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Did the first chordates organize without the organizer?

2005; Elsevier BV; Volume: 21; Issue: 9 Linguagem: Inglês

10.1016/j.tig.2005.07.002

1362-4555

Matthew J. Kourakis, William C. Smith,

Echinoderm biology and ecology

Models of vertebrate development frequently portray the organizer as acting on a largely unpatterned embryo to induce major components of the body plan, such as the neural plate and somites. Recent experiments examining the molecular and genetic basis of major inductive events of vertebrate embryogenesis force a re-examination of this view. These newer observations, along with a proposed revised fate map for the frog Xenopus laevis, suggest a possible reconciliation between the seemingly disparate mechanisms present in the ontogeny of the common chordate body plan of vertebrate and invertebrate chordates. Here, we review data from vertebrates and from an ascidian urochordate and propose that the organizer was not present at the base of the chordate lineage, but could have been a later innovation in the lineage leading to vertebrates, where its role was more permissive than instructive. Models of vertebrate development frequently portray the organizer as acting on a largely unpatterned embryo to induce major components of the body plan, such as the neural plate and somites. Recent experiments examining the molecular and genetic basis of major inductive events of vertebrate embryogenesis force a re-examination of this view. These newer observations, along with a proposed revised fate map for the frog Xenopus laevis, suggest a possible reconciliation between the seemingly disparate mechanisms present in the ontogeny of the common chordate body plan of vertebrate and invertebrate chordates. Here, we review data from vertebrates and from an ascidian urochordate and propose that the organizer was not present at the base of the chordate lineage, but could have been a later innovation in the lineage leading to vertebrates, where its role was more permissive than instructive. also called lancelets, or amphioxus, these are the closest living non-craniate relatives of vertebrates, and contain all chordate features, yet lack the cranial and sensory complexity of the vertebrate clade. Their position in chordate phylogeny makes these organisms ideal for comparisons with vertebrates. the deuterostome phylum that includes urochordates, cephalochordates and vertebrates. Chordates have a notochord flanked by muscle, a dorsal nervous system, pharyngeal gill slits, and a postanal tail. These characters have been modified or lost in some lineages, but are usually present at some point in development. also called tunicata, because of an outer covering, or 'tunic', in adults, urochordates are the most basal chordates. Adults filter-feed and bear few visible traces of chordate affinity aside from a pharyngeal basket perforated with gill slits. Other chordate features, notably a dorsal nervous system and notochord, are present in the larvae. Ascidian urochordates, such as Ciona, Halocynthia, Molgula and Styela, have been used as developmental models for more than a century. characterized by a vertebral column, this group includes the jawless lamprey and all jawed vertebrates including cartilaginous fishes (e.g. sharks), ray-finned fishes (e.g. teleost fish) and lobe-finned fishes (e.g. coelacanth, tetrapods). Vertebrates, together with the hagfish, comprise the craniata, a group characterized by a bony or cartilaginous skull as well as numerous sensory innovations. Neural crest and ectodermal placodes are thought to be craniate novelties, but this view has come under scrutiny recently.