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      An Amphioxus Gli Gene Reveals Conservation of Midline Patterning and the Evolution of Hedgehog Signalling Diversity in Chordates

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          Abstract

          Background

          Hedgehog signalling, interpreted in receiving cells by Gli transcription factors, plays a central role in the development of vertebrate and Drosophila embryos. Many aspects of the signalling pathway are conserved between these lineages, however vertebrates have diverged in at least one key aspect: they have evolved multiple Gli genes encoding functionally-distinct proteins, increasing the complexity of the hedgehog-dependent transcriptional response. Amphioxus is one of the closest living relatives of the vertebrates, having split from the vertebrate lineage prior to the widespread gene duplication prominent in early vertebrate evolution.

          Principal Findings

          We show that amphioxus has a single Gli gene, which is deployed in tissues adjacent to sources of hedgehog signalling derived from the midline and anterior endoderm. This shows the duplication and divergence of the Gli gene family, and hence the origin of vertebrate Gli functional diversity, was specific to the vertebrate lineage. However we also show that the single amphioxus Gli gene produces two distinct transcripts encoding different proteins. We utilise three tests of Gli function to examine the transcription regulatory capacities of these different proteins, demonstrating one has activating activity similar to Gli2, while the other acts as a weak repressor, similar to Gli3.

          Conclusions

          These data show that vertebrates and amphioxus have evolved functionally-similar repertoires of Gli proteins using parallel molecular routes; vertebrates via gene duplication and divergence, and amphioxus via alternate splicing of a single gene. Our results demonstrate that similar functional complexity of intercellular signalling can be achieved via different evolutionary pathways.

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          Most cited references59

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          Tunicates and not cephalochordates are the closest living relatives of vertebrates.

          Tunicates or urochordates (appendicularians, salps and sea squirts), cephalochordates (lancelets) and vertebrates (including lamprey and hagfish) constitute the three extant groups of chordate animals. Traditionally, cephalochordates are considered as the closest living relatives of vertebrates, with tunicates representing the earliest chordate lineage. This view is mainly justified by overall morphological similarities and an apparently increased complexity in cephalochordates and vertebrates relative to tunicates. Despite their critical importance for understanding the origins of vertebrates, phylogenetic studies of chordate relationships have provided equivocal results. Taking advantage of the genome sequencing of the appendicularian Oikopleura dioica, we assembled a phylogenomic data set of 146 nuclear genes (33,800 unambiguously aligned amino acids) from 14 deuterostomes and 24 other slowly evolving species as an outgroup. Here we show that phylogenetic analyses of this data set provide compelling evidence that tunicates, and not cephalochordates, represent the closest living relatives of vertebrates. Chordate monophyly remains uncertain because cephalochordates, albeit with a non-significant statistical support, surprisingly grouped with echinoderms, a hypothesis that needs to be tested with additional data. This new phylogenetic scheme prompts a reappraisal of both morphological and palaeontological data and has important implications for the interpretation of developmental and genomic studies in which tunicates and cephalochordates are used as model animals.
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            Genetic transformation of Drosophila with transposable element vectors.

            Exogenous DNA sequences were introduced into the Drosophila germ line. A rosy transposon (ry1), constructed by inserting a chromosomal DNA fragment containing the wild-type rosy gene into a P transposable element, transformed germ line cells in 20 to 50 percent of the injected rosy mutant embryos. Transformants contained one or two copies of chromosomally integrated, intact ry1 that were stably inherited in subsequent generations. These transformed flies had wild-type eye color indicating that the visible genetic defect in the host strain could be fully and permanently corrected by the transferred gene. To demonstrate the generality of this approach, a DNA segment that does not confer a recognizable phenotype on recipients was also transferred into germ line chromosomes.
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              Sonic hedgehog mediates the polarizing activity of the ZPA.

              The zone of polarizing activity (ZPA) is a region at the posterior margin of the limb bud that induces mirror-image duplications when grafted to the anterior of a second limb. We have isolated a vertebrate gene, Sonic hedgehog, related to the Drosophila segment polarity gene hedgehog, which is expressed specifically in the ZPA and in other regions of the embryo, that is capable of polarizing limbs in grafting experiments. Retinoic acid, which can convert anterior limb bud tissue into tissue with polarizing activity, concomitantly induces Sonic hedgehog expression in the anterior limb bud. Implanting cells that express Sonic hedgehog into anterior limb buds is sufficient to cause ZPA-like limb duplications. Like the ZPA, Sonic hedgehog expression leads to the activation of Hox genes. Sonic hedgehog thus appears to function as the signal for antero-posterior patterning in the limb.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                PLoS ONE
                plos
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2007
                12 September 2007
                : 2
                : 9
                : e864
                Affiliations
                [1 ]Department of Zoology, University of Oxford, Oxford, United Kingdom
                [2 ]Medical Research Council Functional Genetics Unit, Department of Human Anatomy and Genetics, University of Oxford, Oxford, United Kingdom
                [3 ]Developmental Neurobiology, National Institute for Medical Research, London, United Kingdom
                Katholieke Universiteit Leuven, Belgium
                Author notes
                * To whom correspondence should be addressed. E-mail: sebastian.shimeld@ 123456zoo.ox.ac.uk

                Conceived and designed the experiments: JB SS Mv. Performed the experiments: JB SS Mv RD. Analyzed the data: JB SS Mv. Contributed reagents/materials/analysis tools: SS. Wrote the paper: JB SS Mv.

                Article
                07-PONE-RA-01676
                10.1371/journal.pone.0000864
                1955834
                17848995
                911beab3-8147-4d95-b5d8-5523e64ee5cf
                Shimeld et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                History
                : 6 July 2007
                : 15 August 2007
                Page count
                Pages: 10
                Categories
                Research Article
                Developmental Biology
                Evolutionary Biology
                Developmental Biology/Developmental Evolution
                Developmental Biology/Pattern Formation
                Evolutionary Biology/Developmental Molecular Mechanisms

                Uncategorized
                Uncategorized

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