The evolvability of segment number in vertebrates

Thesis

Understanding the developmental origins of evolvability, the capacity of a lineage to evolve novel heritable and adaptive phenotypes, is a central question in evolutionary developmental biology. Vertebrates show a high degree of evolvability in the number of vertebrae dividing their axial skeleton, which has allowed vertebrates to adopt different lifestyles and habitats across the tree of life. Vertebral number is determined by the number of somites that form in the early embryo. Somites are paired blocks of mesoderm which form sequentially in an anterior-to-posterior fashion via a process known as somitogenesis. The total number of somites formed depends on the frequency at which somites appear, and the duration of somitogenesis. The frequency of somitogenesis depends on the frequency of oscillations of a gene regulatory network known as the segmentation clock. While somitogenesis is a paradigmatic example of pattern formation in development, it is unknown which properties make somitogenesis evolvable.

In this thesis I aim to uncover the sources of evolvability in somitogenesis. I use computational modelling to predict that the frequency and duration of somitogenesis exhibit developmental modularity in Zebrafish, suggesting that vertebral number may be evolvable because large changes in vertebral number can be created by small changes in both frequency and duration. This modularity depends on properties of the segmentation clock. I compare somitogenesis in two closely related species of Cichlidae with divergent vertebral number. I find that the difference in vertebral number is due to differing duration of somitogenesis and not frequency, suggesting that duration is a more evolvable component of somitogenesis than frequency. By perturbing the segmentation clock I find that modularity of somitogenesis is conserved across most of the bony fishes, approximately 34% of extant vertebrate species. This thesis suggests that evolvability of vertebral number is underpinned by modularity of the frequency and duration of somitogenesis.

Authors: Hammond, J

• DOI: 10.5287/ora-o5wp6eawo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: evolvability; segmentation clock; evo-devo; somitogenesis; somite; pre-somitic mesoderm; Cichlid; vertebra
• Subjects: Computational biology; Evolutionary developmental biology