REQUIREMENT FOR EGF RECEPTOR SIGNALING IN NEURAL RECRUITMENT DURING FORMATION OF DROSOPHILA CHORDOTONAL SENSE ORGAN CLUSTERS

Background: Drosophila proneural genes act in the process of selecting neural precursors from undifferentiated ectoderm, The proneural gene atonal is required for the development of precursors of both chordoton al organs (stretch receptors) and photoreceptors. Although these types of sensory element are dissimilar in structure and function, they bot h occur as organized arrays of neurons. Previous studies have shown th at clustering of photoreceptors involves local recruitment, and that s ignalling by the Drosophila epidermal growth factor receptor (DER) pat hway is involved in the recruitment process, We present evidence that a similar mechanism is required for the clustering of embryonic chordo tonal organs, Results: We have examined the expression patterns of ato nal and genes of the DER pathway in wild-type and mutant backgrounds. Expression of atonal was restricted to a subset of the atonal-requirin g chordotonal precursors, which we call founder precursors, The remain ing precursors required DER signalling for their selection, Signalling by the founder precursors was initiated by atonal activating, directl y or indirectly, rhomboid expression in these cells. Signalling by the se founder precursors then provoked a response in the surrounding ecto dermal cells, as shown by the activation of expression of the DER targ et genes pointed and argos. The signal and response then led to recrui tment of some of the ectodermal cells to the chordotonal precursor cel l fate. DER hyperactivation by misexpression of rhomboid resulted in e xcessive chordotonal precursor recruitment, Conclusions: Increased num bers of chordotonal precursors are recruited by homeogenetic induction involving signalling via DER from founder precursors to surrounding e ctodermal cells. We suggest that the reason chordotonal organs and pho toreceptors share a requirement for the proneural gene atonal is that this gene activates a common pathway leading to neural aggregation.