Expression of evolutionarily conserved eye specification genes during Drosophila embryogenesis

Eye specification in Drosophila is thought be controlled by a set of seven nuclear factors that includes the Pax6 homolog, Eyeless. This group of genes is conserved throughout evolution and has been repeatedly recruited for eye specification. Several of these genes are expressed within the developing eyes of vertebrates and mutations in several mouse and human orthologs are the underlying causes of retinal disease syndromes. Ectopic expression in Drosophila of any one of these genes is capable of inducing retinal development, while loss-of-function mutations delete the developing eye. These nuclear factors comprise a complex regulatory network and it is thought that their combined activities are required for the formation of the eye. We examined the expression patterns of four eye specification genes, eyeless (ey), sine oculis (so), eyes absent (eya), and dachshund (dac) throughout all time points of embryogenesis and show that only eyeless is expressed within the embryonic eye anlagen. This is consistent with a recently proposed model in which the eye primordium acquires its competence to become retinal tissue over several time points of development. We also compare the expression of Ey with that of a putative antennal specifying gene Distal-less (Dll). The expression patterns described here are quite intriguing and raise the possibility that these genes have even earlier and wide ranging roles in establishing the head and visual field.     

The lens in focus: a comparison of lens development in Drosophila and vertebrates

The evolution of the eye has been a major subject of study dating back centuries. The advent of molecular genetics offered the surprising finding that morphologically distinct eyes rely on conserved regulatory gene networks for their formation. While many of these advances often stemmed from studies of the compound eye of the fruit fly, Drosophila melanogaster, and later translated to discoveries in vertebrate systems, studies on vertebrate lens development far outnumber those in Drosophila. This may be largely historical, since Spemann and Mangold's paradigm of tissue induction was discovered in the amphibian lens. Recent studies on lens development in Drosophila have begun to define molecular commonalities with the vertebrate lens. Here, we provide an overview of Drosophila lens development, discussing intrinsic and extrinsic factors controlling lens cell specification and differentiation. We then summarize key morphological and molecular events in vertebrate lens development, emphasizing regulatory factors and networks strongly associated with both systems. Finally, we provide a comparative analysis that highlights areas of research that would help further clarify the degree of conservation between the formation of dioptric systems in invertebrates and vertebrates.     

EGF receptor and Notch signaling act upstream of Eyeless/Pax6 to control eye specification

The Drosophila compound eye is specified by the concerted action of seven nuclear factors that include Eyeless/Pax6. These factors have been called "master control" proteins because loss-of-function mutants lack eyes and ectopic expression can direct ectopic eye development. However, inactivation of these genes does not cause the presumptive eye to change identity. Surprisingly, we find that several of these eye specification genes are not coexpressed in the same embryonic cells-or even in the presumptive eye. We demonstrate that the EGF Receptor and Notch signaling pathways have homeotic functions that are genetically upstream of the eye specification genes, and show that specification occurs much later than previously thought-not during embryonic development but in the second larval stage.     

Control of photoreceptor cell morphology, planar polarity and epithelial integrity during Drosophila eye development

We report that the hindsight (hnt) gene, which encodes a nuclear zinc-finger protein, regulates cell morphology, cell fate specification, planar cell polarity and epithelial integrity during Drosophila retinal development. In the third instar larval eye imaginal disc, HNT protein expression begins in the morphogenetic furrow and is refined to cells in the developing photoreceptor cell clusters just before their determination as neurons. In hnt mutant larval eye tissue, furrow markers persist abnormally posterior to the furrow, there is a delay in specification of preclusters as cells exit the furrow, there are morphological defects in the preclusters and recruitment of cells into specific R cell fates often does not occur. Additionally, genetically mosaic ommatidia with one or more hnt mutant outer photoreceptor cells, have planar polarity defects that include achirality, reversed chirality and misrotation. Mutants in the JNK pathway act as dominant suppressors of the hnt planar polarity phenotype, suggesting that HNT functions to downregulate JUN kinase (JNK) signaling during the establishment of ommatidial planar polarity. HNT expression continues in the photoreceptor cells of the pupal retina. When an ommatidium contains four or more hnt mutant photoreceptor cells, both genetically mutant and genetically wild-type photoreceptor cells fall out of the retinal epithelium, indicating a role for HNT in maintenance of epithelial integrity. In the late pupal stages, HNT regulates the morphogenesis of rhabdomeres within individual photoreceptor cells and the separation of the rhabdomeres of adjacent photoreceptor cells. Apical F-actin is depleted in hnt mutant photoreceptor cells before the observed defects in cellular morphogenesis and epithelial integrity. The analyses presented here, together with our previous studies in the embryonic amnioserosa and tracheal system, show that HNT has a general role in regulation of the F-actin-based cytoskeleton, JNK signaling, cell morphology and epithelial integrity during development.     

Continuity versus split and reconstitution: exploring the molecular developmental corollaries of insect eye primordium evolution

Holometabolous insects like Drosophila proceed through two phases of visual system development. The embryonic phase generates simple eyes of the larva. The postembryonic phase produces the adult specific compound eyes during late larval development and pupation. In primitive insects, by contrast, eye development persists seemingly continuously from embryogenesis through the end of postembryogenesis. Comparative literature suggests that the evolutionary transition from continuous to biphasic eye development occurred via transient developmental arrest. This review investigates how the developmental arrest model relates to the gene networks regulating larval and adult eye development in Drosophila, and embryonic compound eye development in primitive insects. Consistent with the developmental arrest model, the available data suggest that the determination of the anlage of the rudimentary Drosophila larval eye is homologous to the embryonic specification of the juvenile compound eye in directly developing insects while the Drosophila compound eye primordium is evolutionarily related to the yet little studied stem cell based postembryonic eye primordium of primitive insects.     

The novel plant homeodomain protein rhinoceros antagonizes Ras signaling in the Drosophila eye

The sequential specification of cell fates in the Drosophila eye requires repeated activation of the epidermal growth factor receptor (EGFR)/Ras/MAP kinase (MAPK) pathway. Equally important are the multiple layers of inhibitory regulation that prevent excessive or inappropriate signaling. Here we describe the molecular and genetic analysis of a previously uncharacterized gene, rhinoceros (rno), that we propose functions to restrict EGFR signaling in the eye. Loss of rno results in the overproduction of photoreceptors, cone cells, and pigment cells and a corresponding reduction in programmed cell death, all phenotypes characteristic of hyperactivated EGFR signaling. Genetic interactions between rno and multiple EGFR pathway components support this hypothesis. rno encodes a novel but evolutionarily conserved nuclear protein with a PHD zinc-finger domain, a motif commonly found in chromatin-remodeling factors. Future analyses of rno will help to elucidate the regulatory strategies that modulate EGFR signaling in the fly eye.     

Perturbing Nuclear Transport in Drosophila Eye Imaginal Discs Causes Specific Cell Adhesion and Axon Guidance Defects

To study nucleocytoplasmic transport during multicellular development, we developed a sensitive nuclear protein import assay in living blastoderm embryos. We show that dominant negative truncations of the human nuclear transport receptor karyopherinbeta/Importinbeta (DNImpbeta) disrupt mRNA export and protein import in Drosophila. To test the sensitivity of different developmental processes to nuclear trafficking perturbations, we expressed DNImpbeta behind the morphogenetic furrow of the eye disc, at a time when photoreceptors are patterned and project their axons to the brain. DNImpbeta expression does not disrupt the correct specification of different photoreceptors, but causes a defect in cell adhesion that leads to some photoreceptors descending below the layer of ommatidia. The photoreceptors initially project their axons correctly to the posterior, but later their axons are unable to enter the optic stalk en route to the brain and continue to project an extensive network of misguided axons. The axon guidance and cell adhesion defects are both due to a disruption in the function of Ketel, the Drosophila ortholog of Importinbeta. We conclude that cell adhesion and axon guidance in the eye have specific requirements for nucleocytoplasmic transport, despite involving processes that occur primarily at the cell surface.