The developing dorsal ganglion of the salp Thalia democratica, and the nature of the ancestral chordate brain

The development of the dorsal ganglion of the salp, Thalia democratica, is described from electron microscope reconstructions up to the stage of centr al neuropile formation. The central nervous system (CNS) rudiment is initia lly tubular with an open central canal. Early developmental events include: (i) the formation of a thick dorsal mantle of neuroblasts from which paire d dorsal paraxial neuropiles arise; (ii) the differentiation of clusters of primary motor neurons along the ventral margin of the mantle; and (iii) th e development from the latter of a series of peripheral nerves. The dorsal paraxial neuropiles ultimately connect to the large central neuropile, whic h develops later. Direct contact between neuroblasts and muscle appears to be involved in the development of some anterior nerves. The caudal nerves r esponsible for innervating more distant targets in the posterior part of th e body develop without such contacts, which suggests that a different patte rning mechanism may be employed in this part of the neuromuscular system. The results are compared with patterns of brain organization in other chord ates. Because the salp CNS is symmetrical and generally less reduced than t hat of ascidian larvae, it is more easily compared with the CNS of amphioxu s and vertebrates. The dorsal paraxial centres in the salp resemble the dor solateral tectal centres in amphioxus in both position and organization; th e central neuropile in salps likewise resembles the translumenal system in amphioxus. The neurons themselves are similar in that many of their neurite s appear to be derived from the apical surface instead of the basal surface of the cell. Such neurons, with extensively developed apical neurites, may represent a new cell type that evolved in the earliest chordates in conjun ction with the formation of translumenal or intralumenal integrative centre s. In comparing the salp ganglion with vertebrates, we suggest that the mai n core of the ganglion is most like the mes-metencephalic region of the ver tebrate brain, i.e. the zone occupied by the midbrain, isthmus, and anterio r hindbrain. Counterparts of more anterior regions (forebrain) and posterio r ones (segmented hindbrain) appear to be absent in salps, but are found in other tunicates, suggesting that evolution has acted quite differently on the main subdivisions of the CNS in different types of tunicates.