An Update on the Progress of the TACF Breeding Program

By Dr. Fred Hebard, TACF Chief Scientist

TACF's Wagner Farm at Meadowview Research Farms.

A major activity at Meadowview Research Farms this past fall was culling the seed orchards that produce our Restoration Chestnuts 1.0. For the first time since 2010, we are caught up on our culling. Since 2002, we have planted 62,343 seeds in our two seed orchards. As of December 20, 2013, only 8,900 trees remained, down from 25,361 in May of 2012 and a peak of 38,358 in May of 2009. The blight resistance of the Restoration Chestnuts 1.0 trees produced by these seed orchards should increase above current levels as we continue to cull to our ultimate target of 540 selections.

The current level of blight resistance in the Restoration Chestnut 1.0 trees is similar, on average, to that of first hybrids between American and Chinese chestnut. On the family level, average resistance ranges from similar to that of American chestnut to intermediate between first hybrids and pure Chinese. On an individual tree basis, the percentage of Restoration Chestnut 1.0 trees as resistant as Chinese chestnut varies from 10 to 30%, depending on the test year. Thus our trees have enough resistance to survive blight for extended times.

Dr. Stacy Clark of the US Forest Service has been testing the forest performance of our Restoration Chestnut 1.0 trees since 2009. Currently, quite a few are the fastest growing trees on site, still exceeding the size of competing yellow poplar seedlings. Most families of Restoration Chestnut 1.0 trees are similar to American chestnut controls in appearance and growth. Thus it appears we have recovered the American type during backcrossing.

In addition to blight resistance and American type, a third component needed to restore the American chestnut is genetic diversity. Computer simulations indicate that progeny derived from the two seed orchards at Meadowview will have an inbreeding effective population size of about 70. Our current state chapter efforts will increase the effective size to about 250. If each chapter can add one new source of blight resistance, the effective population size will be greater than 500.

Nearly 20 feet tall, this Restoration Chestnut 1.0 was planted on National Forest land in TN, in 2009.
 

To summarize, TACF already has trees that are similar enough to American chestnut to grow well in our forests, have sufficient blight resistance to survive for extended times, and have adequate genetic diversity to survive short term (a few centuries). Our state chapter seed orchards should add enough genetic diversity to allow the species to persist indefinitely. Likewise, we expect the blight resistance of our Restoration Chestnut 1.0 trees will improve beyond the level present today.

There is still a lot of hard work ahead for TACF to reach this goal. Our chapters are only starting to plant most of their seed orchards. Selecting trees highly homozygous for blight resistance genes using progeny tests will be a large chore. At Meadowview, we are trying to lessen the magnitude of that chore by identifying more precisely the location of genes for blight resistance and developing DNA-based genetic markers for those locations. The markers would be used to identify trees homozygous for blight resistance. We see no technical reason these makers cannot be developed, but, until they are, we need to continue using progeny tests to identify homozygotes. Progeny tests also will be used to verify results obtained with genetic markers.

An important part of the overall TACF breeding plan is using 20 different sources of blight resistance. These sources will include Chinese chestnut as well as additional species. The main purpose of using 20 sources of blight resistance is to counter the fungus with a diverse collection of resistance genes rather than a limited number, since a limited number of resistance genes could be overcome by a limited number of changes in the blight fungus. Using 20 different sources of blight resistance also will minimize inbreeding in the genes from the source linked to resistance. Additionally, it will increase the genetic diversity of our breeding population for American chestnut. Finally, it will be possible to pyramid (stack) resistance genes by growing Restoration Chestnut 1.0 trees from different sources together in one location and arranging for them to reproduce. This may lead to higher levels of blight resistance than we can achieve with a single source, should that higher resistance be desirable.

 
Chestnut root infected by Phytophthora cinnamomi.
 

Due to Phytophthora root rot (PRR), our blight-resistant, genetically diverse, American-type chestnut trees may be unable to grow in much of the piedmont of the southeastern US. To address this problem, Joe James and Steve Jeffers have been systematically evaluating the PRR resistance of TACF’s breeding lines since 2004. Their results indicate that a single PRR resistance gene in the ‘Graves’ source of blight resistance persists in about one-fourth of the trees in our ‘Graves’ seed orchard. That single gene, or, more properly, locus, will spread into all our ‘Graves’ family lines in the Restoration Chestnut 1.0 generation. It could be fixed in all those family lines with one additional cycle of crossing. The number of individuals needed after selection for the Restoration Chestnut 1.0 and subsequent generation is about 800, and those individuals will occur at frequencies of about one-fourth. So, this is a very tractable process that can be accomplished in less than 10-15 years. Already, one-fourth of our ‘Graves’ Restoration Chestnut 1.0 trees should be able to grow in the southern piedmont as well as the mountains. After fixation, all our ‘Graves’ trees and their progeny will thrive throughout the native range (unless Phytohthora cinnamomi and Cryphonectria parastica can reach down into their genetic bag of tricks to overcome our resistances).

Overall, we are quite far along the path to restoring the American chestnut. We have much work still to do, but our progress is very encouraging, and newer tools under development should smooth the path. Regardless of how smooth the path, restoring the American chestnut will require hard, sustained effort from the entire foundation; but we can do it!