Chapter seven - A Dynamic Network of Morphogens and Transcription Factors Patterns the Fly Leg
Introduction
Animal appendages are external projections from the body wall that are used for very diverse functions including locomotion, grooming, and feeding. In the thorax of diptera, such as the fruit fly Drosophila melanogaster, there are dorsal appendages required for flight—a pair of wings in the second thoracic (T2) segment and a pair of halteres in T3—and three pairs of legs used for walking and grooming. The fly leg, the subject of this review, is composed of 10 morphologically unique segments: coxa, trochanter, femur, tibia, tarsal segments 1–5, and the claw. Together, these segments comprise the proximodistal (PD) axis, in which the proximal coxa is closest to the body and the claw is furthest from the body (Fig. 7.1).
Unlike the two other primary body axes (anteroposterior, AP; dorsoventral, DV), for each appendage, the PD axis is established during embryogenesis de novo. In contrast, at all stages of development, even in the unfertilized egg, rudimentary AP and DV axes exist. Thus, in this respect, the PD axis is unique among the main body axes. This topic, how so-called secondary developmental fields are established from preexisting developmental information, has been debated for decades both from theoretical perspectives and by classical developmental biologists (reviewed by Baker, 2011). Data generated over the past several years have provided novel mechanistic and molecular insights that build upon these earlier studies, providing interesting connections between cell division, secreted morphogens, and the use of dedicated cis-regulatory modules (CRMs) for transcriptional regulation of genes expressed along the PD axis. It is the goal of this review to summarize our current understanding of the intimate interplay between these components, orchestrated over developmental time, which establishes, elaborates, and fine-tunes the leg's PD axis.
Section snippets
The Molecular Players in PD Axis Formation
As for much of the adult fly, fly legs are derived from imaginal discs, elliptical sheets of epithelia that are highly folded by the end of larval development. The fate map of the leg disc is such that cells at its center will give rise to distal-most structures, while cells further away from the center generate more proximal structures (Fig. 7.1). Imaginal discs do not only give rise to appendages: cells at the periphery of the leg disc, for example, generate the ventral portion of the adult
The Initial Establishment of the PD Axis is Encoded in the cis-Regulatory Architecture of Dll
A common principle that has emerged from studying transcriptional regulatory mechanisms in many developmental genes is that dedicated CRMs drive small subsets of complex expression patterns (reviewed in Maeda and Karch, 2011). Dissection of the Dll locus revealed a similar level of CRM dedication, but with the additional finding that distinct Dll CRMs control Dll expression in cells that have different degrees of developmental potential (Fig. 7.3). The discovery of these CRMs has allowed
The Role of Sp1 in Distinguishing Ventral Appendage from Dorsal Appendage Fates
Although the above focus on Dll regulation reveals how the initial PD axis and leg fate map are established, several questions remain concerning this early stage of leg development. Some of these questions are answered by two paralogous genes, buttonhead (btd) and Sp1 (Estella and Mann, 2010, Estella et al., 2003, Wimmer et al., 1996). Both genes encode Sp family zinc-finger transcription factors that share a similar expression pattern throughout development. Despite their similar expression
Elaboration of the PD Axis: The Role of brk
By the end of embryogenesis, a rudimentary PD axis of the leg is apparent in the expression patterns of Dll (via DllLT, DllDKO, and DllLP), hth, and tsh. How are these initial patterns elaborated upon to create the mature PD axis present in the third larval instar stage? The separation between these two time points is huge both in terms of time (96 h) and tissue growth (from ∼ 60 cells to ∼ 10,000 cells). Also, at the end of embryogenesis, dac has not yet been activated but begins to be expressed
Elaboration of the PD Axis: The Role of a Transcription Factor Cascade and Cross-regulation
In addition to positing that Wg and Dpp are critical for initiating the PD axis, Lecuit and Cohen proposed a gradient model to account for the PD axis expression patterns of dac and Dll. According to this model, the expression of dac and Dll along the PD axis depends on the levels of Wg and Dpp a cell perceives: high concentrations of both Wg and Dpp activate Dll and repress dac in the center of the leg disc; intermediate levels activate dac but not Dll in medial regions of the disc; and low
Patterning the DV Axis
If gradients of Wg and Dpp are not used to establish the PD axis, what purpose might they serve? The posterior expression of the homeodomain transcription factors engrailed (en) and invected (inv) divides the leg into anterior and posterior compartments, which have distinct cell lineages (Morata and Lawrence, 1975). In contrast to strict lineage restrictions along the AP axis, the distinction between dorsal and ventral fates is controlled by the secreted molecules Wg and Dpp in a
EGFR Signaling Patterns the Tarsus
While Wg and Dpp play an important role in initiating the PD axis, by the early third instar, Wg and Dpp are no longer required for the PD axis and the role of further elaborating this axis is handed off to the EGFR signaling pathway (Campbell, 2002, Galindo et al., 2002; Fig. 7.7). Shifts of the temperature-sensitive mutant Egfr[ts] (Egfr[tsla]/Egfr[null]) to the restrictive temperature in the beginning of the third larval stage lead to development of legs without pretarsus and one or more
Leg Segmentation and Growth
The process of leg segmentation, or forming the joints that separate each of the leg segments, is one critical downstream consequence of PD gene expression (Rauskolb, 2001). Several genes and pathways required for forming the joints have been defined (Bishop et al., 1999, Chu et al., 2002, Ciechanska et al., 2007, de Celis Ibeas and Bray, 2003, de Celis et al., 1998, Galindo et al., 2005, Greenberg and Hatini, 2009, Greenberg and Hatini, 2011, Hao et al., 2003, Kerber et al., 2001, Mishra et
Concluding Remarks
The above review reveals that a molecular framework of PD axis formation is now emerging. Yet, many questions remain. For one, the initial stages of dorsal and ventral primordia establishment are not well understood. How, for example, does Sp1 block dorsal appendage fates? Second, although graded levels of Wg and Dpp activities may not be relevant to elaborating the PD axis, it remains an open question whether a gradient of EGFR signaling is used to turn on its targets in the tarsus. Third, it
Acknowledgments
We are grateful to members of the Mann, Struhl, and Johnston labs for comments, and funding from the NIH (GM058575). R. V. is a CDP Fellow of the Leukemia and Lymphoma Society.
References (112)
- et al.
Distinct functions of homothorax in leg development in Drosophila
Mech. Dev.
(2002) - et al.
Cell lineage, growth, and determination in the imaginal leg discs of Drosophila melanogaster
Dev. Biol.
(1969) Regulation of gene expression in the distal region of the Drosophila leg by the Hox11 homolog, C15
Dev. Biol.
(2005)- et al.
Transducing the Dpp morphogen gradient in the wing of Drosophila: Regulation of Dpp targets by brinker
Cell
(1999) - et al.
Axis specification in the developing Drosophila appendage: The role of wingless, decapentaplegic, and the homeobox gene aristaless
Cell
(1993) - et al.
The role of Teashirt in proximal leg development in Drosophila: Ectopic Teashirt expression reveals different cell behaviours in ventral and dorsal domains
Dev. Biol.
(1999) - et al.
Molecular integration of wingless, decapentaplegic, and autoregulatory inputs into Distalless during Drosophila leg development
Dev. Cell
(2008) - et al.
Control of Distal-less expression in the Drosophila appendages by functional 3′ enhancers
Dev. Biol.
(2011) - et al.
Specificity of Distalless repression and limb primordia development by abdominal Hox proteins
Dev. Cell
(2002) - et al.
Establishment of medial fates along the proximodistal axis of the Drosophila leg through direct activation of dachshund by Distalless
Dev. Cell
(2011)
Essential roles for lines in mediating leg and antennal proximodistal patterning and generating a stable Notch signaling interface at segment borders
Dev. Biol.
Systematic expression and loss-of-function analysis defines spatially restricted requirements for Drosophila RhoGEFs and RhoGAPs in leg morphogenesis
Mech. Dev.
Drosophila Tbx6-related gene, Dorsocross, mediates high levels of Dpp and Scw signal required for the development of amnioserosa and wing disc primordium
Dev. Biol.
The odd-skipped family of zinc finger genes promotes Drosophila leg segmentation
Dev. Biol.
The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation
Cell
Complementary and mutually exclusive activities of decapentaplegic and wingless organize axial patterning during Drosophila leg development
Cell
The pleiohomeotic gene is required for maintaining expression of genes functioning in ventral appendage formation in Drosophila melanogaster
Dev. Biol.
A concerted action of a paired-type homeobox gene, aristaless, and a homolog of Hox11/tlx homeobox gene, clawless, is essential for the distal tip development of the Drosophila leg
Dev. Biol.
Gene expression in time and space: Additive vs hierarchical organization of cis-regulatory regions
Curr. Opin. Genet. Dev.
Spatial regulation of DELTA expression mediates NOTCH signalling for segmentation of Drosophila legs
Mech. Dev.
decapentaplegic overexpression affects Drosophila wing and leg imaginal disc development and wingless expression
Dev. Biol.
The activities of two Ets-related transcription factors required for Drosophila eye development are modulated by the Ras/MAPK pathway
Cell
The 11-aminoacid long Tarsal-less peptides trigger a cell signal in Drosophila leg development
Dev. Biol.
Tarsal-less peptides control Notch signalling through the Shavenbaby transcription factor
Dev. Biol.
Notch-mediated segmentation and growth control of the Drosophila leg
Dev. Biol.
Nuclear translocation of extradenticle requires homothorax, which encodes an extradenticle-related homeodomain protein
Cell
Notch signaling relieves the joint-suppressive activity of Defective proventriculus in the Drosophila leg
Dev. Biol.
Organizing activity of wingless protein in Drosophila
Cell
Generation of multiple antagonistic domains along the proximodistal axis during Drosophila leg development
Development
Capicua DNA-binding sites are general response elements for RTK signaling in Drosophila
Development
Proximodistal patterning in the Drosophila leg: Models and mutations
Genetics
Compartment boundaries and the control of Drosophila limb pattern by hedgehog protein
Nature
Sp8 is crucial for limb outgrowth and neuropore closure
Proc. Natl. Acad. Sci. USA
Composite signalling from Serrate and Delta establishes leg segments in Drosophila through Notch
Development
An extensive 3′ cis-regulatory region directs the imaginal disk expression of decapentaplegic, a member of the TGF-beta family in Drosophila
Development
Distal-less functions in subdividing the Drosophila thoracic limb primordium
Dev. Dyn.
Antagonistic interactions between wingless and decapentaplegic responsible for dorsal-ventral pattern in the Drosophila Leg
Science
The ETS domain protein pointed-P2 is a target of MAP kinase in the sevenless signal transduction pathway
Nature
Distalization of the Drosophila leg by graded EGF-receptor activity
Nature
The roles of the homeobox genes aristaless and Distal-less in patterning the legs and wings of Drosophila
Development
How the Hox gene Ultrabithorax specifies two different segments: The significance of spatial and temporal regulation within metameres
Development
Limb type-specific regulation of bric a brac contributes to morphological diversity
Development
dAP-2 and defective proventriculus regulate Serrate and Delta expression in the tarsus of Drosophila melanogaster
Genome
Multiple RTK pathways downregulate Groucho-mediated repression in Drosophila embryogenesis
Development
Specification of limb development in the Drosophila embryo by positional cues from segmentation genes
Nature
Proximal-distal pattern formation in Drosophila: Cell autonomous requirement for Distal-less gene activity in limb development
EMBO J.
Distal-less encodes a homoeodomain protein required for limb development in Drosophila
Nature
Allocation of the thoracic imaginal primordia in the Drosophila embryo
Development
The bric a brac locus consists of two paralogous genes encoding BTB/POZ domain proteins and acts as a homeotic and morphogenetic regulator of imaginal development in Drosophila
Development
Bowl is required downstream of Notch for elaboration of distal limb patterning
Development
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Current Address: Fundación Instituto Valenciano de Infertilidad (FIVI), Valencia University, and Instituto Universitario IVI/INCLIVA, Valencia, Spain