Physiological Performance in Clarkia Sister Taxa with Contrasting Mating Systems: Do Early-Flowering Autogamous Taxa Avoid Water Stress Relative to Their Pollinator-Dependent Counterparts?

Abstract

Consistent differences in the physiological performance of wild populations of closely related plant taxa may be the result of environmentally induced phenotypic plasticity or adaptive evolution (or a combination of the two). Here we report the results of a field study of physiological and fitness-related traits in geographically proximate sister taxa in the annual wildflower genus Clarkia (Onagraceae) and interpret the differences between them in light of their ecological and reproductive differences. Within two pairs of taxa, the predominantly autogamous (self-fertilizing) taxon flowers and completes its life cycle before its pollinator-dependent (predominantly outcrossing) counterpart growing in sympatry or at similar elevations in the southern Sierra Nevada. Selfers generally exhibited higher rates of photosynthesis and transpiration than their outcrossing sister taxa, and, except for the earliest-flowering (autogamous) taxon, both photosynthetic and transpiration rates tended to decline as the season progressed. Within taxa, high photosynthetic rates were positively correlated with lifetime fruit production, and selfers had lifetime fruit production equivalent to or higher than that of outcrossers, despite the fact that the latter had higher aboveground stem biomass. These patterns are consistent with the hypothesis that natural selection has favored higher gas exchange rates in selfers to allow them to achieve their faster life cycles and so escape seasonal late-spring drought. An alternative explanation is that the differences in gas exchange rates represent environmentally induced plastic responses to the cooler temperatures and higher soil moisture content in early spring. Further experimental work is necessary to distinguish between these hypotheses.