Ribosomal DNA organization and composition in American and Chinese chestnuts
The American chestnut (Castanea dentata, 2n = 2x = 24), once a foundation forest species over 800,000 km2 in eastern North America, was decimated by chestnut blight caused by an introduced fungal pathogen, Cryphonectria parasitica. The devastating disease was first reported in 1904 by Hermann Merkel, a forester at the New York Zoological Park (Murril 1906). The disease spread rapidly, covering the entire species range by the early 1950s and killing nearly 4 billion trees. (Hepting 1974). Chinese chestnut (Castanea mollissima), a species closely related to American chestnut, is relatively resistant to the blight pathogen. Efforts are underway to transfer resistance from Chinese chestnut to American chestnut, including a backcross breeding program operated by the American Chestnut Foundation (Hebard 2006; www.acf.org) and a biotechnology-based program sponsored by the Forest Health Initiative (Nelson et al. 2014; www.foresthealthinitiative.org). Recently an integrated genetic/physical map of Chinese chestnut was published (Kubisiak et al. 2013) and the species genome has been sequenced (Staton et al. 2020); however, little cytogenetic data are available to confirm and complement these genomic resources. Fluorescence in situ hybridization (FISH) is an important cytogenetic technique for assigning and orienting genetic markers to specific chromosomes. In this study we assign the major 35S rDNA to LG_H and compare this linkage group chromosome between American and Chinese chestnuts.