Two U.S.
Department of Agriculture (USDA) scientists
may have discovered “the map to El Dorado” for the American elm – a
previously hidden population of elms that carry genes for resistance to Dutch
elm disease. The disease kills individual branches and eventually the entire
tree within one to several years.
It has been
accepted for 80 years that American elms (Ulmus
americana)
are tetraploids, trees with four copies of each chromosome. But there have also
been persistent but dismissed rumors of trees that had fewer copies – triploids,
which have three copies of chromosomes, or diploids, which have two copies.
Now botanist
Alan T. Whittemore and geneticist Richard T. Olsen with USDA’s AgriculturalResearch Service (ARS) have
proven beyond question that diploid American elms exist as a subset of elms in
the wild. Their findings were published in the April edition of the AmericanJournal of Botany.
Whittemore and Olsen work at the U.S. National Arboretum operated
by ARS in Washington, D.C.
American elms
once lined the country’s streets and dominated eastern forests until they
succumbed by the millions after Dutch elm disease arrived in the United States
in 1931. Yet elms are still one of the most important tree crops for the $4.7
billion-a-year nursery industry, especially since the introduction of a very
few new trees with some tolerance to the disease. American elms remain popular
because of their stately beauty, their rapid leaf litter decay and their
ability to stand up to city air pollution.
It was one of
the disease-tolerant elm trees – Jefferson,
released jointly by ARS and the National Park Service in 2005 – that put
Whittemore and Olsen on the trail of the diploid.
“Jefferson is a triploid. To get a triploid elm, we
thought there had to be a diploid parent out there somewhere in the wild that
had crossed with a tetraploid,” Whittemore says.
To settle the
question, the two scientists tested elm trees from across the species’ eastern
and central U.S.
range. About 21 percent of the wild elms sampled were diploid; some grew in
stands with tetraploids, while others were larger groupings of diploids.
The small amount
of genetic data now available suggests that at least some tetraploid and diploid
elm populations have diverged significantly from one another, which strengthens
the possibility of the diploid trees having genes for disease resistance that
the tetraploids don’t have, Whittemore says.
“We can’t
say yet whether this is a distinct race of U.
americana
or if we are really talking about a separate species,” he says.
“That’s a job we will tackle this summer.”
