A pet fish adorned with tooth-like scales is helping biologists tackle a longstanding debate about the origin of teeth, and explore how body structures can be lost and regained during evolution.
The suckermouth armoured catfish is commonly found in pet shops and, unusually for a bony fish, has tooth-like structures called odontodes covering its skin. These physically resemble teeth, erupting from thickened patches of skin to form layered structures of pulp, dentine and enamel, and similar genes appear to be active in both during development. But which evolved first, and how did tissues gain or regain them?
Their evolutionary history is complicated, because while ancient fish had similar structures, they were lost in most bony fish, but retained in fish with cartilage-based skeletons, like sharks. They re-emerged again independently in four different bony fish groups, including armoured catfish.
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To find out more, biologists needed the ability to study and manipulate genes in a fish with skin odontodes, but zebrafish, a common model animal for these kinds of experiments, don鈥檛 have them.
Now Shunsuke Mori and Tetsuya Nakamura at Rutgers, the State University of New Jersey, have analysed gene activity in developing suckermouth armoured catfish skin odontodes. They uncovered a network of genes very similar to those found in developing teeth. 鈥淢ost of the genes are shared,鈥� says Nakamura.
One of these genes, pitx2, is needed for the first steps of tooth development, yet is absent from the skin odontodes of sharks. So Mori and Nakamura used gene silencing techniques to reduce the activity of pitx2 in the catfish and found that the odontodes didn鈥檛 develop properly. This showed the gene is needed for catfish skin odontode development, suggesting that evolution had redeployed the tooth genes to recreate skin odontodes in the catfish.
鈥淭he repeated re-emergence of dermal teeth in fishes is fascinating, because once lost, traits do not usually reappear,鈥� says Jake Daane at Northeastern University in Nahant, Massachusetts. 鈥淭his study lays the groundwork to look across at other catfish species to learn how this tooth-like armour has independently reappeared, and in further untangling the evolutionary relationships between teeth and body armour.鈥�
bioRxiv
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