Jarero, Francesca; (2018) Wnt, Hedgehog and Notch signalling in relation to tapeworm anteroposterior polarity and segmentation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Tapeworms are a group of parasitic flatworms whose uniquely segmented body plan has raised questions regarding their anteroposterior polarity and homology to other animals for more than a century. The Wnt, Hedgehog and Notch pathways are conserved developmental signalling pathways involved in embryonic patterning and segmentation in the Metazoa. In this thesis, these three pathways were explored in a tapeworm for the first time. Spatial expression patterns for components belonging to each of the pathways were investigated in the mouse bile duct tapeworm, Hymenolepis microstoma. Wnt pathway factors showed polarised, conserved patterns of expression of ‘posterior’ ligands (Wnts) and ‘anterior’ inhibitors (Sfrps). The expression of Wnt inhibitors, in particular, identified the true developmental anterior of H. microstoma larvae, answering a previously unresolved question of tapeworm development. In adult worms, expression patterns of these Wnt inhibitors confirmed the scolex (a region of the worm presumed to be anterior) as truly anterior. Expression of inhibitors and ‘posterior’ Wnts during strobilation shows polarised patterns along the anteroposterior boundaries of individual segments. These expression patterns indicate that the role of the Wnt pathway in specifying anteroposterior axes is conserved amongst tapeworms. Furthermore, a tapeworm-­specific Wnt11 paralog is hypothesised to have taken on a novel function during adult development, likely acting as the effector of strobilation. The expression of Hmic-­Wnt11a is highly restricted between the neck and strobila and defines a newly recognised region of the adult body, dubbed the ‘transition zone’. Hedgehog signalling in adult H. microstoma is linked with the nervous system (as it is in free-­living flatworms). During larval development Hedgehog shows canonical specification of the midline. Notch signalling was found to be involved in a number of developmental processes. The expression pattern of Hmic-­Notch1 was of particular interest as it showed polarised posterior expression within all segments. Finally, this study developed single and double fluorescent in situ hybridisation techniques using tyramide signal amplification and lead the first steps towards an in vivo approach to RNA interference method in H. microstoma.

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