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  • Author or Editor: Ruth L. Dix x
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The historic Japanese flowering cherry trees planted around the Tidal Basin in Washington, D.C., were given to the United States in 1912 as a gift from Japan, and have become a popular tourist attraction. Unfortunately, only a small portion of the original trees remain, and these trees are in various states of decline due to old age and stress. In cooperation with the National Park Service, we have propagated from cuttings nine trees that are known to be original and 10 trees that are thought to be original. DNA from these and other P. × yedoensis were compared using RAPD markers. Twenty-one 10-nucleotide primers yielded 80 repeatable bands that were used to assess genetic distance among the accessions. Twenty of these bands were monomorphic across all 28 accessions tested, so were not informative. The frequency of the remaining 60 bands varied from 0.04 to 0.96, with an average frequency of 0.58. Thirteen of the accessions, including six of the nine that are known to be original germplasm, were identical at all loci tested. Other accessions that are thought to be original trees were similar, with similarity values of 0.93 to 0.99. The most genetically dissimilar trees were P. × yedoensis accessions from our collection that were collected as seed in Japan. Accessions obtained from commercial nurseries including `Afterglow', `Akebono” and Yoshino were also dissimilar to the Tidal Basin trees. This study indicates that most of the older trees planted around the Tidal Basin are genetically very similar, but that variability in P. × yedoensis exists, especially in accessions collected as seed from Japan.

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Interspecific hybridizations among members of the genus Hamamelis (the witchhazels) and Corylopsis were carried out in 1993, 1994, 1995, and 1996 at the U.S. National Arboretum. Specifically, crosses involving the native witchhazel (H. vernalis and H. virginiana) and the Asian taxa (H. mollis, H. japonica, and H. × intermedia) were attempted in order to combine the ornamental qualities of the Asian species with the adaptability and fall blooming characteristics of the native species. Additionally, C. platypetala, a hardy species with small inflorescences, was crossed with C. himalaica, which has large showy inflorescences but is less hardy. Approximately 50 seedlings resulting from these crosses have been analyzed using randomly amplified polymorphic DNA (RAPD) markers to verify interspecific hybridization. Based on these assays, we report the first incidence of controlled interspecific hybridization between the Asian and native witchhazel taxa.

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The issue of invasive plants has become a concern to a variety of groups, including environmentalists, policymakers, and nurserymen. Although many surveys of invasive plants have been made, little research on the biology of hybridization has been conducted. Bittersweet (Celastrus) species serve as a good model system to test the effects of interspecific hybridizations since native and introduced species are found in the U.S. The American bittersweet (Celastrus scandens L.) is a deciduous climbing or twining shrub native to eastern and central North America. Although the bark has been used for medicinal purposes, the plant is cultivated as a nursery crop primarily for its bright red berries. In its natural habitat, native bittersweet is also an important source of food and cover for wildlife. Over the past several decades, populations of native bittersweet have declined to such low levels that some states are considering listing it as a threatened species. One reason for the rarity of American bittersweet in the wild is thought to be competition and possibly hybridization with an aggressive introduced species, oriental bittersweet (Celastrus orbiculatus Thunb.), which was introduced from Asia into the U.S. in 1860 as an ornamental. This plant can form dense, tangled, impenetrable thickets or climb small trees to girdle and smother them. It has been seen in at least 21 states since it was first recorded as an escape plant in 1912. Our objective was to determine whether oriental bittersweet can hybridize with native bittersweet, thus contributing to the loss of native populations in the United States. We performed controlled pollinations using C. scandens as the female parent and C. scandens or C. orbiculatus as the male parent. Although the intraspecific pollinations resulted in significantly more germinating seedlings than the interspecific crosses, the seedlings from the interspecific crosses had less seed dormancy and were more vigorous and more quick to vine than the intraspecific seedlings. These results indicate that the decline of the American bittersweet may be due to interspecific hybridizations with the invasive introduced species.

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