title: Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes.
abstract: In Xenopus, the canonical Wnt-signaling pathway acting through beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate Amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of Amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Wnts, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in Amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution.
authors.affiliation: Marine Biology Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0202, USA. lzholland@ucsd.edu
authors.forename: Linda Z
authors.initials: LZ
authors.lastname: Holland
mesh_terms: D000818:Animals; D001755:Blastocyst / Q000502:physiology / Q000648:ultrastructure; D019521:Body Patterning / Q000235:genetics / Q000502:physiology*; D019070:Cell Lineage; D016764:Cell Polarity; D002642:Chick Embryo; D002816:Chordata, Nonvertebrate / Q000196:embryology* / Q000235:genetics / Q000648:ultrastructure; D003598:Cytoskeletal Proteins / Q000502:physiology; D029721:Drosophila Proteins / Q000235:genetics / Q000502:physiology; D004447:Echinodermata / Q000196:embryology; D004625:Embryo, Nonmammalian / Q000502:physiology / Q000648:ultrastructure; D004627:Embryonic Induction; D005260:Female; D005326:Fetal Proteins; D005775:Gastrula / Q000502:physiology / Q000648:ultrastructure; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D005801:Genes, Homeobox; D018398:Homeodomain Proteins / Q000235:genetics / Q000502:physiology; D007814:Larva; D008297:Male; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology; D008954:Models, Biological; D009865:Oocytes / Q000648:ultrastructure; D009928:Organ Specificity; D010802:Phylogeny; D011506:Proteins / Q000235:genetics / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology; D051880:Receptors, Notch; D015398:Signal Transduction; D013045:Species Specificity; D020825:T-Box Domain Proteins / Q000235:genetics / Q000502:physiology; D015534:Trans-Activators; D014157:Transcription Factors; D014561:Urochordata / Q000196:embryology; D014714:Vertebrates / Q000196:embryology / Q000235:genetics; D051153:Wnt Proteins; D000071818:Wnt-5a Protein; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology / Q000235:genetics; D029961:Zebrafish Proteins; D051176:beta Catenin; D000098908:Brachyury Protein
chemical_list: D003598:Cytoskeletal Proteins; D029721:Drosophila Proteins; D005326:Fetal Proteins; D018398:Homeodomain Proteins; D008565:Membrane Proteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; D051880:Receptors, Notch; D020825:T-Box Domain Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D051153:Wnt Proteins; D000071818:Wnt-5a Protein; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; D000098908:Brachyury Protein; C495266:CTNNB1 protein, Xenopus; C085096:Wnt5a protein, Xenopus; C084564:cad protein, Drosophila