Requirement for EGF receptor signalling 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 chordotonal organs (stretch receptors) and photoreceptors. Although these types of sensory element are dissimilar in structure and function, they both occur as organized arrays of neurons. Previous studies have shown that clustering of photoreceptors involves local recruitment, and that signalling by the Drosophila epidermal growth factor receptor (DER) pathway is involved in the recruitment process. We present evidence that a similar mechanism is required for the clustering of embryonic chordotonal organs.
RESULTS: We have examined the expression patterns of atonal and genes of the DER pathway in wild-type and mutant backgrounds. Expression of atonal was restricted to a subset of the atonal-requiring chordotonal precursors, which we call founder precursors. The remaining precursors required DER signalling for their selection. Signalling by the founder precursors was initiated by atonal activating, directly or indirectly, rhomboid expression in these cells. Signalling by these founder precursors then provoked a response in the surrounding ectodermal cells, as shown by the activation of expression of the DER target genes pointed and argos. The signal and response then led to recruitment of some of the ectodermal cells to the chordotonal precursor cell fate. DER hyperactivation by misexpression of rhomboid resulted in excessive chordotonal precursor recruitment.
CONCLUSIONS: Increased numbers of chordotonal precursors are recruited by homeogenetic induction involving signalling via DER from founder precursors to surrounding ectodermal cells. We suggest that the reason chordotonal organs and photoreceptors share a requirement for the proneural gene atonal is that this gene activates a common pathway leading to neural aggregation.