Genotyping Bromus tectorum using SNP Markers from a Partially Sequenced Transcriptome to Detect Cryptic Invasion in Western North America

We have partially sequenced the Bromus tectorum (cheatgrass or downy brome) transcriptome and mined it for SNPs to genetically assess population diversity. Bromus tectorum is a cleistogamous species of Eurasian origin that is dominant on millions of hectares of western North American rangelands. Our objective has been to investigate the role of independent introductions in recent range expansions. Secondary range expansion into new habitats often follows establishment and spread of invasive species within an ecosystem. The introduction of pre-adapted genotypes into new habitats is one mechanism accounting for secondary range expansion. After its introduction in the late 1800s, rapid establishment of B. tectorum within sagebrush-steppe habitats of western North America has been followed by expansion into more xeric as well as mesic environments. Using our SNP markers we have compared the genetic composition of North American sagebrush-steppe populations of B. tectorum with those found in more xeric desert environments of the southwest. We also compared the genetic composition of western North American populations with populations from the native Eurasian range. We found some desert haplotypes are genetically distinct from those found in the sagebrush-steppe habitat. We also discovered that common haplotypes in North American habitats are most similar to haplotypes from Northern European populations, whereas the distinct North American desert haplotypes are most similar to populations in south-central Asian deserts. Our data suggest the recent invasion of arid habitats of North America may be a consequence of the independent introduction of pre-adapted biotypes from climatically similar parts of Eurasia.