The blood/vascular system in a phylogenetic perspective.

The genetically and experimentally accessible organs of Drosophila, such as the heart or blood-forming tissues, have become a fertile ground for systematic projects of gene discovery and for functional studies of gene networks and signaling pathways. One argument justifying this approach is the often-tacit assumption that clear-cut homologies can be established between the Drosophila organs and their vertebrate counterparts. Here we investigate this assumption by surveying pertinent aspects of vascular structure and development in different invertebrate phyla, in the hope that this information will help to reveal the ancestral condition of the vascular system. Evolutionary scenarios that derive the structure of the cardiovascular system of extant animal taxa from the ancestral condition will be used to qualify hypotheses regarding homologies that are based on molecular similarities.