Martinez P.1, Perea-Atienza E.1, Gavilán B.1, Fernandez C.1, Sprecher S.2 2017. The study of xenacoelomorph nervous systems. Molecular and morphological perspectives // Invertebrate Zoology. Vol.14. No.1: 32–44 [in English].

1 Departament de Genètica, Microbiologia I Estadística, Universitat de Barcelona, Av. Diagonal, 645, E-08028 Barcelona, Spain. e-mail:

2 Department of Biology, University of Fribourg, 10, ch. du Musée, 1700 Fribourg, Switzerland. E-mail:

ABSTRACT: The evolution of centralized nervous systems (NS) and complex brains are among the most important innovations in the history of life on our planet. Many of the major transitions in animal evolution have been accompanied by significant reorganization of the NS. For instance, during the emergence of bilaterians, the radial–bilateral transition (RBT) not only involved the innovation of orthogonal body axes — anteroposterior or AP; dorsoventral or DV — and of a third embryonic layer, the mesoderm, providing controlled motility, but also the emergence of neuronal aggregates at the anterior pole of the animal (some of these innovations, though, are seen already in the anthozoans). This mass of interconnected neurons allowed better-oriented control and response to external sensory inputs. In this context, several important general questions need to be addressed. What evolutionary changes in the mechanisms of development can lead to a centralized or conversely to a diffuse NS? What constitutes the minimal genetic toolkit for the development of a centralized NS (a brain)? All these questions belong to the realm of neurogenetic-oriented basic research, and therefore, the human brain is treated just as one peculiar case (highly derived) of a centralized NS. To answer these questions, we have chosen to perform a developmental and genomic study of the phylum Xenacoelomorpha, because the different clades within this phylum show distinct degrees of nervous system centralization (from none to the presence of a ganglionic, true brain), and because we expect that the analysis of their NS will eventually be crucial to understand their phylogenetic position. An (speculative) evolutionary hypothesis for the origin of centralized nervous systems is proposed at the end of this manuscript.

doi: 10.15298/invertzool.14.1.06

KEY WORDS: Xenacoelomorpha, Xenoturbellida, Nemertodermatida, Acoela, Genome Evolution, Gene Families, Brain, Nerve Net, RNA Hypothesis, Origin Bilateria.

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