title: Seed set variation in wildClarkiapopulations: teasing apart the effects of seasonal resource depletion, pollen quality, and pollen quantity.
abstract: In habitats where resource availability declines during the growing season,selection may favor early-flowering individuals. Under such ephemerally favorable conditions, late-blooming species (and individuals) may be particularly vulnerable to resource limitation of seed production. In California, a region prone to seasonal drought, members of the annual genusClarkiaare among the last to flower in the spring. We compared pollen limitation (PL) of seed set and outcrossing rates between early- and late-flowering individuals in two mixed-matingClarkiataxa to detect whether flowering time is associated with changes in seed set due to resource depletion, PL, or increased selfing. In 2008-2010, we hand-pollinated one flower on a total of 1855 individual plants either Early (near the onset of flowering) or Late (near the end of flowering) in the flowering season and compared seed set to adjacent, open-pollinated flowers on the same stem. To assess the contribution of pollen quality to reproduction, we first (2008) used allozymes to estimate outcrossing rates of seeds produced by Early and Late open-pollinated flowers. Second (2009), we conducted an anther-removal experiment to estimate self-pollen deposition. Seed set inClarkia unguiculatawas not pollen-limited.Clarkia xantianassp. xantianawas pollen-limited in 2008 and 2010, but not 2009. PL did not differ between Early and Late treatments. In both taxa, seed set of Early flowers was greater than Late flowers, but not due to PL in the latter. Reproduction was generally pollinator-dependent. Most pollen deposition was xenogamous, and outcrossing rates were >0.7 - and similar between Early and Late periods. These results suggest that pollen receipt and pollen quality remain seasonally consistent. By contrast, the resources necessary to provision seeds decline, reducing the fitness benefits associated with resource allocation to ovules.
authors: Biology Department Warren Wilson College PO Box 9000 Asheville NC 28815-6217.
authors: Alisa A
authors: AA
authors: Hove
authors: Ecology, Evolution, and Marine Biology Department University of California Santa Barbara CA 93106.
authors: Susan J
authors: SJ
authors: Mazer
authors: Department of Biological Sciences California State University Chico Chico CA 95929-0515.
authors: Christopher T
authors: CT
authors: Ivey
keywords: Adaptation; Clarkia unguiculata; Clarkia xantiana ssp. xantiana; outcrossing; pollen limitation; pollination; resource limitation; self‐fertilization