Scobell, Summer , Schultz, Stuart , Soltis, Pamela S. .
Quantifying the birds and the bees of plant sex: Pollinator exclusion experiments and FST values reveal how a change in pollinators can promote the evolution of dioecy.
A common, yet untested hypothesis in the dioecy literature is that a change in pollinators – from specialists to generalists – could promote the evolution of separate sexes in zoophilous plants. The proposed mechanism is that the pollinating behavior of generalists causes large increases in selfing rates (s) of plants. If plants also harbor high levels of deleterious recessive mutations, expression of inbreeding depression (δ) in selfed offspring will then select for single-sex mutants if loss of hermaphrodite fitness (s * δ) < 0.5. Microsatellite analyses were conducted to test this hypothesis. Forty hermaphroditic plants of Echinocereus coccineus (Cactaceae) were caged in one population in the Chiricahua Mountains, AZ. The cage covered half of each plant, creating a Caged treatment (only bee pollination), and an Open treatment (hummingbird and bee pollination). In absence of specialized pollinators (Caged treatment), selfing rates increased 238%. Estimates of early-acting inbreeding depression (fruit set, seed set and germination) were indistinguishable from 1. Because bee pollination creates the condition of (s * δ) < 0.5 in this hermaphroditic population, it is prone to invasion of unisexual mutants. If these conditions persist the population could progress to full dioecy. This hypothesis appears to be supported by the unique distribution of mating systems in E. coccineus: hermaphroditic populations line the center of the plant’s range which overlaps the Rocky Mountain hummingbird migration corridor, dioecious populations occur in lower-elevation areas east and west of the Rockies where hummingbirds are rare or absent. To investigate if hummingbirds were promoting gene flow among, as well as within populations, pairwise FST values were calculated among seven populations throughout the range of E. coccineus. Populations blooming in synchrony with each other and the hummingbird migration had the lowest FST values; asynchronously blooming populations had the highest FST values, regardless of geographic distance
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1 - University of Miami, Biology, 252 D Cox Science Center, Coral Gables, Florida, 33124, USA
2 - University of Zadar, Department of Maritime Studies, Ulica M. Pavlinovica bb, Zadar, 23000, Croatia
3 - University of Florida, Department of Botany, Florida Museum of Natural History, P.O. Box 117800, Gainesville, Florida, 32611-7800, USA
Presentation Type: Oral Paper:Papers for Topics
Date: Wednesday, July 30th, 2008
Time: 4:00 PM