Genetically distinct cardial cells within the Drosophila heart

Although often viewed as a simple pulsating tube, the Drosophila dorsal ves sel is intricate in terms of its structure, cell types, and patterns of gen e expression. Two nonidentical groups of cardial cells are observed in segm ents of the heart based on the differential expression of:transcriptional r egulators. These include sets of four cell pairs that express the homeodoma in protein Tinman (Tin), alternating with groups of two cell pairs that exp ress the orphan steroid hormone receptor Seven Up (Svp). Here we show that these myocardial cell populations are distinct in terms of their formation and gene expression profiles, The Svp-expressing cells are generated by asy mmetric cell divisions of precursor cells based on decreases or increases i n their numbers in numb or sanpodo mutant embryos. In contrast, the numbers of Tin-expressing cardial cells are unchanged in these genetic backgrounds , suggesting they arise from symmetric cell divisions. One function for Svp in the two pairs of cardial cells is to repress the expression of the tin gene and at least one of its targets, the beta3 tubulin gene. Further diffe rences in the cells are substantiated by the identification of separable en hancers for D-mef2 gene transcription in the distinct cardioblast sets. Tak en together, these results demonstrate a greater cellular and genetic compl exity of the Drosophila heart than previously appreciated, (C) 2000 Wiley-L iss, Inc.