No abstract

The Spermann organizer induces neural tissue from dorsal ectoderm and dorsalizes lateral and ventral mesoderm in Xenopus. The secreted factor noggin, which is expressed in the organizer, can mimic the dorsalizing signal of the organizer. Data are presented showing that noggin directly induces neural tissue, that it induces neural tissue in the absence of dorsal mesoderm, and that it acts at the appropriate stage to be an endogenous neural inducing signal. Noggin induces cement glands and anterior brain markers, but not hindbrain or spinal cord markers. Thus, noggin has the expression pattern and activity expected of an endogenous neural inducer.

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The ascidian notochord follows a morphogenetic program that includes convergent extension (C/E), followed by anterior-posterior (A/P) elongation [1-4]. As described here, developing notochord cells show polarity first in the mediolateral (M/L) axis during C/E, and subsequently in the A/P axis during elongation. Previous embryological studies [3] have shown that contact with neighboring tissues is essential for directing M/L polarity of ascidian notochord cells. During C/E, the planar cell polarity (PCP) gene products prickle (pk) and dishevelled (dsh) show M/L polarization. pk and dsh colocalize at the notochord cell membranes, with the exception of those in contact with neighboring muscle cells. In the mutant aimless (aim), which carries a deletion in pk, notochord morphogenesis is disrupted, and cell polarization is lost. After C/E, there is a dynamic relocalization of PCP proteins in the notochord cells with dsh localized to the lateral edges of the membrane, and pk and strabismus (stbm) at the anterior edges. An A/P polarity is present in the extending notochord cells and is evident by the position of the nuclei, which in normal embryos are invariably found at the posterior edge of each cell. In the aim mutant, all appearances of A/P polarity in the notochord are lost.

A common CNS architecture is observed in all chordates, from vertebrates to basal chordates like the ascidian Ciona. Ciona stands apart among chordates in having a complete larval connectome. Starting with visuomotor circuits predicted by the Ciona connectome, we used expression maps of neurotransmitter use with behavioral assays to identify two parallel visuomotor circuits that are responsive to different components of visual stimuli. The first circuit is characterized by glutamatergic photoreceptors and responds to the direction of light. These photoreceptors project to cholinergic motor neurons, via two tiers of cholinergic interneurons. The second circuit responds to changes in ambient light and mediates an escape response. This circuit uses GABAergic photoreceptors which project to GABAergic interneurons, and then to cholinergic interneurons. Our observations on the behavior of larvae either treated with a GABA receptor antagonist or carrying a mutation that eliminates photoreceptors indicate the second circuit is disinhibitory.