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Developmental basis of limblessness and axial patterning in snakes

Abstract

The evolution of snakes involved major changes in vertebrate body plan organization, but the developmental basis of those changes is unknown. The python axial skeleton consists of hundreds of similar vertebrae, forelimbs are absent and hindlimbs are severely reduced. Combined limb loss and trunk elongation is found in many vertebrate taxa1, suggesting that these changes may be linked by a common developmental mechanism. Here we show that Hox gene expression domains are expanded along the body axis in python embryos, and that this can account for both the absence of forelimbs and the expansion of thoracic identity in the axial skeleton. Hindlimb buds are initiated, but apical-ridge and polarizing-region signalling pathways that are normally required for limb development are not activated. Leg bud outgrowth and signalling by Sonic hedgehog in pythons can be rescued by application of fibroblast growth factor or by recombination with chick apical ridge. The failure to activate these signalling pathways during normal python development may also stem from changes in Hox gene expression that occurred early in snake evolution.

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Figure 1: Morphology of python axial and appendicular skeleton.
Figure 2: Whole-mount antibody staining showing Hox gene expression in python and chick embryos.
Figure 3: Antibody staining and scanning electron micrographs comparing apical ectoderm in python and chick limb buds.
Figure 4: Polarizing activity and dorsoventral polarity in python hindlimb buds.
Figure 5: Developmental model for the evolution of snakes.

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Acknowledgements

We thank Drayton Manor Zoo, Edinburgh Zoo, London Zoo, Welsh Mountain Zoo and J. Fletcher for fertile python eggs; M. Caldwell, A. Cohn, S. Evans, M. Ferguson, E. Kochva, M. Lee, C.O. Lovejoy, K. Patel, J. R. D. Stalvey, V. Wilson and L. Wolpert for discussion; M. Turmaine for assistance with scanning electron microscopy; and E. De Robertis, T. Jessell, A. Joyner, G. Martin, A.McMahon, G. Panganiban, C. Tabin, N. Wall, and R. Zeller for reagents. This research was funded by the BBSRC.

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Correspondence to Martin J .Cohn.

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.Cohn, M., Tickle, C. Developmental basis of limblessness and axial patterning in snakes. Nature 399, 474–479 (1999). https://doi.org/10.1038/20944

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