Chordate Evolution and Autonomous Specification of Cell Fate: The Ascidian Embryo Model

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Abstract
SYNOPSIS. Two parallel themes emerge in the history of the investigation of the ascidian tunicate [Urochordata] embryo: the realization that the larval stage is probably a surviving example of the earliest chordate body plan from which vertebrates arose, and secondly the unusual degree of autonomous specification of cell fate involved in the development of ascidian larval parts. Such developmental autonomy in larval structures results in patterns of development referred to as “mosaic.” This paper follows the progress of these two themes from their beginnings in the second half of the nineteenth century to their status at the present time. Romer's concept of vertebrates as a “dual animal” (somatic and visceral) stands out as a landmark perception in support of the theory of vertebrate origin by paedomorphosis through a merger of the pelagic larval and benthic adult stages of a tunicate-like animal. The present contribution attempts to unite the two themes by postulating that autonomous specification further enhanced the modular nature of the developing tunicate embryo and permitted natural selection to act differentially on the largely independent organ systems of larvae and paedomorphs, in what amounts to a mosaic selection pattern. This, in turn, favored the very rapid emergence and radiation of the chordates during the Cambrian explosion.

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