Loricarioid catfish evolved skin denticles that recapitulate teeth at the structural, developmental, and genetic levels (raw RNAseq reads)

The first vertebrate mineralized skeleton was an external bony armor
coated with dental structures. The subsequent emergence of a mineralized
endoskeleton and of teeth are considered key innovations in the
diversification of vertebrates. Although time clouds our understanding of
the initial evolution of these mineralized structures, recent
re-emergences may shed light on the underlying processes. Loricarioid
catfishes are a lineage that, much like the ancestral vertebrates, bear
denticle-clad bony armor from head to tail. Loricarioid denticles (LDs)
and oral teeth are very similar in superstructure. We show here that other
extra-oral dental structures are found as ancestral characters only in
lineages that are distantly related to loricarioids such as sharks or
coelacanth, indicating that LDs have independently re-emerged in
loricarioid catfishes. We investigate whether the similarities between LDs
and teeth extend to their developmental and genetic context, and how their
development compares to that of other vertebrate integument structures.
Our detailed study of the development of LDs, and gene expression analyses
through in situ hybridization confirm that all 12 genes from the
tooth-forming gene regulatory network (oGRN) are expressed in developing
LDs in a similar way as they are expressed in developing teeth. We then
compare the developmental, structural, and genetic aspects of LD and teeth
with that of other integument appendages such as fish scales, shark dermal
denticles, feathers and hairs. We find that LDs share all developmental
cues with teeth and, to a lesser extent, with the other vertebrate
integument structures. Taken together, our results indicate that denticles
have re-emerged on the trunk of loricarioid catfishes through the ectopic
co-option of the oGRN rather than the resurrection of an ancestral
trunk-specific denticle genetic pathway.