MBE Advance Access published online on April 2, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn075
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Research Article |
Plastid-Derived Genes in the Non-Photosynthetic Alveolate Oxyrrhis marina
Department of Botany, University of British Columbia, 3529-6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada
* Correspondence: pkeeling{at}interchange.ubc.ca
Received for publication February 8, 2008. Revision received March 11, 2008. Accepted for publication March 13, 2008.
Reconstructing the history of plastid acquisition and loss in the alveolate protists is a difficult problem because our knowledge of the distribution of plastids in extant lineages is incomplete due to the possible presence of cryptic, non-photosynthetic plastids in several lineages. The discovery of the apicoplast in apicomplexan parasites has drawn attention to this problem, and more specifically to the question of whether many other non-photosynthetic lineages also contain cryptic plastids, or are derived from plastid-containing ancestors. Oxyrrhis marina is one such organism: it is a heterotrophic, early-branching member of the dinoflagellate lineage for which there is no evidence of a plastid. To investigate the possibility that O. marina is derived from a photosynthetic ancestor, we have generated and analysed a large-scale EST database and searched for evidence of plastid-derived genes. Here we describe eight genes whose phylogeny shows them to be derived from plastid-targeted homologues. These genes encode proteins from several pathways known to be localised in the plastids of other algae, including synthesis of tetrapyrrols, isoprenoids and amino acids, as well as carbon metabolism and oxygen detoxification. The 5’ end of 5 cDNAs were also characterised using cap-dependent or spliced leader-mediated RT-PCR, revealing that at least four of these genes have retained leaders that are similar in nature to the plastid-targeting signals of other secondary plastids, suggesting these proteins may be targeted to a cryptic organelle. At least two genes do not encode such leaders, and their products may presently function in the cytosol. Altogether, the presence of plastid-derived genes in O. marina shows that its ancestors contained a plastid, and the pathways represented by the genes and presence of targeting signals on at least some of the genes further suggests that a relict organelle may still exist to fulfil plastid metabolic functions.
Key Words: plastid evolution • Chromalveolates • apicoplast • dinoflagellates • Oxyrrhis
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