title: From dinosaurs to birds: a tail of evolution.
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.
authors.affiliation: Museum of the Rockies, Montana State University, 600 West Kagy Blvd, Bozeman, MT 59717, USA.
authors.forename: Dana J
authors.initials: DJ
authors.lastname: Rashid
authors.affiliation: Department of Biological Sciences, Clemson University, 340 Long Hall, Clemson, SC 29634, USA.
authors.forename: Susan C
authors.initials: SC
authors.lastname: Chapman
authors.affiliation: Redpath Museum, McGill University, 859 Sherbrooke Street W., Montreal, Quebec H3A 0C4, Canada.
authors.forename: Hans Ce
authors.initials: HC
authors.lastname: Larsson
authors.affiliation: Museum of the Rockies, Montana State University, 600 West Kagy Blvd, Bozeman, MT 59717, USA ; Department of Earth Sciences, Montana State University, 226 Traphagen Hall, Bozeman, MT 59717, USA.
authors.forename: Chris L
authors.initials: CL
authors.lastname: Organ
authors.affiliation: Museum of the Rockies, Montana State University, 600 West Kagy Blvd, Bozeman, MT 59717, USA ; Current address: Vaccine and Gene Therapy FL, 9801 Discovery Way, Port Lucie, FL 34987, USA.
authors.forename: Anne-Gaelle
authors.initials: AG
authors.lastname: Bebin
authors.affiliation: Department of Cell Biology & Neuroscience, Montana State University, 513 Leon Johnson Hall, Bozeman, MT 59717, USA.
authors.forename: Christa S
authors.initials: CS
authors.lastname: Merzdorf
authors.affiliation: Department of Cell Biology & Neuroscience, Montana State University, 513 Leon Johnson Hall, Bozeman, MT 59717, USA.
authors.forename: Roger
authors.initials: R
authors.lastname: Bradley
authors.affiliation: Museum of the Rockies, Montana State University, 600 West Kagy Blvd, Bozeman, MT 59717, USA.
authors.forename: John R
authors.initials: JR
authors.lastname: Horner
keywords: Archaeopteryx; Avian; Bird evolution; Confuciusornis; Dinosaur; Jeholornis; Sapeornis; Somitogenesis; Tail