Neural pathways in invertebrate nervous systems

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My lab has pioneered studies on simple invertebrate nervous systems, in particular the visual system of the fruit fly Drosophila melanogaster and the central nervous system (CNS) of the tadpole larva in the ascidian Ciona intestinalis, a close sibling relative of vertebrates. Using electron microscopy, we have generated comprehensive maps, or connectomes, of synaptic networks in these tiny brains, an approach to neural function that has now become far more widely recognized than hitherto. We will continue these approaches, using advanced methods harnessed to biological diversity. The question itself is simple enough. The brain is a network, one formed by synaptic contacts between identified neurons. The complete connectome these form constitutes a formal definition of any brain, essential to know if we are ever to establish the circuit basis for animal behaviour, an ultimate objective in neuroscience. Certainly not new, this idea is now enabled by recent developments in electron imaging and especially by rapid computer 3D reconstruction methods. Functional studies that allow synaptic transmission to be disabled then reveal the role that identified neurons play in specific behaviours, the ultimate value of a connectome. We will apply these ideas to three carefully chosen nervous systems described below. 1) We will compare the visual circuits of 12 identified neuron classes in each column, or cartridge, in the optic lamina of select species of Hawaiian drosophilids, which have previously defined evolutionary relationships. This will reveal how synaptic circuits have evolved among homologous neurons to subserve different visual behaviours in related species. Our work will also address how the evolution of brains has proceeded from the evolution of their synaptic circuits, selected by the visual behaviours these support.2) Complementing our published work on the larval CNS of Ciona we will reconstruct the connectome for parts of the CNS of a related basal chordate, the larvacean, Oikopleura dioica, a species immensely important in the sea's food chains. Like Ciona, its CNS has a constant cell number, but the circuits these form are not known and may be supplemented by epithelial conduction pathways. We will work with larval stages small enough to examine from EM series, and sufficiently transparent for future imaging and other functional studies.3) In an entirely new choice of nervous system that also exploits Dalhousie's unique strength in marine resources, we will document the connectome of the brachial ganglion in the CNS of the pygmy squid, Idiosepius. This tiny cephalopod is small enough to fit in whole-mounts beneath a compound microscope immersion objective, and we will use immunolabelling to identify neurons and EM to examine their synaptic circuits in this important relay station, initially to provide the unit structure of those circuits that regulate a single arm of this tiny cephalopod.