Ontogeny of the appendicularian Oikopleura dioica (Tunicata, Chordata) reveals characters similar to ascidian larvae with sessile adults

Abstract

Appendicularians have always occupied a central role in considerations of tunicate and chordate evolution. Two hypotheses have been proposed – one holds that appendicularia represents the sister taxon to the remaining tunicates, the other suggests that appendicularians were derived from an ascidian-like ancestor. In the present study I report results from electron microscopic investigation of larval tunicates including the first electron microscopic investigation of the tail of the early ontogenetic appendicularian “Streckform” and discuss their phylogenetic implications. The early “Streckform” of Oikopleura dioica Fol, 1872 is invested with an extracellular covering that consists of an inner electron-light layer and an electron-dense outermost layer. In addition, the extracellular covering forms fin blades. Because these traits are shown to be similar to the tunic of different ascidian larvae, the extracellular covering in early appendicularian embryos is suggested to be homologous to the larval tunic of ascidian larvae. Overall, the tail of early developmental stages of appendicularians consists of a mosaic of apomorphic and plesiomorphic features. The straight, continuous endodermal strand was inherited from a common chordate ancestor whereas the finlets of larvae, consisting of extracellular material, were inherited from a common tunicate ancestor. The horizontal orientation of the tail as a whole was inherited from the last common ancestor of appendicularians and aplousobranch ascidians, and the discovered floating extension at the posterior tip of the tail is unique to the holoplanktonic Oikopleura dioica. These findings support the hypothesis that Appendicularia is derived from a sessile, ascidian-like ancestor.