Pollen aperture heteromorphism is not due to unreduced gametophytes

Variations in ploidy level of the sporophyte are usually acknowledged as the main cause of polymorphism in pollen aperture number and pollen size. In heteromorphic species (two or more pollen morphs in all flowers of all plants) the different pollen types could thus be due to the presence of reduced and unreduced gametes. But data from the fossil record indicate that pollen aperture number has increased, suggesting that this trait may have evolved through selection. Our studies of Viola diversifolia have shown that the different aperture morphs have different selective values, also indicating that this character could have evolved and is not always due to ploidy level variations. To test this hypothesis, we measured DNA content of the reproductive nucleus as a test of gametophytic reduction in four heteromorphic species of Viola and in diploid and tetraploid individuals of a Petunia hybrida clone (which also served as a control to test for accuracy of the measurements). Our results show that aperture polymorphism is not due to gametophytic ploidy variations within plants and that variations in the proportions of the different pollen morphs are not correlated with variations of the amount of DNA in the sporophyte.