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Increased inbreeding but not homozygosity in small populations of Sabatia angularis (Gentianaceae)

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Abstract

Understanding how the mating system varies with population size in plant populations is critical for understanding their genetic and demographic fates. We examined how the mating system, characterized by outcrossing rate, biparental inbreeding rate, and inbreeding coefficient, and genetic diversity varied with population size in natural populations of the biennial Sabatia angularis. We found a significant, positive relationship between outcrossing and population size. Selfing was as high as 40% in one small population but was only 7% in the largest population. Despite this pattern, observed heterozygosity did not vary with population size, and we suggest that selection against inbred individuals maintains observed heterozygosity in small populations. Consistent with this hypothesis, we found a trend of lower inbreeding coefficients in the maternal than progeny generation in all of the populations, and half of the populations exhibited significant excesses of adult heterozygosity. Moreover, genetic diversity was not related to population size and was similar across all populations examined. Our results suggest that the consequences of increased selfing for population fitness in S. angularis, a species that experiences significant inbreeding depression, will depend on the relative magnitude and consistency of inbreeding depression and the demographic cost of selection for outcrossed progeny in small populations.

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Acknowledgments

Funding for this study was awarded to R.B.S. through grants from Adkins Arboretum, Georgia Native Plant Society, North Carolina Native Plant Society, and the Department of Plant Biology at the University of Georgia. R.B.S. was supported in part by a National Science Foundation Graduate Research Fellowship. The authors thank M. Boyd, C. Deen, A. Tull, and B. Waitman for lab, field, and greenhouse assistance and Coweeta Hydrological Station and the Wykle and Spoon families for permission to work on their properties. We thank L. Comita for writing the R programs and C. Heckel for help creating the map figure. S. Hubbell, J. Leebens-Mack, R. Mauricio, D. Waller and an anonymous reviewer provided helpful comments on earlier versions of this manuscript.

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Authors and Affiliations

  1. Department of Plant Biology, University of Georgia, 2502 Miller Plant Sciences, Athens, GA, 30602, USA

    Rachel B. Spigler, J. L. Hamrick & Shu-Mei Chang

  2. Department of Biological Sciences, University of Pittsburgh, 4249 Fifth Avenue, Pittsburgh, PA, 15260, USA

    Rachel B. Spigler

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  1. Rachel B. Spigler
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  2. J. L. Hamrick
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Correspondence to Rachel B. Spigler.

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Spigler, R.B., Hamrick, J.L. & Chang, SM. Increased inbreeding but not homozygosity in small populations of Sabatia angularis (Gentianaceae). Plant Syst Evol 284, 131–140 (2010). https://doi.org/10.1007/s00606-009-0245-x

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