title: Evolutionary origin and development of snake fangs.
abstract: Many advanced snakes use fangs-specialized teeth associated with a venom gland-to introduce venom into prey or attacker. Various front- and rear-fanged groups are recognized, according to whether their fangs are positioned anterior (for example cobras and vipers) or posterior (for example grass snakes) in the upper jaw. A fundamental controversy in snake evolution is whether or not front and rear fangs share the same evolutionary and developmental origin. Resolving this controversy could identify a major evolutionary transition underlying the massive radiation of advanced snakes, and the associated developmental events. Here we examine this issue by visualizing the tooth-forming epithelium in the upper jaw of 96 snake embryos, covering eight species. We use the sonic hedgehog gene as a marker, and three-dimensionally reconstruct the development in 41 of the embryos. We show that front fangs develop from the posterior end of the upper jaw, and are strikingly similar in morphogenesis to rear fangs. This is consistent with their being homologous. In front-fanged snakes, the anterior part of the upper jaw lacks sonic hedgehog expression, and ontogenetic allometry displaces the fang from its posterior developmental origin to its adult front position-consistent with an ancestral posterior position of the front fang. In rear-fanged snakes, the fangs develop from an independent posterior dental lamina and retain their posterior position. In light of our findings, we put forward a new model for the evolution of snake fangs: a posterior subregion of the tooth-forming epithelium became developmentally uncoupled from the remaining dentition, which allowed the posterior teeth to evolve independently and in close association with the venom gland, becoming highly modified in different lineages. This developmental event could have facilitated the massive radiation of advanced snakes in the Cenozoic era, resulting in the spectacular diversity of snakes seen today.
authors.affiliation: Institute of Biology, Leiden University, Kaiserstraat 63, PO Box 9516, 2300 RA, Leiden, The Netherlands.
authors.forename: Freek J
authors.initials: FJ
authors.lastname: Vonk
authors.forename: Jeroen F
authors.initials: JF
authors.lastname: Admiraal
authors.forename: Kate
authors.initials: K
authors.lastname: Jackson
authors.forename: Ram
authors.initials: R
authors.lastname: Reshef
authors.forename: Merijn A G
authors.initials: MA
authors.lastname: de Bakker
authors.forename: Kim
authors.initials: K
authors.lastname: Vanderschoot
authors.forename: Iris
authors.initials: I
authors.lastname: van den Berge
authors.forename: Marit
authors.initials: M
authors.lastname: van Atten
authors.forename: Erik
authors.initials: E
authors.lastname: Burgerhout
authors.forename: Andrew
authors.initials: A
authors.lastname: Beck
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Mirtschin
authors.forename: Elazar
authors.initials: E
authors.lastname: Kochva
authors.forename: Frans
authors.initials: F
authors.lastname: Witte
authors.forename: Bryan G
authors.initials: BG
authors.lastname: Fry
authors.forename: Anthony E
authors.initials: AE
authors.lastname: Woods
authors.forename: Michael K
authors.initials: MK
authors.lastname: Richardson
mesh_terms: D000818:Animals; D018507:Gene Expression Regulation, Developmental; D053823:Hedgehog Proteins / Q000235:genetics / Q000378:metabolism; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D010802:Phylogeny; D012911:Snakes / Q000033:anatomy & histology / Q000145:classification / Q000196:embryology* / Q000235:genetics; D014070:Tooth / Q000033:anatomy & histology / Q000196:embryology*
chemical_list: D053823:Hedgehog Proteins