Résumé
Pollen heteromorphism is defined as the production by a single plant of different fertile pollen types in all its anthers, and thus all flowers, throughout its life cycle. Eight cultivars of Nicotiana tabacum, as well as its ancestors (N. tabacum is an amphiploid hybrid 4 × from a cross between N. sylvestris and N. tomentosiformis) and recent hybrids were analyzed. Most cultivars and the hybrids are heteromorphic (producing 3‐ and 4‐aperturate pollen grains), whereas both parent species are homomorphic (3‐aperturate). Heteromorphism is a common consequence of polyploidization and these results agree with this interpretation. There is a significant variation in the proportions of the two pollen types among cultivars (genetic component), but also (with a much lower component of variance) within each cultivar, between plants (genets), flowers of a plant, and even anthers of a flower. This is interpreted as a release of the selective pressure: the cultivars of N. tabacum were obtained after several generations of selfing and are themselves selfers. Selfing, by removing pollen mixtures on a stigma, removes pollen competition, which is the drive for heteromorphism, and allows for a large variation of the proportions of the different pollen types.