Robertson, Helen Elizabeth; (2017) Molecular approaches for studying the evolution of the Xenacoelomorpha. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Xenacoelomorpha comprises the genus Xenoturbella and two related groups of acoelomorph worms – the Acoela and Nemertodermatida. The phylogenetic position of the Xenacoelomorpha is debated. Previous work suggested that they are deuterostomes; alternative publications suggest they are outside the main group of bilaterians (protostomes and deuterostomes). All members of the Xenacoelomorpha have a simple body plan: they are unsegmented; lack a coelomic cavity; and are commonly assumed to lack any of the organs commonly associated with the Bilateria. In the first part of this thesis, I develop and apply gene visualisation protocols (in situ hybridisation and immunohistochemistry) to investigate the expression patterns of genes commonly associated with ultrafiltration in Xenoturbella and the acoel Symsagittifera roscoffensis. Limited molecular protocols have previously been applied to members of the Acoelomorpha, but none have been successful in Xenoturbella. Given their simple morphology, the Xenacoelomorpha have been assumed to lack any structures specialised for filtration or excretion (nephrocytes). The main bilaterian protostome and deuterostome grouping has thus been termed the Nephrozoa, implying that nephrocyte-like structures, required for ultrafiltration, are an innovation of this clade. Understanding the role of these genes in members of the Xenacoelomorpha could shed light on the origin and homology of filtratory structures, which remain unclear. More broadly, establishing reliable gene visualisation approaches in Xenoturbella is helpful for better understanding their morphology and organisation. In the second part, I apply novel RNA-Seq approaches in Xenoturbella. These comprise whole-organism single cell sequencing, to investigate organisational complexity and tissue specification in Xenoturbella; and 'Tomoseq', for investigating spatially resolved transcriptomics across the anteroposterior body axis. In association with my gene visualisation protocols, these techniques are valuable approaches to enhance our understanding of the biology and complexity of Xenoturbella.

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