The ommatidia of Arca noae: a three-tier structure with a central light-guiding element for the receptor cell.

The compound eyes of ark clams appear to function as an optical system to trigger shell closure against predators. We have analyzed the structure of the ommatidia of Arca noae by thin section electron microscopy and serial sectioning, Concanavalin A-gold labeling and acid phosphatase cytochemistry. Our results demonstrate that the ommatidia are a three-tier structure composed of a central single receptor cell, surrounded and covered by proximal pigment cells followed by rows of distal pigment cells. The receptor cells of Arca noae have no lens and the disks of their receptive segment are derived from sensory cilia. The distal mitochondrial segment in the cytoplasm between the nucleus and the receptive segment is surrounded by a mass of Concanavalin A-reactive glycogen particles. Although both, proximal and distal pigment cells have numerous microvilli, only those of the proximal pigment cells form a well-aligned brush border. The microvilli of the latter are ≈9-11 µm long and have a diameter of ≈70-80 nm. Numerous microlamellar bodies cover them. The microlamellar bodies are stored in acid phosphatase-negative secretory granules of the pigment granule-free apical cytoplasm of proximal pigment cells before their secretion. Observation of living compound eyes indicated that the apex of proximal pigment cells transmitted significantly more light than the surrounding distal pigment cells. Hence, the regular geometry of the brush border seems to be a light-guiding structure for receptor cells similar to an optical fiber.

Misexpression screen in Drosophila melanogaster aiming to reveal novel factors involved in formation of body parts.

To identify novel factors that lead a fly imaginal disc to adopt its developmental fate, we carried out a modular dominant misexpression screen in imaginal discs. We have identified two factors that appear to change the fate of the respective body structure and appear to lead to the transformation of a body part. In one mutant line, notum tissue, normally derived from wing imaginal tissue, formed close to the site of the sternopleural bristles, which are leg disc derivatives. In the other line, the arista is transformed into a tubular structure, resembling an abnormal leg. We found that ectopic expression of abrupt was responsible for this potential transformation of the arista.

Expression of COPI components during development of Drosophila melanogaster.

In a P{lArB} enhancer detector collection, a line was found that showed upregulated expression within centrally to posteriorly located germarial cysts. It was inserted in the gammaCOP locus on chromosome 3R. GammaCOP is a component of the COPI coatomer involved in membrane traffic. Most of the other known components of the COPI coatomer also showed higher expression in the posterior half of the germarium. Not only meiotic germline cysts but also migrating follicle cells upregulate the COPI subunits. During embryonic and larval development, the COPI subunits are expressed ubiquitously as expected for genes required for cell viability. In addition, they are strongly expressed in the salivary glands and the proventriculus. Whether tissue-specific transcriptional upregulation of COPI subunits is required for the reorganization of membranous compartments that are needed for the developmental processes that confer cyst polarity and follicle maturation will have to be addressed in a genetic study.

Evolution of the Hox gene complex from an evolutionary ground state.

In this chapter, we consider the question of how the ordered clusters of Hox genes arose during evolution. Since ordered Hox clusters are found in all major superphyla, we have to assume that the Hox clusters arose before the Cambrian "explosion" giving rise to all of these taxa. Based on his studies of the bithorax complex (BX-C) in Drosophila Lewis considered the ground state to be the mesothoracic segment (T2) since the deletion of all of the genes of the BX-C leads to a transformation of all segments from T3 to A8/9 (the last abdominal segment) into T2 segments. We define the developmental ground state genetically, by assuming that loss-of-function mutants lead to transformations toward the ground state, whereas gain-of-function mutants lead to homeotic transformations away from the ground state. By this definition, T2 also represents the developmental ground state, if one includes the anterior genes, that is, those of the Antennapedia complex. We have reconstructed the evolution of the Hox cluster on the basis of known genetic mechanisms which involve unequal crossover and lead from an urhox gene, first to an anterior and a posterior gene and subsequently to intermediate genes which are progressively inserted, between the anterior and posterior genes. These intermediate genes are recombinant due to unequal crossover, whereas the anterior and posterior genes are not affected and therefore had the longest time to diverge from the urhox gene. The molecular phylogenetic analysis strongly supports this model. We consider the ground state to be both developmental and evolutionary and to represent the prototypic body segment. It corresponds to T2 and is specified by Antennapedia or Hox6, respectively. Experiments in the mouse also suggest that the ground state is a thoracic segment. Evolution leads from the prototypic segment to segmental divergence in both the anterior and posterior direction. The most anterior head and tail segments are specified by homeobox genes localized outside of the cluster.