Censusing natural microgametophyte populations: variable spatial mosaics and extreme fine-graininess in winter-flowering Helleborus foetidus (Ranunculaceae)

Little is known about patterns and correlates of variation in microgametophyte populations for naturally pollinated plants, yet this information is critical for evaluating the prevalence and potential evolutionary significance of gametophyte competition in the wild. This paper analyzes spatial and temporal variation in microgametophyte populations (= number of pollen tubes per style) for the winter-flowering, perennial herb Helleborus foetidus (Ranunculaceae), based on data from 29 populations in three regions of the Iberian Peninsula collected over two consecutive years. Mean size of microgametophyte populations varied significantly at a wide range of spatial scales, including among regions, among populations within regions, among individual plants within populations, among flowers of the same plant, and among pistils of the same flower (H. foetidus flowers are apocarpous). Differences between regions were quantitatively negligible. Differences between populations in the same region were moderate to low, and their sign and magnitude were inconsistent between years. Roughly half of total variance in microgametophyte numbers was accounted for by variation within individual plants, and the largest part of this component was due to differences between the pistils of the same flower. These results reveal extreme spatial fine-graininess and marked stochasticity in the spatial variation of H. foetidus microgametophyte populations and suggest that opportunities for consistent selection on male gametophyte competitive ability are probably negligible in this species.