Origin of the Chordate Notochord

Abstract

The phylum of Chordata is defined based on the discovery of a coelom-like dorsal notochord in ascidian and amphioxus embryos. Chordata can be classified into three subphylums, Cephalochordata, Urochordata, and Vertebrata, united by the presence of a notochord at some point during development. The origin of the notochord, the signature anatomical structure of chordates, has been under debate since the publication of Alexander Kovalevsky’s work in the mid-19th century that placed ascidians close to the vertebrates on the phylogenetic tree. During the late 20th century, the development of molecular and genetic tools in biology brought about a revival of studies on the evolutionary path of notochord development. Two main hypotheses for the origin of the notochord were proposed, the de novo theory and the axochord theory. The former states that notochord has developed de novo from the mid-dorsal archenteron of a chordate ancestor with simple morphology and no central nervous system nor notochord homolog. The putative notochord along the dorsal side of the animal is proposed to take on the signal functions later from the endoderm and ectoderm. An alternative hypothesis, the axochord theory, proposes that notochord has evolved from the mid-line muscle tissue, the so-called axochord, in annelids. Structural and molecular evidence point to the midline muscle of annelids as a distant homolog of the notochord. This hypothesis thus suggests a notochord-like structure in the urbilaterian ancestor, opposed to the consensus that notochord is a chordate-specific feature. In this review, we introduce the history of the formation of these views and summarize the current understandings of embryonic development, molecular profile, and gene regulatory networks of notochord and notochord-like structures.