The ascidian is an attractive model for the study of genes involved in development. The ascidian tadpole larva possesses a prototypical chordate body plan including a notochord, dorsal nerve cord and lateral muscle. They share many features with vertebrates, especially in the patterning and organization of the nervous system, yet the entire larva is composed of only ∼2600 cells. Development of the embryo is rapid, and the pattern of cell cleavage is invariant. In addition to their morphological simplicity, the genome of the ascidian is roughly 180 Mb, which is 20 times smaller than the human genome allowing for simplified genetic analysis.

A screen for N-ethyl-N-nitrosourea (ENU)-induced mutations affecting larval development was performed in Ciona savignyi with a mutation frequency of 5%. Two mutant lines from this screen were isolated for further study. The spotless mutant is unable to produce melanin due to a mutation in the tyrosinase gene. The resulting lack of pigmentation in the sensory organs of the brain (the otolith and the ocellus) makes the larvae unable to respond properly to gravity and illumination cues while settling. The vagabond mutant is caused by a mutation in the Doublesex/Mab-3 related gene CsDmrt-1. Mutant larvae lack the adhesive organs necessary for settlement and metamorphosis and have pattering defects in the central and peripheral nervous systems. These two mutants further our understanding of the development of the ascidian nervous system and its role in larval behavior.