de Vos, Jurriaan M. , Keller, Barbara , Isham, Samuel T. , Kelso, Sylvia , Conti, Elena .
Do homostylous species outcross? Variation of anther-stigma separation and mating system in Primula halleri.
Angiosperms have evolved many spectacular adaptations to outcrossing. Distyly, characterized by reciprocal herkogamy and self- and intra-morph incompatibility, has received considerable attention since Darwin, who interpreted it as a dimorphic breeding system that enhances outcrossing. While shifts from outcrossing to selfing have occurred repeatedly, selfing might represent an evolutionary dead end. In Primula, homostylous species are monomorphic and self-compatible and evolved from distyly multiple times. The actual amount of selfing in such species is a matter of longstanding controversy evident in the literature, but never tested experimentally. We analyze variation in anther-stigma separation (herkogamy) and its effect on the mating system in Primula halleri, a homostylous Alpine species. Herkogamy varies at different levels: during development of individual flowers, between individuals within populations, and between populations. Flowers have 1-4 mm more herkogamy at the beginning than at the end of anthesis. Nevertheless, just before wilting, some individuals still have up to 3.5 mm herkogamy, whereas in others the stigma touches the anthers. These inter-individual differences enable the assignment of plants to herkogamy classes. Between populations, the frequencies of these classes differ. With pollinator exclusion and emasculation experiments, we measured how herkogamy influences the speciesí potential for selfing and outcrossing. We found that seed set after emasculation remains considerable, regardless of herkogamy, indicating the speciesí potential for outcrossing. A very strong negative effect of herkogamy on seed set was detected when pollinators were excluded by caging plants. Our results reveal the complexity of reproductive strategies in P. halleri. Minute differences in anther-stigma separation in a homostyle have a large effect on mating, causing some plants to be mostly selfing, some to be mostly outcrossing, and some to have a mixed mating system. This leads to the conclusion that homostylous species are not necessarily selfing, as commonly assumed.
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1 - University of Zurich, Institute of Systematic Botany, Zollikerstrasse 107, Zurich, CH-8008, Switzerland
2 - Colorado College, Department of Biology, 14 E. Cache la Poudre, Colorado Springs, CO, 80903, USA
3 - Colorado College, Biology Department, 14 E. Cache La Poudre, Colorado Springs, Colorado, 80903, USA
Presentation Type: Oral Paper:Papers for Topics
Location: 551B/Convention Center
Date: Wednesday, August 4th, 2010
Time: 8:45 AM