Fig. 1. The PDGF/VEGF signaling superfamily. PDGFRs (A), PDGFs (B) and related proteins (A,B) identified in different organisms. Vertebrate VEGFs and VEGFRs are not included in this figure, but a prototypical mammalian VEGFR is shown for comparison (A). Mouse, human, Drosophila and C. elegans proteins are discussed in the text (see text for references). In some teleosts, including zebrafish and pufferfish, the Pdgfrb genomic locus appears to have been duplicated, giving these species three PDGF receptors, PDGFR, PDGFRß1.1, and PDGFRß1.2 (Williams et al., 2002). There are three known splice variants of chick PDGFA, which correspond to the short and long forms identified in mouse, as well as an additional short form (Ataliotis, 2000; Hamada et al., 2000; Horiuchi et al., 2001). Only one receptor (PDGFR) and two splice variants of PDGFA have been identified in Xenopus (Mercola et al., 1988; Bejcek et al., 1990; Jones et al., 1993). C. elegans and Drosophila melanogaster each have a single PDGF/VEGF-like signaling network. C. elegans has four VEGFR-like proteins, VER-1 to VER-4, but no definitive VER ligand has been identified or characterized. Drosophila has three PDGF/VEGF ligands, PVF-1, 2 and 3, and one receptor, PVR. Two splice forms of PVF-1 have been identified that differ at their N termini. Three splice variants of PVR have also been identified: two differ in the region between the second and third immunoglobulin (Ig) domains, and the third is truncated after the second Ig domain. Mm, Mus musculus (mouse); Hs, Homo sapiens (human); Dr, Danio rerio (zebrafish); Gg, Gallus gallus (chick); Xl, Xenopus laevis (frog); Dm, Drosophila melanogaster (fruit fly); Ce, Caenorhabditis elegans (nematode).