Selective illumination of single photoreceptors in the house fly retina: local membrane turnover and uptake of extracellular horseradish peroxidase (HRP) and Lucifer Yellow

Summary Single photoreceptor cells in the compound eye of the housefly Musca domestica were selectively illuminated and subsequently compared electron-microscopically with the unilluminated photoreceptors in the immediate surroundings. The rhabdomeres of the illuminated cells remain largely unaffected, but the cells show an increase in the number of coated pits, various types of vesicles, and degradative organelles; some of the latter organelles are described for the first time in fly photoreceptors. Coated pits are found not only at the bases of the microvilli, but also in other parts of the plasma membrane. Degradative organelles, endoplasmic reticulum (ER) and mitochondria aggregate in the perinuclear region. The rough ER and smooth ER are more elaborate, the number of Golgi stacks, free ribosomes and polysomes is increased, and the shape and distribution of heterochromatin within the nuclei are altered. Illuminated photoreceptors also interdigitate extensively with their neighbouring secondary pigment cells. These structural changes in illuminated fly photoreceptor cells indicate an increase in membrane turnover and cellular metabolism. When applied to the eye, Lucifer Yellow spreads into the extracellular space and is taken up only by the illuminated photoreceptor cells. These cells show the same structural modifications as above. Horseradish peroxidase applied in the same way is observed in pinocytotic vesicles and degradative organelles of the illuminated cells. Hence, the light-induced uptake of extracellular compounds takes place in vivo at least partially as a result of an increase in pinocytosis.     

Ultrastructure of carotenoid deprivation in photoreceptors of Manduca sexta: myeloid bodies and intracellular microvilli

Summary The ultrastructural effects of carotenoid (vitamin A) deprivation were studied in the adult photoreceptors of the tobacco hornworm moth Manduca sexta. Moths were reared on a deprived diet, which lacked the carotenoid sources of the photopigment chromophore, 3-hydroxy retinal, or on a control, fortified diet, containing ample carotenoid. The latter supported normal levels of visual function, whereas visual pigment and sensitivity were reduced by at least 3 log units in moths reared on the deprived diet. Myeloid bodies, massed cisternae of hypertrophied smooth endomembrane, filled the cytoplasm in the receptors of deprived animals. The myeloid bodies assumed various configurations that included lamellate stacks of parallel cisternae, and tubular networks in a paracrystalline form. Freeze-fracture preparations of myeloid membranes revealed a high density of P-face particles. Vacuoles containing microvilli similar to those of the rhabdomere were also present in deprived photoreceptors. We suggest that the elaboration of smooth endoplasmic reticulum as myeloid bodies in chromophore-deprived photoreceptors may stem from the hypertrophy of a biochemical system for processing the chromophore or the interruption of the intracellular pathway that normally carries visual pigment to the rhabdomere.     

The plasma membrane and generative cell organization in pollen of the mimosoid legume,Acacia retinodes

Summary Cytological changes associated with the final maturation, and dehiscence of the 16-grain compound pollen (polyads) have been followed in anthers at female and male phase of flowering. InAcacia retinodes, the transition from female to male phase takes approximately 24 h. The spherical generative cell at female phase is connected with the vegetative cell plasma membrane by a junction zone. This is sited adjacent to a germinal aperture, comprising wall ingrowths and membrane labyrinths. By male phase, the generative cell has elongated into a spindle-shape, and its surface is characteristically scalloped. The membrane labyrinths, especially those at the apertures, now contain masses of coated vesicles, implicated in the transport and secretion of proteins. Two-dimensional projections indicate that the generative cell and vegetative nucleus are closely associated forming a male germ unit.     

Localization of the pupil trigger in insect superposition eyes

Summary In the superposition eyes of the sphingid moth Deilephila and the neuropteran Ascalaphus, adjustment to different intensities is subserved by longitudinal migrations of screening pigment in specialized pigment cells. Using ophthalmoscopic techniques we have localized the light-sensitive trigger that controls pigment position.In both species, local illumination of a small spot anywhere within the eye glow of a dark-adapted eye evokes local light adaptation in the ommatidia whose facets receive the light. Details of the response pattern demonstrate that a distal light-sensitive trigger is located axially in the ommatidium, just beneath the crystalline cone, and extends with less sensitivity deep into the clear zone. The distal trigger in Deilephila was shown to be predominantly UV sensitive, and a UV-absorbing structure, presumably the distal trigger, was observed near the proximal tip of the crystalline cone.In Ascalaphus we also found another trigger located more proximally, which causes local pigment reaction in the ommatidia whose rhabdoms are illuminated (the centre of the eye glow). The light-sensitive trigger for this response appears to be the rhabdom itself.     

Identificaton of UV,green and red receptors,and their projection to lamina in the cabbage butterfly,Pieris rapae

Summary The photoreceptors in the compound eye of a cabbage butterfly, Pieris rapae, were examined by conventional and intracellular-labeling electron microscopy by the use of the cobalt(III)-lysine complex as an ionized marker. Five types of spectral sensitivity were recorded intracellularly in electrophysiological experiments. They peaked at about 340, 380, 480, 560 and 620 nm, respectively. One of the distal retinula cells (R2) was a UV receptor, whereas the R4 distal retinula cell was a green receptor. The basal retinula cell, R9, was found to be a red receptor; it was localized near the basement membrane, having a bilobed cell body with an individual nucleus in each lobe. A small number of rhabdomere microvilli were present in a narrow cytoplasmic bridge connecting the two lobes. The axons of six retinula cells (R3–R8) in each ommatidium terminated at the cartridge in the lamina (short visual fiber), whereas those of the other three retinula cells, R1, R2 and R9, extended to the medulla (long visual fiber). The information from the UV and red receptors is therefore probably delivered directly to the medulla neurons, independent of that from the other spectral receptor types.     

Interneuronal and glial-neuronal gap junctions in the lamina ganglionaris of the compound eye of the housefly,Musca domestics

Summary The cell-body layer of the lamina ganglionaris of the housefly, Musca domestica, contains the perikarya of five types of monopolar interneuron (L1–L5) along with their enveloping neuroglia (Strausfeld 1971). We confirm previous reports (Trujillo-Cenóz 1965; Boschek 1971) that monopolar cell bodies in the lamina form three structural classes: Class I, Class II, and midget monopolar cells. Class-I cells (L1 and L2) have large (8–15 m) often crescentshaped cell bodies, much perinuclear cytoplasm and deep glial invaginations. Class-II cells (L3 and L4) have smaller perikarya (4–8 m) with little perinuclear cytoplasm and no glial invaginations. The midget monopolar cell (L5) resides at the base of the cell-body layer and has a cubshaped cell body. Though embedded within a reticulum of satellite glia, the L1–L4 monopolar perikarya and their immediately proximal neurites frequently appose each other directly. Typical arthropod (-type) gap junctions are routinely observed at these interfaces. These junctions can span up to 0.8 m with an intercellular space of 2–4 nm. The surrounding nonspecialized interspace is 12–20 nm. Freezefracture replicas of monopolar appositions confirm the presence of -type gap junctions, i.e., circular plaques (0.15–0.7 m diam.) of large (10–15 nm) E-face particles. Gap junctions are present between Class I somata and their proximal neurites, between Class I and Class II somata and proximal neurites, and between Class II somata. Intercartridge coupling may exist between such monopolar somata. The cell body and proximal neurite of L5 were not examined. We also find that Class I and Class II somata are extensively linked to their satellite glia via gap junctions. The gap width and nonjunctional interspace between neuron and glia are the same as those found between neurons. The particular arrangement and morphology of lamina monopolar neurons suggest that coupling or low resistance pathways between functionally distinct neurons and between neuron and glia are probably related to the metabolic requirements of the nuclear layer and may play a role in wide field signal averaging and light adaptation.     

Lobula plate and ocellar interneurons converge onto a cluster of descending neurons leading to neck and leg motor neuropil in Calliphora erythrocephala

Summary In the fly, Calliphora erythrocephala, a cluster of three Y-shaped descending neurons (DNOVS 1–3) receives ocellar interneuron and vertical cell (VS4–9) terminals. Synaptic connections to one of them (DNOVS 1) are described. In addition, three types of small lobula plate vertical cell (sVS) and one type of contralateral horizontal neuron (Hc) terminate at DNOVS 1, as do two forms of ascending neurons derived from thoracic ganglia. A contralateral neuron, with terminals in the opposite lobula plate, arises at the DNOVS cluster and is thought to provide heterolateral interaction between the VS4–9 output of one side to the VS4–9 dendrites of the other. DNOVS 2 and 3 extend through pro-, meso-, and metathoracic ganglia, branching ipsilaterally within their tract and into the inner margin of leg motor neuropil of each ganglion. DNOVS 1 terminates as a stubby ending in the dorsal prothoracic ganglion onto the main dendritic trunks of neck muscle motor neurons. Convergence of VS and ocellar interneurons to DNOVS 1 comprises a second pathway from the visual system to the neck motor, the other being carried by motor neurons arising in the brain. Their significance for saccadic head movement and the stabilization of the retinal image is discussed.     

Sexual dimorphism in the visual system of flies: The divided brain of male Bibionidae (Diptera)

Summary The mapping of the compound eyes onto the visual neuropils and the cell types in the lamina and the lobula complex of Bibionidae (Diptera) were studied by means of extracellular cobalt injections and Golgi impregnations. Dorsal and ventral eyes in males map into separate dorsal-and ventral neuropils up to the level of the lobula complex. The dorsal-eye lamina is unilayered, while the ventral-eye lamina in males and the lamina in females are multilayered: layers A and C are invaded by en-passant terminals of long visual fibres, layer B by the terminals of short visual fibres. Long visual fibres have a short and a long terminal in the ventral medulla with terminal specialisations in three distinct layers. Only one type of receptor ending exists in the dorsal medulla, the terminal branches of which are restricted to one layer only. Arrays of contralateral neurones are found in the medial part of the dorsal lobula, which receives input from the zone of binocular vision of the ipsilateral dorsal eye, and in the posterior dorsal lobula and lobula plate. The dorsal lobula plate contains large tangential neurones, the dendritic arborisations of which are revealed by cobalt injection into the thoracic ganglia. The divided brain of male bibionids offers the opportunity to investigate separately the nervous systems involved in sex-specific visually guided flight behaviour and in general visually guided flight control.     

Effects of compound eye-removal on the photoperiodic response of the band-legged ground cricket, Pteronemobius nigrofasciatus

The band-legged ground cricket Pteronemobius nigrofasciatus shows a clear photoperiodic response at 25°C with respect to the control of the induction of embryonic diapause. When crickets were reared under a short-day (LD 12 12) photoperiod and then transferred to a long-day (LD 16 8) photoperiod upon adult emergence, the adults mainly laid nondiapause eggs. However, adults maintained continuously under short-day conditions laid dispause eggs. When compound eyes were bilaterally removed after adult emergence, the crickets mainly laid nondiapause eggs, irrespective of the photoperiod. Thus, the adults completely lost their sensitivity to photoperiod after bilateral removal of their compound eyes. Unilateral removal of the compound eye also affected the crickets under a short-day photoperiod, and the incidence of diapause eggs was intermediate between that laid by intact adults and that laid by adults after the bilateral removal of compound eyes. The incidence of diapause eggs in sham-operated crickets was not significantly different from that in intact crickets under both sets of photoperiodic conditions. These results show that P. nigrofasciatus perceives the photoperiod through its compound eyes.     

The effects of activators of G-proteins,mastoparan and compound 48/80, on the G-protein/adenylate cyclase system in cultured melanophores of the black-moor goldfish,Carassius auratus

 To investigate the functions of GTP-binding protein(s) in the melanosome-aggregating response in fish melanophores, the effects of activators of G-proteins, namely, mastoparan and compound 48/80, were examined in cultured melanophores of the balck-moor goldfish, Carassius auratus. Both mastoparan and compound 48/80 induced an approximately 40% increase in the GTP-hydrolyzing activity in the melanophore membranes compared to the basal level. In intact melanophores, these compounds inhibited the effect of 3-isobutyl-1-methylxanthine, which induced the accumulation of intracellular cAMP. Pretreatment of melanophores with pertussis toxin at 1 μg ⋅ ml^-1 for 15 h attenuated the inhibitory effect of mastoparan on the accumulation of cAMP. However, pretreatment with the toxin only slightly attenuated the inhibitory effect of compound 48/80 on the accumulation of cAMP. In addition, compound 48/80 at 1 mg ⋅ ml^-1 induced full aggregation of the melanosomes in melanophores, though mastoparan at 5 μmol ⋅ l^-1 induced only 10–20% aggregation of melanophores. These results suggest that mastoparan and compound 48/80 can each activate the inhibitory G-protein in goldfish melanophores, which results in inhibition of adenylate cyclase activity. This signal-transduction pathway is involved in the aggregation of melanosomes in these cells. Accepted: 3 June 1996