A shared role for sonic hedgehog signalling in patterning chondrichthyan gill arch appendages and tetrapod limbs.
Chondrichthyans (sharks, skates, rays and holocephalans) possess paired appendages that project laterally from their gill arches, known as branchial rays. This led Carl Gegenbaur to propose that paired fins (and hence tetrapod limbs) originally evolved via transformation of gill arches. Tetrapod limbs are patterned by asonic hedgehog(Shh)-expressing signalling centre known as the zone of polarising activity, which establishes the anteroposterior axis of the limb bud and maintains proliferative expansion of limb endoskeletal progenitors. Here, we use loss-of-function, label-retention and fate-mapping approaches in the little skate to demonstrate that Shh secretion from a signalling centre in the developing gill arches establishes gill arch anteroposterior polarity and maintains the proliferative expansion of branchial ray endoskeletal progenitor cells. These findings highlight striking parallels in the axial patterning mechanisms employed by chondrichthyan branchial rays and paired fins/limbs, and provide mechanistic insight into the anatomical foundation of Gegenbaur's gill arch hypothesis.
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA jag93@cam.ac.uk.
J Andrew
JA
Gillis
Department of Biology, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia, Canada B3H 4R2.
Brian K
BK
Hall
D000818:Animals; D005121:Extremities / Q000196:embryology; D015870:Gene Expression; D005880:Gills / Q000196:embryology*; D053823:Hedgehog Proteins / Q000378:metabolism*; D018878:Limb Buds / Q000166:cytology; D015398:Signal Transduction; D012860:Skates, Fish / Q000196:embryology* / Q000235:genetics
D053823:Hedgehog Proteins
Appendage patterning; Evolution; Gill arch; Leucoraja erinacea; Skate; Sonic hedgehog