Lepidium is a genus with about 175 species distributed worldwide. Although the basic floral structure is conserved throughout most of the Brassicaceae, there are many deviations within Lepidium . Over half of Lepidium species have only two medial stamens rather than four medial and two lateral stamens. And one-eighth have four medial stamens and lack lateral stamens. Furthermore, species with a reduced number of stamens tend to have reductions in petal stature. Those species with reduced floral structures are mostly native to the Americas and Australia. To understand the frequency and evolutionary pathway of floral reduction, PISTILLATA first intron sequences from 43 species were used in phylogenetic analyses. The resulting gene tree suggests a complicated history of reticulation within the American and derived Australian Lepidium species. Based on the number of loci of PITILLATA introns and known chromosome numbers, many New World Lepidium species appear to be allopolyploids. An artificial hybrid experiment demonstrated phenotypic dominance of reduced stamen numbers. From genetic and molecular phylogenetic studies, we propose a model in which the reduced stamen number phenotype dominated in the New World Lepidium via allopolyploid hybridization and subsequent speciation

Key words: Allopolyploid, Lepidium, PISTILLATA