A Dynamic Network of Morphogens and Transcription Factors Patterns the Fly Leg

Abstract

Animal appendages require a proximodistal (PD) axis, which forms orthogonally from the two main body axes, anteroposterior and dorsoventral. In this review, we discuss recent advances that begin to provide insights into the molecular mechanisms controlling PD axis formation in the Drosophila leg. In this case, two morphogens, Wingless (Wg) and Decapentaplegic (Dpp), initiate a genetic cascade that, together with growth of the leg imaginal disc, establishes the PD axis. The analysis of cis-regulatory modules (CRMs) that control the expression of genes at different positions along the PD axis has been particularly valuable in dissecting this complex process. From these experiments, it appears that only one concentration of Wg and Dpp are required to initiate PD axis formation by inducing the expression of Distal-less (Dll), a homeodomain-encoding gene that is required for leg development. Once Dll is turned on, it activates the medially expressed gene dachshund (dac). Cross-regulation between Dll and dac, together with cell proliferation in the growing leg imaginal disc, results in the formation of a rudimentary PD axis. Wg and Dpp also initiate the expression of ligands for the EGFR pathway, which in turn induces the expression of a series of target genes that pattern the distal-most portion of the leg.

Hexapoda: Comparative Aspects of Later Embryogenesis and Metamorphosis

“…(1) hth in the presumptive coxa; (2) a domain coexpressing hth , dac , and the proximal ring of Dll in the trochanter; (3) a dac -only domain in the femur and proximal tibia; (4) a domain coexpressing dac and Dll in the distal tibia and proximal tarsus; and (5) a Dll -only domain in the distal tarsus and pretarsus (reviewed in Estella et al 2012 ). Regulation of Dll in the leg disc primordia is complex, with a series of enhancers driving expression fi rst in a broad embryonic domain that encompasses both the leg and wing disc primordia, then in a narrower domain encompassing the intermediate and distal leg, and fi nally in a domain restricted to the distal leg (Cohen et al 1993 ;McKay et al 2009 ;Galindo et al 2011 ).…”

The evolutionarily conserved transcription factor Sp1 controls appendage growth through Notch signaling

“…Appendage formation starts in early embryos by the activation of Dll (through its early enhancer, Dll-304), btd and Sp1 by Wg, and their expression is repressed posteriorly by the abdominal Hox genes Vachon et al, 1992). Some hours later, there is a molecular switch from the early Dll enhancer (Dll-304) to the late enhancer (Dll-LT) to keep Dll expression throughout the embryo-larvae transition restricted to the cells that will form the leg (reviewed by Estella et al, 2012). At this developmental stage, Sp1 and btd play redundant roles in Dll activation, as only the elimination of both genes suppresses Dll expression and Dll-LT activity in the leg primordia.…”

“…Therefore, the early elimination of btd and Sp1 completely abolishes leg formation and, in some cases, causes a legto-wing homeotic transformation (Estella and Mann, 2010). As the leg imaginal disc grows, a proximo-distal (PD) axis is formed by the differential expression of three leg gap genes, Dll, dachshund (dac) and homothorax (hth), which divides the leg into distal, medial and proximal domains, respectively (reviewed by Estella et al, 2012). Once these genes have been activated, their expression is maintained, in part through an autoregulatory mechanism, and no longer relies on Wg and Dpp (Diaz-Benjumea et al, 1994;Galindo et al, 2002).…”

Genomic Control Processes in Adult Body Part Formation

“…C1). (C and D from Estella et al (2012)). (E, F), Specification of pre-tarsus and of the five tarsal segments.…”

“…The cis-regulatory module driving this initial spatial and temporal phase of dll expression, "dll304", essentially defines these imaginal discs. This cis-regulatory module requires Wg input, is repressed in the abdomen by Ubx and AbdA, is repressed dorsally downstream of Dpp signaling, and is repressed ventrally downstream of EGF signaling (McKay et al, 2009;Estella et al, 2012). But dll304 activity shortly fades out.…”