REVIEWS Chordate roots of the vertebrate nervous system: expanding the molecular toolkit Linda Z. Holland Abstract | The vertebrate brain is highly complex with millions to billions of neurons. During development, the neural plate border region gives rise to the neural crest, cranial placodes and, in anamniotes, to Rohon-Beard sensory neurons, whereas the boundary region of the midbrain and hindbrain develops organizer properties. Comparisons of developmental gene expression and neuroanatomy between vertebrates and the basal chordate amphioxus, which has only thousands of neurons and lacks a neural crest, most placodes and a midbrain–hindbrain organizer, indicate that these vertebrate features were built on a foundation already present in the ancestral chordate. Recent advances in genomics have provided insights into the elaboration of the molecular toolkit at the invertebrate –vertebrate transition that may have facilitated the evolution of these vertebrate characteristics. Tunicates Over the past two centuries, many groups of inverte- to which a neural crest, most neurogenic placodes and The sister group of the brates have been nominated as the immediate ancestors an MHB organizer were added. This raises the question vertebrates. Tunicates include of vertebrates1. Recently, however, molecular biology data of what were the changes in genetic mechanisms at the the appendicularians or have supported only one of these older views — that the invertebrate chordate–vertebrate transition that allowed larvaceans, ascidians and the thaliaceans. In phylogenetic proximate invertebrate ancestor of the vertebrates most evolution of such vertebrate-specific structures. Recent 2–4 analyses, appendicularians closely resembled a modern amphioxus (or lancelet) . advances in genomics and epigenetics are providing some typically fall basal to ascidians Within the chordates, amphioxus is basal to tunicates and answers to this question. and thaliaceans, although vertebrates (FIG. 1)2,4. In the past 20 years, comparisons of This Review considers the evolutionary origins of the branch length is long. The the expression patterns of developmental genes, together the vertebrate central and peripheral nervous systems thaliaceans, thought to have evolved from ascidians, include with detailed microanatomy, have indicated that the ante- emphasizing the genetic and epigenetic mechanisms that three taxa: doliolids, salps and rior expanded portion of the amphioxus CNS is probably may have facilitated the evolution of vertebrate placodes, pyrosomes. equivalent to a diencephalic forebrain plus a small mid- the neural crest and an MHB organizer. The focus is brain, whereas the remainder of the CNS is homologous on the invertebrate chordate amphioxus, but tunicates to the vertebrate hindbrain and spinal cord. However, and hemichordates are also discussed (FIG. 1). The evo- amphioxus has a much simpler CNS than vertebrates lution of bilaterian nervous systems from prebilaterian with approximately 20,000 neurons5, no telencephalon ancestors7,8 is not covered here. and no neural crest6. Amphioxus has numerous ectoder- mal sensory neurons (although it is thought that it has The neural plate border region homologues of only the adenohypophyseal and olfactory In vertebrates, the neural plate border region gives rise Marine Biology Research Division, Scripps Institution placodes). Even so, expression of genes in the neural plate to the neural crest, cranial placodes and, in anamniotes, to of Oceanography, and those encoding transcription factors that specify the Rohon-Beard sensory neurons. Neural crest cells undergo University of California San neural plate border and position the midbrain–hindbrain an epithelial–mesenchymal transition and migrate Diego, La Jolla, California (MHB) boundary are comparable in amphioxus and ver- throughout the body differentiating into many cell types 92093-0202, USA. tebrates. However, the two groups differ in their expres- including the glia and most of the neurons in the peripheral e-mail: [email protected] 9 (FIG. 2) doi:10.1038/nrn2703 sion of late neural crest specifiers and genes encoding nervous system . In the cranial region, additional 10 Published online proteins that confer organizer properties on the vertebrate peripheral neurons develop from neurogenic placodes . 9 September 2009 MHB. Thus, amphioxus probably reflects the foundation Although it has been argued that the neural crest and 736 | OCTOBER 2009 | VOLUME 10 www.nature.com/reviews/neuro © 2009 Macmillan Publishers Limited. All rights reserved FOCUS ON CNS EVOLUTREVIEWSION Amphioxus Appendicularian Ascidian Vertebrate Neural crest- More gene like cells losses (e.g. Hox3, More gene Neural crest; Hox5, Hox6) losses MHB organizer; (e.g. Hox9, lateral line, otic, lens, Hox11) epibranchial, trigeminal Loss of Gbx, and profundal placodes Wnt1, Hox7, Hox8 Two whole genome and other genes duplications followed by loss of many duplicate genes MHB with organizer properties? MHB lacking organizer properties; dorsal hollow nerve cord; adenohypophyseal and olfactory placodes Figure 1 | Major events in nervous system evolution mapped onto the phylogenetic tree of the chordates. Although Organizer independent gene duplications and losses have occurred in all lineages, the two genome duplicationsNatur ate Re thevie basews | ofNeur theoscienc e An embryonic tissue that, when ectopically transplanted, vertebrates and the considerable losses of developmental genes in the tunicates (that is, appendicularians and ascidians plus can redirect the fate of the thaliceans) are noted. Nothing is known about developmental genes in the third group of tunicates, the thaliaceans. A third recipient tissue. Organizers in whole-genome duplication occurred at the base of the teleost fish. MHB, midbrain–hindbrain. The amphioxus image is courtesy vertebrate embryos include of M. Dale Stokes; the ascidian image is courtesy of R.W. Zeller; the vertebrate image is courtesy of F. Holland Morris. The the dorsal blastopore lip or appendicularian image is reproduced, with permission, from REF. 131 (2007) Macmillan Publishers Ltd. All rights reserved. Spemann’s organizer in amphibians and its equivalent in other vertebrate embryos, 10,15 and the tissue spanning the placodes have a common evolutionary origin, as they arise transition . In addition, the genetic mechanisms that midbrain–hindbrain boundary. from a population of cells in the neural plate boundary specify the neural plate border region and migrating region and give rise to several similar cell types, the pre- ectodermal sensory cells in amphioxus or placodal neuro- Rohon-Beard sensory vailing view is that they evolved independently9,11,12. blasts in vertebrates seem to be comparable (FIG. 3), which neurons Large, mechanosensory Unequivocal homologues of the neural crest and pla- suggests a common evolutionary heritage. neurons in the dorsal portion codes, other than the olfactory and adenohypophyseal of the spinal cord of larval placodes, are lacking in amphioxus, although intramed- Evolutionary origin of the CNS anamniote vertebrates. They ullary sensory cells in the dorsal portion of the amphioxus The genetic basis for initial specification of the neural typically degenerate later in CNS are thought to be homologous to Rohon-Beard sen- plate border region is the evolutionarily ancient mecha- development. sory neurons6. Amphioxus does have tissues that could nism that distinguishes neuroectoderm from non-neural Protostomes be considered harbingers of the neural crest and other ectoderm. This involves a change from a high level of One of the two main groups of placodes — namely, the tissue at the border of the neural BMP (bone morphogenic protein) signalling on the non- bilaterally symmetrical plate that detaches from the neural plate, develops lamel- neural side, to a low level on the neural side that is medi- animals. The protostomes are divided into the Ecdysozoa, lipodia and migrates over it as sheets that ultimately fuse ated by secreted BMP antagonists. This mechanism is 13 including nematodes and in the dorsal midline , and the ectodermal sensory cells, found in all bilaterians with a CNS that have been studied arthropods (insects, spiders, which originate from ectodermal cells along the ventral to date (the only known exceptions are some nematodes crustaceans and some smaller side of the embryo, lose their cilia and migrate dorsally in the protostomes, and tunicates in the deuterostomes, groups) and the beneath the ectoderm14. However, the tissue bordering the both of which have modified development)16–21. Lophotrochozoa (annelids, molluscs and some smaller amphioxus neural plate remains epithelial, and the amphi- Most data argue for a common evolutionary origin 8 groups). oxus ectodermal sensory cells develop axons that extend of the CNS in protostomes and chordates . This idea to the CNS, regrow their cilia and microvilli and reinsert has its roots in the famous drawing by Goeffroy Saint- Deuterostomes themselves into the ectoderm14. Amphioxus ectodermal Hilaire22, which shows that an upside down lobster has One of the two main groups of bilaterally symmetrical sensory cells do not contribute to ganglia (the dorsal roots essentially the same dorso–ventral organization as a ver- 6 animals. The deuterostomes of the amphioxus CNS lack ganglia ) and do not resem- tebrate. This principle of unity of organization formed the include the Ambulacraria ble hair cells of the vertebrate lateral line or otic placode, basis of theories proposing