Increased seed dispersal potential towards ...

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Title Increased seed dispersal potential towards geographic range limits in a Pacific coast dune plant
Author(s) Emily Darling, Karen E. Samis, Christopher G. Eckert
Journal New Phytology
Date 2008
Volume 6
Issue 12
Start page 424
End page 435
Abstract • Dispersal may be favoured at geographic range edges by unstable population
and metapopulation dynamics. However, dispersal may also evolve in response to
geographic variation in other life-history traits, especially the mating system. Here,
increased dispersal at range margins was tested for with a range-wide analysis of
seed dispersal and mating system traits in Abronia umbellata, a plant endemic to
Pacific coastal dunes of North America.
• Seeds disperse within winged anthocarps. Anthocarps from 34 populations varied
widely in wing size (mass-corrected wing index). Wing index correlated negatively
with threshold wind velocity for dispersal in wind tunnel tests, suggesting that wings
facilitate tumbling over open sandy substrate. As predicted, wing index increased
and threshold velocity decreased towards both range limits.
• Flower size, herkogamy and self-incompatibility declined towards range limits,
indicating a shift to self-fertilization, and flower size and wing index correlated
negatively. However, the increase in wing index towards range limits remained after
statistically controlling flower size.
• These results are consistent with selection favouring dispersal at range margins.
The evolutionary lability of dispersal across the range may affect the interaction
between selection and gene flow in the establishment and maintenance of
geographic range limits.

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