Paired appendages are considered to have evolved either from the gill arch or from paired lateral fin folds. Evidence for the latter has been derived from the shared structural and molecular features of the unpaired and paired fins, suggesting that the developmental program generating paired fins was co-opted from median fins during evolution. It is hypothesised that a lateral fin fold would have been an intermediate state between the two, but there is scant evidence for this in the fossil record or extant species. Furthermore, despite commonalities in development, median fins and paired fins derive strictly from distinct mesodermal compartments, namely paraxial and lateral plate mesoderm respectively. Thus, the fin developmental program must have been copied from the paraxial to lateral plate at some unknown point. Using a combination of RNA in situ staining and transgenic cell lineage tracing methods we here show that fish surprisingly possess a lateral plate derived median fin and suggest that this fin is an intermediate in the median to paired fin transition. We show that this is conserved through evolution from cyclostomes to tetrapods, and further show by gene knockdown that it possesses latent and conserved capacity for being bifurcated into paired structures. Our work thus provides support for the lateral fin fold hypothesis.
Identification of an intermediate fin in the median to paired fin transition
Abstract ID: 212
Event: The 3rd AsiaEvo Conference
Topic: Early evolution of vertebrates from evo-devo and paleontological perspectives
Presenter Name: Tom Carney