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Ecological Section

Scobell, Summer [1], Schultz, Stewart [1].

Testing Causal Models for the Evolution of Dioecy using GIS, SEM and Microsatellites: The birds and bees of plant sex.

There are two hotly debated hypotheses for why dioecy evolves: 1) The Inbreeding Avoidance Hypothesis: Dioecy evolves as a response to the deleterious effects of uniparental inbreeding. 2) The Resource Allocation Hypothesis: If resources (e.g. water) are limited, evolving dioecy allows more efficient allocation to separated reproductive systems.Echinocereus coccineus (Cactaceae) is an excellent model system to test these hypotheses because it possesses both hermaphroditic and dioecious populations growing over a wide precipitation gradient.E. coccineus flowers appear specialized to hummingbirds, but are also pollinated by Halictid bees. These pollinator types (i.e. birds, bees) have been shown to produce different rates of inbreeding when pollinating other species. Distribution and abundance of these pollinator types varies across the geographic range of E. coccineus. Dioecy appears to be associated with areas of low hummingbird abundance and lower precipitation. In order to investigate the relative influence of these factors, A Geographic Information System (GIS) database was compiled containing data on sex of flowers, annual rainfall, and hummingbird abundance. Structural equation modeling (SEM) was used to assess the relative direct and indirect influences of pollinator type and annual rainfall on the presence of dioecy.Models containing only precipitation were not significant, whereas models with hummingbird abundance as the main causal variable were. If the assumption that hummingbirds produce lower rates of inbreeding when pollinating is valid, this evidence supports the inbreeding avoidance hypothesis for the evolution of dioecy. This assumption was tested by developing 5 new microsatellite markers and analyzing progeny arrays produced through either hummingbird or bee pollination. By examining the interaction of pollinators, resources and E. coccineuspopulations at the plant, population, and landscape scale this will be the first study to concurrently test both the Inbreeding Avoidance and Resource Allocation hypotheses in a species undergoing incipient evolution of dioecy.


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1 - University of Miami, Biology, 252 D Cox Science Center, Coral Gables, Florida, 33124, USA

Keywords:
pollination evolution
dioecy
coevolution
GIS
Structural Equation Modeling
microsatellites
cacti
hummingbirds.

Presentation Type: Oral Paper
Session: 37-7
Location: Salon A - Gov Ballroom/Hilton
Date: Tuesday, August 16th, 2005
Time: 3:45 PM
Abstract ID:459


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