The lateral and dorsal neurons of Drosophila melanogaster: new insights about their morphology and function

This chapter summarizes our present knowledge about the master clock of the fruit fly at the neuronal level. The clock is organized in distinct groups of interconnected pacemaker neurons with different functions. All of these neurons appear to communicate with one another in order to produce the species-specific activity rhythm, which is organized in morning (M) and evening (E) activity bouts. These two activity components are differentially influenced by distinct groups of pacemaker neurons reminiscent of the Pittendrigh-Daan dual oscillator model. In the original work (Grima et al. 2004; Stoleru et al. 2004), the ventrolateral (LN(v)) and dorsolateral (LN(d)) plus some dorsal groups (DN) of clock neurons have been defined as M and E cells, respectively. We further specify that the clock neurons belong to the M and E oscillators and define a more complex picture of the Drosophila brain clock.

Expression of achaete-scute homologues in discrete proneural clusters on the developing notum of the medfly Ceratitis capitata, suggests a common origin for the stereotyped bristle patterns of higher Diptera

The stereotyped positioning of sensory bristles in Drosophila has been shown to result from complex spatiotemporal regulation of the proneural achaete-scute genes, that relies on an array of cis-regulatory elements and spatially restricted transcriptional activators such as Pannier. Other species of derived schizophoran Diptera have equally stereotyped, but different, bristle patterns. Divergence of bristle patterns could arise from changes in the expression pattern of proneural genes, resulting from evolution of the cis-regulatory sequences and/or altered expression patterns of transcriptional regulators. Here we describe the isolation of achaete-scute homologues in Ceratitis capitata, a species of acalyptrate Schizophora whose bristle pattern differs slightly from that of Drosophila. At least three genes, scute, lethal of scute and asense have been conserved, thus demonstrating that gene duplication within the achaete-scute complex preceded the separation of the families Drosophilidae and Tephritidae, whose common ancestor goes back more than 100 million years. The expression patterns of these genes provide evidence for conservation of many cis-regulatory elements as well as a common origin for the stereotyped patterns seen on the scutum of many Schizophora. Some aspects of the transcriptional regulation have changed, however, and correlate in the notum with differences in the bristle pattern. The Ceratitis pannier gene was isolated and displays a conserved expression domain in the notum.

The HLH domain of a zebrafish HE12 homologue can partially substitute for functions of the HLH domain of Drosophila DAUGHTERLESS

We have identified a zebrafish homologue of the human E12 protein, which we have called ZFE12. It shows a high degree of identity to HE12 throughout its entire sequence, particularly in the bHLH domain (93%); amino acid sequence identity of the bHLH domain of ZFE12 to Drosophila DAUGHTERLESS is also very high (75%). During early embryogenesis, expression of ZfE12 is widespread. Following gastrulation, ZfE12 transcripts can be detected in all embryonic tissues with the exception of the notochord, although zones with relatively higher densities of transcripts are present in the developing brain and in the somites. To assay biological activities associated with the ZFE12 protein, two P-element constructs were made, each carrying a Drosophila daughterless gene that had been modified by replacing either the HLH domain or the entire C-terminus including the bHLH domain, by the equivalent domains of ZfE12. These constructs were used to transform flies and tested for their ability to rescue the daughterless mutant phenotype. Complete rescue of the neural phenotype and of the embryonic lethality was obtained. However, the daughterless function could not be completely restored. Although fertile flies transheterozygous for a hypomorphic and an amorphic mutation and carrying the construct encoding the zebrafish HLH domain emerged, they lacked various types of sensory organs on head and thorax and showed slight wing defects. Mutants carrying the second construct occasionally reached pupal stages but died subsequently. These data demonstrate that the HLH domain of ZFE12 can carry out most, but not all, functions performed by the corresponding region of the DAUGHTERLESS protein in Drosophila.