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Embryonic Development
Snake embryonic development initially follows similar steps as any vertebrate embryo. The snake embryo begins as a zygote, undergoes rapid cell division, forms a germinal disc, also called a blastodisc, then undergoes gastrulation, neurulation, and organogenesis[1]. Cell division and proliferation continues until an early snake embryo develops and the typical body shape of a snake can be observed[1]. Multiple features differentiate the embryologic development of snakes from other vertebrates, two significant factors being the elongation of the body and the lack of limb development.
The elongation in snake body is accompanied by a significant increase in vertebra count (Mice have 60 vertebra, whereas snakes may have over 300)[2]. This increase in vertebra is due to an increase in somites during embryogenesis, leading to an increased number of vertebra which develop[2]. Somites are formed at the presomitic mesoderm due to a set of oscillatory genes called the somitogensis clock. The snake somitogensis clock operates at a frequency 4 times that of a mouse (after correction for developmental time), creating more somites, and therefore creating more vertebra[2]. This difference in clock speed is believed to be caused by differences in Lunatic fringe gene expression, a gene involved in the somitogensis clock[3].
There is ample literature focusing on the limb development/lack of development in snake embryos and the gene expression associated with the different stages. In basal snakes, such as the python, embryos in early development exhibit a hind limb bud that develops with some cartilage, and a cartilaginous pelvic element, however this degenerates before hatching [4]. This presence of vestigial development suggests that snakes underwent hind limb reduction before elimination [5]. There is no evidence in basal snakes of forelimb reduction and no examples of snake forelimb bud initiation in embryo, so little is known regarding the loss of this trait[5]. Recent studies suggests that hind limb reduction could be due to mutations in enhancers for the SSH gene[5], however other studies suggested that mutations within the Hox Genes or their enhancers could contribute to snake limblessness [2]. Since multiple studies have evidence suggesting different genes played a role in the loss of limbs in snakes, it is likely that multiple gene mutations had an additive effect leading to limb loss in snakes[6]
- ^ a b Zehr, David R. (20 July 1962). "Stages in the Normal Development of the Common Garter Snake, Thamnophis sirtalis sirtalis". Copeia. 1962 (2): 322. doi:10.2307/1440898.
- ^ a b c d Woltering, Joost M. "From Lizard to Snake; Behind the Evolution of an Extreme Body Plan". Current Genomics. 13 (4): 289–299. doi:10.2174/138920212800793302.
- ^ Gomez, Céline; Özbudak, Ertuğrul M.; Wunderlich, Joshua; Baumann, Diana; Lewis, Julian; Pourquié, Olivier (2008-07-17). "Control of segment number in vertebrate embryos". Nature. 454 (7202): 335–339. doi:10.1038/nature07020. ISSN 0028-0836.
- ^ Boughner, Julia C.; Buchtová, Marcela; Fu, Katherine; Diewert, Virginia; Hallgrímsson, Benedikt; Richman, Joy M. (June 2007). "Embryonic development of Python sebae – I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs". Zoology. 110 (3): 212–230. doi:10.1016/j.zool.2007.01.005. ISSN 0944-2006.
- ^ a b c Leal, Francisca; Cohn, Martin J. (January 2018). "Developmental, genetic, and genomic insights into the evolutionary loss of limbs in snakes". genesis. 56 (1): e23077. doi:10.1002/dvg.23077.
- ^ Kvon, Evgeny Z.; Kamneva, Olga K.; Melo, Uirá S.; Barozzi, Iros; Osterwalder, Marco; Mannion, Brandon J.; Tissières, Virginie; Pickle, Catherine S.; Plajzer-Frick, Ingrid; Lee, Elizabeth A.; Kato, Momoe (October 2016). "Progressive Loss of Function in a Limb Enhancer during Snake Evolution". Cell. 167 (3): 633–642.e11. doi:10.1016/j.cell.2016.09.028. PMC 5484524. PMID 27768887.
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