Distal spinal nerve development and divergence of avian groups

Abstract

The avian transition from long to short, distally fused tails during the Mesozoic ushered in the Pygostylian group, which includes modern birds. The avian tail embodies a bipartite anatomy, with the proximal separate caudal vertebrae region, and the distal pygostyle, formed by vertebral fusion. This study investigates developmental features of the two tail domains in different bird groups, and analyzes them in reference to evolutionary origins. We first defined the early developmental boundary between the two tail halves in the chicken, then followed major developmental structures from early embryo to post-hatching stages. Differences between regions were observed in sclerotome anterior/posterior polarity and peripheral nervous system development, and these were consistent in other neognathous birds. However, in the paleognathous emu, the neognathous pattern was not observed, such that spinal nerve development extends through the pygostyle region. Disparities between the neognaths and paleognaths studied were also reflected in the morphology of their pygostyles. The ancestral long-tailed spinal nerve configuration was hypothesized from brown anole and alligator, which unexpectedly more resembles the neognathous birds. This study shows that tail anatomy is not universal in avians, and suggests several possible scenarios regarding bird evolution, including an independent paleognathous long-tailed ancestor. Vertebrate tails are highly divergent in form and function. In birds, tails have evolved adaptations specific for flight and sexual selection. Extant bird tails are composed of the proximal region, characterized by unfused ('free') caudal vertebrae, and the distal region harboring the pygostyle, a bony structure formed from fusion of the distal caudals. The observation that pygostyle fusion occurs progressively after hatching 1 led us to question how the unique avian tail morphology is rooted in early developmental events, and whether these events are consistent across all three major groups of extant birds (neoaves, galloanseriforms, and paleognaths). The modern bird tail originated in the Mesozoic era, at the long-to short-tailed transition 2. Archaeopteryx, the earliest known bird, sported a long, reptilian-like tail with greater than 20 caudal vertebrae 3,4. While Archaeopteryx fossil specimens were discovered in Germany, the greatest cache of Mesozoic bird fossils has been found in the Jehol beds in China, representing a period between 131 to 120 million years ago 5. Jehol specimens indicate that both long and short-tailed birds coexisted at this critical time in avian evolution. The short-tailed birds exhibited a number of unique morphologies, among them the occurrence of the pygostyle and additional bone fusions throughout the axial and peripheral skeleton 6. With the exception of Rahonavis, a specimen from Madagascar that may or may not have been avian 7 , in the last 120 million years, only short-tailed birds have been

Somite development and regionalisation of the vertebral axial skeleton

“…The caudal domain, post-sacrally, represents the tail in most mammals and the rudimentary coccyx in humans. In birds, the caudal area includes a few small vertebrae and the pygostyle, formed by fused vertebrae [44][45][46]. Distinctive molecular networks produce these anatomical domains (see section 5, Figure 1C).…”

TGFβ signaling is required for sclerotome resegmentation during development of the spinal column in Gallus gallus

“…At E6.5, evidence of resegmentation was observed in the lateral cartilages between muscle fibers (Fig 4F, blue brackets = cartilage) where half of each cartilage element was stained far red and half green. To better image the midline, E6.5 embryos that had been injected with DiD and DiO were sectioned and the vertebrae and IVDs were identified by counterstaining with rhodamine conjugated peanut agglutinin (PNA) (Rashid et al, 2020). Sections were simultaneously imaged for all three fluorophores (rhodamine, DiD, and DiO) and the developing vertebrae were outlined by using the ImageJ threshold tool to decrease background noise in PNA stained images and then the ROI function was used to outline the VB (Fig.…”

The long and the short of tails

“…Spinal nerve anatomy can also vary at the very end of the tail. Emus and likely other paleognathous birds have distinct distal spinal nerve anatomy that may point to a long‐tailed ancestor not shared with other birds 31 . In these examples, applying an evo‐devo approach is helping to establish universal vs. convergent evolution processes, and to evaluate the relatedness of different species and their branch points on the evolutionary tree.…”

“…[27][28][29][30] One strategy to unambiguously define the tail throughout ontogeny is to trace vertebral development from the somite stage onwards, and equate early development with postnatal cloaca/anus position. This is an arduous process, 26,31 and of course, is not possible for extinct species; consequently, tail definition is not always consistent and not always precise.…”