The evolution of dorsoventral pattern formation in the chordate neural tube

Abstract

SYNOPSIS. Living members of ^Phylum Chordata are divided into three groups: the Urochordata, the Cephalochordata (amphioxus) and the Craniata (vertebrates). These animals are united by a common body plan, a key component of which is the development of a neural tube dorsal to a notochord. Studying the genetics and embryology of these animals allows evolutionary comparison to be made between the mechanisms controlling the development of homologous body parts in different taxa. This paper focuses specifically on the evolution of dorsovcntral pattern in the neural tube. In vertebrate embryos external inductive signals, originating from the notochord and the dorsal ectoderm, initiate a program of cell differentiation that subdivides the neural tube into a stereotyped pattern of neurons and glia. To understand the evolution of this pattern I have been characterising amphioxus members of the gene families involved, including genes from the HNF-3, Msx, Hh, Gli and Netrin families. Coupled with similar analyses of urochordate development, analysis of these genes shows that the signalling functions of the notochord and lateral ectoderm seem to predate vertebrate origins, and have not increased in complexity in vertebrates despite duplication of the gene families involved. Conversely, expansion of gene families downstream of these signals has increased the complexity of gene expression and function in vertebrate embryos. These data therefore provide an indication of how gene duplication and divergence may have provided the raw material for the evolution of the complex pattern of cell types that develops in the vertebrate neural tube.