Comparative neuroanatomy of Mollusks and Nemerteans in the context of deep metazoan phylogeny

Despite the plenty of molecular phylogenetic studies that emerged in recent years the position of major animal clades within the phylogenetic tree is still controversial. Therefore, the field of neurophylogeny that links the morphology of the nervous system to early evolutionary events emerged as a discipline that can be helpful for the inference of phylogenetic relationships as well as for the retracing of central nervous system evolution. So far, comparative neuroanatomical investigations in arthropods and annelids have suggested that certain brain centers are highly conserved during evolution. If structures like the mushroom bodies are indeed ancestral features of the bilaterian brain, we would expect to find similar neuropils in other invertebrate phyla as well. However, so far in depth studies of the detailed neuroanatomy of numerous invertebrate taxa are lacking. The present thesis therefore focuses on adding new comparative immunohistochemical data on the nervous system of the two lophotrochozoan phyla Mollusca and Nemertea. The first part of the thesis presents an extensive survey on the detailed neuroanatomy of the lesser-known branches of the phylum Mollusca: Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda. By comparing the neuroanatomy of those different taxa it is demonstrated that the presence of two pairs of nerve cords of the medullary type is the main unifying feature of the three non-conchiferan taxa. In other respects, the great diversity of the phylum Mollusca likewise results in a considerable variability of the nervous system. Taking the results of both aplacophoran taxa, the brain as well as the nerve cords of Caudofoveata and Solenogastres are built in a conspicuous different manner. Structures that are similar in the nervous system of Caudofoveata and Solenogastres are as well present in the Polyplacophora. Thus a monophyly of Aplacophora could not be confirmed. Comparing the nervous system of the Caudofoveata, Solenogastres, and Polyplacophora, the caudofoveate brain seems to be most complex among the non-conchiferan taxa because it exclusively shows a division into discrete sections. The neuroanatomical data presented here, are therefore not in accordance with the Adenopoda hypothesis placing Caudofoveata basal within Mollusca. The second part of the present thesis concentrates on the neuroanatomy of one representative of the phylum Nemertea. The nervous system of Lineus viridis basically consists of a well developed brain that reveals no compartmentalized neuropils. Paired medullary cords emanate posteriorly from the ventral lobes of the brain while paired cerebral organs are posteriorly attached to the dorsal lobes of the brain. By presenting a detailed view on the peripheral nervous system as well, this study reveals that nemerteans possess four distinct but interconnected nerve plexus. The results indicate that a subepidermal plexus most likely belongs to the ground pattern of Lophotrochozoa. Higher brain centers like the mushroom bodies that are typical for the arthropod and annelid brain could not been identified neither in the investigated molluscan taxa nor in the nemertean species. However, the nervous system of the investigated caudofoveate, scaphopod, and nemertean species exhibit clusters of cells resembling the globuli cells that characterize the arthropod and annelid mushroom body. Those globuli-like cell clusters are located in the precerebral ganglia of caudofoveates and the cerebral organs of nemerteans, respectively. Like the mushroom bodies of arthropods and annelids the precerebral ganglia as well as the cerebral organs are involved in the processing of olfactory information. Assuming that these cell clusters are indeed homologous, the present findings argue for the existence of mushroom body-like precursor structures in the last common ancestor of all protostomes. Besides the contribution to the evolution of certain brain centers in protostomes, the neuroanatomical data obtained in the present study are used for the construction of a data matrix that will result in a phylogenetic tree that is exclusively based on neuroanatomical data.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2012

Author dc:creator

• Faller, Simone

Contributors dc:contributor

• Loesel, Rudolf

• info:eu-repo/classification/ddc/570
• Zoologie
• Morphologie <Biologie>
• Systematik
• Neuroanatomie
• Gehirn
• Weichtiere
• Schnurwürmer
• Konfokale Mikroskopie
• Biowissenschaften, Biologie
• Mollusca
• Nemertea
• Neurophylogenie
• neurophylogeny
• neuroanatomy
• brain

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124415%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62940