Specificity and Promiscuity among Proneural Proteins

fails to resolve, demonstrating that, as with the selection of other neural precursors, the resolution of amos expression involves lateral inhibition. In the antennal imagi-nal disc, amos mRNA is detected in three semicircular The peripheral nervous system (PNS) is made up of an bands that correspond to the sites where olfactory sen-array of different sensory structures that detect aspects sillae arise. of the environment—for example, the proprioceptive Confirmation that amos has proneural activity comes and mechanoreceptive sensory neurons in vertebrates. from loss-and gain-of-function experiments, although In Drosophila, there is a similar array of sensory organs, these were hampered by the absence of mutations in including the external sensory bristles (mechanosen-amos. One approach was to disrupt amos function using sory), chordotonal organs (stretch receptors), multiden-double-stranded RNA interference. Injection of amos dritic neurons, and olfactory sensillae. The development dsRNA into embryos resulted in the loss of MD neurons, of these structures involves two processes, the acquisi-whereas control experiments using atonal dsRNA had tion of neural competence and the specialization appro-no effect on these structures (Huang et al., 2000). Be-priate for each sensory organ type. Initially, it appeared cause Amos, like other proneural proteins, dimerizes that the regulation of these two aspects of development with Daughterless (the Drosophila E12/E47 homolog) to was separate, with proneural genes conferring neural bind to DNA (Huang et al., 2000), another way to assay potential and other transcription factors specifying the amos function was to see whether defects in PNS devel-precise structures to be formed. However, more recently opment were revealed when the levels of both daughter-it has become clear that proneural proteins can confer less and amos were reduced in heterozygous animals. sensory organ identity as well. Two papers in this issue This was done by making a transheterozygous combina-of Neuron (Goulding et al., 2000; Huang et al., 2000) tion between a chromosomal deficiency that eliminates further support this, as they describe the isolation of a amos and a null allele of daughterless. A convincing new Drosophila proneural gene, amos, that is involved decrease in olfactory sensillae was observed in these in the development of the olfactory sensillae and multi-transheterozygotes in comparison to heterozygotes of dendritic neurons (MD). either mutant chromosome alone (Goulding et al., 2000). Neural development in both vertebrates and invertebrates involves the establishment of a zone of compe-The proneural function of amos was also evident from targeted misexpression, which resulted in extra MD neu-tent …