Establishment of a natural floral variant of Shepherd's purse in the wild: analysis of life-history traits in `Capsella apetala' (Brassicaceae)

Abstract

Background and aims - Molecular studies in model systems have pushed forward our understanding of floral developmental genetics, but the evolutionary significance of such modifications in natural populations is rather unexplored. To improve our knowledge in this field, the sympatric occurrence of two floral variants of Capsella bursa-pastoris (L.) Medik. in the wild appears to be an ideal model system. In our study, wild type plants showing small white petals and the homeotic variant `Starnenoid petals' (Spe also known as Capsella apetala Opiz), in which petals are replaced by additional stamens, will be compared to evaluate potential ecological differentiation. Material and methods - Progenies from field collections were used in common garden experiments to detect possible differences in several life-history traits involved in the reproductive fitness of both variants. A second experiment was intended to shed light on the relative hybridization rate among floral variants, using the enzyme aspartate amino transferase (AAT) as a molecular marker. Results - Comparing fitness revealed that the two variants invested differently into their progeny. Wild-type plants showed more fruits per plant, whereas Spe showed higher investment in seeds per fruit. However, the overall reproductive output (seeds/plant) is almost equal. Wild-type donates more pollen for cross-fertilization, because floral visits are more common in this variant. Furthermore, both variants are separated in the onset of flowering, with Spe having a significant later onset of flowering. Conclusion - We conclude that the maintenance of the floral variant within a broad wild-type population is driven by complementary mechanisms including high rates of self-fertilization and ecological differentiation. Taking into account that fitness is not reduced in Spe, the floral variant might in fact have the potential to be a promising model to study speciation at an early stage.