Life-history traits and ploidy levels in the genus Capsella (Brassicaceae)

In a random block field experiment, life-history traits associated with colonizing ability of diploid and tetraploid cytotypes of Capsella (Brassicaceae) were compared. These were germination, flowering, growth-form parameters, and leaf shape. Data are not in favour of differences in germination behaviour between the diploid and tetraploid Capsella species, as germination rate and capacity are highly influenced by inception and release of seed dormancy. Although our data at first glance seem to suggest that diploid C. rubella start flowering later than tetraploid C. bursa-pastoris, considerable ecotypic variation for flowering in both species makes it difficult to assign an effect specifically to ploidy level. We also conclude that plant height, rosette diameter, and branching number are not directly determined by ploidy level. In contrast however, leaf shape is clearly determined by ploidy level. In the light of all available data including data of previous experiments, we suggest that gene duplication by polyploidization may have been a key element that provided C. bursa-pastoris with additional genetic flexibility. It is not primarily the gain of colonization ability, as both species are weeds and colonizers. Rather, the greater genetic flexibility enabled C. bursa-pastoris to extend its range beyond that of C. rubella. Key words: ploidy level, germination, flowering, growth form, leaf morphology, Capsella.