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Genomic Data Suggests Tooth Enamel Started Out on Skin

NEW YORK (GenomeWeb) – A new Nature study hinging on genomic data coupled with paleontology patterns suggests tooth enamel evolved from a tissue that once armored ancient fish.

In an effort to understand the roots of tooth enamel, researchers from Sweden and China began by looking for genes used by an existing fish — the spotted gar, Lepisosteus oculatus — to produce an enamel-like scale and dermal bone covering called ganoine. The search led to ambn and enam, genes involved in tooth enamel formation in other animals, that were expressed in the gar's skin tissue.

The team's analysis of two fossil fish further supported the notion that tooth enamel started out as a covering for scales and skull bones, rather than the teeth: A Silurian bony fish known as Andreolepis had scales with an enamel-like covering but teeth made up of naked dentine, for example, while remains from a bony fish called Psarolepis romeri from the Early Devonian period had enamel-like material on skulls and tooth-like denticles, though their teeth were enamel-free.

"Psarolepis and Andreolepis are among the earliest bony fishes, so we believe that their lack of tooth enamel is primitive and not a specialization," senior author Per Erik Ahlberg, an evolution and development researcher at Uppsala University, said in a statement.

"It seems that enamel originated in the skin, where we call it ganoine," he explained, "and only colonized the teeth at a later point."

Past studies have shown that tooth enamel formation relies on a matrix made up of ameloblastin, enamelin, and other enamel matrix proteins encoded by the ambn, enam, and amel genes — a protein platform that's gradually degraded as a hardened, mineralized material forms in their place.

Although cartilaginous fish are missing enamel and enamel matrix protein-coding genes, the researchers explained, at least some fish from the actinopterygian ray-finned bony fish lineage have a compound known as ganoine on their scales and dermal bones that may be akin to tooth enamel in lobe-finned bony fish and other animals falling in the so-called sarcopterygian group.

Even so, the team turned up ambn and enam genes in the recently sequenced spotted gar genome. The genes, which fell in a cluster of genes coding for other hard tissue components, were expressed in gar skin tissue, according to RNA sequence data.

Such findings support the notion that the ganoine material in gar and related fish is homologous to tooth enamel in tetropods, the researchers reported. On the other hand, the apparent absence of an amel gene coding for the enamel matrix protein ameloblastin indicated that enamel and ganoine likely form somewhat differently.

Coupled with histological patterns in the Andreolepis and Psarolepis fossils assessed in the study, the genetic data points to the presence of enamel-like material in ancient fish that originally appeared on dermal tissue and progressively moved to skull bones and teeth.

The study's authors noted that more detailed phylogenetic analyses are needed to tease apart details of the transition from enamel from skin and scales to the teeth.