Scientists from the US Department of Agriculture believe they may have found a hidden population of American elm trees that carry genes that are resistant to Dutch elm disease. If so, the find could eventually lead to production of new elms that could once again populate American cities.
Dutch elm disease was introduced into the United States in the 1930's and it wiped out millions of trees that lined city streets and populated eastern forests. In recent years, elms with some disease resistance, including one called 'Jefferson', have been introduced, and they are very popular because they retain the tree's most important characteristics: a stately form, filtered shade, and the ability to withstand urban pollution.
For years, it's been thought that all American elms are tetraploids, trees with four copies of each chromosome. But now, botanist Alan Whittemore and geneticist Peter Olsen of the USDA's Agricultural Research Service have found some elms in the wild that are diploids, with two copies of each chromosome. The two tested elms from across the tree's central and eastern range and found that about 21 percent of the trees in the wild are diploid — some growing in stands along with the tetraploids. According to the scientists, the data suggests that the two types of elms have diverged from each other in a significant way, strengthening the possibility that the wild diploid elms have some resistance to Dutch elm disease.
The diploid elms were found in forests from Ohio east to Maryland and Virginia south to Tennessee and South Carolina, and also in Oklahoma, Texas and Missouri.
(photo: ARS)
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