2.01 - Evolution of the Deuterostome Central Nervous System: An Intercalation of Developmental Patterning Processes with Cellular Specification Processes

B Fritzsch; J.C Glover

doi:10.1016/B0-12-370878-8/00125-7

The morphological, histological, and molecular basis of deuterostome and craniate nervous system organization is reviewed in the context of cladistic evidence of relationships and basic principles of neuroanatomical comparisons. The central nervous system (CNS) of deuterostomes shows a variety of forms, from a hardly specialized basiepithelial nerve plexus (hemichordates), to a few small ganglia with a tail nerve cord (urochordates; or a tailless head in adult sessile urochordates), to a swimming spinal cord with a hardly recognizable cerebral vesicle (cephalochordates), to a fully developed brain and spinal cord (craniates). Despite these overall differences, similarities related to a certain degree of rostrocaudal and dorsoventral patterning are present. These may be directly related to the overall rostrocaudal patterning of the body and to the fact that the neural tube evolved only once in ancestral chordates and has maintained a molecularly identical dorsoventral patterning scheme. This evidence implies that deep homology or homocracy exists among deuterostome (CNS) but that distinct evolutionary adaptations exist that render morphological comparisons of nervous tissue organization among deuterostomes difficult. For example, comparisons at a cellular level show that the motoneurons of deuterostomes share certain basic similarities across taxa but also exhibit unique taxon-specific features that do not appear to follow a progressive evolutionary transformation but rather indicate independently derived transformations from a potentially common root in the basiepithelial nerve plexus of primitive deuterostome taxa. Likewise, comparative analysis of common neuronal cell types identifiable by their specific neurotransmitters demonstrates a variety of patterns of topology and homology. Enough similarities exist to suggest that most of the apparent differences probably represent variations on a theme that may already have been set up in the last common ancestor of all deuterostomes. Moving on to the molecular specification of the cellular topology of the nervous system, the available data provide a promising beginning toward a phylogenetic synthesis but also indicate that the comparison of gene expression domains does not provide a magic bullet to pinpoint basic neuromeric homologies. Evolutionary modifications of gene expression patterns and of patterns of neuronal differentiation within given neuromeres make the road ahead as difficult as the road was until here. Still, the emerging framework of nested gene expression patterns, combined with more sophisticated neuronal tracing studies, will clearly facilitate tracing the origins of region-specific neuronal phenotypes and relating this to specific gene expression domains through gain-of-function and loss-of-function analyses.

2.01 - Evolution of the Deuterostome Central Nervous System: An Intercalation of Developmental Patterning Processes with Cellular Specification Processes

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