Comparative molecular and morphogenetic characterisation of larval body regions in the polychaete annelid Platynereis dumerilii
The aim of this thesis was the molecular and morphogenetic characterisation of the larval body regions of the polychaete Platynereis dumerilii for evolutionary comparison with the body regions of other protostome and deuterostome model species. I have described the expression of several conserved...
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|Zusammenfassung:||The aim of this thesis was the molecular and morphogenetic characterisation of the larval body regions of the polychaete Platynereis dumerilii for evolutionary comparison with the body regions of other protostome and deuterostome model species. I have described the expression of several conserved homeobox transcription factors that broadly mark the neuroectoderm of the different larval body regions and can therefore be used as regionalisation markers in the trochophore larva of Platynereis dumerilii. A six3 orthologue is a marker gene for the prostomium, the most anterior region of the trochophore larva. The peristomium that harbours the mouth region and the prototroch ciliary band expresses an otx orthologue as regionalisation marker in Platynereis. The subdivision of the metastomium into larval segments has been analysed by Pdu-engrailed expression. It is found to give rise to four larval segments, of which the first one is reduced and bears the first tentacular cirri. It is furthermore characterised as the most anterior gbx-expressing segment and is innervated by axons that connect to the circumoesophageal connectives left and right of the mouth opening. The second larval segment develops into the first chaetiferous segment and is characterised by the anterior-most hox1 expression. The molecular and morphological comparison of the Platynereis anterior CNS with arthropods brains has allowed homologisation of the polychaete prostomial ganglia to the “archicerebrum” (putative most anterior part of the protocerebrum of arthropods), the peristomial ganglia with the “prosocerebrum” (putative posterior part of the protocerebrum), the ganglia of the first larval segment with the deutocerebrum and the second larval segment (first chaetiferous) ganglia with the tritocerebrum. Comparison with enteropneust (basal deuterostomes) larvae suggests homology and evolutionary conservation in Bilateria of the following larval body regions: prostomium/prosoma (six3), peristomium/mesosoma (otx) and metastomium/metasoma (gbx and hox1). This is supported by the origin and localisation of the different mesodermal populations in Platynereis that I have characterised by the expression of six3, fgfr, myoD, twist, mef2 and troponin I. I have found that the Platynereis “brain mesoderm” expresses six3 as does the enteropneust prosoma. The mesodermal sheath around the stomodaeum in Platynereis has morphological similarities to the enteropneust mesosomal coelomic pouches and expresses fgfr and twist in Platynereis. The trunk mesoderm in Platynereis dynamically expresses myoD, twist, and mef2, forms differentiated muscles cells as described by troponin I expression and originates from a similar position as the enteropneust metasomal mesoderm. The molecular comparison of Platynereis neuroectodermal regions with lower vertebrates’ brain regions suggests homology of prostomium/forebrain (six3), peristomium/midbrain (otx) and trunk CNS/hindbrain (gbx and hox1). In vertebrates, the midbrain-hindbrain boundary (positioned at the otx/gbx boundary) is a morphogenetic boundary between the posterior hindbrain/spinal chord region that undergoes convergent extension and the anterior forebrain-/midbrain regions that do not extend. I have found that reminiscent to vertebrate convergent extension in the hindbrain, the gbx-/hox1-expressing neural plate in Platynereis undergoes convergent extension by mediolateral cell intercalation that is possibly controlled by the non-canonical Wnt pathway. Similar to the vertebrate midbrain, the otx-expressing peristomium does not extend. Yet, these movements take place along a slit-like blastopore that develops into both mouth and anus in Platynereis, but occur in front of the blastopore that develops exclusively into the anus in vertebrates. This suggests that convergent extension movements are ancestral in Bilateria and have evolved in early bilaterians to relocate the blastopore-derived mouth and anus to opposite ends of the elongating body axis.|