Distribution of F-actin in the compound eye of the blowfly, Calliphora erythrocephala (Diptera, Insecta)

Abstract

Summary.Filaments that probably consist of actin have been observed by electron microscopy in various cells and structures of the compound eye of insects and crustaceans. The overall distribution of F-actin in the compound eye of Calliphora erythrocephala was studied using fluorescent phalloidins as specific probes for F-actin. F-actin is localized (1) in the rhabdomeres, (2) in the Semper cells, (3) in the basement membrane, and (4) associated with desmosomes and membranes. It is concluded that the actin filaments present in the compound eye are involved in mechanical stabilization (e.g., within microvillar processes of the Semper cells and in association with intercellular contacts) and in motile phenomena (e.g., in the rhabdomeric microvilli). Key words: Compound eyesThe cytoskeleton in the cells of the ommatidia of the insect compound eye has only partially been examined and characterized (Blest 1988 ;Blest and Eddey 1984;Walz 1983). Most studies have focused on the axial filaments within the rhabdomeric microvilli of the photoreceptors (for example Blest et al. 1982a, b ; el-Gammalet al. 1987). In contrast to microtubules, microfilaments can be specifically characterized only by immunohisto-or cytochemical methods. Using these techniques, Arikawa and coworkers (1990) have shown in Drosophila melanogaster, that the axial filaments of the photoreceptive microvilli are composed of F-actin. However, a systematic study of the distribution of actin filaments within the insect compound eye is still lacking. In the present study it is shown that actin filaments in Calliphora erythrocephala occur within (1) the rhabdomeric microvilli, (2) the basement membrane, (3) the microvillar projections of the Semper cells, and are (4) associated with microtubules or with desmosomes or hemidesmosomes. Materials and methodsThe wild-type and the white-eyed "chalky" mutant of Calliphora erythrocephala (Diptera) were obtained from local cultures.Preparation. Specimens were fixed in a mixture of 0.2% glutaraldehyde and 3% paraformaldehyde in 0.1 M phosphate buffer, pH 7.2, for 3 h at 4°C. They were subsequently washed in cold buffered 10% sucrose for 1 h. followed by infiltration with buffered 25% sucrose for 12 h at 4°C. The specimens were surrounded by boiled liver, embedded (medium: Reichert-Jung, Nussloch, FRG) and cryofixed in melting iso-pentane. They were sectioned at 6-12 µm thickness in a cryostate (Dittes, Heidelberg, FRG) between -15°C and -25°C, and placed on coverslips precoated with 0.05% aequous poly-L-lysine.Phalloidin staining. Rhodamine-coupled phalloidin (a generous gift from Dr. H. Faulstich, M P Ifor Medicine, Heidelberg) and FITCcoupled phalloidin (Sigma, Deisenhofen, FRG) were used as specific probes for filamentous actin (Wulf et al. 1979;Faulstich et al. 1988). Each cryosection was incubated with 7 µl of one of the fluorescent phalloidins in phosphate-buffered saline, pH 7.2, (PBS) (0.01 mg/ml and 0.1 mg/ml. respectively) for 1 h at room temperature and washed 3 times in PBS and one time in H2O to remove u...

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