Sideritis L. subgenus Marrubiastrum (Lamiaceae) comprises 24 species of Macaronesian endemic perennials. The constituent taxa encompass a wide array of life forms that have exploited all ecological zones present in the Macaronesian archipelagos of Madeira and the Canary Islands. Though recently revised, 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 DNA restriction site analysis to elucidate the pattern of evolution within and among the islands. The insular taxa form a strongly supported monophyletic group, indicating a single colonization of Macaronesia. Monophyly of the sections is not supported. Integrating geographical and ecological distribution with the cpDNA phylogeny reveals a pattern of diversification within the islands consistent with adaptive radiation. Most other Macaronesian plant groups studied thus far have diversified by inter-island colonization between similar ecological zones. An analysis of chromosome number suggests a bimodal pattern of chromosomal change in the island subgenus, with one clade showing a decreasing aneuploid series and a second clade exhibiting aneuploid increase. The rate of change appears markedly amplified relative to that of the continental subgenus. This extreme level of chromosomal diversity stands in stark contrast to a general pattern of chromosomal stasis in island plants.

Communicating Editor: John V. Freudenstein