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 flow ers of all plants) the different pollen types could thus be due to the presence of reduced and unreduced gametes. But data from the fossil r ecord indicate that pollen aperture number has increased, suggesting t hat this trait may have evolved through selection. Our studies of Viol a diversifolia have shown that the different aperture morphs have diff erent selective values, also indicating that this character could have evolved and is not always due to ploidy level variations. To test thi s 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 measuremen ts). Our results show that aperture polymorphism is not due to gametop hytic ploidy variations within plants and that variations in the propo rtions of the different pollen morphs are not correlated with variatio ns of the amount of DNA in the sporophyte.