Variation in resource limitation of plant reproduction influences natural selection on floral traits of Asclepias syriaca
The availability of both pollen and resources can influence natural selection on floral traits, but their relative importance in shaping floral evolution is unclear. We experimentally manipulated pollinator and resource (fertilizer and water) availability in the perennial wildflower Asclepias syriaca L. Nine floral traits, one male fitness component (number of pollinia removed), and two female fitness components (number of pollinia inserted and number of fruits initiated) were measured for plants in each of three treatments (unmanipulated control, decreased pollinator access, and resource supplementation). Although decreasing pollinators’ access to flowers did result in fewer pollinia inserted and removed, fruit set and phenotypic selection on floral traits via female and male fitness did not differ from the control. In contrast, resource supplementation increased fruit set, and phenotypic selection on seven out of nine floral traits was stronger via female than male fitness, consistent with the prediction that selection via female fitness would be greater when reproduction was less resource-limited. Our results support the hypothesis that abiotic resource availability can influence floral evolution by altering gender-specific selection.
KeywordsFemale fitness Male fitness Phenotypic selection Pollen limitation
We thank B. Casper, H. Maherali, A. Parachnowitsch, and three anonymous reviewers for their comments on this manuscript. D. Black (Jasper County Conservation Board) provided permission to work on public land and J. Brown provided access to a digital imaging system. I. Smith provided the drawing for Fig. 1. This work was supported by a National Science Foundation AIRE grant (awarded to Grinnell College) and the Grinnell College Committee in Support of Faculty Scholarship. During the writing of this manuscript, C. M. Caruso was supported by an operating grant from the Natural Science and Engineering Research Council of Canada.
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