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Naman Kanakiya
Vonk, Freek J., Jeroen F. Admiraal, Kate Jackson, Ram Reshef, Merijn A. G. de Bakker, Kim Vanderschoot, Iris van den Berge, et al. Evolutionary Origin and Development of Snake Fangs. Nature 454:7204, 630-633 (2008).

正文：
Introduction
Under the class of Squamata, snakes are ectothermic amniotic vertebrates covered in scales. All of the continents except for Antarctica host some species of snakes, and most of these places have species which have evolved fangs. These fangs deliver poisonous venom to their prey. This venom is useful in a variety of ways, such as immobilization and digestion of prey.These fangs are located in different places depending on the snake species. For example, some species have fangs located in the anterior portion of the upper jaw such as Viperidae,Atractaspis and Elapidae, while all of the other species are either fangless or have fangs located in the posterior portion of the upper jaw (Vidal 2002, Young 1996). Snake tooth development is unique in that the front and back teeth are formed from two separate dental lamina (Vonk 2008).
（a, Phylogeny from ref. 16. b, c, Adult skulls (Supplementary Table 4): lateral views (b); palate, schematic ventral views (c; maxilla colored, fangs circled). Asterisks indicate species studied by electron microscopy (Supplementary Fig. 5, Supplementary Table 3). The evolutionary changes leading from an unmodified maxillary dentition to the different fang types in advanced snakes are indicated at the nodes: (1) continuous maxillary dental lamina, no specialized sub regions—ancestral condition for advanced snakes; (2) evolution of posterior maxillary dental lamina—developmental uncoupling of posterior from anterior teeth; (3) starting differentiation of the posterior teeth with the venom gland; (4) loss of anterior dental lamina and development of front fangs. (Figure 1, Vonk 2008)）
Some hypotheses postulate that fangs developed independently, but this idea was challenged due to the similarity and complexity of anterior and posterior fangs. The paper seeks to address what evolutionary path was undertaken by snakes in the development of the fangs. This would lead to a better understanding of what caused the major radiation in the Cenozoic era of snakes, and in turn a better understanding of modern day snakes (Kuch 2006, Fry 2007).

Sonic Hedgehog – A Review
Sonic hedgehog (SHH) refers to a protein of the signaling protein pathway hedgehog. It is widely known for its regulatory role in vertebrate organogenesis, and is the best established example of a morphogen. Within snakes, it is known for its function in the development of fangs and thus is used to track fang development (Cobourne 2004). More general information can be found here.

Methods
In situ hybridization – The RNA probe used for hybridization was based off of a cDNA PCR construct of sonic hedgehog, and hybridization was done using normal standards.
Histology – Histology is the microscopic study of the anatomy. The embryos in this study were cut into 5-7 micrometer sections and stained.
Scanning electron microscopy – The bone of interest (maxillary bone) was dissected, and prepared for electron microscopy. The microscope used was a JEOL JSM-T300 scanning electron microscope at a voltage of 15kV.

Anterior Fangs
In situ hybridization showed that anterior fangs first develop in the back of the mouth and then are displaced by the growth of the snake jaw bone.
（a–d, Palate, ventral view: top, anterior; scale bar, 0.5 mm; dotted lines, upper jaw (posterior margin of premaxilla to attachment of the mandible); boxes, schemes of maxillary odontogenic band (purple, shh expression; grey, no shh expression). Positions of fangs in b–d were identified histologically (Fig. 3, Supplementary Fig. 3). The odontogenic band in the front-fanged species is located posterior in the upper jaw (b, d). In the non-fanged outgroup (a) and the rear-fanged Natrix (c), the odontogenic band extends along the entire upper jaw. f, fang; mx, maxillary odontogenic band; pa, palatine odontogenic band; pt, pterygoid odontogenic band. e, Ontogenetic allometry in the fang in the front-fanged Causus displaces the fang along the upper jaw (Supplementary Figs 5–9, Supplementary Tables 5–9). Scale bars, 1 mm. We note the change in relative size of the upper jaw subregions: i, anterior; ii, fang; iii, posterior. d.a.o., days after oviposition. (Figure 2, Vonk 2oo8)）

Posterior Fangs
In situ hybridization showed that posterior fangs first develop in the dental lamina which functions as the tissue that forms teeth.
（a–c, e–f, Sagittal sections, anterior to the left, of L. mackloti (Boidae) 22 d.a.o. (a), N. natrix (Natricidae) 22 d.a.o. (b), Calloselasma rhodostoma (Viperidae) 8 d.a.o. (c), N. natrix 22 d.a.o. (e), Naja siamensis (Elapidae) 23 d.a.o. (f). d, Transverse section, medial to the left, of Trimeresurus hageni (Viperidae) 8 d.a.o. The posterior maxillary dental laminae in b and e are similar in morphogenesis to the maxillary dental laminae in all front-fanged species examined (c, d, f; see also Fig. 4). Arrowheads, shh expression; amdl, anterior maxillary dental lamina; dr, dental ridge; e, eye; f, fang; mdl, maxillary dental lamina; pa, palatine dental lamina; pmdl, posterior maxillary dental lamina; t, tooth bud; vd, primordium of venom gland; scale bars, 300 microm. (Figure 3, Vonk 2008)）

Discussion/Summary:
The results show that only snakes in the ‘advanced’ snake lineages have a consistent differentiation in the anterior versus posterior tooth morphologies. However, the maxillary teeth of the boids examined don’t show any morphological differences between the anterior and posterior teeth. This provides evidence to the model suggested by the paper in that the posterior teeth develop independent from the anterior, and in close association with the venom glands. This theory is further evidenced by a lack of shh expression anterior in the upper jaws, as this implies that the front-fanged elapids and viperids have independently lost the anterior dental lamina. Thus, the paper claims that the evidence supports their model that the posterior teeth and venom gland became modified over the course of time and and formed the fang-gland complex.

Strengths, Weaknesses and the Future:
The paper addresses the question it seeks to address in a proper manner. The question of whether the evolution of fangs is convergent evolution or independent evolution is further made clear through tracking the expression of shh. The experiment supports the theory of independent evolution, however there are some issues which need to be further addressed to cement this position.
First, the model proposed in the experiment is only an explanation of a few snake species, and thus should be tested upon other species on different branches of a phylogenetic tree. Secondly, other markers for fang development other than shh should be tested to verify the results published by this paper, as shh could have other roles in that area.
The paper mentions that this new proposed model of theirs could explain the massive radiation of advanced snakes in the Cenozoic era. For that, a study would need to be done on either the fossils or the snakes most similar to those of the Cenozoic era.