phyllopod

DEVELOPMENTAL BIOLOGY

Embryonic

phyl is normally found in both the central nervous system and the PNS. Mutants reveal a variable phenotype with neurons missing in each of the major groups of PNS neurons, including the multiple dendrite neurons, the chordotonal neurons, and the external sensory organs (Chang, 1995).

Larval

PHYL transcripts are express within the morphogenetic furrow coinciding with the stripe of decapentaplegic expression. PHYL mRNA can be detected in regularly spaced clusters of about 12 cells. This pattern is similar to the expression pattern of scabrous, suggesting that these clusters correspond to the proneural clusters from which the R8 cell is specified. The staining intensity is drastically reduced immediately behind the furrow and then disappears before becoming more intense, once again approximately six rows behind the furrow. Posterior to the furrow, PHYL mRNA is mostly confined to a thin stripe, more or less spanning the region between ommatidial columns 4-8. Within this stripe, PHYL mRNA accumulates in clusters that at high magnification appear to consist of three cells: R1, R6 and R7. Within the R1, R6, and R7 photoreceptors, phyl expression mimics activation of the Raf pathway in its ability to induce photoreceptor rather than cone cell development. The transformation of cone cells into R7 cells in response to Raf activation is both accompanied by and dependent upon ectopic phyl expression. phyl expression is elevated in cone cell precursors in constitutively active raf (raftorY9) and yan mutant discs. phyl thus represents a possible target gene of the Raf pathway during eye development, controlling the fates of a novel subset of photoreceptors (Dickson, 1995).

Effects of mutation or deletion

A range of bristle defects is observed in phyllopod clones. Notal clones often lack macrochaetae and microchaetae, although in some instances these bristles are twinned or triplicated. Twinned bristles within phyl mutant clones lack neurons, suggesting a possible fate transformation from neuronal precursor cell to a trichogen (shaft cell). Clonal regions that are void of microchaetae also lack neurons. Retinal mechanosensory bristles are also either absent, twinned, or triplicated. Occasional sockets display an aberrant morphology in which a crescent-shapped socket partially envelops an apparently normal socket. Bristle neurons are absent within the clones, independent of the presence or absence of the bristle shaft. Homozygous phyl flies die as larvae, but their cuticles appear normal (Chang, 1995).

phyllopod mutation suppresses in a dose dependent manner the rough eye phenotype caused by expression of a constitutively active raf (raftorY9). phyl also suppresses the identical phenotype caused by constitutive activation of Ras1. phyl is epistatic to constitutively active rolled (rlSem) and yan. In other words, formation of additional R7 cells in both rlSem and yan mutants requires phyl action. phyllopod regulates the fates of a subset of cells in the developing Drosophila eye; in the absence of phyl function, the R1, R6, and R7 photoreceptors are transformed into additional cone cells, whereas ectopic phyl expression in the cone cell precursors transforms these cells into additional R7 cells (Dickson, 1995).

In the absence of a functional phyl gene product, ommatidia are usually missing three photoreceptor cells: two outer cells with large rhabdomeres (the light-sensitive apparatus of the photoreceptor), and one inner cell with a small rhabdomere. Thus, phyl is required for fate determination of photoreceptors R1, R6, and R7, the last three photoreceptors to be recruited into the ommatidia of the developing Drosophila eye. These three cells are born during the second of two waves of cell division that generate the population of cells from which the eye is assembled. Genetic data suggests that phyl acts downstream of Ras1, raf, and yan to promote neuronal differentiation in this subset of photoreceptors. Expression of phyl under control of sevenless regulatory sequences generates extra R7 photoreceptors. Sevenless is active in R3, R4, R7, the cone cell precursors, the mystery cells, and (at low levels) R1 and R6. Ectopic expression of phyl in the cone cell precursors mimics the effect of ectopic activation of Ras1, suggesting that phyl expression is regulated by Ras1. Such ectopic expression induces the expression of elav. phyl is also required for embryonic nervous system and sensory bristle development (Chang, 1995).

The pattern of connections between R1-R6 neurons and their targets in the lamina is one of the most extraordinary examples of connection specificity known. An interwoven set of connections precisely maps R cells in different ommatidia (that 'see' the same point in space) onto the same group of postsynaptic cells, the lamina cartridge. R1-R6 cells that see the same point in space are distributed over six neighboring ommatidia as a consequence of the curvature of the eye and the angular placement of their light-sensing organelles. Conversely, each of the R1-R6 axons from a single ommatidium sees a different point in space and connects to a different set of lamina target neurons arranged in an invariant pattern. Each cartridge is innervated by a complete set of R1-R6 neurons from six different ommatidia (i.e., an R1 from one ommatidium, an R2 from another, and so on). By superimposing multiple inputs from the same point in visual space upon a single synaptic unit, the signal-to-noise ratio of the response to a signal in the visual field is enhanced. This phenomenon is called neural superposition (Clandinin, 2000 and references therein).

The R1-R6 projection pattern develops in two temporally distinct stages. During the third larval stage, R cells extend axons into the brain, where they terminate between two layers of glia, forming the lamina plexus. These glia act as intermediate targets for R1-R6 neurons. R cell axons induce the differentiation and organization of lamina target neurons and glia. At this stage of development, R cell axons from the same ommatidium form a single fascicle. A column of lamina neurons forms above the lamina plexus, in tight association with a single R cell axon fascicle. By the sequential addition of ommatidial bundles and their associated columns of lamina neurons, a precise retinotopic map forms in which fascicles from neighboring ommatidia terminate adjacent to each other. As lamina neurons differentiate, they send axons along the surface of R cell axons through the plexus and fasciculate with R7 and R8 as they project into the medulla. Although lamina neurons are in close association with R cell axons at this early stage, no synaptic contacts are formed (Clandinin, 2000).

In the second phase of development, ~30 hr after reaching the lamina plexus, R cell axons defasciculate from each ommatidial bundle and project across the surface of the lamina to their synaptic partners, making the pattern of connections characteristic of neural superposition. Growth of R cell axons toward their targets occurs approximately simultaneously in all ommatidial bundles and is presaged by an invariant sequence of contacts between R cell growth cones. This reorganization of terminals converts a strictly anatomical retinotopic map that reflects neighbor relationships between ommatidia into a new topographic map that reflects R cell visual response and reconstructs visual space in the first layer of the optic ganglion (Clandinin, 2000 and references therein).

R cell projections from a single ommatidium display two prominent features. (1) Each R cell axon terminates in an invariant position relative to the other axons from the same ommatidial fascicle. (2) The projection is oriented with respect to the dorsoventral midline of the eye (i.e., the equator), with the R3 axon extending toward the equator -- as a result, the projection patterns on opposite sides of the dorsoventral midline of the eye are mirror images. Using mutations that eliminate specific subsets of R cells or alter ommatidial polarity, tests were performed to see whether R cell synaptic specificity requires interactions among neighboring afferent axons or reflects independent navigation of each axon to its target. It has been demonstrated that interactions between specific R cells are required for target selection, and it is proposed that the precise composition of R cell axons within a fascicle plays a critical role in target specificity (Clandinin, 2000).

Neural superposition was first noted 90 years ago and the R1-R6 connection pattern in the lamina was first described using serial reconstruction of electron microscopic images in 1965. This pattern is cited as a classic example of extreme connection specificity. However, mechanistic analysis of this pattern was prevented by the absence of a rapid method for assessing R cell projections. In particular, the complexity of the pattern precludes conventional approaches based on visualizing all R cell axons in the target region, yet the assessment of connection specificity requires visualization of all R cell axons from one ommatidium. A method has been developed to label individual ommatidia with DiI and visualize the projection pattern using confocal microscopy. R1-R6 axons form a single bundle as they project into the brain. They defasciculate, project across the surface of the lamina, and then turn 90° and extend into the lamina cartridge. R cell axons elaborate a complex en passant presynaptic structure with lamina interneurons within the lamina cartridge. The axons of R7 and R8 project through the lamina, into the medulla. The relative positions of lamina targets chosen by each R1-R6 growth cone are invariant between ommatidia. This labeling method facilitates analysis of R1-R6 projections in various genetic backgrounds and creates a unique experimental system in which synaptic partner choices made by identified neurons can be directly assessed (Clandinin, 2000).

Serial electron microscopic reconstruction studies have revealed that, during pupal development, individual R cell axons leave their original bundle and migrate outward, in the precise direction of their final targets. This process was visualized using confocal microscopy. Early in pupal development, each ommatidial bundle forms a compact mass of expanded growth cones in the lamina plexus. This spherical mass then flattens, as distinct filopodial extensions corresponding to individual R cell axons become visible. This pattern of connections forms within a spatially patterned environment containing lamina target neurons and glial cells, as well as R cell axons. Since extension from the bundle is not preceded by extensive filopodial exploration, interactions between axons within ommatidial bundles may specify the initial trajectory of each growth cone. To address whether cell intrinsic mechanisms or interactions between R cell growth cones or both control target specificity, R cell projections were examined in mutant animals lacking specific subsets of R1-R6 cells. R cell axons from single ommatidia were labeled with DiI and visualized by confocal microscopy. In this series of experiments, animals were analyzed in which the eye was genetically mutant and the lamina neurons and glia in the target were wild type. Three mutant backgrounds were examined: (1) phyllopod, in which R1, R6, and R7 are transformed into nonneuronal cone cells; (2) lozenge^sprite, in which R3 and R4 are transformed into R7 cells; and (3) seven-up, in which R1, R3, R4, and R6 are transformed into R7 cells (Clandinin, 2000).

The first step of lamina target innervation is the coordinated defasciculation of R cell axons from bundles comprising axons from the same ommatidium. To determine whether interactions between specific subsets of R1-R6 axons are necessary for this defasciculation, R cell projections were assessed in phyllopod, seven-up, and lozenge^sprite mutants. In all three of the R cell transformation mutants examined, R cell axons migrated outward from the bundle. In particular, 4 R cell fibers in the lamina of 14/15 phyllopod mutant animals (missing R1, R6, and R7) and 20/24 lozenge^sprite mutants (missing R3 and R4) defasciculated from the bundle and projected to local targets. Similarly, in 17/23 seven-up mutants (missing R1, R3, R4, and R6), it was observed that the two remaining R cell axons defasciculated from the ommatidial bundle and innervated separate cartridges. In some cases, additional R cell axons also defasciculated, consistent with the reported incomplete expressivity of cell fate transformations in these mutants. In each case, axons projected to lamina targets in the local environment of the fascicle terminus. It is concluded that each R cell subtype is programmed to initiate a search for targets in a local region of the lamina target, independent of interactions between other R cell subtypes. In the following sections, whether interactions between specific R1-R6 cells regulate target specificity is assessed (Clandinin, 2000).

In phyllopod mutants, R1, R6, and R7 are transformed into nonneuronal cone cells. The remaining R cells made normal projections: a single long projection corresponding to R3 was observed, and R2, R4, and R5 made projections of appropriate relative lengths compared to wild type. In 1 of these 15 phyllopod mutant ommatidia, an additional short R cell projection was also observed, consistent with an incomplete cell fate transformation of either R1 or R6. In phyllopod mutant animals, the pattern of targets chosen by R3 and R4 were invariably normal, while those chosen by R2 and R5 were usually correct. In 4/15 animals, R2 and R5 made projections of the appropriate length, but the targets they chose were misoriented with respect to the equator. Therefore, R3 and R4 do not require R1, R6, and R7 to target correctly, while in some cases R2 and R5 are affected by their loss. These effects are not caused by the loss of R7; a sevenless mutation that specifically eliminates R7 has no effect on R cell targeting in the lamina (Clandinin, 2000).

A gain-of-function mutation, lozenge^sprite, which transforms R3 and R4 into R7 cells was examined. In this mutant, the Lozenge gene product is ectopically expressed in R3 and R4. In such mutant animals, ~73% of ommatidia have both R3 and R4 transformed into R7 cells; in most of the remaining ommatidia (20% of the total), only R4 is transformed; the remaining ommatidia are missing one R cell. Since the reduction in the number of R cells projecting to specific cartridges roughly corresponds to the fraction of R3 and R4 cells transformed into R7, it is presumed that transformation is complete in ommatidia where four fibers are observed in the lamina. In cases in which five R cell axons are observed, it is inferred that R4 but not R3 was transformed into R7. In 20/24 lozenge^sprite ommatidia injected, four R cell projections were observed in the lamina, with R1, R2, R5, and R6 all making projections of appropriate length, while transformed R3 and R4 cells projected through the lamina into the medulla. In the remaining 4/24 cases, five projections were seen in the lamina, one of which was a long projection characteristic of R3. In completely transformed lozenge^sprite ommatidia, the relative positions of the targets chosen by R1, R2, R5, and R6 were frequently highly aberrant. In the remaining 9/20 fully transformed lozenge^sprite animals, the pattern of targets chosen was not grossly distorted, though minor irregularities were seen. The effects seen in lozenge^sprite do not result from defects in ommatidial orientation: ommatidia are normally oriented in this mutant (Clandinin, 2000).

In seven-up mutants, R1, R3, R4, and R6 are frequently transformed into R7, while R2 and R5 are unaffected. Moreover, the transformation of individual seven-up ommatidia is variable and complex, making detailed reconstruction of many ommatidia impossible. However, in the majority of seven-up ommatidia (17/23), two short R cell projections, characteristic of R2 and R5, are seen in the lamina, while the transformed R1, R3, R4, and R6 cells project into the medulla (as R7 cells normally do). The targets chosen by the presumptive R2 and R5 were invariably misoriented with respect to the equator. In 4/23 ommatidia, there were either three or four short R cell projections in the lamina, while the remaining R cells projected into the medulla. In 2/23 cases, a single, relatively long, R3-like projection was observed in the lamina, flanked by either two or three short projections. In summary, these data establish that R2 and R5 project to a local region within the lamina independent of R1, R3, R4, and R6 but require interactions with these neurons to specify their correct targets (Clandinin, 2000).

The defects in R cell projections seen in seven-up and lozenge^sprite animals are not due to effects on the differentiation of neurons in the target region as assessed using multiple markers; lamina neuron differentiation was not assessed in phyllopod. The defects seen in lozenge^sprite and seven-up are also not due to extra R7 cells; a gain-of-function mutation in the Raf gene recruits extra R7 cells to each ommatidium without affecting the differentiation and targeting of R1-R6 neurons. It is possible, however, that the effects seen in these mutants reflect altered composition of axons within the ommatidial fascicle caused by ectopic R7 axons in abnormal positions within the bundle (Clandinin, 2000).

Two models could explain the mechanisms that determine the precise projection of R3 and R4 axons toward the dorsoventral midline and, by extension, the relative orientations of the other R cell axons. The growth cones of R3 and R4 may respond to an orienting cue in the lamina that promotes extension toward the dorsoventral midline. Alternatively, the orientation of R cell bodies in the retina may determine the orientation of R cell growth cones in the lamina, independent of any environmental cues. To assess the role of ommatidial polarity on projection specificity, projections from misoriented ommatidia were assessed (Clandinin, 2000).

If a lamina cue can promote equatorial extension of the R3 and R4 axons, ommatidia that rotate incorrectly should project their axons normally, toward the equator. Alternatively, if ommatidial orientation determines the direction of axon projection in the lamina, incorrectly oriented ommatidia should project their R3 and R4 axons away from the equator (Clandinin, 2000).

In wild-type animals, ommatidia are mirror image reflected about the dorsoventral equator of the eye. R cell projections are also mirror image symmetric about the equator but are rotated 180° with respect to the retina. That is, while the R3 cell body is oriented toward the pole in each ommatidium, its axon projects toward the equator in the lamina. This rotation is generated by a twist in the axon fascicle that occurs between the retina and the lamina (Clandinin, 2000).

To test the effects of large changes in ommatidial orientation, two mutations, spiny legs (in homozygous animals) and frizzled (in somatic mosaic animals in which a mutant eye projects to a wild-type target), were examined. In these mutants, ommatidia frequently adopt orientations that are 180° rotated; that is, the R3 cell body is frequently oriented toward the equator in the eye. In these two mutant backgrounds, the orientation of projections from ommatidia that were correctly oriented was normal. Therefore, neither gene is required for R cell axons to respond to orienting cues in the target. However, almost 90% of the ommatidia that were ~180° misoriented in the eye made projections that were also 180° misoriented in the lamina. Rare, abnormal projections of single R cell axons in both of these mutant backgrounds were observed, irrespective of ommatidial orientation. Therefore, the orientation of R cell projections along the dorsoventral axis of the lamina is largely determined by the orientation of ommatidia in the retina (Clandinin, 2000).

Three exceptional cases, in which misoriented ommatidia projected axons toward the equator, were observed. Thus, a cue in the lamina may reinforce the ommatidial orientation cue to ensure the correct direction of outgrowth along the dorsoventral axis. To test whether such a cue contributes to directionality of R cell projections, a mutation that causes a more moderate defect in ommatidial orientation was examined. In nemo mutant animals, ommatidia are misoriented up to 45°. If ommatidial orientation directly determines the directionality of R cell projections, they would be misoriented 45° with respect to the equator; the angle between ommatidial orientation and the axon projection pattern would remain 180°. However, while ommatidial orientation was disrupted in nemo, R cell projections were normal with respect to the equator. This observation suggests that in addition to ommatidial polarity, a cue in the lamina can influence R cell projection orientation (Clandinin, 2000).

It is concluded that interactions between R cell afferents play a crucial role in target specification, and it is proposed that the spatial relationships between axons within a fascicle influence synaptic specificity. It is hypothesized that the interactions between R cell subtypes that are required for target specificity are mediated by direct contacts between specific growth cones. R3 and R4 are required for the remaining R cell axons to choose their normal targets. R1 and R6 are required for R2 and R5 projections but are not required for the projections of R3 and R4. These interactions could occur between growth cones from the same or neighboring ommatidial bundles. The characteristic morphological changes of these growth cones as revealed through electron microscopic reconstruction studies are consistent with the notion that precise spatial relationships between specific growth cones within the lamina plexus are required for these critical interactions to occur. This sequence of interactions determines the relative positions of targets chosen by R cell axons from the same ommatidium (Clandinin, 2000).

R cell transformation mutants could disrupt these interactions in two ways. First, transformation of specific R cells could directly disrupt the instructive signals between R cell growth cones within the plexus that determine growth cone trajectories. Alternatively, these mutations could affect the interactions indirectly, by disrupting the spatial relationships between the remaining R cell axons. That is, outgrowth trajectory could be determined passively by the position each growth cone occupies as it leaves the ommatidial fascicle. In this view, these mutant backgrounds alter the composition of axons within each ommatidial bundle and, hence, disrupt the precise packing of axons within the fascicle. The differential requirements for particular R cell subtypes would reflect their specific roles in directing the spatial relationships between growth cones within the fascicle, rather than interactions between specific growth cones in the target region (Clandinin, 2000).

The cellular mechanisms described here provide a conceptual framework for understanding the molecular basis of synaptic specificity. While the DiI method facilitates the analysis of R1-R6 specificity on a scale sufficient to analyze many mutants, it is too laborious to accommodate large-scale screening. Hence, a genetic screen based on visual behavior driven specifically by R1-R6 is required to extend these studies to the molecular level. A wealth of visual behaviors have been described in Drosophila, one of which, the optomoter response, is mediated by these cells. Techniques that generate mosaic flies in which only R cells are made homozygous for randomly induced mutations, while the rest of the fly is heterozygous, have recently been described. Currently, projects are underway, combining this specific behavioral screen with genetic mosaics, in order to screen for genes controlling R1-R6 synaptic specificity (Clandinin, 2000).

Phyllopod (Phyl) is one of the most downstream nuclear components identified in the Sevenless receptor tyrosine kinase-RAS1 signaling pathway. Using the eye-specific expression vector pGMR, which contains a multimerized binding site for the zinc-finger protein Glass placed upstream of the basal hsp70 promoter, Phyl was expressed in all cells posterior to the morphogenetic furrow during larval development, and in all cells except cone cells in the pupal eye. This results in a rough eye phenotype that was used to screen for dominant modifiers. The lilliputian gene corresponds to one of the complementation groups that strongly suppress the rough eye phenotype of GMR-phyl. Complementation analyses revealed that many lilli alleles have been identified as suppressors in a number of different GMR-based dominant modifier screens. For example, lilli alleles were isolated in a GMR-sina screen and in a GMR-Yan^ACT screen. Mutations in lilli suppress the rough eye phenotypes generated by overexpression of either positive (Sina and Phyl) or negative (Ttk and Yan) components of the RAS1 signaling pathway under GMR control, as well as other GMR constructs from different signaling pathways. In addition, lilli mutants dominantly suppress the rough eye phenotypes of many sE transgenes, in which the sevenless enhancer is placed upstream of the hsp70 basal promoter. These observations suggest that lilli is required, either directly or indirectly, for proper transcription from the GMR and sE expression constructs. Further supporting this hypothesis, it was found that the levels of CAT activity from a GMR-CAT reporter construct are decreased by ~40% in third instar larvae heterozygous for lilli. Similar results were obtained when one copy of glass, a known activator of GMR transcription, is removed. These results suggest that lilli acts as a transcriptional regulator for GMR transgenes (Tang, 2001).

A dual function of phyllopod in Drosophila external sensory organ development: cell fate specification of sensory organ precursor and its progeny

During Drosophila external sensory organ development, one sensory organ precursor (SOP) arises from each proneural cluster and then undergoes asymmetrical cell divisions to produce an external sensory (es) organ made up of different types of daughter cells. phyllopod (phyl), known to be essential for R7 photoreceptor differentiation, is required in two stages of es organ development: the formation of SOP cells and cell fate specification of SOP progeny. Loss-of-function mutations in phyl result in failure of SOP formation, which leads to missing bristles in adult flies. At a later stage of es organ development, phyl mutations cause the first cell division of the SOP lineage to generate two identical daughters (IIb cells are transformed into IIa cells), leading to the fate transformation of neuron and sheath cells to hair cells and socket cells. Conversely, misexpression of phyl promotes ectopic SOP formation, and causes opposite fate transformation in SOP daughter cells. Thus, phyl functions as a genetic switch in specifying the fate of the SOP cells and their progeny. seven in absentia (sina), another gene required for R7 cell fate differentiation, is also involved in es organ development. Genetic interactions among phyl, sina and tramtrack (ttk) suggest that phyl and sina function in bristle development by antagonizing ttk activity, and ttk acts downstream of phyl. Notch (N) mutations induce formation of supernumerary SOP cells, and transformation from hair and socket cells to neurons. phyl acts epistatically to N. phyl is expressed specifically in SOP cells and other neural precursors, and its mRNA level is negatively regulated by N signaling. Thus, these analyses demonstrate that phyl acts downstream of N signaling in controlling cell fates in es organ development (Pi, 2001).

Genetic analyses show that phyl functions together with sina to promote SOP formation by antagonizing ttk activity. These results suggest that degradation of the Ttk protein is a major function of Phyl in the cell fate specification of SOP cells. Consistent with this idea, misexpression of ttk can inhibit the formation of SOP cells and suppress the ectopic bristle phenotype caused by misexpression of phyl. Several lines of evidence also indicate that Ttk functions as a repressor to inhibit SOP cell fate. (1) Ttk is expressed ubiquitously in the pupal notum except in SOP cells. (2) In embryos, overexpression of ttk inhibits the formation of es organs. (3) Injection of ttk dsRNA results in extensive increase of neurons in embryonic PNS, a phenotype observed in neurogenic mutants. All of these results suggest that ttk might play a negative role in the fate specification of SOP cells, and phyl promotes SOP fate specification by degrading Ttk (Pi, 2001).

phyl is essential for formation of pupal SOP cells, but is partly required for embryonic SOP cells. In senseless mutants, the larval SOP cells fail to form, but the embryonic SOP cells form and divide to generate daughter cells that fail to differentiate. These results suggest that there are some distinctions between the SOP cell fate specification of embryos and larvae (Pi, 2001).

These studies show that phyl is required for IIb cell fate specification. In phyl mutants, more than half of the adult notal microchaetes and more than 80% of the embryonic dm es organs exhibited IIb to IIa cell fate transformation. Cell fate transformation from hair cells to socket cells also occurs in adult bristle (four-socket phenotype seen in phyl² clone), but much less frequent in embryonic dm es organs (less than 5%). Transformation from neurons to sheath cells in es organs was rarely seen in phyl embryos. These results suggest that unlike numb and N, phyl is mainly required for the binary cell fate determination between IIa and IIb cells in both adult and embryonic es organs. In support of this conclusion, misexpression of phyl in both adult and embryonic sensory organ lineages most often resulted in a two-neuron/two-sheath cell phenotype (Pi, 2001).

In phyl¹/phyl² mutant embryos, most of the neurons and sheath cells in chordotonal organs are also lost, indicating that phyl is also required for the formation of neurons and sheath cells in chordotonal organ lineage. In weaker mutant embryos (phyl²²⁴⁵/phyl²²⁴⁵), more sheath cells than neurons were often observed in the regions where chordotonal organs lch5 and vchA and B are located. Therefore, phyl may be required for cell fate determination between neurons and sheath cells in the development of chordotonal organs (Pi, 2001).

During lateral inhibition, the N pathway is essential to single out the SOP cells. In situ analyses indicate that phyl expression is negatively regulated by N. Also, the supernumerary bristles in N mutants are suppressed by phyl mutation. These results strongly suggest that down-regulation of phyl expression is a major function of N signaling to suppress SOP cell fate (Pi, 2001).

In the SOP lineage, N also plays an important role in the cell fate specification of SOP progeny. Several components of the N pathway, such as Delta, Serrate, Su(H), Hairless, and proteins of the Bearded and E(spl) families, have been shown to be involved in the cell fate specification of all or subsets of progeny. phyl acts epistatically to N in the cell fate specification of SOP daughter cells and is expressed in IIb cells. Also, misexpression of phyl rescues the defects caused by N^ACT, indicating that N regulates the phyl activity in sensory organ lineage at the transcriptional level (Pi, 2001).

Genetic analyses of phyl, sina and ttk are mostly consistent with the model that Phyl functions together with Sina to promote es organ development by degrading Ttk. In embryos, strong defects are detected only when both maternal and zygotic sina transcripts are removed, suggesting that maternally contributed sina transcript play an essential role in the development of embryonic es organs. Consistently, no genetic interaction between zygotic sina and phyl, and between zygotic sina and ttk was detected in embryonic PNS development. In adults, the bristle phenotypes in sina mutants are weaker than in phyl mutants. One possible reason is that the perdurance of sina gene products from maternal transcripts might supply activity for some adult bristles to develop normally. Another possibility is that phyl is able to down-regulate ttk activity in a sina-independent manner. In the Drosophila genome, a sequence (CG 13030) is located next to sina in the genome and encodes a putative protein with 50% identity and 70% similarity with Sina. It might be possible that sina functions redundantly with this gene in bristle development (Pi, 2001).

These studies of phyl/sina/ttk in es organ development and previous studies in photoreceptor differentiation indicate that the Drosophila eye and es organs depend on the same protein complex to specify their cell fate. In both cases, phyl mutations transform neural cells to non-neural cells. Both studies also show that phyl expression is tightly regulated by the upstream signaling pathways. The expression of phyl is activated by the Ras pathway in photoreceptor cells. In SOP cells, the transcription of phyl is likely activated by the proneural genes ac and sc, and is repressed by N signaling. Interestingly, it has been shown that the Egrf/Ras/Raf pathway acts antagonistically with the N pathway in SOP formation of adult macrochaetes and chordotonal organs. Whether these two pathways converge on phyl expression to regulate sensory organ formation remains to be examined (Pi, 2001).

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date revised: 25 September 2005 

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