title: Fin development in a cartilaginous fish and the origin of vertebrate limbs.
abstract: Recent fossil finds and experimental analysis of chick and mouse embryos highlighted the lateral fin fold theory, which suggests that two pairs of limbs in tetrapods evolved by subdivision of an elongated single fin. Here we examine fin development in embryos of the primitive cartilaginous fish, Scyliorhinus canicula (dogfish) using scanning electron microscopy and investigate expression of genes known to be involved in limb positioning, identity and patterning in higher vertebrates. Although we did not detect lateral fin folds in dogfish embryos, Engrailed-1 expression suggests that the body is compartmentalized dorso-ventrally. Furthermore, specification of limb identity occurs through the Tbx4 and Tbx5 genes, as in higher vertebrates. In contrast, unlike higher vertebrates, we did not detect Shh transcripts in dogfish fin-buds, although dHand (a gene involved in establishing Shh) is expressed. In S. canicula, the main fin axis seems to lie parallel to the body axis. 'Freeing' fins from the body axis and establishing a separate 'limb' axis has been proposed to be a crucial step in evolution of tetrapod limbs. We suggest that Shh plays a critical role in this process.
authors.affiliation: Division of Cell and Developmental Biology, Wellcome Trust Biocentre, University of Dundee, Dow Street, Dundee DD1 5EH, UK. mtanaka@dundee.ac.uk
authors.forename: Mikiko
authors.initials: M
authors.lastname: Tanaka
authors.forename: Andrea
authors.initials: A
authors.lastname: Münsterberg
authors.forename: W Gary
authors.initials: WG
authors.lastname: Anderson
authors.forename: Alan R
authors.initials: AR
authors.lastname: Prescott
authors.forename: Neil
authors.initials: N
authors.lastname: Hazon
authors.forename: Cheryll
authors.initials: C
authors.lastname: Tickle
mesh_terms: D000818:Animals; D030161:Avian Proteins; D005075:Biological Evolution; D002356:Cartilage / Q000196:embryology* / Q000648:ultrastructure; D004284:Dogfish / Q000033:anatomy & histology / Q000196:embryology* / Q000235:genetics; D005121:Extremities / Q000196:embryology; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020825:T-Box Domain Proteins / Q000235:genetics; D014714:Vertebrates / Q000196:embryology
chemical_list: D030161:Avian Proteins; D018398:Homeodomain Proteins; D020825:T-Box Domain Proteins; C103172:T-box transcription factor 5; C491196:TBX4 protein, Gallus gallus