Evolutionary constrains in the tetrapod neck: how evolution shapes cervical vertebrae variability

• Autor(a) principal: Martins, Rita Neto
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/34478
• topic: Evolution; Hox genes; Intraspecific variation; Development; Presacral vertebrae; Cervical vertebrae

The vertebral column is composed of repetitive skeletal elements, organized in homologous series (the vertebrae) that are of paramount importance for evolutionary analysis. Their patterning is regulated by an extremely conserved group of Hox genes that regulate the body plan formation during animal embryonic development. The clade Mammalia and Reptilia show great divergences in axial development plasticity, since the beginning of tetrapod evolution. Currently, several studies tried to explain the causes of vertebral count variation as a whole, while others focused in explaining the conservation of cervical regionalization of mammals. To clarify how cervical regionalization has evolved over millions of years, we conducted a comprehensive survey of presacral vertebrae counts focusing on intraspecific and interspecific variations of 488 tetrapod taxa (extinct and extant). We then mapped vertebral counts to amniotic phylogeny as well as more detailed phylogenetic trees within the group. This approach allowed us to test several hypotheses about the relationships between the number of cervical and presacral vertebrae versus: total body length, total body mass, and neck length. We also calculate the intraspecific variation to find new evolutionary patterns. Finally, we used embryos from three model species Gallus gallus, Anas platyrhyncos and Coturnix coturnix to analyze the frequency with which these meristic changes occurred in early ontogeny. Our results demonstrate a substantial variation in the number of cervical vertebrae in several distantly related species, suggesting the absence of strong selection in some groups. Contrary to the initial hypothesis, our results suggest an inverse correlation between intraspecific variation and number of cervical vertebrae. We hypothesize that developmental constraints are limiting intraspecific variation in long-neck species.