Heart Position is Associated with Vertebral Regionalization in Two Species of Garter Snakes (Thamnophis)
A long-standing question regarding the evolution of the snake body plan is to what extent does axial regionalization and organ position correspond with that of generalized tetrapod vertebrates. Here, we evaluated the position of shifts in vertebral morphology with respect to heart location in 2 species and 13 specimens of garter snakes (Thamnophis). From dissections, geometric morphometrics, and segmented regressions on principal component scores describing shape of the cranial aspect of vertebrae, we determined a consistent morphological transition at approximately 17% of the pre-cloacal vertebral column. The transition was strongly coincident with the position of the heart, suggesting a developmental link between the first major transition in vertebral regions and the longitudinal position of the heart in garter snakes. Our novel discovery has implications for further recognizing the pre-cloacal vertebral column of snakes as regionalized, and that these regions are positionally linked with organogenesis of the viscera.Abstract
(a) cranial aspect of a representative trunk vertebra of Thamnophis elegans, showing the landmark scheme used in this study; (b) density histogram of distance between atrial and transitional vertebrae as a percentage of the length of the vertebral column, based on 1,000 resamples of the original data set; (c) thin-plate spline from a representative T. marcianus specimen, modeling shape change from two standard deviations below to two standard deviations above the mean shape for principal component (PC) 1 (red dots represent position at −2 SD and blue dots represent position at +2 SD, with arrows representing vector of shape change); (d) thin-plate spline from a representative T. elegans specimen, modeling shape change from two standard deviations below to two standard deviations above the mean shape for PC 1 (red dots represent position at −2 SD and blue dots represent position at +2 SD, with arrows representing vector of shape change); (e) PC 1 scores as a function of vertebral position along the length of the vertebral column for all specimens included in the separate PC analyses. “A” represents 95% CI for atrial position, “T” represents 95% CI for transitional vertebral position, and gray and red smoothers and symbols represent data for T. elegans and T. marcianus, respectively. Inset photographs of the vertebrae are exemplary of T. elegans at approximately 5% (left) and 50% (right) of the pre-cloacal vertebral column.
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