Sally Stowe

Rapid synthesis of photoreceptor membrane and assembly of new microvilli in a crab at dusk

SummaryLarge areas of photoreceptor membrane are synthesized in the retinula cells of the crab Leptograpsus variegatus at dusk. Initially, new membrane differentiates from rough endoplasmic reticulum (ER) as large tubules of smooth ER. These tubules transform to concentric ellipsoids of closely apposed pairs of membranes (“doublet ER”), sometimes passing through an intervening crenate form. The new membrane is transported through bridges of cytoplasm that cross the palisade to the rhabdom region, from which the remains of the rhabdomeres that were built during the previous dusk have been dissolved. The degradation of the old microvilli of one rhabdomere is accomplished without affecting neighbouring rhabdomeres of other cells. New microvilli are assembled in situ from sheets of doublet ER, which are converted to tubules oriented in the same direction as the future microvilli. The cytoplasmic face of the ER remains the cytoplasmic face of the tubules, which become progressively narrower, partly by further longitudinal division, until the final diameter of the microvillus is reached. A central core is often seen in transverse sections of mature microvilli. It may be involved in the final consolidation, but rhabdomeric microvilli are not formed in the same manner as those of intestinal brush border cells. There is no evidence that new membrane passes through the Golgi compartment before incorporation into the rhabdom, as is the case for rod outer segment membrane in vertebrate photoreceptors.

Butterfly wing colours: scale beads make white pierid wings brighter

The wing–scale morphologies of the pierid butterflies Pieris rapae (small white) and Delias nigrina (common jezabel), and the heliconine Heliconius melpomene are compared and related to the wing–reflectance spectra. Light scattering at the wing scales determines the wing reflectance, but when the scales contain an absorbing pigment, reflectance is suppressed in the absorption wavelength range of the pigment. The reflectance of the white wing areas of P. rapae, where the scales are studded with beads, is considerably higher than that of the white wing areas of H. melpomene, which has scales lacking beads. The beads presumably cause the distinct matt–white colour of the wings of pierids and function to increase the reflectance amplitude. This will improve the visual discrimination between conspecific males and females.

Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage

Neuroanatomical studies have demonstrated that the architecture and organization among neuropils are highly conserved within any order of arthropods. The shapes of nerve cells and their neuropilar arrangements provide robust characters for phylogenetic analyses. Such analyses so far have agreed with molecular phylogenies in demonstrating that entomostracans+malacostracans belong to a clade (Tetraconata) that includes the hexapods. However, relationships among what are considered to be paraphyletic groups or among the stem arthropods have not yet been satisfactorily resolved. The present parsimony analyses of independent neuroarchitectural characters from 27 arthropods and lobopods demonstrate relationships that are congruent with phylogenies derived from molecular studies, except for the status of the Onychophora. The present account describes the brain of the onychophoran Euperipatoides rowelli, demonstrating that the structure and arrangements of its neurons, cerebral neuropils and sensory centres are distinct from arrangements in the brains of mandibulates. Neuroanatomical evidence suggests that the organization of the onychophoran brain is similar to that of the brains of chelicerates.

Carbon-Dioxide Sensing Structures in Terrestrial Arthropods

Sensory structures that detect atmospheric carbon dioxide have been identified and described to the subcellular level in adults of Lepidoptera, Diptera, Hymenoptera, Isoptera, Chilopoda, and Ixodidae, as well as in lepidopteran larvae. The structures are usually composed of clusters of wall‐pore type sensilla that may form distinct sensory organs, often recessed in pits or capsules. In insects, they are located on either the palps or the antennae, in chilopods on the head capsule, and in ixodids on the forelegs. In the two cases where the central projections have been examined (Lepidoptera and mosquitoes), the clustering is preserved to the level of second order neurons, which are located in the deutocerebrum. Individual sensilla usually contain a single receptor neuron that is sensitive to CO2; it may be accompanied by other neurons that respond to other olfactory qualities. The distal dendritic processes of CO2‐sensitive neurons invariably show an increased surface area, dividing into many cylindrical branches or into lamellar structures. Lamellar membranes are often closely linked to arrays of microtubules. Fine pore canal tubules are usually associated with the cuticular pores. Microsc. Res. Tech. 47:416–427, 1999.

A labile, Ca2+-dependent cytoskeleton in rhabdomeral microvilli of blowflies

SummaryRhabdomeral microvilli of photoreceptors of the blowfly Lucilia are shown to contain a cytoskeleton. An axial filament (∼ 6–11 nm) in each microvillus is inserted into a terminal cap distally, and into a plug filling the narrow neck of the microvillus proximally. In some states, the axial filament projects beyond the neck; within the microvillus it is surrounded by amorphous material. Together, they form an axial complex, which supports side-arms linking it to the plasma membrane. Conventional fixation for examination with the electron microscope destroys the cytoskeleton. To preserve it, retinae are pre-treated with a Ringers solution buffered with 20 mM imidazole and containing, minimally, the following components: (i) a protease inhibitor, usually phenylmethylsulphonyl-fluoride (PMSF); (ii) either the Ca2+-chelator EGTA, or the calmodulin-blocking agent trifluoperazine (TFP); and (iii) a source of divalent cations to preserve the side-arms. When EGTA is used, Mg2+, Sr2+, Ba2+, Mn2+ and Co2+ are effective, Ba2+ giving the most satisfactory contrast, and Mg2+ and Co2+ the best preservation. It is inferred that the cytoskeletal complex includes at least one Ca2+-activated protease, and possibly calmodulin. Microvilli are bonded together by intermicrovillar bridges with a periodicity of 11–17nm. The cytoskeleton is destroyed by pretreatment with 1 mM dithiothreitol (DTT), possibly by the activation of a thiol protease. It does not survive osmication unless treated with low molecular weight tannic acid (LMWT). The evidence does not discriminate between actin and intermediate filaments as the basis of the cytoskeleton. Attention is drawn to similarities and differences between the rhabdomeral cytoskeleton and that of vertebrate intestinal brush-borders. The extreme lability of the rhabdomeral cytoskeleton to conventional methods of fixation is attributed in part to the Ca2+ fluxes experienced by invertebrate photoreceptors, and in part to the effects of osmication.

Anisotropic imaging in the dragonfly median ocellus: a matched filter for horizon detection

Abstract. It is suggested that the dragonfly median ocellus is specifically adapted to detect horizontally extended features rather than merely changes in overall intensity. Evidence is presented from the optics, tapetal reflections and retinal ultrastructure. The underfocused ocelli of adult insects are generally incapable of resolving images. However, in the dragonfly median ocellus the geometry of the lens indicates that some image detail is present at the retina in the vertical dimension. Details in the horizontal dimension are blurred by the strongly astigmatic lens. In the excised eye the image of a point source forms a horizontal streak at the level of the retina. Tapetal reflections from the intact eye show that the field of view is not circular as in most other insects but elliptical with the major axis horizontal, and that resolution in the vertical direction is better than in the horizontal. Measurements of tapetal reflections in locust ocelli confirm their visual fields are wide and circular and their optics strongly underfocused. The ultrastructure suggests adaptation for resolution, sensitivity and a high metabolic rate, with long, widely separated rhabdoms, retinulae cupped by reflecting pigment, abundant tracheoles and mitochondria, and convoluted, amplified retinula cell plasma membranes.

The sensitivity of receptors in the posterior median eye of the nocturnal spider,Dinopis

Summary1.Intracellular and extracellular (ERG) recordings were made of receptor responses in the posterior median eye of the nocturnal spiderDinopis subrufus. These are among the largest single lens eyes described among the arthropods and are used during prey capture at night.2.The receptor potential is a depolarising wave, typical of the responses recorded from other spiders, and from rhabdomeric photoreceptors in general. By comparison with other spiders, the receptor responses are slow in time course. Large (4–12 mV) discrete potentials are seen at very low intensities. The evidence suggests that bumps are responses to single photons. If this is the case the quantum capture efficiency for receptors shown to be in their diurnal state of photoreceptive membrane depletion is 7%, measured relative to photons incident on the cornea.3.Large responses to single photons and the known light gathering capacity of the lens (Blest and Land, 1977) make individual dark adapted photoreceptors very sensitive. A corneal flux of 5×105 photons·cm−2·s−1 produces an initial responses of half maximal amplitude. This corresponds to an intensity midway between starlight and moonlight under clear sky conditions. 2×105 more light is required to produce the same response in the diurnal jumping spiderPlexippus (Hardie and Duelli, 1978).4.The spectral sensitivities of single photoreceptors are almost identical to that of the massed receptor response (ERG), and this indicates a retina dominated by photoreceptors containing a single 517 nm rhodopsin. UV sensitivity is depressed by selective attenuation in the lens matrix.5.The mean receptor angular sensitivity function has a half-width of 2.3°, larger than expected from the optical data, but still sufficient for the resolution of spatial detail whose fineness matches the receptor mosaic.6.ERG recordings fail to detect the sensitivity shifts that should be associated with the daily cycle of photoreceptive membrane (Blest, 1978). Perhaps such shifts may still exist because we find that our test lights block the synthetic phase of photoreceptive membrane turnover.7.By comparison with vertebrate rods and cones,Dinopis photoreceptors exhibit the following properties of a scotopic system: large discrete responses to single photons, a slow time course of response, the possibility of electrical coupling between receptors and a spectral sensitivity in the blue/green. The resolving power ofDinopis retina is inferior to human rod vision at the same intensities, as expected of a smaller eye.

Effects of illumination changes on rhabdom synthesis in a crab

SummaryThe rhabdoms ofLeptograpsus variegatus photoreceptors are several times larger at night than in the day. In animals kept in the laboratory and accustomed to a cycle of fluorescent light turned on and off at approximately natural dawn and dusk times, photoreceptors generally form new rhabdomeres within 30–60 min of the beginning of the dark period. At the beginning of the light period a large proportion of these rhabdomeres are removed by pinocytosis, leaving the smaller day rhabdomeres which persist until the following ‘dusk’, when they are rapidly broken down and replaced by newly synthesized phototransductive membrane. It is shown that;(i)Synthesis occurs if crabs are dark adapted several hours before the usual dark period, but the final rhabdom size is reduced.(ii)Crabs maintained in light through the usual dark period synthesize a new rhabdom.(iii)Synthesis as a reaction to early darkness occurs unilaterally if one eye is blinded.An unusual turnover schedule recently proposed forLimulus lateral eye is discussed in relation to some earlier studies onLimulus light and dark adaptation and recent work on turnover in other arthropods. It is concluded thatLimulus very probably complies with the turnover schedule emerging as normal for animals that have enlarged rhabdoms at night.

The Local Deletion of a Microvillar Cytoskeleton from Photoreceptors of Tipulid Flies during Membrane Turnover

The distal regions of the photoreceptor microvilli of tipulid flies are shed to extracellular space during membrane turnover. Before abscission, the microvillar tips undergo a transformation: they become deformed, and after conventional fixation for electron microscopy are relatively electron-lucent compared to the stable, basal microvillar segments. We now show that the electron-lucent segment is an empty bag of membrane whose P-face after freeze–etch preparation appears as densely particulate as the remainder of the microvillus. Transformation is achieved by the local deletion of a microvillar cytoskeleton which consists of a single, axial filament linked to the plasma membrane by side-arms. The filament may be partially preserved by the chelation of Ca2+; the provision of a divalent cation (Mg2+ or Ba2+) stabilizes the side-arms during subsequent fixation, as has been shown previously for the rhabdomeral cytoskeleton of blowflies. Incubation of the isolated retina in the presence of 0.25 mM Ca2+ at room temperature for 10–20 min causes proteolysis of the cytoskeleton which is blocked by as little as 0.5 mM of the thiol protease inhibitors Ep-475 and Ep-459. Loss of the cytoskeleton is accompanied by deformation of all regions of the microvilli. Local deletion of the cytoskeleton from the transformed zone of the normal rhabdom is sufficient to explain deformation of the microvillar tips, but not their subsequent abscission. The intimate association between a Ca2+-activated thiol protease and the cytoskeleton implied by the great rapidity of proteolysis calls for a reassessment of published studies of membrane turnover by radioautography, and of the nature of light-induced damage to arthropod photoreceptor membranes.

The sources of acid hydrolases for photoreceptor membrane degradation in a grapsid crab

SummaryDawn photoreceptor breakdown in the crab Leptograpsus variegatus is analysed at the ultrastructural level. Coated vesicles derived from microvilli are assembled as multivesicular bodies (mvbs), which degrade to multilamellar bodies (mls) and are lysed. Cytochemical markers for hydrolases were a fluorideinhibited β-glycerophosphatase and a fluoride-insensitive p-nitrophenyl phosphatase, with indistinguishable distributions when localised at pH 5.0. These enzymes are injected into the secondary lysosomes from two sources: (i) Immediately after dawn Golgi bodies are highly active, and differentiate a transtubular network, from which tubules and vesicles detach, and can be seen fusing with mvbs and mlbs. (ii) Saccules derived from the rough endoplasmic reticulum (RER) provide a second source and are most often seen in association with late mlbs. Both kinds of primary lysosome rarely give AcPh-positive responses when free in the cytosol, but are seen to do so as they make contact with their secondary lysosomal targets. Lipid droplets and lipofuscin bodies are interpreted as the residual products of breakdown. These results are discussed in relation to previous findings on photoreceptor membrane breakdown in a dinopid spider. Attention is drawn to the implied diversity of organisation of lysosomal compartments in receptors which internalise membranes of similar compositions.