| Ascidians are an amenable research organism in both developmental biology and evolutionary studies. As marine invertebrates in the chordate phylum, they are the closest invertebrate relatives to vertebrates; however, they have greatly reduced gene families and much simpler body plan. The ascidian larva has a simple nervous system consisting of roughly 130 neurons and 230 glial cells. The most prominent feature of the anterior CNS is the sensory vesicle, which is the putative homolog of the vertebrate fore- and mid-brain. The sensory vesicle contains two pigmented sensory organs, the ocellus and otolith. Other prominent neuroectodermal derivatives in the anterior head of the embryo include the oral siphon primordium (OSP), or mouth, and the palps, sensory adhesive organs. The anterior sensory vesicle, along with the OSP and palps, are derived from the anterior neural plate cells, from the a4.2 lineage. This lineage contributes to both anterior neural structures and epidermis. A mutant was identified in our lab to be missing palps, the OSP, and anterior sensory vesicle, including the pigmented cells. These cells are found to initially be properly specified, but later lose their identity and revert to a basal epidermal fate. Using genetic mapping techniques revealed the causal mutation to be a four base pair deletion interrupting the canonical splice donor site in a gene encoding a connexin protein. Understanding how the connexin is required to maintain proper cell identity in the CNS will provided needed insight into later mechanisms of neural specification and stability. |
