Understanding the ecological and evolutionary processes that underlie phenotypic and genetic variation among invasive plant populations and the capacity of such populations to colonize a broad range of environments is a major goal in ecology. Rapid adaptation to local environmental conditions often leads to latitudinal clines in both phenotypes and genotypes of invasive plant species. Previous studies suggest that natural selection influenced a latitudinal cline in phenotypic traits of the invasive plant Spartina alterniflora in China. However, little is understood of the relationship between genetic and phenotypic differentiation and environmental factors among S. alterniflora populations. Here, we studied whether S. alterniflora exhibits a latitudinal cline in genetic diversity across the east coast of China and whether genetic and phenotypic differentiation among populations were correlated with environmental factors. The Restriction-site Associated DNA (RAD-seq) sequencing was used to obtained genomic information of 17 populations of S. alterniflora in China. A redundancy analysis (RDA) was performed to assess whether environmental variables were corelated with genetic and phenotypic differentiation among the populations. A univariate environmental association analysis (implemented through Latent Factor Mixed Model; LFMM) and a principal component analysis (PCAdapt) were used to test for signature of local adaptation. 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 characteristics of S. alterniflora were revealed, suggesting that sexual reproduction traits might be stable and genetically heritable. Outlier loci that were identified by both PCAdapt and LFMM methods were significantly correlated to soil phosphorus content, soil salinity, and precipitation. These results provide evidence of molecular signatures of local adaptation of S. alterniflora and suggest that rapid adaptive evolution may facilitate invasiveness of S. alterniflora in China.

A total of 17 populations of S. alterniflora that occurred along a coastal line in China were studied. Leaf tissues for DNA analysis were obtained from a total of 253 individuals (14-15 individuals per population) between August and September 2018, with the distance between any two individuals maintained at more than 30 m. In situ measurements of four phenotypic traits, including plant height, basal diameter, length of mature inflorescence, and the number of spike-like branches on one mature inflorescence, were also taken from the same S. alterniflora individuals that were sampled for DNA analysis.