Evolution of endemic Sideritis (Lamiaceae) in macaronesia: Insights from achloroplast DNA restriction site analysis

Sideritis L. subgenus Marrubiastrum (Lamiaceae) comprises 24 species of Mac aronesian endemic perennials. The constituent taxa encompass a wide array o f life forms that have exploited all ecological zones present in the Macaro nesian archipelagos of Madeira and the Canary Islands. Though recently revi sed, an explicit phylogeny of the subgenus is still lacking. This group is especially intriguing because it exhibits the largest aneuploid series ever reported in oceanic island plants. The present study uses a chloroplast DN A restriction site analysis to elucidate the pattern of evolution within an d among the islands. The insular taxa form a strongly supported monophyleti c group, indicating a single colonization of Macaronesia. Monophyly of the sections is not supported. Integrating geographical and ecological distribu tion with the cpDNA phylogeny reveals a pattern of diversification within t he islands consistent with adaptive radiation. Most ether Macaronesian plan t groups studied thus far have diversified by inter-island colonization bet ween similar ecological zones. An analysis of chromosome number suggests a bimodal pattern of chromosomal change in the island subgenus, with one clad e showing a decreasing aneuploid series and a second clade exhibiting aneup loid increase. The rate of change appears markedly amplified relative to th at of the continental subgenus. This extreme level of chromosomal diversity stands in stark contrast to a general pattern of chromosomal stasis in isl and plants.