Tracing cell fate in brain formation during embryogenesis of the ascidian Halocynthia roretzi

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Abstract
Ascidian eggs develop into tadpole larvae. They have a simple central nervous system (CNS) at the dorsal midline. The CNS is formed through neural tube formation at the neurula stage, as in vertebrates. The total number of cells in the CNS is approximately 300. In Halocynthia roretzi, the anterior part of the CNS, which consists of the brain (sensory vesicle) and the visceral ganglion, is exclusively derived from 10 blastomeres at the 110‐cell stage. The anterior CNS is relatively complex and shows remarkable left–right asymmetry, with the lumen of the sensory vesicle, the otolith, and the ocellus on the right side, and the presumed hydrostatic pressure organ on the left side. We labeled these 10 precursor blastomeres – six in the animal hemisphere (a‐line) and four in the vegetal hemisphere (A‐line) – with lineage tracer, and examined the fates in swimming larvae. The clonal organization of the anterior CNS is essentially invariant among individuals, although slight variation in the clonal boundary was observed. There was no extensive mixing between descendants of each precursor. We observed no evidence of cell migration except for two neuronal cells derived from a8.25 blastomeres. The eventual fates of the bilateral blastomeres produced extensive left–right asymmetry. The results suggest that the anterior neural tube rotates in a clockwise direction when viewed from the posterior pole. Staged observation indicated that this rotation takes place during the last 5 h of embryogenesis. We describe detailed positions of descendants of each precursor blastomere. In particular, specific cells of sensory structures were identified by their morphology and staining with specific antibodies and probes. The otolith and ocellus pigment cells were derived from left and right a8.25 blastomeres. Lens cells of the ocellus have a right A8.8 origin, and most of the photoreceptor cells originated from the right A8.7. The presumed pressure organ was formed by descendants of left and right a8.19 and left a8.17 blastomeres. The description of cell lineages of the CNS would facilitate future research to analyze the mechanisms of development of the simple CNS of ascidian tadpole larvae.