Characterization of a novel subset of cardiac cells and their progenitors in the Drosophila embryo

The Drosophila heart is a simple organ composed of two major cell types: ca rdioblasts, which form the simple contractile tube of the heart, and perica rdial cells, which flank the cardioblasts. A complete understanding of Dros ophila heart development requires the identification of all cell types that comprise the heart and the elucidation of the cellular and genetic mechani sms that regulate the development of these cells. Here, we report the ident ification of a new population of heart cells: the Odd skipped-positive peri cardial cells (Odd-pericardial cells). We have used descriptive, lineage tr acing and genetic assays to clarify the cellular and genetic mechanisms tha t control the development of Odd-pericardial cells, Odd skipped marks a pop ulation of four pericardial cells per hemisegment that are distinct from pr eviously identified heart cells. We demonstrate that within a hemisegment, Odd-pericardial cells develop from three heart progenitors and that these h eart progenitors arise in multiple anteroposterior locations within the dor sal mesoderm, Two of these progenitors divide asymmetrically such that each produces a two-cell mixed-lineage clone of one Odd-pericardial cell and on e cardioblast, The third progenitor divides symmetrically to produce two Od d-pericardial cells. All remaining cardioblasts in a hemisegment arise from two cardioblast progenitors each of which produces two cardioblasts. Furth ermore, we demonstrate that numb and sanpodo mediate the asymmetric divisio ns of the two mixed-lineage heart progenitors noted above.