Fixed cervical count and the origin of the mammalian diaphragm
Version of Record online: 5 SEP 2012
© 2012 Wiley Periodicals, Inc.
Evolution & Development
Volume 14, Issue 5, pages 399–411, September / October 2012
How to Cite
Buchholtz, E. A., Bailin, H. G., Laves, S. A., Yang, J. T., Chan, M.-Y. and Drozd, L. E. (2012), Fixed cervical count and the origin of the mammalian diaphragm. Evolution & Development, 14: 399–411. doi: 10.1111/j.1525-142X.2012.00560.x
- Issue online: 5 SEP 2012
- Version of Record online: 5 SEP 2012
- NSF. Grant Number: 0842507
Why is mammalian cervical count fixed across the historically long and ecologically broad mammalian radiation? Multiple lines of evidence, considered together, suggest a link between fixed cervical count and the muscularization of the diaphragm, a key innovation in mammalian history. We test this hypothesis by documenting the anteroposterior (AP) movement of the diaphragm, a lateral plate derivative, relative to that of the somitic thoracolumbar transition in unusually patterned mammals, by comparing the temporal occurrence of an osteological proxy for the diaphragm and fixed cervical counts in the fossil record, and by quantifying morphological differentiation within the mammalian cervical series. We then integrate these anatomical observations with details of diaphragm function and development to propose a sequence of innovations in mammalian evolution that could have led to fixed cervical count. We argue that the novel commitment of migratory muscle precursor cells (MMPs) of mid-cervical somites to a fate in the abaxial diaphragm defined these somites as a new and uniquely mammalian modular subunit. We further argue that the coordination of primaxial abaxial patterning constrained subsequent AP migration of the forelimb, thereby secondarily fixing cervical count.