Introgression between invasive saltcedars (Tamarix chinensis and T. ramosissima) in the USA

Abstract

Saltcedars (Tamarix ramosissima and T. chinensis) are native to Asia, but since introduction into the USA have become common and invasive in many western riparian habitats. Recent molecular analysis of a single locus nuclear DNA sequence marker has shown that in their native range the two species are genetically distinct, but within the USA populations many of the plants (23%) are novel hybrids. Here, we used multilocus DNA markers (amplified fragment length polymorphisms) to determine the levels of introgression in USA plants. Species-specific diagnostic markers, principal coordinates analysis, and a Bayesian model-based clustering analysis all indicate a much higher incidence of hybridization (83–87%) than was revealed by the single locus marker, with USA plants forming a genetic continuum between the two parental types. Additionally, the level of introgression toward Tamarix ramosissima or T. chinensis was strongly correlated with latitude. Concordance of level of introgression was highest between principal coordinates analysis and the Bayesian analysis. The high percentage of novel hybrids may have implications for classical biological control efforts.