Abstract
Understanding of the present-day genetic diversity, population structure, and evolutionary history of tree species can inform resource management and conservation activities, including response to pressures presented by a changing climate. Cercis canadensis (Eastern Redbud) is an economically valuable understory tree species native to the United States (U.S.) that is also important for forest ecosystem and wildlife health. Here, we document and explain the population genetics and evolutionary history of this deciduous tree species across its distributed range. In this study, we used twelve microsatellite markers to investigate 691 wild-type trees sampled at 74 collection sites from 23 Eastern U.S. states. High genetic diversity and limited gene flow were revealed in wild, natural stands of C. canadensis with populations that are explained by two major genetic clusters. These findings indicate that an ancient population bottleneck occurred coinciding with the last glacial maximum (LGM) in North America. The structure in current populations likely originated from an ancient population in the eastern U.S. that survived LGM and then later diverged into two contemporary clusters. Data suggests that populations have expanded since the last glaciation event from one into several post-glacial refugia that now occupy this species’ current geographic range. Our enhanced understanding benchmarks the genetic variation preserved within this species and can direct future efforts in conservation, and resource utilization of adaptively resilient populations that present the greatest genetic and structural diversity.
Highlights
The genetic structure and demographics of many North American plant species have been greatly influenced by climate fluctuations that occurred during the Pleistocene epoch[1,2]
The role that glaciation has played in the distribution, range, genetic variation, and spatial genetic structure of outcrossing tree species that span a geographically wide range are not well understood, especially across the eastern U.S To better understand the role of last glacial maximum (LGM) in structuring current species distributions and population structure of temperate tree species in the eastern U.S, we evaluated the spatial population structure of widely distributed forest understory tree Cercis canadensis L. (C. canadensis var. canadensis L.; Fabaceae; eastern redbud)
Wild populations of C. canadensis that were sampled across its native range in the U.S revealed high levels of genetic diversity and population differentiation, the presence of population structure, limited gene flow, and an ancient bottleneck that temporally coincides with the last glacial period in North America
Summary
The genetic structure and demographics of many North American plant species have been greatly influenced by climate fluctuations that occurred during the Pleistocene epoch[1,2]. During the last glacial maximum (LGM), which occurred approximately 18,000–21,000 years a go[3,4,5], the Laurentide Ice Sheet extended from the northernmost portions of North America to 39°N3 These events reduced the range of many temperate tree species, forcing them into glacial refugia, which included unglaciated southern regions and suitable micro-environments that were present in northern glaciated r egions[1,6]. Cercis canadensis is a self-incompatible[20], deciduous tree native to the midwestern and eastern U.S, as well as northeastern Mexico[21,22] This species grows well in partial shade, is well adapted to a wide-range of climate conditions and elevations, and can be found in the USDA hardiness zones 4 through 9 23,24. This relatively small ornamental tree is characterized by its wide, colorful, umbrella-shaped c anopy[25], and is a popular landscape tree due to its heart-shaped foliage, compact form, and early spring flowers[23]
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