Diversification and differentiation of the cardioblasts along the anterior-posterior axis of the heart-tube. R. Ponzielli 1, M. Astier 1, A. Chartier 1, A. Gallet 2, M. Semeriva 1. 1) Institut de Biologie et Developpement de Marseille. Laboratoire de Genetique et Physiologie du Developpement, Faculte de Sciences de Luminy, Marseille, France; 2) Institut of Signaling, Developmental Biology and Cancer Research. CNRS UMR 6543, Centre de Biochimie, Parc Valrose, Nice, France.
The cardiomyocytes of the Drosophila mature heart display a segmental genetic diversity. For example, the homeobox gene tinman (tin) is expressed in four of the six pairs of cardioblasts in each segment, while the two posterior most pairs of cardioblasts express the nuclear receptor gene seven-up (svp) (the homologue of the vertebrates COUP-TFII). Differentiation along the anterior-posterior axis of the heart tube is initiated at the end of embryogenesis and proceeds further in larval stages to eventually form the aorta in the anterior region of the heart tube and the heart proper in the posterior region. By using confocal microscopy analysis, we have described the morphological changes associated to the differentiation of the various subtypes of cardioblasts. We have also been able to follow in living embryos the acquisition of specific heart physiological functions such as heart beating and ostia functioning. The three pairs of functional ostiae that are formed in the heart originate from the svp-expressing cardioblasts. The segmentally repeated expression of svp is directed by the Hedgehog signaling, the morphogen being secreted by the overlying ectoderm. In svp mutants, ostiae do not form in the heart tube that is by all criterions normally differentiated. Differentiation of functional ostiae also requires the superimposition of a more general positional information along the anterior-posterior axis, which is provided, at least in part, by the genes of the Ultrabithorax Complex. Particularly, abdominal A function is required for the identity of heart versus aorta cardioblasts, including cardioblasts that will give rise to ostiae.