Evol Ecol Res 1: 97-109 (1999)     Full PDF if your library subscribes.

The distribution of pollen heteromorphism in Viola: Ecological and morphological correlates

Isabelle Dajoz

Laboratoire d’Ecologie, CNRS UMR 7625, Bâtiment A, 7ème étage, case 237, 7 Quai St. Bernard, Université Pierre et Marie Curie, F-75252 Paris Cedex 05, France

e-mail: Isabelle.Dajoz@snv.jussieu.fr


Pollen grains have apertures that affect their life histories. Pollen heteromorphism, defined as the production by flowers of several pollen morphs that differ by their aperture number, is frequent in modern angiosperms. In species of Viola, it has been demonstrated experimentally that more apertures lead to faster germination, but to shorter duration of viability. The number of apertures is under genetic control and many species of Viola are heteromorphic for aperture number. In this study, the occurrence of pollen heteromorphism was measured in 28 species of Viola that belong to the European flora. The genus Viola is divided into two groups, violets and pansies, which differ in corolla morphology. Pollen heteromorphism was detected in more than 85% of the pansies, but in only 40% of the violets. In contrast to violets, pollen heteromorphism in pansies was not due to variation in the ploidy level of the sporophyte. In pansies, mean aperture number decreased with the elevation at which the plants were collected. This suggests that pollinator-mediated selection due to variation in pollinator fauna along the altitudinal gradient, in conjunction with other physical characteristics of the environment (temperature, potential for pollen dehydration), may affect the proportions of the different pollen types. In violets, mean aperture number increased with elevation, due to polyploid species, which exhibit pollen heteromorphism, being more abundant at higher elevation than diploid species, which are not pollen-heteromorphic. Finally, significant differences in corolla size and spur length were observed among species of pansies and, to a lesser extent, among species of violets. These differences may in part be due to differences in the pollinating fauna among the different species.

Keywords: corolla size, heteromorphism, ploidy level, pollen, pollination biology, Viola.

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