Coordinated animal locomotion depends on the development of functional proprioceptors. While early cell-fate determination processes are well characterized, little is known about the terminal differentiation of cells within the proprioceptive lineage and the genetic networks that control them. In this work we describe a gene regulatory network consisting of three transcription factors–Prospero (Pros), D-Pax2, and Delilah (Dei)–that dictates two alternative differentiation programs within the proprioceptive lineage in Drosophila. We show that D-Pax2 and Pros control the differentiation of cap versus scolopale cells in the chordotonal organ lineage by, respectively, activating and repressing the transcription of dei. Normally, D-Pax2 activates the expression of dei in the cap cell but is unable to do so in the scolopale cell where Pros is co-expressed. We further show that D-Pax2 and Pros exert their effects on dei transcription via a 262 bp chordotonal-specific enhancer in which two D-Pax2- and three Pros-binding sites were identified experimentally. When this enhancer was removed from the fly genome, the cap- and ligament-specific expression of dei was lost, resulting in loss of chordotonal organ functionality and defective larval locomotion. Thus, coordinated larval locomotion depends on the activity of a dei enhancer that integrates both activating and repressive inputs for the generation of a functional proprioceptive organ.

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In this work we describe a gene regulatory network consisting of three transcription factors- Prospero (Pros), D-Pax2/Shaven (Sv) and Delilah/Taxi (Dei/Tx)- that dictates two alternative differentiation programs within the proprioceptive lineage in Drosophila. D-Pax2 and Pros control the differentiation of cap versus scolopale cells in the chordotonal organ lineage by, respectively, activating and repressing the transcription of dei. Normally, D-Pax2 activates the expression of dei in the cap cell but is unable to do so in the scolopale cell where Pros is co-expressed. If D-Pax2 activity is lost, the cap cell fails to express dei as well as additional proteins, such as αTub85E, that characterize a fully differentiated cap cell. In contrast, if Pros activity is lost, dei is ectopically expressed in the scolopale cell that, as a consequence, adopts some cap cell features, including the expression of αTub85E. We further show that D-Pax2 and Pros exert their effects on dei transcription via a 262 bp chordotonal-specific regulatory module (deiChO-262) in which two D-Pax2- and three Pros-binding sites were identified experimentally. When this regulatory element was removed from the fly genome, the cap- and ligament-specific expression of dei was lost. Ectopic expression of D-Pax2, or the elimination of pros activity, did not lead to upregulation of dei in the scolopale cell in the absence of the D-Pax2-responsive deiChO-262 enhancer. Finally, we show that the regulation of dei expression via the D-Pax2/Pros-responsive element is critical for chordotonal organ functionality and coordinated larval locomotion.