Widespread Triploidy in Western North American Aspen (Populus tremuloides)

Abstract

We document high rates of triploidy in aspen (Populus tremuloides) across the western USA (up to 69% of genets), and ask whether the incidence of triploidy across the species range corresponds with latitude, glacial history (as has been documented in other species), climate, or regional variance in clone size. Using a combination of microsatellite genotyping, flow cytometry, and cytology, we demonstrate that triploidy is highest in unglaciated, drought-prone regions of North America, where the largest clone sizes have been reported for this species. While we cannot completely rule out a low incidence of undetected aneuploidy, tetraploidy or duplicated loci, our evidence suggests that these phenomena are unlikely to be significant contributors to our observed patterns. We suggest that the distribution of triploid aspen is due to a positive synergy between triploidy and ecological factors driving clonality. Although triploids are expected to have low fertility, they are hypothesized to be an evolutionary link to sexual tetraploidy. Thus, interactions between clonality and polyploidy may be a broadly important component of geographic speciation patterns in perennial plants. Further, cytotypes are expected to show physiological and structural differences which may influence susceptibility to ecological factors such as drought, and we suggest that cytotype may be a significant and previously overlooked factor in recent patterns of high aspen mortality in the southwestern portion of the species range. Finally, triploidy should be carefully considered as a source of variance in genomic and ecological studies of aspen, particularly in western U.S. landscapes.

Imaging spectroscopy links aspen genotype with below-ground processes at landscape scales

“…They occur in mixed forests as well as monospecific stands that can extend continuously over broad areas; the largest individual organism known to science, the Pando genet, covers 44.3 ha in southern Utah, USA [42]. Western US aspen are often triploid and form expansive genets of genetically identical ramets [43][44][45]. Conversely, aspen in the Great Lakes region of the USA are rarely triploid [44], and form small genets that are often interspersed with mixed northern hardwood species.…”

“…During a 1 year period from 2005 to 2006, the San Juan National Forest in Colorado experienced a 58% increase in area of recent aspen mortality [50]. That aspen decline and triploidy co-occur is not inconsequential; physiological traits associated with polyploidy may influence drought susceptibility, making regional declines in aspen a potential threat to genetic variation and persistence in aspen [44].…”

“…This may be the case in aspen-dominated systems of the species depauperate north-temperate zone, and losses in the genetic diversity in aspen systems could constitute a significant loss of ecological diversity and evolutionary potential. For instance, the rapid decline of aspen owing to long-term drought conditions may be a significant threat to the genetic diversity of aspen [44], with unknown consequences for above-ground herbivore communities and below-ground processes. That is, the loss of genotypic diversity represents a loss of focal genotypes, as well as the loss of the associated above-and below-ground patches of ecosystems.…”

Forest stand structure, productivity, and age mediate climatic effects on aspen decline

“…We restricted the sample area for three reasons: (1) in the western United States, most of the aspen populations are found in these eight states; (2) these populations appear to be genetically distinct from populations in Canada and the northeastern United States (Mock et al 2012);and (3) all FIA plots in this region were sampled by the Rocky Mountain Research Station, minimizing differences in sampling protocols. To maximize the opportunity of observing climatic impacts on mortality indices, we examined FIA plots measured after 2002, when regional declines were observed to increase (Hogg et al 2008, Worrall et al 2008).…”

Breeding triploid aspen and poplar clones for biomass production

“…The first North American triploid quaking aspen (P. tremuloides) were found in northeastern Minnesota ( VAN BUIJTENEN et al, 1957;EINSPAHR et al, 1963) and later in Utah (EVERY and WIENS, 1971). In recent studies, high rates of triploidy (up to 69 % of genets) were demonstrated in P. tremuloides across the western USA (MOCK et al, 2012). Natural well-growing triploids of Populus tomentosa were first discovered in China (ZHU et al, 1998;YAO and PU, 1998).…”