coracle

DEVELOPMENTAL BIOLOGY

Embryonic

See the embryonic expression pattern of cora at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site

Protein expression was not detectable on whole mount embryos before stage 12, just at the onset of germ band retraction. Coracle appears simultaneously in the epidermis, hindgut and foregut. As the germ band retracts, expression increases in these tissues and also appears in the tracheal branches and salivary glands. Coracle is not expressed within the CNS although it is expressed within some peripheral nervous system sensory neural cells such as the scolopidia of the pentascolopidial chordotonal organ. Once established, this pattern of expression is maintained throughout embryogenesis. Differential distribution of the different transcripts are not observed (Fehon, 1994).

Coracle is closely associated with the cell membrane in all expressing cells. In tall columnar epithelia, such as hindgut, there is an obvious polarity of Coracle expression toward the apicolateral epithelial surface (Fehon, 1994).

Larval

Imaginal disc cells show intense staining in regions of cell contact just below the apical epithelial surface, just as was observed in embryonic epithelia. Filamentous actin is largely localized to the most apical region of the cell, while the majority of Coracle localization is clearly more basal. In addition, there is a significant amount of filamentous actin in the basolateral domain of these cells, but only background levels of Coracle. Antibodies against Drosophila Spectrin have shown that Spectrin has a similar apical and basal location to that of actin, although the basolateral component is more predominant for spectrin. Thus, the Coracle protein does not appear to colocalize to a great extent with either filamentous actin or spectrin in the apical-basal axis of the cell (Fehon, 1994).

The distribution of proteins in the apico-lateral cell junctions in Drosophila imaginal discs was examined. The subcellular distribution of these proteins in normal and mutant proliferating cells was analyzed with marker antibodies and confocal microscopy. Antibodies to phosphotyrosine (PY), Armadillo (Arm) and Drosophila E-cadherin (DE-cad) as well as FITC phalloidin marking filamentous actin, labeled the site of the adherens junction, whereas antibodies to Discs large (Dlg), Fasciclin III (FasIII) and Coracle (Cor) labeled the more basal septate junction. The junctional proteins labeled by these antibodies underwent specific changes in distribution during the cell cycle. A loss-of-function dlg mutation, which causes neoplastic imaginal disc overgrowth, leads to loss of the septate junctions and the formation of what appear to be ectopic adherens junctions (Woods et al., 1996).

The current study was expanded to examine the effects of mutations in other genes that also cause imaginal disc overgrowth. Based on staining with PY and Dlg antibodies, the apico-lateral junctional complexes appear normal in tissue from the hyperplastic overgrowth mutants fat facets, discs overgrown, lethal (2) giant discs and warts. However, imaginal disc tissue from the neoplastic overgrowth mutants dlg and lethal (2) giant larvae show abnormal distribution of the junctional markers including a complete loss of apico-basal polarity in loss-of-function dlg mutations. These results support the idea that some of the proteins of apico-lateral junctions are required both for apico-basal cell polarity and for the signaling mechanisms controlling cell proliferation, whereas others are required more specifically in cell-cell signaling (Woods, 1997).

Invasive cell behavior during Drosophila imaginal disc eversion is mediated by the JNK signaling cascade

Drosophila imaginal discs are monolayered epithelial invaginations that grow during larval stages and evert at metamorphosis to assemble the adult exoskeleton. They consist of columnar cells, forming the imaginal epithelium, as well as squamous cells, which constitute the peripodial epithelium and stalk (PS). A new morphogenetic/cellular mechanism for disc eversion has been uncovered. Imaginal discs evert by apposing their peripodial side to the larval epidermis and through the invasion of the larval epidermis by PS cells, which undergo a pseudo-epithelial-mesenchymal transition (PEMT). As a consequence, the PS/larval bilayer is perforated and the imaginal epithelia protrude, a process reminiscent of other developmental events, such as epithelial perforation in chordates. When eversion is completed, PS cells localize to the leading front, heading disc expansion. The JNK pathway is necessary for PS/larval cells apposition, the PEMT, and the motile activity of leading front cells (Pastor-Pareja, 2004).

One hallmark of epithelial cells is their distinct apico-basal cell polarity. This polarity depends on a set of intercellular connections, which encircle epithelial cells at the border of the apical and basal-lateral membrane domains. The cells in insect epithelial tissues are interconnected by zonula adherens (ZAs), which function in both cellular adhesion and signaling. DE-cadherin is the major constituent of the ZAs in a complex with Armadillo (Arm, ß-catenin) and Dalpha-catenin. In addition, epithelia of flies and other invertebrates exhibit septate junctions, which are located basally to the ZAs. Septate junctions prevent diffusion through the pericellular space and are functionally equivalent to vertebrate tight junctions (Pastor-Pareja, 2004).

All imaginal disc cells at the third instar larval stage presented ZAs in an apical belt. During disc eversion, however, it was found that ZAs components delocalize from the free edges of the PS cells, remaining cytoplasmic at the edges of the perforations arising through the PS/larval bilayer and in those PS cells leading the spreading of the discs over the larval tissues. As a consequence, ZAs are lost in these cells. Moreover, septate junction components, such as Coracle and Disc Large are also found to be missing from the membranes of leading front cells (Pastor-Pareja, 2004).

The loss of apico/basal polarity and adhesion of the PS cells during disc eversion is reminiscent of an epithelial-mesenchymal transition (EMT), as described for mesoderm and neural crest cells in vertebrates, and for the acquisition of the invasive phenotype in carcinomas (Pastor-Pareja, 2004).

In summary, the evagination of imaginal disc can be divided into the following sequential steps: (1) an overall positional change of the imaginal discs leading to the confrontation and apposition of the PS and the larval epidermis; (2) a regulated modulation (PEMT) of PS cells, which involves the downregulation of their cell-cell adhesion systems and allows them to move into their local neighborhood and invade the larval epithelium; (3) the fenestration of the peripodial/larval bilayer and the formation of an unbound peripodial leading front, which will direct imaginal spreading by planar cell intercalation, and (4) a bulging of the imaginal tissue (Pastor-Pareja, 2004).

Once the hole is opened, the planar intercalation of PS cells ensures that, first in the hole and later in the leading front, all four dorsal, ventral, anterior, and posterior compartments of the wing disc are represented. This mechanism also guarantees the maintenance of a continuous epithelial barrier (Pastor-Pareja, 2004).

Transient apical polarization of Gliotactin and Coracle is required for parallel alignment of wing hairs

In Drosophila, wing hairs are aligned in a distally oriented, parallel array. The frizzled pathway determines proximal-distal cell polarity in the wing; however, in frizzled pathway mutants, wing hairs remain parallel. How wing hairs align has not been determined. A novel role for the septate junction proteins Gliotactin (Gli) and Coracle (Cora) in this process has been demonstrated. Prior to prehair extension, Gli and Cora are restricted to basolateral membranes. During pupal prehair development, Gli and Cora transiently form apical ribbons oriented from the distal wing tip to the proximal hinge. These ribbons are aligned beneath prehair bases and persisted for several hours. During this time, Gli was lost entirely from the basolateral domain. A Gliotactin mutation alters the apical polarization Gli and Cora and induces defects in hair alignment in pupal and adult stages. Genetic and cell biological assays demonstrate that Gli and Cora function to align hairs independently of frizzled. Taken together, these results indicate that Gli and Cora function as the first-identified members of a long-predicted, frizzled-independent parallel alignment mechanism. A model is proposed whereby the apical polarization of Gli and Cora functions to stabilize and align prehairs relative to anterior-posterior cell boundaries during pupal wing development (Venema, 2004).

The Gliotactin null phenotype is paralysis and death due to an open blood-nerve barrier at the end of embryogenesis. No adult-viable mutant Gli genotypes have been reported. In order to extend the analysis of Gliotactin function postembryogenesis, a mutagenesis screen was performed and a number of novel, ethylmethane-sulfonate (EMS)-induced Gli alleles were isolated that failed to complement the null allele Gli^AE2Δ45. One novel allele, Gli^dv5, was homozygous viable. Sequencing of this allele revealed a single, nonconservative mutation in the extracellular serine esterase-like domain. Several of the novel and previously identified homozygous-lethal Gli alleles generated adult escapers in trans to Gli^dv5, resolving these alleles into an allelic series. Structural changes underlying this allelic series were determined by sequencing each allele. Gli^RAR77 was found to contain a nonsense mutation in the intracellular carboxyl-terminal region. This homozygous-lethal allele generated more adult escapers in trans to Gli^dv5 than Gli^dv5 itself, indicating that the extracellular and intracellular domains of Gli can function together when on separate proteins. The Gli^dv1 allele results from a premature stop codon within the serine esterase like domain; this allele escaped in trans to Gli^dv5 at a low frequency. Two alleles (Gli^P34 and Gli^dv3) rarely escaped in trans to Gli^dv5; both alleles are premature stop codons early in the Gli open reading frame (Venema, 2004).

The availability of Gli genotypes that survive to adult stages allowed an examination of adult epithelia for mutant phenotypes. Adult wings of Gli mutants were examined for defects. Gli mutants display an increasing severity of wing hair orientation defects that parallels the Gli allelic series. The primary Gli wing hair phenotype was disruption of parallel alignment between neighboring hairs such that adjacent hairs converge in a chevron pattern. This phenotype was present in Gli wings not mounted under a coverslip, as well as in Gli wings examined using scanning electron microscopy, and as such is not a mounting artifact. To quantify the Gli phenotype, veins were used to divide the wing into seven main regions, 20 wings of each genotype were scored, and the number of regions that contained patches of nonparallel hairs were counted. The Gli⁺ chromosome used for mutagenesis had the wild-type pattern of parallel, distally oriented hairs. In weak Gli genotypes, only small numbers of hairs are affected, resulting in small, infrequent mutant patches. Random samples of 20 Gli^dv5/Gli^RAR77 or Gli^dv5/Gli^dv5 wings ranged from wings with disrupted hairs in every wing region to completely wild-type wings. On average, Gli^dv5/Gli^RAR77 wings had 2.9 regions with altered alignment compared to 3.4 for Gli^dv5/Gli^dv5 wings. In stronger Gli genotypes, mutant patches were more frequent and larger. All 20 Gli^dv5/Gli^dv1 wings sampled had at least one region of disrupted alignment, with an average of 4.1 regions affected. In the severe Gli^dv5/Gli^dv3 genotype, mutant patches covered large portions of the wing and were consistently present. In a random sample of 20 Gli^dv5/Gli^dv3 wings, eight had an area of disrupted alignment in six wing regions; the remaining 12 wings had disrupted alignment in all seven intervein areas, giving an average of 6.7 disrupted regions per wing. Despite this consistent phenotype, hairs with correct alignment remained in even the most severely affected Gli^dv5/Gli^dv3 wings. In addition to nonparallel alignment, patches of deformed hairs were occasionally observed in all adult Gli genotypes, often at the anterior-posterior boundary. Deformed hairs were thinner than normal and were often bent. A third phenotype, blistering at the distal margin, was observed in the strongest Gli genotype, Gli^dv5/Gli^dv3. In a sample of 20 Gli^dv5/Gli^dv3 wings, 7 contained blisters. Such blisters were always in contact with the distal margin (Venema, 2004).

In Drosophila, septate junctions are initially formed during embryogenesis: these septate junctions persist in epithelia derived from the epidermis, such as imaginal structures. In all previously examined tissues, septate junctions, once formed, appear static in the absence of cell division. In contrast, Gli and Cora are extensively remodeled during wing development at several time points in postmitotic cells. The initial Gli/Cora pattern in the wing before prehair extension is identical to that of the embryonic epidermis. Indeed, Gli and Cora remain stable until after prehair extension at approximately 32 h APF. Beginning at 35 h APF, Gli and Cora are apically polarized at the wing margins. Althought the initial stages of Cora polarization have been noted previously, previous studies did not assay Cora localization past 35 h APF or examine mutant cora genotypes. By 37 h APF, Gli and Cora colocalize to continuous apical ribbons across the pupal wing. These ribbons run along anterior-posterior cell boundaries and between proximal and distal vertices, passing just basal to prehair bases. At this stage, Gli is absent from basolateral membranes, yet Cora retains its basolateral pattern. This pattern remains stable until at least 40 h APF. By 47 h APF, Cora has lost its apical polarization and is present only around the basolateral cell periphery. In contrast, Gli is lost apically and basolaterally with only minimal Gli present on basolateral membranes, which is not predominantly tricellular at 47 h APF. It may be that the weak tricellular staining of Gli at this time point represents the start of relocalizing Gli to the tricellular septate junction; examination of Gli at stages after 47 h APF is underway to test this possibility (Venema, 2004).

The dynamic rearrangement of these junction components is without precedent in the literature. While septate junction components such as Discs-large, Coracle, Neurexin IV, Neuroglian, and the Na⁺/K⁺ ATPase are expressed in wing discs, these studies have not examined the localization of septate junction components past 30 h APF nor examined mutant genotypes for wing hair or adhesion phenotypes. Thus, it is an open question whether these septate junction proteins also are dynamic during later stages of wing development. It remains to be seen whether these components are recruited to the apical Gli/Cora ribbon and if their basolateral localization at septate junctions is modified during Gli and Cora polarization (Venema, 2004).

Loss of Dlg function in wing discs causes an overgrowth phenotype that includes loss of apical-basal polarity. In contrast to Dlg, cora null clones do not survive to adult stages. Additionally, clones for null alleles of Nrg and Nrv2 do not survive, consistent with the results that null Gli clones are cell lethal. Given the essential nature of the septate junction for both proliferation control and cell viability, it will not be possible to assess the adult wing phenotype for complete loss of septate-junction function. Attempts to induce null Gli clones after 30 h APF have not proved successful since cell division in the wing ceases around this stage. Null clones induced during the last round of cell division did not survive long enough to examine the start of Gli/Cora polarization at 35 h APF (Venema, 2004).

The polarization of Gli and Cora to apical ribbons is compromised in Gli mutant wings, which in turn leads to unstable prehairs later in development. The close proximity of the Gli ribbon to the prehair base, and the orientation of the ribbon along the axis the hairs will align to, is compelling evidence for a physical connection between prehairs and Gli/Cora ribbons that is required for hair alignment. In Gli mutant wings, the polarization of apical Gli and Cora is incomplete; additionally, it does not persist for the correct length of time. In wild-type wings, Gli and Cora remain polarized until at least 40 h APF; in contrast, in Gli mutant wings Gli is lost altogether by 40 h APF and Cora is no longer polarized. Thus, the Gli nonparallel phenotype seems to result from either a lack of apically polarized Gli/Cora or an insufficient time span of their polarization, or perhaps a combination of both. In either case, the nonparallel phenotype seems to arise from a failure to properly polarize apical Gli and Cora, which in turn leads to prehair instability later in development (Venema, 2004).

Though apical Gli and Cora define the distal-to-proximal axis in the wing, how they are connected to the prehair for its stabilization remains unresolved. The separation between the prehair base and the apical ribbon of Gli and Cora passing beneath it suggests an intermediate link between the two structures. A possible candidate to connect the Gli/Cora ribbon to the prehair pedestal is the cytoskeleton. Physical and genetic links between septate junction components and the cytoskeleton have previously been reported. Both subunits of the Na⁺/K⁺ ATPase are localized to the septate junction and physically interact with ankyrin, which acts as an adapter between transmembrane proteins and the spectrin/F-actin network. In Drosophila, the subcellular location of the Na⁺/K⁺ ATPase is disrupted in β spectrin mutants. Additionally, Neuroglian clustering assembles ankyrin at cell-cell contacts through physical association. Interactions through either component could physically connect the Gli/Cora ribbon to the F-actin prehair through spectrin; it remains to be seen, however, if the Na⁺/K⁺ ATPase and/or Neuroglian are apically polarized in conjunction with Gli and Cora. Previous studies in the wing have demonstrated that actin polymerization is necessary for prehair formation and cell viability. Disruption of F-actin regulation also causes wing hair phenotypes: perturbation of RhoA and its downstream effector Drok causes multiple hair phenotypes. Application of F-actin-disrupting drugs to pupal wings also causes a multiple hair phenotype. The Gli hair phenotype does not include multiple hairs per cell, suggesting that prehair F-actin is correctly regulated in Gli mutants (Venema, 2004).

The tubulin cytoskeleton is also a potential candidate for connecting the septate junction to the apical prehair. Microtubules form a web that contacts the cell periphery at the level of Cora staining at 30 h APF; additionally, microtubules are present inside the prehair after elongation. Experiments are underway to determine if the tubulin cytoskeleton is polarized to prehair bases in conjunction with Gli and Cora (Venema, 2004).

A relationship between septate junction components and the cytoskeleton may also explain the Gli/septate-junction marginal adhesion defects. Adhesion between wing layers is dependent on integrins at basal-basal contacts between wing layers and on the transalar cytoskeleton connecting them. Interestingly, this dependence does not extend to the wing margin: Clones lacking β-integrin adjacent to the wing margin do not cause blisters, even when the clone spans the wing margin, placing cells lacking β-integrin into basal-basal contact. Gli mutations are the first described that blister specifically at the wing margin. Adhesion in this area is thus dependent on Gli and independent of integrins. While the ultrastructure of the transalar cytoskeleton in this region has not been reported, the results suggest the possibility that the septate junction may be an alternative to integrin-based focal adhesions in this area of the wing (Venema, 2004).

When a mechanism functioning to align wing hairs in parallel was first suggested, it was predicted to be frizzled independent. The results have borne this prediction out. Gli and Cora are the first factors necessary for hair orientation shown to function independently of frizzled signaling: The localization and apical polarization of Gli and Cora does not require fz function; conversely, the fz pathway is unaffected by mutations in Gli. Gli and Cora are also the first factors shown to polarize in line with the distal-proximal axis in the pupal wing, the converse pattern of the fz pathway components. Thus, it appears that proximal-to-distal hair polarity is controlled by factors polarized in line with the anterior-posterior axis, while parallel alignment is controlled by factors polarized in line with the proximal-distal axis. Additionally, the Gli nonparallel phenotype appears to arise from unstable prehairs as a result of compromised, but partially functional, apically polarized Gli and Cora. The formation of the final wing hair pattern thus has two phases: the specification of the site of prehair emergence by the fz signaling pathway, and subsequent alignment and stabilization of extended prehairs through apically polarized septate junction components. Future experiments are necessary to determine the precise link between the apical Gli/Cora ribbon and the prehair pedestal; however, the demonstrated interactions between F-actin/ankyrin and the septate junction markers Neuroglian and the Na⁺/K⁺ ATPase are likely candidates for further investigation (Venema, 2004).

Effects of Mutation or Deletion

The primary effect of the two known truncation mutations in the coracle gene is a failure in Dorsal closure. Both alleles cause recessive embryonic lethaity, with the stronger allele causing abnormalities in head involution. Other Drosophila genes whose mutants are defective in dorsal closure include basket (also known as JNK), hemipterous, neurexin, Rac1, and zipper (also known as Myosin II). In addition, coracle mutations dominantly suppress Ellipse, a hypermorphic allele of the Drosophila EGF-receptor homolog. (Fehon, 1994).

Although extensively studied biochemically, members of the Protein 4.1 superfamily have not been as well characterized genetically. A phenotypic analysis of coracle has been carried out. Screens for new coracle alleles confirm the null coracle phenotype of embryonic lethality and failure in dorsal closure, and they identify additional defects in the embryonic epidermis and salivary glands. Hypomorphic coracle alleles reveal functions in many imaginal tissues. Analysis of coracle mutant cells indicates that Coracle is a necessary structural component of the septate junction required for the maintenance of the transepithelial barrier but is not necessary for apical-basal polarity, epithelial integrity, or cytoskeletal integrity. In addition, coracle phenotypes suggest a specific role in cell signaling events. Complementation analysis suggests that the conserved amino terminal region (CNTR) of Coracle constitutes a functional domain and that sequences C-terminal to this domain may also be functionally significant (Lamb, 1998).

Several studies have suggested that coracle and some of the other genes required for dorsal closure may play a role in epithelial morphogenesis. To better assess the functions of coracle in the embryonic epidermis, phenotypes of null and strongly hypomorphic mutant embryos have been examined. Cuticle preparations of coracle mutant embryos display characteristic epidermal phenotypes in addition to dorsal closure defects. In coracle mutant embryos the cuticle appears to be thinner than normal and often appears to split into two layers in cuticle preparations. The cuticular thinning is most obvious on the ventral surface where the denticle belts appear faint in cuticle preparations. Although the overall segmental pattern is normal, denticle belts contain fewer than normal denticles, and there are correspondingly fewer hairs on the dorsal side. Ultrastructural examination of the epidermis and cuticle reveals that the apparent delamination of the cuticle results from a failure of the epicuticle to adhere to the procuticle. All of the embryonic lethal alleles also display salivary gland defects, which are apparent as necrotic material that remains in cuticle preparations. This salivary gland defect is observed in embryos that have been aged beyond the stage at which wild-type embryos hatch as larvae, suggesting that this necrosis is a very late effect (Lamb, 1998).

The salivary glands express coracle at high levels. To study the effects of loss of coracle function at the cellular level in epithelial cells, the morphology of salivary gland epithelia in cor⁵ mutant embryos was studied using several molecular markers for the plasma membrane. These experiments were performed in midembryogenesis (stage 14), before the salivary gland necrosis described earlier becomes apparent. Markers for the adherens junction (Armadillo and Notch) and the apical membrane (Crumbs) all displayed normal subcellular localizations in cor⁵ mutant cells. It is important to note that these embryos have no detectable Coracle protein and that coracle is not expressed maternally. These results indicate that the apical-basal polarity of epithelial cells is not grossly affected by loss of Coracle function (Lamb, 1998).

It has been reported that in cor⁵ mutant embryos, Neurexin is mislocalized, raising the possibility that the integrity of the septate junction is compromised in coracle mutant embryos. To investigate this idea, an ultrastructural examination of the epidermis and internal epithelia of late stage 17 mutant embryos was carried out. Similar to the observations regarding loss of Neurexin, the septate junction in cor⁵ mutant epithelia is disrupted. Although the adherens junction appears unaffected, the individual septae that characterize the pleated septate junction are always absent in cor⁵ mutant tissue. To examine the functional consequences of this lack of an organized septate junction, the transepithelial barrier function of coracle mutant epithelia was carried out using a 10-kDa rhodamine-labeled dextran. This dye was injected into the hemocoel of stage 16 embryos. Although in wild-type embryos the dextran is confined to the hemocoel even 1 h after injection, in cor⁵ mutant embryos the dextran rapidly crosses the salivary gland epithelium and fills the luminal space. The epidermis, hindgut, and tracheae are similarly compromised. Taken together, these results demonstrate the essential requirement for coracle in the integrity of the transepithelial barrier function of the septate junction (Lamb, 1998).

To examine further the effects of loss of coracle function in epithelial cells, somatic mosaic analysis was used to generate cor minus cells in the imaginal epithelium. cor⁴ somatic clones were generated ~76 h after egg laying using the FLP recombinase/FRT target system. coracle mutant clones were observed in late third instar wing imaginal discs using anti-Coracle antibodies to identify mutant cells. The cor⁴ allele was chosen for this analysis because it is a strong allele that disrupts the ability of Coracle to associate with the plasma membrane. Cells within the mutant clones are contiguous with the rest of the imaginal epithelium and appear normally shaped. Loss of coracle function in imaginal epithelial cells results in disruption of Neurexin localization. In addition, Neurexin protein is not readily detectable in the center of cor^- clones, suggesting that in imaginal epithelia Neurexin is not stable in the absence of coracle function. To assess apical-basal polarity and cytoskeletal organization in cor minus cells, imaginal discs were stained with antibodies for Notch, beta_H-Spectrin, and Moesin. As in the embryonic epidermis, these markers for the apical junctional region are localized normally in mutant cells. In addition, the distribution of filamentous actin was examined using rhodamine phalloidin and also found to be normal. These results indicate that coracle function is not necessary for overall apical-basal polarity or cytoskeletal organization in imaginal tissues; however, when adult flies were examined for the presence of cor minus tissue, no mutant clones could be observed either in the eye or in the thorax, indicating that cor minus cells do not persist to the adult stage (Lamb, 1998).

The majority of coracle alleles are embryonic lethal; however, some weak alleles and several hypomorphic allelic combinations produce adult escapers. For example, cor⁷ is mostly embryonic lethal, but it produces 7% of expected viable, fertile adults and can be maintained as a homozygous stock at 25°C. cor¹⁴ is almost exclusively larval lethal, but 0.3% of the expected homozygous class survived to viable adults. cor¹⁵ is completely viable, with 100% of the expected homozygotes surviving to adulthood. All of the coracle alleles that produced adult escapers display a similar range of adult defects, although the severity and penetrance of the phenotypes varies with genotype. All of these alleles are also cold sensitive (Lamb, 1998).

All coracle escapers display some degree of a rough eye phenotype. These eye defects range from a slight roughening of the eye, especially across the equator, to eyes in which part of the eye tissue in the anterior equatorial region is replaced by head cuticle. The penetrance of severe phenotypes increases with the strength of the coracle allele, to include up to 60% of the mutant adults. Histological sections show that the roughened appearance of coracle mutant eyes is caused by abnormally shaped, fused, improperly spaced, and occasionally misoriented ommatidia, rather than abnormal photoreceptor cell differentiation. Essentially all of the ommatidia in the mutant eyes display normal numbers and positions of photoreceptors. Interommatidial bristles are also frequently lost in the roughened area. Another very penetrant phenotype seen in coracle mutant animals is partial or total loss of the ocelli and associated bristles. In addition to the eye defects, coracle hypomorphic escapers display a range of other pleiotropic defects. These include wing vein phenotypes (interrupted cross veins and truncated fifth veins); rotational defects in the male genital apparatus; kinked or curved bristles, and leg abnormalities. Also both male and female escapers displayed partial or complete sterility (Lamb, 1998).

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date revised: 20 April 2005 

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