14
Developmental Mechanisms
Underlying the Origin and
Evolution of Chordates
NORI SATon, KOHn HOTTA, GOUKI SATOH, SHUNSUKE T AGUCHI, HITOYOSHI Y ASUO,
KUNI T AGA W A, HIROKI TAKAHASHI, AND Y OSHITO HARADA
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto
606-8502, Japan
Abstract
Chordates evolved from a common ancestor shared by two other non-chordate deuterostome groups, hemichordates and echinoderms. The notochord is the most defining feature of chordates, and therefore the elucidation of molecular mechanisms
underlying the formation of notochord will lead to a better understanding of developmental mechanisms that permitted and/or accelerated the origin and evolution of
chordates. Brachyury encodes a transcription factor with the T DNA-binding domain and exerts a master control over the formation of notochord in ascidian embryos. However, Brachyury is conserved by non-chordate deuterostomes which do
not have a notochord. We show here that at least the hemichordate Brachyury has
the potential to induce the formation of notochord when the gene is ectopically
expressed in chordate (asci dian) embryos. This strongly suggests that an acquisition of new function(s) of the pre··existing developmental gene is closely associated
with the occurrence of a chordate-specific organ. Furthermore, we present data on
the upstream control of ascidian Brachyury genes that is required for notochordspecific expression of the genes. In addition, we described an isolation of downstream genes, of which expression is regulated by the ascidian Brachyury to form
the notochord.
Key words. chordates, deuterostomes, cephalochordates, urochordates, hemichordates, echinoderms, evolution, developmental mechanisms, notochord, BrachYllry, expression patterns,
transcriptional factor, T-domain, master control, functional conservation
209
Developmental Mechanisms
Underlying the Origin and
Evolution of Chordates
NORI SATon, KOHn HOTTA, GOUKI SATOH, SHUNSUKE T AGUCHI, HITOYOSHI Y ASUO,
KUNI T AGA W A, HIROKI TAKAHASHI, AND Y OSHITO HARADA
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto
606-8502, Japan
Abstract
Chordates evolved from a common ancestor shared by two other non-chordate deuterostome groups, hemichordates and echinoderms. The notochord is the most defining feature of chordates, and therefore the elucidation of molecular mechanisms
underlying the formation of notochord will lead to a better understanding of developmental mechanisms that permitted and/or accelerated the origin and evolution of
chordates. Brachyury encodes a transcription factor with the T DNA-binding domain and exerts a master control over the formation of notochord in ascidian embryos. However, Brachyury is conserved by non-chordate deuterostomes which do
not have a notochord. We show here that at least the hemichordate Brachyury has
the potential to induce the formation of notochord when the gene is ectopically
expressed in chordate (asci dian) embryos. This strongly suggests that an acquisition of new function(s) of the pre··existing developmental gene is closely associated
with the occurrence of a chordate-specific organ. Furthermore, we present data on
the upstream control of ascidian Brachyury genes that is required for notochordspecific expression of the genes. In addition, we described an isolation of downstream genes, of which expression is regulated by the ascidian Brachyury to form
the notochord.
Key words. chordates, deuterostomes, cephalochordates, urochordates, hemichordates, echinoderms, evolution, developmental mechanisms, notochord, BrachYllry, expression patterns,
transcriptional factor, T-domain, master control, functional conservation
209