The expression of pannier and achaete-scute homologues in a mosquito suggests an ancient role of pannier as a selector gene in the regulation of the dorsal body pattern.

The Drosophila gene pannier (pnr) has recently been assigned to a new class of selector genes (Calleja, M., Herranz, H., Estella, C., Casal, J., Lawrence, P., Simpson, P. and Morata, G. (2000). Development 127, 3971-3980; (Mann, R. S. and Morata, G. (2000). ANNU: Rev. Cell Dev. Biol. 16, 243-271). It specifies pattern in the dorsal body. On the dorsal notum it is expressed in a broad medial domain and directly regulates transcription of the achaete-scute (ac-sc) genes driving their expression in small discrete clusters within this domain at the sites of each future bristle. This spatial resolution is achieved through modulation of Pnr activity by specific co-factors and by a number of discrete cis-regulatory enhancers in the ac-sc gene complex. We have isolated homologues of pnr and ac-sc in Anopheles gambiae, a basal species of Diptera that diverged from Drosophila melanogaster (Dm) about 200 million years ago, and examined their expression patterns. We found that an ac-sc homologue of Anopheles, Ag-ASH, is expressed on the dorsal medial notum at the sites where sensory organs emerge in several domains that are identical to those of the pnr homologue, Ag-pnr. This suggests that activation of Ag-ASH by Ag-Pnr has been conserved. Indeed, when expressed in Drosophila, Ag-pnr is able to mimic the effects of ectopic expression of Dm-pnr and induce ectopic bristles. These results are discussed in the context of the gene duplication events and the acquisition of a modular promoter, that may have occurred at different times in the lineage leading to derived species such as Drosophila. The bristle pattern of Anopheles correlates in a novel fashion with the expression domains of Ag-pnr/Ag-ASH. While precursors for the sensory scales can arise anywhere within the expression domains, bristle precursors arise exclusively along the borders. This points to the existence of specific positional information along the borders, and suggests that Ag-pnr specifies pattern in the medial, dorsal notum, as in Drosophila, but via a different mechanism.