Recovery of American chestnut characteristics following hybridization and backcross breeding to restore blight-ravaged Castanea dentata

Morphological features of leaves and twigs of American chestnut, Chinese chestnut, their F₁ hybrid, and three successive generations of backcrosses between hybrid populations and American chestnut were examined to determine rate of recovery of the American chestnut morphology after hybridization to capture Chinese chestnut genes for blight resistance. In aggregate morphology, as measured by a composite index of species identity (ISI), 96% of trees in the third backcross generation (BC₃) resembled American chestnut and were distinctly different from Chinese chestnut. The majority of BC₃ trees also differed from Chinese chestnut in every individual characteristic measured for this study. Thus, recovery of American chestnut characteristics is largely achieved after three generations of backcrossing. If progeny of the BC₃ hybrids can be made homozygous for blight-resistance alleles, as expected, and if the trees equally resemble American chestnut in important ecological attributes, then backcross breeding appears to be a workable strategy for restoring this species as a important component of eastern U.S. forests.