Evol Ecol Res 11: 787-801 (2009)     Full PDF if your library subscribes.

A comparison of pollen-siring ability and life history between males and hermaphrodites of subdioecious Silene acaulis

Marianne Philipp, Ruth Bruus Jakobsen and Gösta Nachman

Department of Biology, University of Copenhagen, Copenhagen, Denmark

Correspondence: M. Philipp, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark.
e-mail: marianp@bio.ku.dk


Background: Silene acaulis is an evergreen, very long-lived cushion plant. Populations of S. acaulis in Greenland are subdioecious, consisting of female, hermaphrodite, and male individuals. The sex expression of males and hermaphrodites can vary over years for the same individual, while females are always females. Previous studies have shown that outcrossed seeds from females become seedlings with higher survival and growth rates than those from outcrossed seeds from hermaphrodites.

Questions: (1) Do pollen grains from males exhibit some advantage over pollen from hermaphrodites? In particular, do they sire more seeds than hermaphrodites? (2) Is the reproductive system of S. acaulis stable or is it evolving towards one with fewer morphs (i.e. dioecy or gynodioecy)?

Hypothesis: Pollen from male plants is better at siring seeds on females than pollen from hermaphrodites.

Study system: A subdioecious population of S. acaulis in Greenland, containing male individuals that produce pollen and rarely or never set seed, hermaphrodites that produce both pollen and seeds, and females that produce only seeds.

Methods: A pollen-competition experiment was performed in which females were hand pollinated with a mixture of pollen from males and hermaphrodites, all with known isozyme alleles, which allowed determination of who sired each seed. We recorded plant size, flower morphology, fruit and seed set, as well as pollen per anther for the individuals used in the experiment, as well as for a large number of other individuals to allow us to make comparisons between the three types of individuals.

Conclusions: Well-developed pollen grains from males or hermaphrodites did not differ in their seed-siring capacity, although males had significantly more well-developed pollen grains per anther than hermaphrodites. Hence, on a per flower basis, males would be predicted to sire more seeds than hermaphrodites. However, given that males and hermaphrodites produce similar numbers of flowers per square centimetre of cushion and that male plants were significantly smaller than hermaphrodites, hermaphrodites have the potential to contribute relatively more to the seed pool than males. Calculations based on our quantitative results suggest that the dynamics of the population should develop slowly towards gynodioecy. However, this conclusion applies only as long as the population does not experience a significant change in the environmental conditions over time.

Keywords: Arctic, gynodioecy, pollen competition, Silene acaulis, subdioecy.

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