Analysis of a botryllid enriched-full-length cDNA library: insight into the evolution of spliced leader trans-splicing in tunicates
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In some animals, mRNA may be modified after transcription by the addition of a 5′ spliced leader sequence. This is known as spliced leader (SL) trans-splicing, and is of uncertain function and evolutionary origin. Here, we report the identification of SL trans-splicing in the colonial ascidian Botryllus schlosseri. Combining our own expressed sequence tag (EST) data with additional data from GenBank, we identify the dominant spliced leader sequence and show it to be similar to that of other ascidians and to that of Oikopleura dioica, a basally diverging tunicate. Gene Ontology analysis of B. schlosseri ESTs with and without a 5′ spliced leader shows that genes encoding ribosomal proteins tend not to be trans-spliced, a character shared with the ascidian Ciona intestinalis. We also examine individual cases of genes that produce mRNAs that are SL trans-spliced in B. schlosseri but not in C. intestinalis. We conclude that SL trans-splicing evolved early in the tunicate lineage and shows stability over considerable evolutionary time. However, SL trans-splicing may be gained or lost in individual genes.
KeywordsAscidian Botryllus Transcription Splicing Spliced leader
The authors wish to thank Michael Kube, Richard Reinhardt and Sven Klages for the support in preparing the library during the NoE “Marine Genomics Europe” training course “Generation of cDNA Libraries by Primer Extension” (20 November–1 December, 2006) attended to one of us (FG) at the Max Planck Institute for Molecular Genetics, Berlin (Germany). We also thank two anonymous referees for their thoughtful comments. FG acknowledges the support of the Ministero della Università e Ricerca Scientifica e Tecnologica and the Università degli Studi di Padova. SMS acknowledges the support of the Royal Society and the British Council. The authors declare that the experiments comply with the current laws of the country in which they were performed. The authors declare that they have no conflict of interest. Additional data are available at http://www.fabiogasparini.org/Data/DGEsuppl/info.htm. (1) The 765 singleton clones and contigs derived from the assembled ESTs generated in this study; (2) accessions and sequences of the 4,477 B. schlosseri ESTs extracted from GenBank and used in this study; (3) dbEST and GenBank accession numbers of the 1,259 B. schlosseri ESTs generated in this study.
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