title: Neurogenesis in directly and indirectly developing enteropneusts: of nets and cords.
abstract: Concerning the evolution of deuterostomes, enteropneusts (acorn worms) occupy a pivotal role as they share some characteristics with chordates (e.g., tunicates and vertebrates) but are also closely related to echinoderms (e.g., sea urchin). The nervous system in particular can be a highly informative organ system for evolutionary inferences, and advances in fluorescent microscopy have revealed overwhelming data sets on neurogenesis in various clades. However, immunocytochemical descriptions of neurogenesis of juvenile enteropneusts are particularly scarce, impeding the reconstruction of nervous system evolution in this group. We followed morphogenesis of the nervous system in two enteropneust species, one with direct (Saccoglossus kowalevskii) and the other with indirect development (Balanoglossus misakiensis), using an antibody against serotonin and electron microscopy. We found that all serotonin-like immunoreactive (LIR) neurons in both species are bipolar ciliary neurons that are intercalated between other epidermal cells. Unlike the tornaria larva ofB. misakiensis, the embryonic nervous system ofS. kowalevskiilacks serotonin-LIR neurons in the apical region as well as an opisthotroch neurite ring. Comparative analysis of both species shows that the projections of the serotonin-LIR somata initially form a basiepidermal plexus throughout the body that disappears within the trunk region soon after settlement before the concentrated dorsal and ventral neurite bundles emerge. Our data reveal a highly conserved mode of neurogenesis in enteropneusts that is independent of the developing mode and is inferred to be a common feature for Enteropneusta. Moreover, all detected serotonin-LIR neurons are presumably receptor cells, and the absence of serotonin-LIR interneurons from the enteropneust nervous system, which are otherwise common in various bilaterian central nervous systems, is interpreted as a loss that might have occurred already in the last common ancestor of Ambulacraria.
authors.affiliation: Department of Integrative Zoology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Kaul-Strehlow
authors.affiliation: Takehara Marine Science Station, Setouchi Field Science Center, Graduate School of Biosphere Science, Hiroshima University, 5-8-1 Minato-machi, Takehara, Hiroshima 725-0024 Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Urata
authors.affiliation: Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Aomori 039-3501 Japan.
authors.forename: Takuya
authors.initials: T
authors.lastname: Minokawa
authors.affiliation: Institute for Biology, Humboldt-University Berlin, Philippstr. 13, 10115 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Stach
authors.affiliation: Department of Integrative Zoology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wanninger
keywords: Bipolar receptor cell; Deuterostome; Enteropneusts; Evolution; Hemichordates; Nervous system; Neurogenesis; Plexus; Serotonin