Leading‐edge populations do not show low genetic diversity or high differentiation in a wind‐pollinated tree

Abstract

Climate changes can shift species' ranges. Knowledge on genetic variation of the leading-edge populations provides critical information to understand responses and adaptation of plants to projected climate warming. To date, the research into genetic variation of leading-edge populations has been limited, particularly in the role of wind-mediated pollen flow in maintaining high genetic variation. Castanopsis sclerophylla (Fagaceae) is a wind-pollinated and gravity-dispersed tree. In the present study, we used seven polymorphic microsatellites to genotype 482 samples from five leading-edge and 12 nonedge populations. Significant effects of recent population bottleneck events were found in three of the five leadingedge populations, indicating that the leading-edge populations might have been recolonized after the Last Glacial Maximum. Genetic diversity was higher, though not significantly, in leading-edge than in non-edge populations. Relationship between genetic diversity and latitude indicated an increasing trend of genetic diversity towards leading-edge populations. No significant difference in genetic differentiation was found between leading-edge and non-edge populations. The inconsistence with the general predictions by leading-edge colonization model could be explained by high gene flow via pollen grains. Pollen-mediated gene flow could maintain high genetic diversity within and low differentiation among leadingedge populations. In response to climate warming, high genetic variation may provide leading-edge populations raw materials for evolutionary adaptation to future environmental conditions.

Genetic evidence for central‐marginal hypothesis in a Cenozoic relict tree species across its distribution in China

“…). Similar patterns consistent with the C‐M hypothesis have been reported elsewhere, for example, in Ulmus laevis (Finland and Estonia), Leavenworthia stylosa (USA) and Arenaria provincialis (France) (Vakkari et al ., ; Dixon et al ., ; Pouget et al ., ), but studies in south and east China have not found such evidence (Ge et al ., ; Shi & Chen, ; Shi et al ., ). Then again, different plant species within a given region do not always show genetic patterns consistent with the C‐M hypothesis (Jump et al ., ; Diniz‐Filho et al ., ; Soares et al ., ).…”

“…topography). Shi & Chen () found evidence for post‐glacial expansion to north, but they demonstrated an increasing trend of genetic diversity towards leading‐edge populations of Castanopsis sclerophylla . Second, the high‐latitude‐skewed C‐M pattern may have been shaped by the complex topography in the species' distribution range.…”

“…However, in contrast to Europe and North America, the effect of historical glaciation in East Asia (especially south-central China) has been debated for a long time because no unified ice sheet developed in China during the Quaternary period (Liu et al, 2012). Some studies demonstrated a signature of post-glacial expansion in plants at south-east China (Shi et al, 2014) and the Qinghai-Tibet Plateau (Wang et al, 2009), while others reported only a limited effect of the Last Glacial Maximum (LGM) on forests in other parts of China (Ying, 2001;Chen et al, 2012). However, until now, few empirical studies tested whether and how historical processes affect the C-M pattern of genetic variation of tree species in subtropical mountains of China.…”

Phylogeography of postglacial range expansion in Juglans mandshurica (Juglandaceae) reveals no evidence of bottleneck, loss of genetic diversity, or isolation by distance in the leading-edge populations

“…In reality, such high and uniform levels of genetic diversity across a species' entire range have already been observed for many temperate trees (for a review see Jaramillo-Correa et al, 2009). In wind-pollinated trees, even higher levels of diversity can be reached when pollen-mediated gene flow constantly brings alleles from source populations to the colonization front (Shi and Chen, 2012). Moreover, in the case of a gradual expansion of a migrating front, neither a population bottleneck nor spatial genetic structure caused by isolation by distance (IBD) would be expected to occur because of the substantial gene flow that would effectively homogenize the newly established populations.…”

Long‐distance pollen dispersal during recent colonization favors a rapid but partial recovery of genetic diversity in Picea sitchensis

“…This heterogeneity is represented across studies of tree species. Empirical studies have found both lower genetic diversity (Mimura & Aitken, 2007;Marsico et al, 2009;Kitamura et al, 2015;Johnson et al, 2017) and similar or higher genetic diversity (Born et al, 2008;Pluess, 2011;Shi & Chen, 2012) in leading edge populations of tree species after range expansion. Temperate forest tree species are generally associated with high gene flow via wind-borne pollen across large geographic distances (Kremer et al, 2012), as well as a long life span and juvenile phase.…”