Conservation of genetic diversity in slippery elm (Ulmus rubra) in Wisconsin despite the devastating impact of Dutch elm disease

Authors: Brunet, Johanne; Zalapa, Juan; GURIES, RAYMOND - University Of Wisconsin

Submitted to: Conservation Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2016
Publication Date: 4/7/2016
Publication URL: https://handle.nal.usda.gov/10113/63175
Citation: Brunet, J., Zalapa, J., Guries, R. 2016. Conservation of genetic diversity in slippery elm (Ulmus rubra) in Wisconsin despite the devastating impact of Dutch elm disease. Conservation Genetics. 17(5):1001-1010. doi: 10.1007/s10592-016-0838-1.

Interpretive Summary: Forest pest epidemics can cause serious damage to forest trees by increasing the mortality of trees and by doing so killing the older, larger trees and reducing the size of the trees in the forest. Forest trees tend to be genetically diverse, a condition related to their longevity and ability to cross with other trees, which allow them to maintain high levels of genetic diversity within populations. In principle loss of trees resulting from disease could diminish the genetic diversity of the remnant populations with consequences for the ability of a species to adapt to changing environments. Slippery elm (Ulmus rubra Muhl.) is a long-lived, wind-pollinated forest tree with a native range covering essentially all of eastern North America. Dutch elm disease (DED) caused by an introduced fungal pathogen (Ophiostoma ulmi) devastated North American elm populations, including slippery elm, beginning in the 1930s. Estimates of the numbers of elms lost to DED are unknown but range into the hundreds of millions of trees. In this study, we compared the genetic diversity of slippery elm herbarium specimens collected pre- and post DED to determine whether DED may have affected the genetic diversity of slippery elm. We also examined the genetic diversity and the genetic structure of 5 wild slippery elm populations to determine the potential impact of population fragmentation resulting from DED. We looked for evidence of a genetic bottleneck in each of the wild populations because such an event could decrease the genetic diversity in the wild elm populations. We observed no loss of genetic diversity in slippery elms post-DED. The number of alleles per locus and the expected level of heterozygosity were similar in the pre- and post-DED herbarium specimens. The 5 wild populations were only slightly differentiated and no genetic bottleneck was detected in any of the populations. At least in Wisconsin, slippery elm apparently has maintained high levels of genetic diversity. This is important because despite the serious impact of disease on the population dynamics of this tree species and changes in the size structure of its populations, this species has maintained the genetic diversity that could facilitate adaptation to future climatic and environmental changes. This information benefits forest managers, government officials and the general public, interested in the impact of disease on forest trees and in their current and future preservation.

Technical Abstract: Forest trees tend to be genetically diverse, a condition related to their longevity, outcrossing mating system and extensive gene flow that maintains high levels of genetic diversity within populations. Forest pest epidemics are responsible for many historic and contemporary population declines reported for forest trees. In principle mortality resulting from disease could diminish the genetic diversity and alter the genetic structure of the remnant populations with consequences for the evolutionary potential and ability of a species to adapt to changing environments. Slippery elm (Ulmus rubra Muhl.) is a long-lived, wind-pollinated, forest tree with a native range covering essentially all of eastern North America. Dutch elm disease (DED) caused by an introduced fungal pathogen (Ophiostoma ulmi) devastated North American elm populations, including slippery elm, beginning in the 1930s. Estimates of the numbers of elms lost to DED are unknown but range into the hundreds of millions of trees given their former abundance. In this study, the genotype of 79 herbarium specimens collected between 1890 and 2004 in Wisconsin and of 100 slippery elm trees from 5 wild populations was characterized at 13 microsatellite loci. The levels of genetic diversity were compared between the herbarium specimens collected pre- and post-DED to examine whether DED may have affected the genetic diversity of slippery elm. In addition, to determine the potential impact of population fragmentation resulting from DED on slippery elm populations, we quantified the level of genetic diversity and degree of genetic differentiation and looked for evidence of a genetic bottleneck in the 5 wild populations. The number of alleles per locus and expected level of heterozygosity were similar between the pre- and post-DED herbarium specimens. The 5 wild populations were only slightly differentiated and no genetic bottleneck was detected in any population. While DED is known to have increased the mortality and modified the size structure of slippery elm populations, at least in Wisconsin, slippery elm apparently has maintained the genetic diversity that could facilitate adaptation to future climatic and environmental changes.