-dependent ems cis-regulatory element that in early- to mid-stage embryos is activated only in the eighth abdominal segment. Genetic epistasis tests suggest that both ems and Abd-B are required in combination for the specification of the filzkörper primordia. In a general sense, these results also provide evidence that the hierarchical level immediately downstream of the homeotic genes contains additional homeo domain transcription factors that define subsegmental domain identities.