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Mating system and inbreeding depression in whitebark pine (Pinus albicaulis Engelm.)

Abstract

Mating system and inbreeding depression in quantitative traits of whitebark pine (Pinus albicaulis Engelm.) was determined using isozymes and a seedling common garden experiment. Simultaneous isozyme analysis of embryo and haploid megagametophyes from progeny arrays of families in three distinct geographic regions (Oregon, Montana, and southern British Columbia) was used to estimate parental and progeny inbreeding coefficients, as well as regional and family mean multilocus outcrossing rates (t m). Quantitative trait family means of seedlings from the same families growing in two temperature treatments in a common garden experiment were regressed on the estimated inbreeding coefficient to determine the presence and magnitude of inbreeding depression. Regional estimates of t m ranged from 0.73 to 0.93, with a mean over all regions of 0.86. Family mean t m values indicated predominant outcrossing; however, some individuals experienced substantial inbreeding. The Oregon region had a significant excess of heterozygotes in the parental generation relative to Hardy–Weinberg equilibrium, while both the Oregon and southern BC regions had a heterozygote deficiency in progeny, suggesting selection against inbred individuals. Biomass in the ambient temperature treatment for the southern BC region was the only trait significantly related to inbreeding coefficient. The mean inbreeding coefficient for this region was 0.25, and based on this relationship, mean predicted biomass would be reduced by 19.6% in this region if inbred individuals are not removed by selection. The estimated outcrossing rate of whitebark pine is slightly lower than most wind-pollinated conifers, and while most individuals are highly outcrossing, some experience substantial inbreeding.

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Acknowledgments

We thank the USDA Forest Service regions 1, 5, and 6, and E. C. Manning Provincial Park of British Columbia, for providing seed for this study. Dr. Carol Ritland provided assistance in the isozyme lab, and Dr. Kermit Ritland provided assistance with MLTR. Many other people helped in various aspects of this project, including: Joanne Tuytel, Christine Chourmouzis, Dorothy Watson, Megan Harrison, Dane Szohner, Jodie Krakowski, and all of the members of the Aitken laboratory group at UBC. Isozyme data for the southern BC region were provided by Dr. Yousry El-Kassaby. Funding for this study came from the British Columbia Forestry Investment Account through the Forest Genetics Council of BC to the Centre for Forest Gene Conservation at UBC. Thank you to Drs. Jeannette Whitton, Mike Whitlock, and Alvin Yanchuk, Yousry El-Kassaby, Sean Graham, Diana Tomback, Associate Editor Outi Savolainen and an anonymous reviewer for their helpful comments on an earlier draft of this manuscript.

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Correspondence to Andrew D. Bower.

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Communicated by O. Savolainen

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Bower, A.D., Aitken, S.N. Mating system and inbreeding depression in whitebark pine (Pinus albicaulis Engelm.). Tree Genetics & Genomes 3, 379–388 (2007). https://doi.org/10.1007/s11295-007-0082-4

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Keywords

  • Pinus albicaulis
  • Mating system
  • Inbreeding depression