Abstract
A particularly critical event in avian evolution was the transition from long- to short-tailed birds. Primitive bird tails underwent significant alteration, most notably reduction of the number of caudal vertebrae and fusion of the distal caudal vertebrae into an ossified pygostyle. These changes, among others, occurred over a very short evolutionary interval, which brings into focus the underlying mechanisms behind those changes. Despite the wealth of studies delving into avian evolution, virtually nothing is understood about the genetic and developmental events responsible for the emergence of short, fused tails. In this review, we summarize the current understanding of the signaling pathways and morphological events that contribute to tail extension and termination and examine how mutations affecting the genes that control these pathways might influence the evolution of the avian tail. To generate a list of candidate genes that may have been modulated in the transition to short-tailed birds, we analyzed a comprehensive set of mouse mutants. Interestingly, a prevalent pleiotropic effect of mutations that cause fused caudal vertebral bodies (as in the pygostyles of birds) is tail truncation. We identified 23 mutations in this class, and these were primarily restricted to genes involved in axial extension. At least half of the mutations that cause short, fused tails lie in the Notch/Wnt pathway of somite boundary formation or differentiation, leading to changes in somite number or size. Several of the mutations also cause additional bone fusions in the trunk skeleton, reminiscent of those observed in primitive and modern birds. All of our findings were correlated to the fossil record. An open question is whether the relatively sudden appearance of short-tailed birds in the fossil record could be accounted for, at least in part, by the pleiotropic effects generated by a relatively small number of mutational events.
Highlights
Tails of extant birds are an evolutionary novelty
Despite the many caveats with this approach, our analyses of genetic and fossil evidence suggest the possibility that a single mutation could have occurred in a paravian dinosaur, which both truncated its tail and fused its distal caudal vertebrae into a pygostyle
Whether there were one or multiple mutations it should be noted that of the 37 posterior vertebral body-fusion mutants we examined [see Additional file 3], all but four are known to be caused by single mutational events, and have substantial phenotypic alterations not just to the tail, but to other parts of the skeleton as well
Summary
Tails of extant birds are an evolutionary novelty. They are critical for powered flight, ensure reproductive success by attracting mates, and safeguard relatives by communicating warning signals. Extant bird tails consist proximally of a small series of unfused caudal vertebrae with a high range of motion These articulate to a distal rod-like pygostyle, composed of several fused caudal vertebrae, which supports the retricial bulb and associated muscles and feathers for controlling tail fan and contour shape. We present a comprehensive survey of mouse tail mutants with the purpose of examining conserved patterns of mutation and likely candidates within the tail gene regulatory network that may have played major roles in reducing the bird tail These paleontological, genetic, and developmental analyses are applied to the critical juncture in bird evolution at which tails were truncated and flight was greatly enhanced, in the late Jurassic to Early Cretaceous transition of primitive longtailed to short-tailed birds
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
