The evolution of sensory and neurosecretory cell types in bilaterian brains
This thesis deals with the origin of the photosensory and neurosecretory cell types in the bilaterian brain. As the main experimental system, I used the annelid Platynereis dumerilii. Platynereis is an emerging protostomian model organism that is ideally suited for comparisons with vertebrates becau...
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|Summary:||This thesis deals with the origin of the photosensory and neurosecretory cell types in the bilaterian brain. As the main experimental system, I used the annelid Platynereis dumerilii. Platynereis is an emerging protostomian model organism that is ideally suited for comparisons with vertebrates because it has retained many ancestral cell types, yet has a relatively simple morphology and mode of development. In the first section, I describe my contribution to the reconstruction of the urbilaterian photosensory system. By cloning and analyzing a novel opsin gene from Platynereis, I was able to provide molecular support for the hypothesis that Urbilateria (the last common ancestors of all Bilateria) already possessed two photoreceptor cell types (PRCs) rhabdomeric and ciliary PRCs. I further corroborated this by the comparative analysis of different upstream regulators for each PRC type. Presumably only the rhabdomeric type was ancestrally involved in vision, while the ciliary type was a light detector in the inner brain (like the ciliary PRCs of the vertebrate pineal organ). Both types, however, were recruited into the vertebrate eye, which is thus a compound structure. This finding provides a novel basis for understanding both the molecular similarities and differences between the vertebrate and invertebrate eyes. I extended these analyses in the second section by investigating the molecular and morphological set-up of the median brain of the Platynereis larva and in particular of the apical organ (APO). The APO is a specialized, highly neurosecretory structure. From a detailed analysis of molecular markers and cellular morphologies I concluded that the median brain of trochophora type larvae with the APO as its core structure and the ventral/ median prosencephalon of vertebrates with the hypothalamus as its center share common heritage from their urbilaterian ancestor. Both vertebrate eyes and hypothalamus have been shown to require proper Hedgehog (Hh) signalling for their correct development. From the expression studies and functional experiments in the third part of my thesis, I obtained evidence that Hh signalling might have played an ancestral role on median forebrain development. Moreover, the analysis permits a clear prediction for specific cell types in vertebrates that are likely under the control of the Hh- signaling pathway. Thereby, this study also supports the use of Platynereis as a model organism for Bilaterian-wide comparisons.|