Chloroplast DNA phylogeny and biogeography of Lepidium (Brassicaceae)

Two intergenic spacers, trnT-trnL and trnL-trnF, and the trnL intron of cpD NA were sequenced to study phylogenetic relationships and biogeography of 7 3 Lepidium taxa. Insertions/deletions of greater than or equal to3 bp (base pairs) provided reliable phylogenetic information whereas indels less than or equal to2 bp, probably originating from slipped-strand mispairing, are prone to parallelism in the context of our phylogenetic framework. For the first time, an hypothesis of the genus Lepidium is proposed based on molecu lar phylogeny, in contrast to previous classification schemes into sections and greges (the latter category represents groups of related species withi n a given geographic region), which are based mainly on fruit characters. O nly a few of the taxa as delimited in the traditional systems represent mon ophyletic lineages. The proposed phylogeny would suggest three main lineage s, corresponding to (1) sections Lepia and Cardaria, (2) grex Monoplocoidea from Australia, and (3) remaining taxa, representing the bulk of Lepidium species with more or less resolved sublineages that sometimes represent geo graphical correspondence. The fossil data, easily dispersible mucilaginous seeds, widespread autogamous breeding systems, and low levels of sequence d ivergence between species from different continents or islands suggest a ra pid radiation of Lepidium by long-distance dispersal in the Pliocene/Pleist ocene. As a consequence of climatic changes in this geological epoch, arid/ semiarid areas were established, providing favorable conditions for the rad iation of Lepidium by which the genus attained its worldwide distribution.