BACKGROUND: Little is known about the control of the development of vertebrate unpaired
appendages such as the caudal fin, one of the key morphological specializations of
fishes. Recent analysis of lamprey and dogshark median fins suggests the co-option
of some molecular mechanisms between paired and median in Chondrichthyes. However,
the extent to which the molecular mechanisms patterning paired and median fins are
shared remains unknown. RESULTS: Here we provide molecular description of the initial
ontogeny of the median fins in zebrafish and present several independent lines of
evidence that Sonic hedgehog signaling emanating from the embryonic midline is essential
for establishment and outgrowth of the caudal fin primordium. However, gene expression
analysis shows that the primordium of the adult caudal fin does not harbor a Sonic
hedgehog-expressing domain equivalent to the Shh secreting zone of polarizing activity
(ZPA) of paired appendages. CONCLUSION: Our results suggest that Hedgehog proteins
can regulate skeletal appendage outgrowth independent of a ZPA and demonstrates an
unexpected mechanism for mediating Shh signals in a median fin primordium. The median
fins evolved before paired fins in early craniates, thus the patterning of the median
fins may be an ancestral mechanism that controls the outgrowth of skeletogenic appendages
in vertebrates.
