Basic Nervous System Types One or Many

F Hirth and H Reichert, University of Basel, Basel, Switzerland

© 2007 Elsevier Inc. All rights reserved.

1.04.1 Introduction 56

1.04.2 The Cuvier-Geoffroy Debate 57 A Common Bauplan for Animal Development? 57 From Unity of Composition to Unity of Nervous Systems? 58

1.04.3 Conserved Mechanisms for Anteroposterior Patterning of the CNS 59 Hox Genes Are Involved in the Regional Specification of Neuronal Identity 59 Cephalic Gap Genes in Regionalization of the Anterior Brain: The otd/Otx Genes 61 A Tripartite Organization of the Insect and Chordate Brain? 62

1.04.4 Conserved Mechanisms for Dorsoventral Patterning of the CNS 62 Antagonistic Activity of Dpp/BMP-4 and sog/Chordin 62 vnd/Nkx, ind/Gsh, and msh/Msx: Specification of Longitudinal Columns 63 The CNS Midline: Pattern Formation and Axonal Guidance 64

1.04.5 Evolutionary Origin of the CNS 65 Molecular Phylogeny: Several Possibilities 65 Do Specialized Gene Expression Patterns Predict Specialized Brain Structures? 66 A Simple Nerve Net at the Base of Nervous System Evolution? 68

1.04.6 Conclusions 69


Bilateria blastopore coelom




A monophyletic group of metazoan animals that is characterized by bilat- homeodomain eral symmetry. This group comprises all of the Metazoa except for the

Radiata (Ctenophores and Cnidaria)

and the Parazoa (sponges).

The site of gastrulation initiation.

Fluid-filled body cavity found in ani- homology mals that is lined by cells derived from mesoderm tissue in the embryo and Lophotrochozoa provides for free, lubricated motion of the viscera.

(From the Greek: mouth second) A major group of the Bilateria including echinoderms and chordates. In deuterostomes, the first opening (the blastopore) becomes the anus and the mouth derives from a secondary invagination. Notoneuralia

Major group of protostome animals, including the arthropods (insects, arachnids, crustaceans, and relatives), roundworms, and several smaller phyla, which are characterized by a trilayered cuticle, composed of organic material, which is periodi- phylogeny cally molted as the animal grows by a process called ecdysis. A subdivision of the Bilateria defined by the location of the nerve cord, Gastroneuralia are characterized by a ventral nerve cord and include most protostomes except the Tentaculata. A 60-amino-acid part of proteins that corresponds to the homeobox sequence found in homeobox genes that are involved in the regulation of the development (morphogenesis) of animals, fungi, and plants. Correspondence or relation in type of structure because of shared ancestry. Major group of protostome animals, including mollusks, annelids, nemer-teans, brachiopods, and several other phyla characterized either by the production of trochophore larvae, which have two bands of cilia around their middle, or by the presence of a lopho-phore, a fan of ciliated tentacles surrounding the mouth. A subdivision of the Bilateria defined by the location of the nerve cord, Notoneuralia are characterized by a dorsal nerve cord and include most deuterostomes except the Echinodermata, Chaetognatha, and Enteropneusta.

The origin and evolution of a set of organisms, which reveals ancestral relationships, such as monophyly (common origin) or polyphyly (independent origin), among known species.

Protostomia (From the Greek: first the mouth) A

major group of the Bilateria including the Lophotrochozoa and the Ecdysozoa. In protostomes, the mouth forms at the site of the blasto-pore and the anus forms as a second opening.

Urbilateria The animal that preceded all recent bilateral symmetric animals.

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