Insights into the Origin of the CNS: Early Development of Aminergic Neurons in Molluscs

Abstract

Advances in molecular approaches over the last few decades have offered new insights into animal phylogeny, revitalizing discussions on the origin of the central nervous system (CNS). Evidence of conserved developmental genes over this time has led many to postulate a single origin of the CNS. However, the degree of morphological diversity across the animal kingdom and recent phylogenetic analyses support multiple origins of the CNS. Research on how these conserved genes are structured into gene regulatory networks that lead to morphological change is required to clarify the discrepancy between conserved developmental mechanisms and morphological diversity. The current thesis aims to inform such studies by expanding previous descriptions of larval neurodevelopment in gastropod molluscs, laying the framework for comparative studies among more distant phyla. Among early developmental descriptions, the ontogenesis of serotonergic, FMRFamidergic and catecholaminergic neural elements have been investigated. To expand on these descriptions, techniques were used to investigate the development of histaminergic and octopaminergic neurons in larva of Ilyanassa obsoleta and Aplysia californica. Results in this thesis reveal early histaminergic labelling of the statocysts forming commissural fibers that could potentially pioneer the development of the cerebral commissure and connectives; this result also suggests early integration and functionality of the statocysts. Additionally, later stages of development revealed histaminergic neural elements that project throughout the posterior loop of the developing CNS with neurons located in nearly every adult ganglion and foot. Based on criteria used in this study, octopamine (OA) was absent in the larval nervous system; however, results confirmed the conservation of dopamine β-hydroxylase (DBH) and its colocalization with OA in adult molluscan neurons. Together the current thesis provides novel descriptions of larval neurotransmitter systems and identifies an antibody that targets a conserved region of DBH making it suitable for comparative developmental studies. Additionally, these results offer insights into aspects of early neurogenesis and neuronal diversity in gastropod molluscs laying the framework for future comparisons among distantly related phyla that can be used to inform evolutionary relationships.