Phylogeography, haplotype trees, and invasive plant species

The distribution of genetic variants in plant populations is strongly affected both by current patterns of microevolutionary forces, such as gene flow and selection, and by the phylogenetic history of populations and species. Understanding the interplay of shared history and current evolutionary events is particularly confounding in plants due to the reticulating nature of gene exchange between diverging lineages. Certain gene sequences provide historically ordered neutral molecular variation that can be converted to gene genealogies which trace the evolutionary relationships among haplotypes (alleles). Gene genealogies can be used to understand the evolution of specific DNA sequences and relate sequence variation to plant phenotype. For example, in a study of the RPS2 gene in Arabidopsis thaliana, resistant phenotypes clustered in one portion of the gene tree. The field of phylogeography examines the distribution of allele genealogies in an explicit geographical context and, when coupled with a nested clade analysis, can provide insight into historical processes such as range expansion, gene flow, and genetic drift. A phylogeographical approach offers insight into practical issues as well. Here we show how haplotype trees can address the origins of invasive plants, one of the greatest global threats to biodiversity. A study of the geographical diversity of haplotypes in invasive Phragmites populations in the United States indicates that invasiveness is due to the colonization and spread of distinct genotypes from Europe (Saltonstall 2002). Likewise, a phylogeographical analysis of Tamarix populations indicates that hybridization events between formerly isolated species of Eurasia have produced the most common genotype of the second-worst invasive plant species in the United States.