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Hybridization in Macaronesian Sideritis (Lamiaceae): evidence from incongruence of multiple independent nuclear and chloroplast sequence datasets

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With 23 suffrutescent to woody perennial species distributed in diverse ecological zones, Sideritis subgenus Marrubiastrum constitutes one of the largest plant radiations in the Macaronesian archipelagos. In an earlier study, we investigated the evolution of Sideritis in Macaronesia using chloroplast restriction site (RFLP) data, but inferences were limited by the lack of a nuclear marker. A second study used sequence data to determine the continental origin of the Macaronesian group, but that study included only seven island taxa in a much larger sampling of continental taxa. For the present study, we generated new datasets from sequences of the nuclear ribosomal DNA internal transcribed spacers (ITS) and two plastid regions (trnL intron, trnT-trnL intergenic spacer) in order to reconstruct relationships among all extant island taxa using both nuclear and chloroplast data. Relationships based upon plastid data suggest that there may be a geographical component to cpDNA variation. Individual phylogenies reconstructed from the nuclear and chloroplast sequence data were incongruent, and differing placements of taxa were well supported in each of the two datasets. This incongruence has enabled us to identify several instances of potential cytoplasmic introgression, suggesting that hybridization may have been important in the evolution of Sideritis in Macaronesia. Because the ITS phylogeny concurs with current taxonomic circumscriptions in the Macaronesian subgenus, we reassess patterns of diversification based upon the nuclear tree.
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Document Type: Research Article

Affiliations: 1: Department of Biology, 3507 Laclede Avenue, Saint Louis University, St. Louis, Missouri 63103, U.S.A. 2: Jardín de Aclimatación de La Orotava, Calle Retama Num. 2, E-38400 Puerto de La Cruz, Tenerife, Canary Islands, Spain 3: Section of Integrative Biology and Institute of Cellular and Molecular Biology, University of Texas, Austin, Texas 78712, U.S.A.

Publication date: 01 February 2007

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