Different types of sense organs are present on the larva of Drosophilu. Several genes that specify the type of sense organ that will form at a particular position have been recently identified. Here we review the functional and molecular analyses of these genes, and summarize the evidence which supports a role in the choice of which type of organ will be formed. Most or all of these genes are required for the appropriate specification of adult as well as larval sense organs, suggesting that the larval and adult systems share many gene requirements. Interestingly, the specifying genes identified so far in the peripheral nervous system are also expressed in subsets of cells in the central nervous system, where they might have similar roles.
lntroduct ion
The circuitry of the human brain may well be the most complex system known to man. Yet this intricate array of billions of connections is progressively built up during devclopment, and must somehow be encoded, ultimately, in the ordered expression of, at most, a few tens of thousands of genes. How does this system develop, and what directs different neurons to do different things?
Even in comparatively simple nervous systems such as the retina of vertebrates, information processing depends on the generation of a reproducible array of connections involving several typcs of neurons with very different morphologies. In spite of the fact that such differences were already well recognized a century ago, the question of how they arise has been investigated only recently. Part of the problem is that this question is difficult to address in the CNS-f, because even in a tiny insect like Drosoplzilu the central nervous system comprises tens of thousands of neurons, the vast majority of which cannot be uniquely recognized. The peripheral nervous system (PNS) of the fly, on the other hand, is simpler and might be better suited for this analysis. This review will concentrate on thc PNS of the Urosophilu larva, since most of the results to be discussed were initially -1-ABBREVIATIONS: ch (organs, neurons, lineage): chordotonal; CNS: central nervous systcm; es (organs, neurons, lineage): exkrnal sensory; PNS: peripheral nervous system; SMC: sensory mother cell.