Macroevolutionary dynamics of vertebral column evolution in African cichlids

Thesis

African cichlids comprise over 1,800 species of freshwater teleosts, with spectacular adaptive radiations in Lakes Tanganyika, Malawi, and Victoria giving rise to extraordinary morphological diversity. However, the evolution of the cichlid axial skeleton has remained largely overlooked, despite the importance of the axial skeleton in the diversification of other teleostean clades. Here, we show that elongation of the fusiform body has been critical for adaptation in multiple African cichlid lineages. Although occupation of axial morphospace broadly correlates with the age of the lacustrine radiations, rates of vertebral count evolution are still higher in older lineages, suggesting constraints have limited axial evolution in the lacustrine radiations of African cichlids. Intriguingly, while vertebral addition contributes to body elongation, we find that intraspecific variation in vertebral count has been strongly canalised during African cichlid diversification. Therefore, despite vertebral counts being highly evolvable somitic fidelity has not undergone selection during African cichlid diversification. Furthermore, individuals with increased vertebral counts exhibit no detectable differences in body aspect ratio, indicating that intraspecific variation in vertebral number is decoupled from macroevolutionary patterns of elongation, where the addition of vertebrae has driven body elongation. Within Lake Malawi, we show that vertebral shape modification correlates with ecological divergence. However, much of the identified shape variation is likely being driven by elongation of the body. In addition, neither vertebral centra elongation nor increased intervertebral spacing appears to contribute to overall body elongation, suggesting that these traits have not evolved to modulate vertebral column flexibility. Collectively, our findings demonstrate the importance of comparative approaches to elucidate axial morphological evolution and highlight African cichlid fishes as a powerful model for investigating the evolutionary and developmental dynamics of the teleostean vertebral column. Future work will focus on dissecting the genetic mechanisms underpinning axial evolution in Lake Malawi cichlid hybrid crosses using quantitative trait locus (QTL) analysis.

Authors: Bucklow, C

• Alternative title: Macroevolutionary dynamics of vertebral column evolution in African cichlid fishes
• DOI: 10.5287/ora-8njrbwzre
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: teleosts; cichlidae; macroevolution; vertebral column; vertebrae; cichlids; somitogenesis; segmentation
• Subjects: Evolution (Biology); Evolutionary developmental biology; Macroevolution