Neural differentiation in fly CNS transplants cultured in vivo

Ablation of Drosophila photoreceptor cells by conditional expression of a toxin gene

We have used toxin-mediated ablation to study some aspects of visual system development in Drosophila melanogaster. To devise a method that permits the conditional expression of a cellular toxin, we introduced an amber mutation into the diphtheria toxin-A-chain gene. In transgenic animals, this toxin gene can be activated by providing the gene for an amber suppressor tRNA. By coupling this toxin gene to the photoreceptor cell-specific promoter of the chaoptic gene, photoreceptor cells could be specifically ablated during development. Photoreceptor cell-specific markers normally activated during pupal development failed to appear after midpupation. Photoreceptor cells were absent from the retinas of adult flies at eclosion. We have assessed the consequences of photoreceptor cell ablation for eye and optic lobe development. We suggest that the larval photoreceptor nerve is not essential, in the late larval stages, for retinula photoreceptor cell axons to achieve their proper projection pattern in the brain. Moreover, while retinula photoreceptor innervation is initially required for the development of normal optic ganglia, the ablation of these cells in midpupation has no discernible effect. This approach to cell-specific ablation should be generally applicable to the study of cellular functions in development and behavior.

Postembryonic differentiation of serotonin-immunoreactive neurons in fleshfly optic lobes developing in situ or cultured in vivo without eye discs

The differentiation of serotonin-immunoreactive (5-HTi) neurons in the optic lobes of fleshflies was studied during in situ development and in in vivo cultures. All 5-HTi neurons with cell bodies in the imaginal optic lobes differentiate during postembryonic (pupal) development. These are local anaxonal neurons. In addition there are two large 5-HTi bilateral neurons that connect all optic lobe neuropil regions on both sides of the brain and have their cell bodies in the midbrain proper. Deafferentation of optic lobes cultured in vivo leads to drastic reduction in optic lobe volume and increased cell death. All the 5-HTi neurons differentiate after deafferentation but their morphology changes. The neuropil receiving the photoreceptor inputs, the lamina, degenerates but a disorganized "pseudolamina" is formed by the processes of the two large 5-HTi neurons. The layering of the optic lobe neuropils cannot be distinguished and 5-HTi processes form novel projectional patterns. Hence, the 5-HTi neurons do not require afferent inputs from the retina for their differentiation and survival, but the effect on other optic lobe interneurons is reflected in the morphological plasticity of the 5-HTi neurons.

Development of axon paths of motorneurons after removal of target muscles in a holometabolous insect

The fate of leg motor neurons and the outgrowth of their axons was studied in fleshflies after removal of their target muscles before the onset of axogenesis. When a mesothoracic leg imaginal disc was extirpated in the prepupa the corresponding leg nerve was missing on the operated side of the five-legged adult fly. Horseradish peroxidase and cobalt filling revealed that the target-deprived motorneurons survive and their axons grow out through other nerve roots. The choice of exit of the axons is variable: adjacent pro- or metathoracic leg nerves on the ipsilateral side or nerves supplying thoracic muscles (e.g. indirect flight muscles). In order to see whether factors in the mesothoracic leg disc can attract mesothoracic motorneurons, implantation of supernumerary leg discs was made in prepupae. Motor innervation of the implanted legs was never seen. Our data thus indicate that under the present experimental conditions leg motor neurons are not dependent on their specific targets for survival. They can track along adjacent preexisting axonal paths to innervate other targets. Under normal conditions specific markers may be present in the appropriate targets and the appropriate preexisting pathways (that we disrupt in our experiments) may lead the axons to these markers.

Axonal projections from transplanted ectopic legs in an insect

Ectopic legs were produced in the fleshfly Sarcophaga bullata by transplantation of leg imaginal discs. Cobalt or horseradish peroxidase (HRP) fills from these supernumerary legs showed that their sensory axons invariably innervate the metathoracic leg neuropil via an abdominal nerve. This was so irrespective of whether they differentiated from pro-, meso-, or metathoracic leg disc. Implantation of left leg discs on the right side and vice versa revealed that the site of implantation determined which side (left or right) of the ganglion was innervated. In general, the same results were obtained when the implantation was made after removal of a leg disc from the host, except in a few instances in which the implanted leg directly innervated the deafferented leg neuropil. The results indicate that axonal pathfinding from the imaginal discs in this insect is via contact guidance along preexisting nerves or imaginal disc stalks.

The morphology, physiology, and neural projections of supernumerary compound eyes in Drosophila melanogaster

Supernumerary compound eyes in Drosophila melanogaster produced by the extra eye (ee) mutation were analyzed with regard to their morphology, physiology, and neural projections. Electron and light microscopy revealed that large extra eyes often possess the normal complement of compound-eye cell types and that these cells usually have standard fine structure. In addition, the array of photoreceptor cell rhabdomeres within individual supernumerary ommatidia is standardly trapezoidal, and ommatidial subpopulations having mirror-image configurations of their rhabdomeric trapezoids are separated by an equator in extra eyes. Light stimulation of supernumerary eyes can elicit photoreceptor depolarization potentials as evidenced by electroretinographic recordings from them. In addition, extra-eye photoreceptor cells have a functional pupillary response to light stimulation. Although the supernumerary eyes can be functionally and anatomically standard, examination of serial, silver-stained sections of extra-eye heads has shown that their photoreceptor axons seldom innervate the brain. This situation obtains even in a case in which the normal, ipsilateral compound eye was removed by the eyeless mutation. In contrast, rare supernumerary antennae occasionally found in ee stocks have receptor cells whose axons innervate ventral brain. In addition to duplications of cuticular epithelia, extra glial cells, muscle fibers, and ocellar interneurons are sometimes found in extra-eye bearing flies. Discussion of these results focuses on a polarity guidance hypothesis which models the growth of adult photoreceptor axons into the brain during normal development.

Cell degeneration in the developing optic lobes of the sine oculis and small-optic-lobes mutants of Drosophila melanogaster

In the small-optic-lobes (sol) and sine oculis (so) mutants of Drosophila melanogaster extensive cell death occurs in the optic lobes during the first half of pupal development. Gynandromorph flies show that the sol mutation acts primarily on cells of the medulla cortex. Degeneration of medullar ganglion cells occurs at an early stage of cellular differentiation, when their axons have not yet participated in the formation of the second optic chiasma. The so gene, on the other hand, acts on the eye anlagen. The analysis of chimeric flies demonstrates that degeneration in the optic lobes of so flies is a consequence of eye reduction. At the level of the second optic chiasma extensive axonal degeneration can be observed in the mutant. Neurons seem to die after their failure to establish a sufficient number of functional contacts. In sol;so double mutants, the mutational effects are cumulative causing complete degeneration of columnar cell types in pupae without any eye anlage. The tiny rudiments of the optic lobes in eyeless double mutants still contain tangential neurons of the medulla and of the lobula complex. The central brain is reduced in size due to the missing visual fibers, however, its overall appearance is surprisingly normal.

Neuronal organization in fly optic lobes altered by laser ablations early in development or by mutations of the eye

The role of afferent and efferent connections in the differentiation of optic lobe interneurons was investigated by using laser ablations of neuronal precursors in the brain of Musca domestica and analysis of two eye mutants of the same species. The first mutant, split eye, had no connections between the retina and the optic lobes. In this case the optic lobes were drastically reduced in volume and the neural organization within the neuropil regions was altered. The other mutant, spindle, had reduced retinae that innervated reduced optic lobes with a normal-appearing orderly arrangement of neurons. In addition disordered neuropil, composed of identified visual interneurons, was found that had no afferent innervation. Three main types of alterations resulting from laser ablations were analyzed. These ablations removed entire neuropil regions or parts of these: (1) removal of the first optic neuropil region (the lamina) resulting in receptor axons projecting directly to the second neuropil (the medulla) and sprouting of medulla neurons toward the receptor layer; (2) removal of one part of the third optic neuropil (the lobula plate) and severe alteration of the other part (the lobula) resulting in sprouting of lobula neurons into the medulla neuropil; and (3) removal of the entire optic lobe resulting in reduction of the volume of the lateral midbrain and photoreceptor axons forming a tangle beneath the retina. Our findings confirm that afferent retinal input is essential for normal differentiation and maintenance of many optic lobe interneurons. Furthermore, it was seen that a normal columnar organization of the neuropils and the dendritic patterns of visual interneurons are dependent on afferent inputs. A common response to removal of inputs was a reorganization of axonal and dendritic projections.

Differentiation of fly visual interneurons after laser ablation of their central targets early in development

To test the differentiation of visual interneurons that had their targets removed before axogenesis, we ablated neuronal precursors in brains of first instar fly larvae by using a laser microsurgery unit. We describe ablations that resulted in the elimination of the third neuropil region (the lobula complex) of the optic lobes. Neural differentiation in the more peripheral second and first neuropil regions (the medulla and the lamina) was thus studied in the absence of the lobula complex. It was found that the medulla neuropil differentiated with normal columnar and layered organization. The neuropil, however, folded along its central surface. The only connection between the medulla and more central neuropil (the midbrain) was via a bundle of axons (the Cuccati bundle) present also in the normal optic lobes. Some types of neurons that normally connect the medulla and the lobula complex could be identified. These appeared to end in a disorganized neuropil mass in the center of the folded medulla. The differentiation of the lamina neuropil also appeared normal in flies with the lobula complex eliminated and the medulla folded. Also, in optic lobes where the medulla was severely disorganized and/or reduced due to laser ablations, the lamina neuropil appeared more or less normal. The results suggest that lamina and medulla nerve cells can differentiate and develop normal neuropil patterns in absence of their appropriate targets.