Background: Silene acaulis is an evergreen, very long-lived cushion plant. Populations of

is an evergreen, very long-lived cushion plant. Populations of

S. acaulis in Greenland are subdioecious, consisting of female, hermaphrodite, and male

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

(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

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

A subdioecious population of 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

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

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.