Population and Landscape Genetics Provide Insights Into Species Conservation of Two Evergreen Oaks in Qinghai-Tibet Plateau and Adjacent Regions

Abstract

The combination of population and landscape genetics can facilitate the understanding of conservation strategy under the changing climate. Here, we focused on the two most diverse and ecologically important evergreen oaks: Quercus aquifolioides and Quercus spinosa in Qinghai-Tibetan Plateau (QTP), which is considered as world's biodiversity hotspot. We genotyped 1,657 individuals of 106 populations at 15 nuclear microsatellite loci throughout the species distribution range. Spatial patterns of genetic diversity were identified by mapping the allelic richness (AR) and locally common alleles (LCA) according to the circular neighborhood methodology. Migration routes from QTP were detected by historical gene flow estimation. The response pattern of genetic variation to environmental gradient was assessed by the genotype-environment association (GEA) analysis. The overall genetic structure showed a high level of intra-species genetic divergence of a strong west-east pattern. The West-to-East migration route indicated the complex demographic history of two oak species. We found evidence of isolation by the environment in Q. aqu-East and Q. spi-West lineage but not in Q. aqu-West and Q. spi-East lineage. Furthermore, priority for conservation should be given to populations that retain higher spatial genetic diversity or isolated at the edge of the distribution range. Our findings indicate that knowledge of spatial diversity and migration route can provide valuable information for the conservation of existing populations. This study provides an important guide for species conservation for two oak species by the integration of population and landscape genetic methods.

Keywords: Hengduan Mountains; Qinling Mountains; Quercus aquifolioides; Quercus spinosa; genotype-environment association; migration routes; species conservation.

FIGURE 1

Geographic distribution and sampling sites of Q. aquifolioides and Q. spinosa. Black and red dashed lines indicate the geographic distribution of Q. aquifolioides and Q. spinosa, respectively. Three blue dashed lines represent defined research areas. The black rectangle on left top map represents the whole research area. QTP: Qinghai–Tibet Plateau, HDM: Hengduan Mountains, QM: Qinling Mountains.

FIGURE 2

The allelic richness and locally common alleles map of Q. aquifolioides and Q. spinosa. The light blue dotted lines represent defined three research areas: (a) Qinghai–Tibet Plateau (QTP); (b) Hengduan Mountains (HDM); (c) Qinling Mountains (QM).

FIGURE 3

Individual assignment to two (top), three (middle), and four (below) genetic clusters by STRUCTURE of Q. aquifolioides and Q. spinosa. Each bar represents a single individual, with portions of the bar colored depending on the ancestry proportions estimated. The y-axis quantifies subgroup membership, and the x-axis shows the sample ID for each individual.

FIGURE 4

Genetic covariance of Q. aquifolioides and Q. spinosa. (A) Principal component analysis (PCA) plots based on genetic covariance among individuals. The first two principal components (PCs) are shown; (B) principal coordinate analysis (PCoA) plots of the first two components based on genetic covariance among populations.

FIGURE 5

Relationship of genetic distance (F_ST/(1-F_ST)) and resistance distance based on climatic niche suitability of (A) Q. aquifolioides and (B) Q. spinosa.

FIGURE 6

Variable importance of environmental variables based on analysis of generalized dissimilarity model (GDM) for (A) all populations of Q. aquifolioides, (B) Q. aqu-West lineage, (C) Q. aqu-East lineage, (D) all populations of Q. spinosa, (E) Q. spi-West lineage, and (F) Q. spi-East lineage.