The effect of hermaphrodite density and frequency at three spatial scales on the polen receipt and seed set of gynodioecious Geranium richardsonii (Geraniaceae)

Abstract

Gynodioecy is a mode of sex expression where male sterlie (hereafter female) plants and hermaphroditic plants coexist within a population. In order for the females to be maintained by natural selection, they must have experienced an increased fitness. Fitness is defined as the ability of genotype or phenotype to contribute genes to the pool of seeds for the next generation. Since females only transmit their genes through seed production, their inability to produce pollen has to be compensated for. Experimental evidence from a variety of gynodioecious species have not found the theoretically required fitness advantage for females. This is in part may be due to interactions of the plant with its local frequency and density and not the global population characteristics. Females in particular may become pollen limited as the density and frequency of hermaphrodites declines. I present the results of a study that examined the effect of hermaphrodite density and frequency at three spatial scales on pollen receipt of Geranium richardsonii in four populations in Gothic, CO. In the lower density, patchy populations the smaller spatial scales seem to be more important to female pollination success . As the density of the populations increased the total hermaphrodite density and female frequency become able to explain more of the variation in female pollen receipt. The small spatial scales did not seem to determine the hermaphrodites pollination success as a large portion of the variation could be explained by the total hermaphrodite density.

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