Polyploidy in Odontarrhena bertolonii (Brassicaceae) in relation to seed germination performance and plant phenotype, with taxonomic implications

Odontarrhena bertolonii is an endemic serpentine species of Tuscany and Liguria and a model system for nickel-hyperaccumulation research in plants. Phenotypic, genetic and karyological variability between populations was previously detected, but existence and distribution of cytotypes was unknown. Accordingly, we examined ploidy level in eight populations from the species range and tested the polyploid fitness hypothesis by analysing relationships with plant phenotype and seed germination capacity. Accessions from central-western Tuscany and Liguria resulted diploid, while those from the upper Tiber valley were tetraploid. Autopolyploidy via unreduced gametes is likely the mechanism for the origin of tetraploid plants. Size and mass were higher in tetraploid seeds, which germinated faster and to higher percentages than diploid ones. Tetraploid plants were more robust and taller, with more richly branched inflorescences. Silicle shape and size did not differ, but style was longer in tetraploids and seeds had a larger wing. Overall, these results supported a higher fitness in polyploids. Multivariate analysis showed continuous variation but consistent differentiation between diploid and tetraploid accessions. Taxonomically, the tetraploid populations at the eastern limit of the species range in the upper Tiber valley can be referred to a new subspecies, here described as O. bertolonii subsp. cesalpina.