For a dentition representing the most basal extant gnathostomes, that of the shark can provide us with key insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled the expression of
sonic hedgehog
, a key regulator of tooth induction. We find in the catshark (
Scyliorhinus canicula
) that intense
shh
expression first occurs in a bilaterally symmetrical pattern restricted to broad regions in each half of the dentition in the embryo jaw. As in the mouse, there follows a changing temporal pattern of
shh
spatial restriction corresponding to epithelial bands of left and right dental fields, but also a subfield for symphyseal teeth. Then, intense
shh
expression is restricted to loci coincident with a temporal series of teeth in iterative jaw positions. The developmental expression of
shh
reveals previously undetected timing within epithelial stages of tooth formation. Each locus at alternate, even then odd, jaw positions establishes precise sequential timing for successive replacement within each tooth family.
Shh
appears first in the central cusp, iteratively along the jaw, then reiteratively within each tooth for secondary cusps. This progressive, sequential restriction of
shh
is shared by toothed gnathostomes and conserved through 500 million years of evolution.
The developmental expression of foxl2 in the dogfish Scyliorhinus canicula