History, population structure and evolution of invasive Tamarix L. in the Southwestern U.S.

Biological invasion is a complex process associated with multiple factors such as long distance migration from spatially and temporarily varying sources, multiple introductions, inter and intraspecific hybridization, and local adaptation to novel biotic and abiotic conditions. The history of invasion and the evolutionary mechanisms related to biological invasions provide insights into the invasion process. This dissertation examined the history, adaptive evolution, and the pattern of gene flow under the influence of anthropogenic introductions and landscape features of the US Tamarix invasion. The genus Tamarix L., native to Eurasia, is one of the most successful non-indigenous trees in the western United States. In the first data chapter (Chapter 2), I investigated how genomic diversity of the invasive Tamarix in introduced regions of the Southwestern US is related to genomic diversity in the native range, Eastern and Central Asia. Based on 3,351 single-nucleotide polymorphism (SNP) markers sampled from 348 individuals (3 populations of T. chinensis, 3 populations of T. ramosissima from Asia; 25 populations of hybrids, T. chinensis × T. ramosissima from Southwestern US; 3 populations of T. gallica derived hybrid from the Gulf Coast Texas), I found that the most abundant invasive Tamarix species in the Southwestern US is a hybrid between T. ramosissima and T. chinensis, while the Tamarix populations along the Gulf Coast Texas were comprised of novel T. gallica-derived hybrids that likely reproduce through apomixis or clonal growth. Different hybrid ratios of T. ramosissima and T. chinensis were associated with major river basins, which is consistent with different introductions of Tamarix source genotypes into the different major rivers in the historical documents. In the second data chapter (Chapter 3), I compared quantitative genetic variation of growth-related traits among 64 individuals from six populations from two regions differing in average rainfall and tested whether trait divergence across regions was consistent with expectations from local adaptation. I found a significant interaction between simulated drought and non-drought conditions and the origins from which the genotypes collected, either relatively high or low rainfall regimes on the degree of leaf loss (biomass loss by senesced leaves to total biomass) in T. ramosissima × T. chinensis. The divergence in leaf loss was greater among regions than was expected given the genetic divergence in neutral loci between those same regions indicating local adaptation to drought. In the final data chapter (Chapter 4), I employed a landscape genetics approach to assess how different landscape factors constrain or facilitate migration and gene flow of Tamarix throughout the Southwestern US. Bayesian clustering and landscape models based on 1,997 SNPs revealed that human-mediated introduction of 4 or 5 genetically distinct populations along river systems were influenced the background spatial genetic pattern of Tamarix populations in the Southwestern US. During the range expansion away from these sites of introduction, river pathways were the primary mode of migration, with the secondary influences of roads and wind dispersal. Low temperature regions and the presence of dams between populations showed negative effects on gene flow, limiting the movement of seeds. Collectively, these results demonstrate that the Tamarix invasion into the southwestern US was a complex process influenced by multiple human-mediated introductions, hybridization, adaptive evolution, and range expansion that was influenced by characteristics of the landscape.