title: Exploring mechanisms of adaptive divergence in an exemplar adaptive radiation
Creators: name: McWhinnie, Kirsty
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: Cichlids, evo-devo, craniofacial, mandible, phenotypic plasticity, adaptive divergence, ecological sexual dimorphism, morphometrics, QTL, finite element analysis.

description: Investigating and exploring mechanisms of adaptive divergence is key to understanding how complex morphological traits have evolved. Exemplar systems of adaptive radiation, whereby numerous species have diverged from a common ancestor in a relatively rapid timeframe, can be used to test ideas about adaptive divergence. Adaptations of the trophic morphology are often the focus of divergence in adaptive radiations, but the evolution of such traits is not yet fully understood. With extensive variation in craniofacial shape, the Lake Malawi cichlids are an excellent system which can be used to investigate the evolution of trophic morphology. Traditional studies of divergence tend to focus on the relationship between shape and ecology, but an evo-devo approach which encompasses multiple aspects such as morphology, genetics, function and development can address questions about the evolutionary process in more detail. Furthermore, investigations which look at smaller scale patterns of divergence, such as between ecologically similar species or between sexes, can be especially enlightening as this can uncover more subtle aspects of variation. Ecological sexual dimorphism, whereby sexes diverge in ecologically relevant traits such as the trophic morphology, can represent one such type of subtle variation. This thesis explores the evo-devo of the trophic morphology with an interdisciplinary approach by considering multiple levels of adaptive divergence and their contribution to evolutionary process. Chapter 1 sets out of the context of these investigations and the background for this work. Firstly, the genetic basis of the mandible is explored in Chapter 2 to uncover new candidate genes. The mandible represents the first point of contact with the environment and as such is a key vertebrate trait, yet the complexity means the underlying genetic architecture is not fully understood. By investigating the genotype to phenotype relationship in high detail, I found a candidate gene not previously characterised in cichlid craniofacial studies, zeb1. Furthermore, there was strong evidence of sexual dimorphism in mandible shape and mapping highlighted regions for quantitative trait loci on the sex-determining chromosome. Following on from this, Chapter 3 utilised an engineering technique, finite element analysis, to assess how the mandible copes with external compressive loading that would be expected during feeding. This analysis identified key structural adaptations in both species to enable them to cope with stress during feeding, and notably there was strong dimorphism between sexes. The final experimental chapter, Chapter 4, assessed phenotypic plasticity through a diet treatment experiment with the main aim to investigate sexual dimorphism in plastic response. Despite strong sexual dimorphism in morphology and function, plastic responses did not differ between the sexes. This is in spite of the fact that females are mouthbrooders, but this does not appear to place constraints on phenotypic plasticity. Discussed in detail in Chapter 5, the work presented in this thesis suggests that adaptive divergence between species and sexes could both be important to the evolution of the Lake Malawi radiation. By using an integrative approach which considers multiple mechanisms of divergence, this can enhance our understanding of the evolution of complex traits and the evolutionary process itself.