LOWE, CJ*; ARONOWICZ, J; TERASAKI, M; KIRSCHNER, M; GERHART, J; University of Chicago; University of Chicago; University of Connecticut; Harvard Medical School; University of California Berkeley: Hemichordate development and the evolution of chordate nervous system

How the chordate central nervous system evolved from their invertebrate ancestors has a long history in evolutionary biology, yet it remains one of the greatest puzzles in zoology. Hemichordates are one of the most promising groups for addressing this issue due to their potential morphological affinities to the chordates that originally led Bateson to classify them as chordates. The dorsal and ventral nerve cords have both been proposed to be the homolog of the chordate dorsal nerve cord at various times by a range of investigators. However, more recently, earlier classical descriptions have been largely over-looked, yet show clearly that the nervous system is entirely decentralized, and the two �cords� are best described at axonal tracks rather than true cords. We have been characterizing the development of a direct-developing enteropneust species of hemichordate, Saccoglossus kowalevskii. This work has compared the expression in S. kowalevskii of conserved regulatory genes involved in the specification and patterning of the chordate central nervous system in both the anteropostero and dorsoventral dimensions. The surprising initial results from this work has shown a remarkable conservation of the relative expression of genes involved in the antero/postero patterning of the entire neuraxis of the chordates during the development of S. kowalevskii despite a fundamental difference in neural organization. A more complex story emerges when comparing the genes involved in dorsoventral patterning, but bmp clearly has a fundamental role in dorso/ventral patterning in hemichordates as shown by functional experiments. I will discuss the evolutionary implications of these data to both the evolution of chordates and the evolution of neuroanatomy.