Does hair cell differentiation predate the vertebrate appearance?

It is generally accepted that the three main chordate groups (tunicates, cephalochordates and vertebrates) originated from a common ancestor having the basic features of the chordate body plan, i.e. a neural tube and a notochord flanked by striated musculature. There is now increasing evidence that tunicates, rather than cephalochordates, are the vertebrate sister-group. Correlated with this, tunicates have sensory structures similar to those derived from placodes or neural crest in vertebrates. In this context, we discuss here whether the precursors of vertebrate hair cells, which are placodal in origin, were present in ancestral chordates. The ascidian tunicates possess a coronal organ, consisting of a row of mechanosensory cells that runs around the base of the oral siphon. Its function is to monitor the incoming water flow. The cells are secondary sensory cells, i.e. they lack axons and synapse with neurons whose somata lie in the cerebral ganglion. They are accompanied by supporting cells and, as in vertebrates, have varying morphologies in the species so far examined: in one order (Enterogona), they are multiciliate; in the other (Pleurogona), they may possess an apical apparatus, consisting of one or two cilia accompanied by stereovilli, that are graded in length. Coronal cells thus resemble vertebrate hair cells closely in their morphology, embryonic origin and arrangement, which suggests they originated early in ancestral hordates. We are continuing our study of the coronal organ in other ascidian species, and report new data here on Botrylloides leachi, which conforms with the pattern of Pleurogona and, in particular, with previously published results on other botryllid ascidians.