Rewriting the Dinosaur Tail: Evidence for Dvl2 Deletion and Changes to Tailbud Development in Birds

Abstract

The evolutionary reduction of the tail is one of the most striking anatomical transformations in the lineage leading from non-avian dinosaurs to modern birds. This dissertation investigates the developmental and genomic mechanisms underlying premature tail termination in birds using comparative embryology, genomics and transcriptomics in the context of the fossil record. I identify a bird-specific deletion of Disheveled-2 (Dvl2), a key conductor of Wnt signaling, that results in axial truncations when it is deleted across vertebrates. This is combined with temporally dynamic shifts in the organization and elongation dynamics of tailbud tissues in chicken relative to alligator, the closest extant relative of birds that possesses a tail. Tailbud transcriptomic analyses conserved, species-specific temporal expression dynamics of both Hox genes and Wnt ligands in both alligator and chicken. Alligator tail initiation exhibited steep bursts of expression, while chickens showed a more graded process, and alligator tailbuds showed an overall coupling of Hox and Wnt dynamics that was absent in chicken. Overall, the findings suggest a more robust onset and maintenance of elongation in alligator tail tissues, while a loss of Dvl2 and disruption of Wnt signaling in birds may have resulted in a gradual tail developmental progression that cannot sufficiently maintain progenitors of the axis.