title: Degenerate evolution of the hedgehog gene in a hemichordate lineage.
abstract: The discovery of a set of highly conserved genes implicated in patterning during animal development represents one of the most striking findings from the field of evolutionary developmental biology. Existence of these "developmental toolkit" genes in diverse taxa, however, does not necessarily imply that they always perform the same functions. Here, we demonstrate functional evolution in a major toolkit gene. hedgehog (hh) encodes a protein that undergoes autocatalytic cleavage, releasing a signaling molecule involved in major developmental processes, notably neural patterning. We find that the hh gene of a colonial pterobranch hemichordate, Rhabdopleura compacta, is expressed in a dramatically different pattern to its ortholog in a harrimaniid enteropneust hemichordate, Saccoglossus kowalevskii. These represent two of the three major hemichordate lineages, the third being the indirect developing ptychoderid enteropneusts. We also show that the normally well-conserved amino acid sequence of the autoproteolytic cleavage site has a derived change in S. kowalevskii. Using ectopic expression in Drosophila, we find that this amino acid substitution reduces the efficiency of Hh autocatalytic cleavage and its signaling function. We conclude that the Hh sequence and expression in S. kowalevskii represent the derived state for deuterostomes, and we argue that functional evolution accompanied secondary reduction of the central nervous system in harrimaniids.
authors.affiliation: Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, United Kingdom. atsuko.sato@zoo.ox.ac.uk
authors.forename: Atsuko
authors.initials: A
authors.lastname: Sato
authors.forename: Helen
authors.initials: H
authors.lastname: White-Cooper
authors.forename: Karen
authors.initials: K
authors.lastname: Doggett
authors.forename: Peter W H
authors.initials: PW
authors.lastname: Holland
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D017124:Conserved Sequence; D019143:Evolution, Molecular; D005815:Genetic Code; D053823:Hedgehog Proteins / Q000235:genetics*; D008969:Molecular Sequence Data
chemical_list: D053823:Hedgehog Proteins