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Increased selfing and decreased effective pollen donor number in peripheral relative to central populations in Picea sitchensis (Pinaceae)¹

Centre for Forest Gene Conservation and Department of Forest Sciences, University of British Columbia, 3041-2424 Main Mall, Vancouver, British Columbia, V6T 1Z4, Canada

ABSTRACT

Because mating system can be influenced by effective neighborhood size, density, and isolation, populations at range peripheries may differ from those in the center. The importance of peripheral populations to conservation and evolution is controversial, and additional information about their genetic structure and evolutionary dynamics will inform conservation strategies. In wind-pollinated species, selfing rate is generally negatively correlated with population size and density, and inbreeding may therefore increase toward range peripheries. Picea sitchensis has a long and narrow range along the Pacific Coast of North America that tapers toward the northern and southern peripheries. We investigated whether central and peripheral populations differ in mating system parameters. The results suggest that population position within the range has a strong effect on mating system, and geographic isolation appears to be associated with higher selfing. The estimated effective number of pollen donors was much higher in the center of the range (mean = 18.5) than at the periphery (mean = 3.6), while selfing rate increased from 7.3% in central populations to as high as 35.2% in the northern, isolated population. These strong geographical patterns suggest mating system is influenced by both population size and isolation at range peripheries.

Key Words: biparental inbreeding • isolated populations • mating system • Picea sitchensis • Pinaceae • range periphery • selfing • Sitka spruce