Meeting Abstract

94.4  Sunday, Jan. 6  Evolutionary and Developmental Modularity in the Digits of Vertebrates KAVANAGH, KD*; WINSLOW, B; LEARY, B; Univ. of Massachusetts Dartmouth; Univ. of Massachusetts Dartmouth; Univ. of Massachusetts Dartmouth kkavanagh@umassd.edu

The toe bones of most tetrapods include the metatarsal followed by a series of phalanges bones. In the embryos, these bones develop in sequence as chondrogenic condensations that grow out distally and segment behind the growing tip to position the joints. By the time the tip is formed, the final adult proportions of the toes are achieved. Among taxa, phalanges’ sizes covary in a highly predictable way, with variations ranging from equal-sized to a proximodistal gradient. The metatarsal variation does not follow this rule, indicating separate evolutionary modules. However, evidence of developmental modularity that establishes independence of MT and phalanges has been elusive. Previous analyses of gene expression and morphogenetic processes consistently show no differences between formation of metatarsal and phalanges. Here, we have found evidence of emergent modularity in the digit. Using experimental perturbations and DiI cell tracing in the chick, we establish the timing of very early separation of MT and phalanges compartments. In contrast, formation of individual phalanges remains plastic until late phalangeogenesis. We propose a two-stage evolutionary scenario for the tetrapod digit.