furrowed is expressed in first, second and third instar larvae. Expression is found in eye, wing, leg and haltere imaginal discs of the third instar larvae. In the eye disc, transcripts are present in all cells but are more abundant in cells of the dorsal and ventral margins. In the wing disc, furrowed is expressed more abundantly in the cells of the prospective dorsal and ventral wing surfaces, the ventral pleura and the notum. It is expressed in the antennal, the labial, the clypeo-labral, the humeral and the genital discs. furrowed is also expressed in portions of the digestive system, the gastric caecae, the proventriculus, and is expressed in low levels in the larval brain. The gene is not expressed in other larval tissue such as the salivary glands, fat bodies, the Malpighian tubules, or the cuticle. Protein distribution agrees with the distribution of mRNA (Leshko-Lindsay, 1997).

Effects of mutation or deletion

Mutations in furrowed result in pleiotropic defects in eye development, with deep furrows or folds in the regina of the adult compound eyes. The mutants also have defects in the development of the mechanosensory bristles including the microchaetae, macrochaetae, and the bristles surrounding the eye. Additionally, furrowed mutations result in a shortened head and scutellum, and reduced viability. Analysis of chromosomal deficiencies suggest that the three different alleles examined are hypomorphs and that they do not represent the null furrowed phenotype. This suggests that the furrowed gene product is necessary for developmental processes other than eye and bristle development. Some reduced furrowed function flies have wing defects, such that the wings are not expanded or appear stringy, and show separation of the wing blades. The lethal phase of furrowed occurs in the pupal stages (Leshko-Lindsay, 1997).

The eyes of furrowed mutants are reduced in size, especially in the ventral margin, and show many deep penetrating furrows or crevices suggestive of an effect on the structural integrity of the retina. The furrows penetrate the entire depth of the retina, reaching the basement membrane that separates the retina from the first optic lobe. The ommatidial pattern is severely disorganized and the ommatidia show altered morphology - many lose their typical hexagonal shape and appear to either be flattened, or to have an amorphic structure with no distinct boundary, or to be rounded and bulbous in shape. The interommatidial bristles often show altered morphology and spacing, and are occasionally duplicated. The disturbances in the patterning and morphology of ommatidia suggest that furrowed mutations may affect the recuitment of the cells into the ommatidia in the developing retina (Leshko-Lindsay, 1997).

The basement membrane separating the retina from the lamina layer of the optic lobe is not clearly defined in furrowed mutants. This membrane, which provides openings for photoreceptor axons to pass, is made up of the feet of the cone and the pigment cells. The furrowed mutation may affect the development of these epidermal cells. The irregular shape and depressions observed in the adult lamina of the furrowed mutants could be either a structural compression of the tissue in the lamina due to stresses exerted from the defective retina, or a loss of neural tissue in the lamina due to a disruption in its innervation by the photoreceptors from the developing retina. A study of an enhancer trap line that expresses beta-galactosidase in laminal precursors suggests the latter explanation is better. Thus furrowed mutations might disrupt the photoreceptor neurons or proper innervation of the lamina by these cells (Leshko-Lindsay, 1997).

In wild-type adults, the microchaetae on the thorax are organized into rows, whereas the macrochaete are interspersed in pairs at regular intervals on the thorax and scutellum. All furrowed mutants analyzed show defects in morphology, orientation and patterning of bristles, some of them suggestive of defects in cell determination. Bristle shafts of macrochaetae are shortened, gnarled or bent. Often bristles have altered polarity or orientation such that the shaft does not point to the posterior, as in wild type. Microchaetae are often irregularly spaced on the thorax, and large bald patches are present where bristles are missing. In addition, mutants often show duplications of macrochaetae, both shaft and socket cells, on the scutellum and lower thorax (Leshko-Lindsay, 1997). For mutations causing similar defects, see numb, musashi, and tramtrack).

The Drosophila ommatidia contain two classes of photoreceptor cells (PRs), R1-R6 and R7 and R8, the outer and the inner PRs respectively. An enhancer trapscreen was carried out in order to target genes specifically expressed in PRs. Using the UAS/GAL4 method with enhanced green fluorescent protein (eGFP)as a vital marker, 180,000 flies were screened. Out of 2730 lines exhibiting new eGFP patterns, a focus was placed on 16 lines expressing eGFP inparticular subsets of PRs. In particular, three lines are described with inserts near the spalt major, m-spondin and furrowed (fw) genes, whoserespective expression patterns resemble those genes. These genes had not been reported to be expressed in the adult eye. These examplesclearly show the ability of this screen to target genes expressed in the adult Drosophila eye (Mollereau, 2000). furrowed (fw) mutants have been reported to exhibit a strongmorphological phenotype in the adult eye as well as defectsat the level of the mechanosensory bristles. A possible explanation for the eye phenotype has been derived from the fact that fw is expressed in theeye imaginal disc, suggesting a role for fw in early eye development. However, it is possible that the mutant phenotype isdue to the loss of the strong adult expression that has beenobserved in the primary pigment cells (Mollereau, 2000).


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furrowed: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation

date revised: 20 October 2013 

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