The origin and evolution of chordate nervous systems

TitleThe origin and evolution of chordate nervous systems
Publication TypeJournal Article
Year of Publication2015
AuthorsHolland LZ
JournalPhilosophical Transactions of the Royal Society B-Biological Sciences
Volume370
Date Published2015/12
Type of ArticleReview
ISBN Number0962-8436
Accession NumberWOS:000364861900007
KeywordsAmphioxus; amphioxus larvae; brain; Brain evolution; cell-types; chordate evolution; developmental expression; frontal eye; gene; insights; nervous system evolution; Neural crest; phylostratigraphic profiles; phylostratigraphy; saccoglossus-kowalevskii; vertebrate neural crest
Abstract

In the past 40 years, comparisons of developmental gene expression and mechanisms of development (evodevo) joined comparative morphology as tools for reconstructing long-extinct ancestral forms. Unfortunately, both approaches typically give congruent answers only with closely related organisms. Chordate nervous systems are good examples. Classical studies alone left open whether the vertebrate brain was a new structure or evolved from the anterior end of an ancestral nerve cord like that of modern amphioxus. Evodevo plus electron microscopy showed that the amphioxus brain has a diencephalic forebrain, small midbrain, hindbrain and spinal cord with parts of the genetic mechanisms for the midbrain/hindbrain boundary, zona limitans intrathalamica and neural crest. Evodevo also showed how extra genes resulting from whole-genome duplications in vertebrates facilitated evolution of new structures like neural crest. Understanding how the chordate central nervous system (CNS) evolved from that of the ancestral deuterostome has been truly challenging. The majority view is that this ancestor had a CNS with a brain that gave rise to the chordate CNS and, with loss of a discrete brain, to one of the two hemichordate nerve cords. The minority view is that this ancestor had no nerve cord; those in chordates and hemichordates evolved independently. New techniques such as phylostratigraphy may help resolve this conundrum.

DOI10.1098/rstb.2015.0048
Short TitlePhilos. Trans. R. Soc. B-Biol. Sci.
Student Publication: 
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