Evolutionary Changes in Floral Structure within Lepidium L. (Brassicaceae).

The basic floral ground plan is remarkably constant across Brassicaceae. However, within Lepidium (ca. 175 species), deviations from this ground plan are common, with over half of Lepidium species having only two stamens rather than the usual six and a further eighth of the species having only four stamens. Furthermore, petals are reduced in size in a majority of Lepidium species. In order to determine the frequency and direction of changes in floral structure within Lepidium, we have inferred the phylogeny within the genus from sequences of the internal transcribed spacers of the nuclear ribosomal DNA. On the basis of this inferred phylogeny, we conclude that floral structure within Lepidium is relatively fluid. In order to account for the phylogenetic distributions of the different floral ground plans, at least two independent reductions to the two-stamen condition and at least one reversal to flowers with increased organ numbers are likely to have occurred. To account for the frequency of morphological evolution observed within the genus, we propose that some clades within Lepidium may be predisposed to changes in floral structure. In addition, several transoceanic dispersals are needed to explain the geographic distributions of the clades inferred from the phylogeny.