Ultrastructure of the Ocellar Visual System in Normal and MutantDrosophila Melanogaster

Abstract

Between the two compound eyes on the vertex on the adult head are the three simple eyes, ocelli. Transmission and scanning electron microscopy were used to investigate the corneal lenses, ocellar photoreceptors, and axonal projections in normal and mutant Drosophila melanogaster. In wild type flies, the cornea consists of about 45 lamellae. It has corneal nipples distally and is underlaid with a monolayer of corneagenous cells. Retinula cells have open rhabdomeres of about 2 microns (diameter) x 7 microns (length). Rhabdomeres extend to the distal extent of the cell and do not have caps. Microvilli have a rodlet within. Retinula cells are joined by belt desmosomes on the lateral borders. Eye color pigment granules are housed within the retinula cells of normal flies, not in accessory cells. The granules do not migrate in response to light. No screening pigment granules exist in the white mutant. Each ocellus has about 80 retinula cells whose axons project to corresponding ganglia from which 4 giant afferent interneurons (per ganglion) project to the brain. receptor terminals are invested with capitate projections from glia. Receptors synapse onto dyads of follower cells, usually interneuron processes, at sites of T shaped presynaptic ribbons. These "T bars" are surrounded by indistinct flattened vesicles. Interneurons make feed back synapses onto receptor terminals at T bars clustered with distinct round vesicles. Three mutants with abnormal ocelli were investigated. The none mutant has unusual compound eye and ocellar corneas. The compound eye is devoid of differentiated photoreceptors but some axons from undifferentiated cells from synapses. No receptors were found in the ocelli of none. The oc mutant has no ocelli, although sometimes an ocellar cornea like that of none is seen; the compound eye is normal. The rdo mutant is also specific to ocelli with smaller ocelli having half the wild type allotment of receptor cells; the number of giant afferents is unaffected. Mutants best known for their compound eye defects were examined. The norpA mutant loses its ocellar rhabdomeres with age but has normal feed forward and feed back synapses. This normal synaptology prevails despite the electrophysiological defects in norpA ocelli reported earlier. The rdgABS12 mutant has poorly formed ocellar receptors which show some degeneration with age but synapses survive. The trp. rdgBKS222 and rgdAPC47 mutants are essentially normal with respect to structure and survival of ocellar receptors and synapses.

Glass encodes a site-specific DNA-binding protein that is regulated in response to positional signals in the developing Drosophila eye.

“…Cell identities in the developing eye of Drosophila are determined by induction, and mutations in several genes that encode putative transcription factors have been shown to disrupt normal eye development (for reviews, see Tomlinson 1988;Banerjee and Zipursky 1990). Here, we show that one of these genes, glass, encodes a site-spe1present address: Molecular Biology Section, Department of Biological Sciences, University of Southern California, University Park, Los Angeles, California 90089 USA.cific DNA-binding protein and that glass function, in its broadest sense, is regulated at the protein level.The glass gene is required for the normal development of photoreceptor cells in all three organs in which they occur: the adult compound eye (Johannsen 1924;Garen and Kankel 1983), the adult simple eyes or ocelli (Stark et al 1984;Moses et al 1989;Stark and Sapp 1989), and the larval photoreceptor or Bolwig's organ (Moses et al 1989). In the retina, we demonstrated that only the photoreceptor cells have a cell-autonomous requirement for glass function (Moses et al 1989).…”

TRP, a protein essential for inositide-mediated Ca2+ influx is localized adjacent to the calcium stores in Drosophila photoreceptors

“…Since the TRP protein is expressed in both compound eye and ocellus, it does not appear that TRPprotein is specific to any particular opsin isoform. Ultrastructurally, however, there are several features found in common between the compound eye and the ocellus that are not apparent in the larval photoreceptor (Pollock and Benzer, 1988;Stark et al, 1989). In particular, the structure and organization of the rhabdomeric microvilli and the subrhabdomeric cysternae is quite similar between the two adult visual organs where true rod-like rhabdomeres are formed.…”

Identification of functionalsine oculismotifs in the autoregulatory element of its own gene, in theeyelessenhancer and in the signalling genehedgehog

“…The Drosophila visual system consists of two compound eyes and three ocelli, which are simple eyes located on the adult vertex (Stark et al, 1989). Both types of optical organs develop from a small number of cells that are set aside during development in the early embryo.…”