Sex ratio and reproductive success along elevational gradients of gynodioecious populations of <i>Geranium richardsonii</i>
Abstract
We studied the sex-ratio and reproductive success of gynodioecious populations of Geranium richardsonii along elevational gradients near Gothic, Colorado. Gynodioecious means that the species produces hermaphrodite and female plants. The hermaphrodites have male (anthers and pollen) and female (ovules and seeds) functioning parts, while females produce non- functional anthers. Genetic and ecological factors are known to influence sex ratio and reproductive success of gynodioecious species (Alonso, 2005). We hypothesized low female frequency at high elevations because wetter, cooler environmental conditions would favor the hermaphrodites, which have higher resource demands, but also have higher individual relative fitness compared to females. Also, we expected hermaphrodites to have larger pollen receipt compared to females due to self-pollination and allocation of resources to pollinator attraction (flower size), when pollinators are limiting. We measured the sex ratio of 31 populations and compared 4 of our populations with data collected in 1997. Reproductive success was assessed for 10 out the 31 populations spanning an elevation range from 9400-11500 feet, by counting stigma pollen loads on hermaphrodite and female flowers. There was a significant negative correlation between the proportion of females in populations and elevation. However, there was no significant relationship between elevation and hermaphrodite or female pollination success. Relative female pollination success declined somewhat as population sex ratio increased, as predicted, but this relationship was not significant in the populations studied. Therefore, pollination success does not appear to be the main reason driving the increase in hermaphrodite frequency in higher elevation populations. Sex ratio has not changed over the past 20 years in four populations studied near Gothic, suggesting that individuals may the long-lived, or that selection maintains constant sex ratios over time.
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