In an earlier study, we used a chloroplast DNA restriction site approach to elucidate evolutionary patterns within Sideritis L. subgenus Marrubiastrum (Lamiaceae). This assemblage of 24 species comprises one of the largest endemic groups in Macaronesia. The subgenus contains a wide array of life forms which are found in all ecological zones present in the Macaronesian archipelagos of Madeira and the Canary Islands. A unique characteristic of the group is its high level of aneuploidy, a feature rare in oceanic island plants. We undertook a second analysis of the insular taxa using sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA in order to test the patterns revealed by the chloroplast phylogeny. Placement of several taxa in the earlier cpDNA RFLP analysis suggested that hybridization may have been important in the evolution of the group. Populations of S. canariensis from three different islands appeared in three different places in the cpDNA tree, and infraspecific taxa of two other species were also split up. Our nuclear-based analysis provides support for an interpretation of hybridization. The three populations of S. canariensis are sister to each other in the ITS phylogeny, as are the subspecies of S. cretica and S. gomerae that were split in the chloroplast tree. If incongruence between the two datasets is non-significant, we will conduct rigorous analyses on a combined dataset to test the hybridization hypothesis and to further characterize evolutionary patterns within Macaronesian Sideritis.

Key words: cpDNA RFLP, ITS, Lamiaceae, Macaronesia, Sideritis