Austrognathia microconulifera, Farris, 1977

Gąsiorowski, Ludwik, Bekkouche, Nicolas & Worsaae, Katrine, 2017, Morphology and evolution of the nervous system in Gnathostomulida (Gnathifera, Spiralia), Organisms Diversity & Evolution (New York, N. Y.) 17 (2), pp. 447-475 : 461

Austrognathia microconulifera

SLIR, acetylated α- tubulin-like immunoreactive (αTLIR), FLIR, and synapsin 2-like immunoreactive (Syn2LIR) nervous structures as well as DAPI-stained cell nuclei are visualized with CLSM.

The brain consists of a central neuropil (np, Figs. 9b–h, 10c–h) surrounded dorsally, laterally, and anteriorly by perikarya and sensory structures. The SLI reactivity was detected in 11 brain perikarya (3 × mp, 2 × llp, 4 × plp, 2 × slp, Figs. 9c, 10e) and four brain commissures: one dorso-anterior (dac, Figs. 9c, 10e), one ventro-anterior (vac, Figs. 9d, 10b), and two ventro-posterior (vpc1 and vpc2, Figs. 9d, 10b); the ventro-anterior and the first ventro-posterior commissures form a ring-shaped structure. The FLI reactivity and the Syn2LI reactivity were detected in respectively 30 and eight brain perikarya and in the neuropil ( Fig. 10g, h , see Online Resource 2 for more details).

Three pairs of longitudinal nerve cords extend from the brain: main ventro-lateral, lateral, and dorso-lateral (vlc, lc, dc, Figs. 9, 10a). The two former are SLIR, αTLIR, FLIR, and Syn2LIR (Figs. 9, 10a–d, i–n) and terminate at the posterior commissure (pc, Figs. 9b, f; 10k–n), whereas the dorso-lateral cords (dc, Figs. 9, 10c) are only SLIR and αTLIR and continue into the tail (dtn, Figs. 9a, e, S1k). Some scattered peripheral FLIR perikarya and processes are present along the ventro-lateral cord ( Fig. S 1g –i ). A pair of small SLIR, αTLIR, FLIR, and Syn2LIR postpharyngeal ganglia (ppg, Figs. 9b–h, 10a–d) were detected posteriorly to the pharynx, connecting to the lateral cord and contralaterally to the ventro-lateral cord, thus forming a ganglionated postpharyngeal commissure (ppc, Figs. 9b–h, 10a–i). At the level of that commissure, a midventral nerve cord (mvc, Figs. 5e, 9b–h, 10a–l), being both SLIR and αTLIR, originates from the ventro-lateral nerves and innervates few midventral ciliary gut receptors (c gr, Figs. 5d, e; 9f, h; 10i). Posteriorly, the ventro-lateral and the lateral nerve cords form a posterior commissure (pc, Figs. 9b, f; 10k–n), related to a pair of posterior cell clusters (pcc, Figs. 9f, 10l–n, S1j), which include SLIR posterior perikarya (pp, Figs. 9b, 10a, k, S1k). A single middorsal αTLIR and Syn2LIR perikaryon (mdc, Figs. 9e, S1j) is present above the male opening.

Approximately 35 cells constitute the buccal ganglion (bg, Figs. 5l; 9e, g; 10c, d; S1a, c), related to paired ciliary pharyngeal receptors (cpr, Figs. 5l, 9g) and a pair of buccal nerves (bn, Figs. 5l, 9g, 10c), along which ciliated buccal cells are located (pcl, Figs. 5l, 9g). Some additional cells in the pharyngeal bulb and around the mouth opening show Syn2LI reactivity (see text and Fig. S1a, b in the Online Resource 2). Additionally, FLI reactivity was detected in the buccal nerves and three short neurite bundles within the buccal ganglion ( Figs. 10d, S 1c ).

The rostral sensory structures include four pairs of αTLIR compound cirri (s1–s4, Figs. 9e–h, 10f), an αTLIR anterior sensory organ (ano, Figs. 9h, 10f), a pair of αTLIR spiral organs (so, Figs. 9g, 10c, f), and a pair of SLIR lateral sensory cells (lsc, Figs. 9d, 10b). In the posterior part of the body, a pair of αTLIR and Syn2LIR sensory cells (psc, Fig. 10n ) are associated with a posterior cell cluster.

The nervous system of Austrognathia christianae and Austrognatharia medusifera was very similar with only minor variance (see text in the Online Resource 2).

Figures

Fig. 10 Z-projections of the nervous system of Austrognathia microconulifera. Serotonin-like immunoreactivity in white, α-tubulinlike immunoreactivity in yellow, FRMF-amid-like immunoreactivity in magenta, and synapsin 2-like immunoreactivity in green. Anterior to the left. a Entire body. b–d Nervous structures in the ventro-anterior part of the body. e–h Brain and sensory structures in the head region. i and j Ventral nervous structures in the middle part of the trunk. k–n Ventral nervous structures in the posterior part of the body. Abbreviations: ano anterior sensory organ; b brain; bg buccal ganglion; bn buccal nerve; bp brain perikarya; cgr ciliary gut receptor; dac dorso-anterior commissure; dc dorso-lateral nerve cord; dp dorsal perikarya; dpc dorso-posterior

Fig. 1 Schematic drawings of the nervous system in Gnathostomula paradoxa as inferred from serotonin-like immunoreactivity and CLSM. a Entire body, dorsal structures. b Entire body, ventral structures. c Head and pharyngeal regions, dorsal structures. d Head and pharyngeal regions, ventral structures. Abbreviations: ap anterior perikaryon; asc anterior sensory cell; b brain; bg buccal ganglion; bn buccal nerve; bu bursa; dac1 first dorso-anterior commissure; dac2 second dorso- anterior commissure; dc dorso- lateral nerve cord; dtn dorsal tail nerve; e egg; lc lateral nerve cord; ldp large dorsal perikaryon; llp large lateral perikaryon; lsc lateral sensory cell; mc median connective; mdc middorsal cell; mic mouth innervating cell; mo mouth opening; mp median perikarya; np neuropil; p penis; pb pharyngeal bulb; pc posterior commissure; pp posterior perikaryon; slp small lateral perikaryon; t tail; te testis; vac ventro-anterior commissure; vlc ventro-lateral nerve cord; vpc ventro-posterior commissure; vtn ventral tail nerve; * and + indicate where the dorsal part connects to the ventral one