Abstract
During evolution, new characters are designed by modifying pre-existing structures already present in ancient organisms. In this perspective, the Central Nervous System (CNS) of ascidian larva offers a good opportunity to analyze a complex phenomenon with a simplified approach. As sister group of vertebrates, ascidian tadpole larva exhibits a dorsal CNS, made up of only about 330 cells distributed into the anterior sensory brain vesicle (BV), connected to the motor ganglion (MG) and a caudal nerve cord (CNC) in the tail. Low number of cells does not mean, however, low complexity. The larval brain contains 177 neurons, for which a documented synaptic connectome is now available, and two pigmented organs, the otolith and the ocellus, controlling larval swimming behavior. The otolith is involved in gravity perception and the ocellus in light perception. Here, we specifically review the studies focused on the development of the building blocks of ascidians pigmented sensory organs, namely pigment cells and photoreceptor cells. We focus on what it is known, up to now, on the molecular bases of specification and differentiation of both lineages, on the function of these organs after larval hatching during pre-settlement period, and on the most cutting-edge technologies, like single cell RNAseq and genome editing CRISPR/CAS9, that, adapted and applied to Ciona embryos, are increasingly enhancing the tractability of Ciona for developmental studies, including pigmented organs formation.
Highlights
The sea squirt Ciona intestinalis represents a suitable model system for studying the transcriptional regulatory mechanisms exploited during chordate development (Satoh et al, 2003), due to its phylogenetic position and to a number of peculiar features
The simplicity of Ciona larval structures, in particular of its Central Nervous System (CNS), permits to depict the genetic programs adopted by a single blastomere in order to build up specific structures, tissues, organs present at the larval stage
The results of this study indicated that Group I, Group II and Group III photoreceptor cells originate from the A-lineage blastomeres and, those of Group I and Group II are the descendants of the right A9.14 (Figure 2, blue) cell and those of Group III derive from the right A9.16 cell (Figure 2, yellow)
Summary
The sea squirt Ciona intestinalis represents a suitable model system for studying the transcriptional regulatory mechanisms exploited during chordate development (Satoh et al, 2003), due to its phylogenetic position (belonging to chordates as sister group of vertebrates) and to a number of peculiar features. The simplicity of Ciona larval structures, in particular of its Central Nervous System (CNS), permits to depict the genetic programs adopted by a single blastomere in order to build up specific structures, tissues, organs present at the larval stage This simplicity is undoubtedly a huge advantage compared to the complexity of vertebrates. For the sake of simplicity and to avoid confusion, in this review the general term Ciona will be used for both species
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