Formation and specification of distal leg segments in Drosophila by dual Bar homeobox genes, BarH1 and BarH2

Here, we show that BarH1 and BarH2, a pair of Bar homeobox genes, play essential roles in the formation and specification of the distal leg segments of Drosophila. In early third instar, juxtaposition of Bar-positive and Bar-negative tissues causes central folding that may separate future tarsal segments 2 from 3, while juxtaposition of tissues differentially expressing Bar homeobox genes at later stages gives rise to segmental boundaries of distal tarsi including the tarsus/pretarsus boundary. Tarsus/pretarsus boundary formation requires at least two different Bar functions, early antagonistic interactions with a pretarsus-specific homeobox gene, aristaless, and the subsequent induction of Fas II expression in pretarsus cells abutting tarsal segment 5. Bar homeobox genes are also required for specification of distal tarsi. Bar expression requires Distal-less but not dachshund, while early circular dachshund expression is delimited interiorly by BarH1 and BarH2.     

Specification of Primary Pigment Cell and Outer Photoreceptor Fates byBarH1Homeobox Gene in the DevelopingDrosophilaEye

In the developingDrosophilaeye,BarH1andBarH2, paired homeobox genes expressed in R1/R6 outer photoreceptors and primary pigment cells, are essential for normal eye morphogenesis. Here, we show evidence thatBarH1ectopically expressed under the control of thesevenlessenhancer (sev-BarH1) causes two types of cone cell transformation: transformation of anterior/posterior cone cells into outer photoreceptors and transformation of equatorial/polar cone cells into primary pigment cells.sev-BarH1repressed the endogenous expression of theroughhomeobox gene in R3/R4 photoreceptors, while theBarH2homeobox gene was activated bysev-BarH1in an appreciable fraction of extra outer photoreceptors. In primary pigment cells generated by cone cell transformation, the expression ofcut,a homeobox gene specific to cone cells, was completely replaced with that ofBarhomeobox genes. Extra outer photoreceptor formation was suppressed and enhanced, respectively, by reducing the activity of Ras/MAPK signaling and by dosage reduction ofyan,a negative regulator of the pathway, suggesting interactions betweenBarhomeobox genes (cell fate determinants) and Ras/MAPK signaling in eye development.     

tailup, a LIM-HD gene, and Iro-C cooperate in Drosophila dorsal mesothorax specification

The LIM-HD gene tailup (tup; also known as islet) has been categorised as a prepattern gene that antagonises the formation of sensory bristles on the notum of Drosophila by downregulating the expression of the proneural achaete-scute genes. Here we show that tup has an earlier function in the development of the imaginal wing disc; namely, the specification of the notum territory. Absence of tup function causes cells of this anlage to upregulate different wing-hinge genes and to lose expression of some notum genes. Consistently, these cells differentiate hinge structures or modified notum cuticle. The LIM-HD co-factors Chip and Ssdp are also necessary for notum specification. This suggests that Tup acts in this process in a complex with Chip and Ssdp. Overexpression of tup, together with araucan, a 'pronotum' gene of the iroquois complex (Iro-C), synergistically reinforces the weak capacity of either gene, when overexpressed singly, to induce ectopic notum-like development. Whereas the Iro-C genes are activated in the notum anlage by EGFR signalling, tup is positively regulated by Dpp signalling. Our data support a model in which the EGFR and Dpp signalling pathways, with their respective downstream Iro-C and tup genes, converge and cooperate to commit cells to the notum developmental fate.     

Prepatterning the Drosophila notum: the three genes of the iroquois complex play intrinsically distinct roles

The Drosophila thorax exhibits 11 pairs of large sensory organs (macrochaetes) identified by their unique position. Remarkably precise, this pattern provides an excellent model system to study the genetic basis of pattern formation. In imaginal wing discs, the achaete-scute proneural genes are expressed in clusters of cells that prefigure the positions of each macrochaete. The activities of prepatterning genes provide positional cues controlling this expression pattern. The three homeobox genes clustered in the iroquois complex (araucan, caupolican and mirror) are such prepattern genes. mirror is generally characterized as performing functions predominantly different from the other iroquois genes. Conversely, araucan and caupolican are described in previous studies as performing redundant functions in most if not all processes in which they are involved. We have addressed the question of the specific role of each iroquois gene in the prepattern of the notum and we clearly demonstrate that they are intrinsically different in their contribution to this process: caupolican and mirror, but not araucan, are required for the neural patterning of the lateral notum. However, when caupolican and/or mirror expression is reduced, araucan loss of function has an effect on thoracic bristles development. Moreover, the overexpression of araucan is able to rescue caupolican loss of function. We conclude that, although retaining some common functionalities, the Drosophila iroquois genes are in the process of diversification. In addition, caupolican and mirror are required for stripe expression and, therefore, to specify the muscular attachment sites prepattern. Thus, caupolican and mirror may act as common prepattern genes for all structures in the lateral notum.     

odd-skipped genes and lines organize the notum anterior-posterior axis using autonomous and non-autonomous mechanisms

The growth and patterning of Drosophila wing and notum primordia depend on their subdivision into progressively smaller domains by secreted signals that emanate from localized sources termed organizers. While the mechanisms that organize the wing primordium have been studied extensively, those that organize the notum are incompletely understood. The genes odd-skipped (odd), drumstick (drm), sob, and bowl comprise the odd-skipped family of C(2)H(2) zinc finger genes, which has been implicated in notum growth and patterning. Here we show that drm, Bowl, and eyegone (eyg), a gene required for notum patterning, accumulate in nested domains in the anterior notum. Ectopic drm organized the nested expression of these anterior notum genes and downregulated the expression of posterior notum genes. The cell-autonomous induction of Bowl and Eyg required bowl, while the non-autonomous effects were independent of bowl. The homeodomain protein Bar is expressed along the anterior border of the notum adjacent to cells expressing the Notch (N) ligand Delta (Dl). bowl was required to promote Bar and repress Dl expression to pattern the anterior notum in a cell-autonomous manner, while lines acted antagonistically to bowl posterior to the Bowl domain. Our data suggest that the odd-skipped genes act at the anterior notum border to organize the notum anterior-posterior (AP) axis using both autonomous and non-autonomous mechanisms.     

Dpp signalling is a key effector of the wing-body wall subdivision of the Drosophila mesothorax

During development, the imaginal wing disc of Drosophila is subdivided along the proximal-distal axis into different territories that will give rise to body wall (notum and mesothoracic pleura) and appendage (wing hinge and wing blade). Expression of the Iroquois complex (Iro-C) homeobox genes in the most proximal part of the disc defines the notum, since Iro-C(-) cells within this territory acquire the identity of the adjacent distal region, the wing hinge. Here we analyze how the expression of Iro-C is confined to the notum territory. Neither Wingless signalling, which is essential for wing development, nor Vein-dependent EGFR signalling, which is needed to activate Iro-C, appear to delimit Iro-C expression. We show that a main effector of this confinement is the TGFbeta homolog Decapentaplegic (Dpp), a molecule known to pattern the disc along its anterior-posterior axis. At early second larval instar, the Dpp signalling pathway functions only in the wing and hinge territories, represses Iro-C and confines its expression to the notum territory. Later, Dpp becomes expressed in the most proximal part of the notum and turns off Iro-C in this region. This downregulation is associated with the subdivision of the notum into medial and lateral regions.     

DNA binding activity of the BarH1 homeodomain of Drosophila.

Using a series of deletion mutants of BarH1, a Drosophila homeobox gene required for eye morphogenesis, the DNA-binding region of the BarH1 protein was determined. Not only homeodomain but also its upstream sequence were found to be necessary for binding, whereas about a half of the conserved downstream sequence (Bar domain) was dispensable.     

Distinct expression patterns for two Xenopus Bar homeobox genes

The BarH1 and BarH2 homeobox genes are coexpressed in cells of the fly retina and in the central and peripheral nervous systems. The fly Bar genes are required for normal development of the eye and external sensory organs. In Xenopus we have identified two distinct vertebrate Bar-related homeobox genes, XBH1 and XBH2. XBH1 is highly related in sequence and expression pattern to a mammalian gene, MBH1, suggesting that they are orthologues. XBH2 has not previously been identified but is clearly related to the Drosophila Bar genes. During early Xenopus embryogenesis XBH1 and XBH2 are expressed in overlapping regions of the central nervous system. XBH1, but not XBH2, is expressed in the developing retina. By comparing the expression of XBH1 with that of hermes, a marker of differentiated retinal ganglion cells, we show that XBH1 is expressed in retinal ganglion cells during the differentiation process, but is down-regulated as cells become terminally differentiated. Received: 12 August 1999 / Accepted: 5 October 1999     

Mutual repression between msh and Iro-C is an essential component of the boundary between body wall and wing in Drosophila

During development, the imaginal wing disc of Drosophila is subdivided into territories separated by developmental boundaries. The best characterized boundaries delimit compartments defined by cell-lineage restrictions. Here, we analyze the formation of a boundary that does not rely on such restrictions, namely, that which separates the notum (body wall) and the wing hinge (appendage). It is known that the homeobox genes of the Iroquois complex (Iro-C) define the notum territory and that the distal limit of the Iro-C expression domain demarks the boundary between the notum and the wing hinge. However, it is unclear how this boundary is established and maintained. We now find that msh, a homeobox gene of the Msx family, is strongly expressed in the territory of the hinge contiguous to the Iro-C domain. Loss- and gain-of-function analyses show that msh maintains Iro-C repressed in the hinge, while Iro-C prevents high level expression of msh in the notum. Thus, a mutual repression between msh and Iro-C is essential to set the limit between the contiguous domains of expression of these genes and therefore to establish and/or maintain the boundary between body wall and wing. In addition, we find that msh is necessary for proper growth of the hinge territory and the differentiation of hinge structures. msh also participates in the patterning of the notum, where it is expressed at low levels.