Mapping a floral trait in Shepherds purse – ‘Stamenoid petals’ in natural populations of Capsella bursa-pastoris (L.) Medik

Striking feature of angiosperm diversity is the huge number of variations in corolla morphology including complex innovations like variations in symmetry or the identity and number of floral organs. Throughout the Brassicaceae, the disymmetric flower structure is highly conserved. Still, quite a few floral alterations occur like a variant of Common Shepherds purse (Capsella bursa-pastoris), in which all petals have been transformed into additional stamens. This “decandric” phenotype has been reported for the first time about 200 years ago. In some of the original locations the variant has been recovered recently. The long term persistence indicates the establishment of an evolutionary novelty in wild populations in sympatric occurrence with wild-type plants. Due to this fact the floral variant has become an interesting model for evolutionary studies. The phenotype is heritable and just a single locus, termed “Stamenoid petals” (Spe), is assumed to be involved in the molecular origin. To unravel the chromosomal localization of this locus, a linkage map analysis was carried out using molecular markers (AFLPs, RAPDs). The final map includes 15 linkage groups and the floral trait was integrated on linkage group 12 (CBP12) including six AFLP markers. Out of these, five markers were successfully sequenced and revealed sequence identities with chromosome IV of the A. thaliana genome. Interestingly, AGAMOUS is located on this chromosome, the only class C floral organ identity gene in the A. thaliana genome, which is compatible with the assumption that Spe is an allele of AGAMOUS rather than a regulator of that gene.