High-throughput DNA sequencing identifies population genetic structure and signatures of local adaptation in invasive populations of Spartina alterniflora in China

Understanding the ecological and evolutionary processes of invasive plants is a major goal in ecology. Invasive plant species often exhibit intraspecific phenotypic clines along geographic and climatic gradients, indicating potential adaptive evolution through spatial variation in selection regimes. However, rarely have genetic studies of biological invasions employed population genomics together with quantitative trait analysis to robustly test the role of adaptive evolution in alien plant invasiveness. We employed Restriction site-associated DNA sequencing (RADSeq) population genomics to analyze 17 populations of the invasive plant Spartina alterniflora in China. Our main goals were to test whether: S. alterniflora exhibits latitudinal clines in traits and genetic diversity; climatic and soil factors correlate with genetic and trait variation; and natural selection drives trait variation. Significant correlations existed between genetic diversity and quantitative traits of S. alterniflora and the three climatic regions (northern, mid-latitude, and southern) where the invader occurred. Basal diameter, spike-like branches, observed heterozygosity and allele frequency exhibited significant latitudinal clines. A total of 3, 310 SNPs showed a clear genetic structure among populations in three climatic regions (northern, mid-latitude, and southern). Positive correlations between genetic differentiation and variation in reproductive traits suggested the heritability of these traits. Outlier loci identified by both principal component analysis (PCAdapt) and latent factor mixed model (LFMM) were significantly correlated to soil nitrogen content and precipitation. These results provide evidence of molecular signatures of local adaptation of S. alterniflora and suggest that rapid adaptive evolution may facilitate the invasiveness of S. alterniflora in China.