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.
Highlights
Species distributions are the manifestation of complex evolutionary and ecological histories
Proportions of triploid genets within sampling sites ranged from 0–69%
Triploidy was highest in portions of the range west of the continental divide, south of the last glacial maximum, and high in southern Utah and western Colorado (Figure 1)
Summary
Species distributions are the manifestation of complex evolutionary and ecological histories. Evolutionary and ecological effects are often nested, such that the distribution and diversity of a foundation species becomes a matrix influencing the distribution and diversity of dependent species [1]. Understanding the factors driving the distribution of widespread, foundation species can be insightful in understanding and predicting distributions of associated species and communities. One of the intrinsic factors influencing plant distributions is clonality. Clonal size often increases with latitude [5], elevation [6,7,8], and position at range edges [9], and is thought to allow persistence in harsh environments where sexual recruitment may be limited or episodic [10,11,12]. Clonal persistence can be dramatic, on the order of millennia [13,14,15]
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