Meeting Abstract
144.6 Monday, Jan. 7 Conservation of primaxial regionalization in the evolution of the snake body form HEAD, J.J. *; POLLY, P.D.; University of Nebraska-Lincoln; Indiana University jhead2@unl.edu
Shifts and reduction in Hox gene expression domains have been proposed as a primary mechanism in the evolution of the elongate, “deregionalized” axial skeleton of snakes and other squamates. Mapped domains do not show a consistent change in expression in the axial skeleton of snakes, however, and the extent of morphological homogenization has not be examined separately for the primaxial and abaxial regions of the skeleton. To test for morphological changes along the primaxial skeleton and their implications for inferring Hox patterning in the snake body form, we quantified vertebral shape in a sample of amniotes including taxa with highly differentiated axial regions and resolved Hox boundaries (Mus, Alligator) and representatives of all major squamates clades including elongate taxa. Geometric morphometric analyses of intracolumnar changes in vertebral morphology along the anterior-posterior axis were used to test against models of differing regionalization and to search for regional boundaries. Testing the method on Mus and Alligator produced perfect correspondence between Hox boundaries and quantified shape variation for a four-region model representing cervical, anterior thoracic, posterior thoracic and lumbar regions. Morphometric variation in squamates, including elongate taxa, also best fit a four-region model, despite the absence of additional regional morphologies. Comparisons of morphometric regions in the snake skeleton with mapped domains revealed an exact correspondence between Hox gene expression and morphometric boundaries in the cervical and thoracic regions and a loose correspondence in the lumbar region. These results strongly suggest that primaxial regionalization is retained in the evolution of elongate body forms, and that “deregionalization” results from reduction or loss of the abaxial skeleton.
