POLLEN HETEROMORPHISM IN NICOTIANA-TABACUM (SOLANACEAE)

Pollen heteromorphism is defined as the production by a single plant o f different fertile pollen types in all its anthers, and thus all flow ers, throughout its life cycle. Eight cultivars of Nicotiana tabacum, as well as its ancestors (N. tabacum is an amphiploid hybrid 4x from a cross between N. sylvestris and N. tomentosiformis) and recent hybrid s were analyzed. Most cultivars and the hybrids are heteromorphic (pro ducing 3- and 4-aperturate pollen grains), whereas both parent species are homomorphic (3-aperturate). Heteromorphism is a common consequenc e of polyploidization and these results agree with this interpretation . There is a significant variation in the proportions of the two polle n types among cultivars (genetic component), but also (with a much low er component of variance) within each cultivar, between plants (genets ), flowers of a plant, and even anthers of a flower. This is interpret ed 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 po llen competition, which is the drive for heteromorphism, and allows fo ra large variation of the proportions of the different pollen types.