Compensating for the corolla? Pollen exposure is not associated with pollen collecting hair length.

BACKGROUND AND AIMS: Secondary pollen-presentation, the relocation of pollen from the anthers to elsewhere on the flower, has evolved multiple times across many plant families. While hypotheses suggest it evolved to promote outcrossing, a byproduct of relocation may be protection of pollen from loss due to abiotic factors. In Campanulaceae pollen is presented on pollen-collecting hairs along the style and the hairs retract over time and release pollen for transfer. Campanulaceae taxa vary in the degree to which pollen is exposed to environmental factors due to variation in the corolla shape and size. We tested the protective function of pollen-collecting hairs by assessing whether there was a tradeoff between the protection provided by the corolla and the pollen-collecting hairs. METHODS: We used phylogenetic comparative methods to test for associations between pollen-collecting hair length, floral shape and size metrics, and pollen exposure traits across 39 species. RESULTS: We anticipated longer pollen-collecting hairs in taxa with more exposed pollen presentation but found there was no relationship between estimates of pollen exposure and pollen-collecting hair length. However, pollen-collecting hair length scaled allometrically with floral size, and variation in pollen-collecting hairs, as well as most floral traits, was phylogenetically structured. CONCLUSIONS: These results indicate that variation in pollen exposure across species does not structure variation in the pollen-collecting hairs, rather hair length scales allometrically and is phylogenetically constrained, therefore pollen-collecting hairs are unlikely to facilitate protection from environmental pressures.

Phylogeography and paleoclimatic range dynamics explain variable outcomes to contact across a species' range.

Replicability of divergence after contact is a poorly characterized process, particularly in the contexts of phylogeography and postglacial range dynamics within species. Using contact zones located at the leading-, mid- and rear-edges of a species' range, we examined variation in outcomes to contact between divergent lineages of Campanula americana. We investigated whether contact zones vary in quantity and directionality of gene flow, how phylogeographic structure differs between contact zones, and how historic range dynamics may affect outcomes to contact. We found that all contact zones formed at similar times via primary contact yet detected significant admixture in only the rear-edge (RE) contact zone. In the northern leading-edge contact zone and the mid-range Virginia contact zone, gene flow was minimal and asymmetric. In the southern RE contact zone, gene flow was strong and symmetric. Asymmetric admixture in the leading-edge and Virginia contact zones matches the directionality of a known cosmopolitan cytonuclear incompatibility between lineages of C. americana. Our results emphasize the dependence of speciation processes on phylogeographic structure, evolutionary history and range dynamics.

Support for Baker's law: Facultative self-fertilization ability decreases pollen limitation in experimental colonization.

PREMISE: The ability to self-fertilize is predicted to provide an advantage in colonization because a single individual can reproduce and establish a next generation in a new location regardless of the density of mates. While there is theoretical and correlative support for this idea, the strength of mate limitation as a selective agent has not yet been delineated from other factors that can also select for self-fertilization in colonization of new habitats. We used known mating-system variation in the American bellflower (Campanula americana) to explore how plants' ability to self-fertilize can mitigate density-dependent reproduction and impact colonization success. METHODS: We created experimental populations of single individuals or a small number of plants to emulate isolated colonization events. These populations were composed of plants that differed in their ability to self-fertilize. We compared pollen limitation of the single individuals to that of small populations. RESULTS: Experimental populations of plants that readily self-fertilize produced consistent seed numbers regardless of population size, whereas plants with lower ability to self-fertilize had density-dependent reproduction with greater seed production in small populations than in populations composed of a single individual. CONCLUSIONS: We experimentally isolated the effect of mate limitation in colonization and found that it can select for increased self-fertilization. We show the benefit of self-fertilization in colonization, which helps to explain geographic patterns of self-fertilization and shows support for Baker's law, a long-held hypothesis in the field of mating-system evolution.