Long-distance cell migration during larval development in the appendicularian, Oikopleura dioica.

The appendicularian, Oikopleura dioica, is a planktonic chordate. Its simple and transparent body, invariant cell lineages and short life cycle of 5 days make it a promising model organism for studies of chordate development. Here we describe the cell migration that occurs during development of the O. dioica larva. Using time-lapse imaging facilitated by florescent labeling of cells, three cell populations exhibiting long-distance migration were identified and characterized. These included (i) a multinucleated oral gland precursor that migrates anteriorly within the trunk region and eventually separates into the left and right sides, (ii) endodermal strand cells that are collectively retracted from the tail into the trunk in a tractor movement, and (iii) two subchordal cell precursors that individually migrate out from the trunk to the tip of the tail. The migration of subchordal cell precursors starts when all of the endodermal strand cells enter the trunk, and follows the same path but in a direction opposite to that of the latter. Labeling of these cells with a photoconvertible fluorescent protein, Kaede, demonstrated that the endodermal strand cells and subchordal cell precursors have distinct origins and eventual fates. Surgical removal of the trunk from the tail demonstrated that the endodermal strand cells do not require the trunk for migration, and that the subchordal cell precursors would be attracted by the distal part of the tail. This well-defined, invariant and traceable long-distance cell migration provides a unique experimental system for exploring the mechanisms of versatile cell migration in this simple organism with a chordate body plan.