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17 July 2008 - The number of vertebrae, and hence the number of segments or 'somites' in the body, is highly variable among different vertebrate species. For instance, frogs have 10 vertebrae, while many snakes have over 300. But what controls vertebra number in a given species and why does it vary so much between species? Gomez et al. propose that the number depends on a balance struck early in embryogenesis between the division of the body into somites and the overall rate of development. They establish this by showing snakes have a much greater segmentation clock speed, relative to embryo development as a whole, than lizards and other vertebrates with fewer somites.
Brief summary and first paragraph written by peers.
17 July 2008 - Snakes have graceful, elongated bodies containing hundreds of vertebrae. This extreme of morphology stems from evolutionary changes in a developmental clock that regulates body patterning.
First paragraph of the article: Evolution has produced an astonishing diversity of body shapes adapted to different lifestyles. An important contributor to the shape of animals with backbones is the number of bones (vertebrae) that make up this structure. Some animals have gone to extremes, none more so than snakes, which have more vertebrae than any other living animal - often more than 300, with some species1 having more than 500. Gomez et al. describe their investigations into how snakes develop this spectacular number of vertebrae. Their data suggest that a developmental 'clock', which regulates key steps in body patterning, ticks faster (relative to growth rate) in snakes than in shorter-bodied animals. This is a dramatic example of heterochrony, in which adjustments in developmental timing lead to morphological change.
Scientific abstract written by the authors of the work.
17 July 2008 - The vertebrate body axis is subdivided into repeated segments, best exemplified by the vertebrae that derive from embryonic somites. The number of somites is precisely defined for any given species but varies widely from one species to another. To determine the mechanism controlling somite number, we have compared somitogenesis in zebrafish, chicken, mouse and corn snake embryos. Here we present evidence that in all of these species a similar 'clock-and-wavefront mechanism operates to control somitogenesis; in all of them, somitogenesis is brought to an end through a process in which the presomitic mesoderm, having first increased in size, gradually shrinks until it is exhausted, terminating somite formation. In snake embryos, however, the segmentation clock rate is much faster relative to developmental rate than in other amniotes, leading to a greatly increased number of smaller-sized somites. Nature
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