A plan to diversify a transgenic blight-tolerant American chestnut population using citizen science

Abstract

Over four billion American chestnut trees have been killed as a result of an introduced pathogen, the chestnut blight fungus. Recently, transgenic blight-tolerant American chestnut trees have been produced by inserting a gene from wheat into the American chestnut genome. Pending federal approval to use these transgenic trees for large-scale forest restoration, this would be the first instance where a transgenic approach has been used to restore a tree species that has been rendered functionally extinct by an introduced pathogen. With the help of citizen scientists, we estimate that large-scale forest restoration using blight-tolerant American chestnut trees is possible within the next few decades. Summary Breeding transgenic blight-tolerant American chestnuts with susceptible wild-type (WT) trees is potentially an efficient method to rescue the genetic diversity and adaptive capacity of the American chestnut population for large-scale restoration. To develop a breeding plan to diversify a transgenic blight-tolerant population, we simulated pedigrees to estimate inbreeding coefficients and effective population size in scenarios involving outcrossing 1–4 transgenic founders to a maximum of 1,500 WT trees over 1–5 generations. We also simulated marker-assisted introgression scenarios to minimize the extent of the transgenic founder genome, especially on the transgene carrier chromosome. Simulations suggest that the effective population size may be increased to >500, and the average inbreeding coefficient reduced to <0.01, by outcrossing a single transgenic founder over five generations to 2, 25, 50, 150, and 450 (677 total) WT parents. Three generations of marker-assisted introgression is predicted to decrease the length of the founder genome to between 7% and 13% of the transgene carrier chromosome length as compared to 42% with event selection (ES) only. Transgenic outcross selections may be intercrossed to select progeny homozygous for the transgene for planting in seed orchards. A diversified population of transgenic blight-tolerant American chestnut is estimated to be available for use in large-scale forest restoration 20–35 years after federal approval to distribute the trees. In contrast, trees from earlier generations would be available almost immediately after federal approval for personal or horticultural plantings. Methods to accelerate pollen production and outcrossing are discussed.