Phenotypic and genetic variation of Bromus tectorum in native and invasive populations

Background/Question/Methods

The genetics of biological invasions sits at the intersection of factors relating to both the invading organism and its new environment, providing a unifying perspective on a complex process.  In the case of the invasive grass Bromus tectorum, overall genetic diversity in North America has been reduced due to sampling effects, but within-population diversity tends to be relatively high due to novel mixing of genotypes.  This makes sense given that B. tectorumis autogamous with relatively low levels of genetic variation expected within native populations.  These overall trends can be more complicated near invasive range margins, and recent studies have explored both phenotypic and genetic variation at high elevation invasive "fronts" in North America with mixed results.

This study examines both phenotypic and genetic variation in B. tectorumpopulations collected across elevation gradients in Armenia/Georgia where it is native and California/Nevada where it is invasive.  The objectives of the study are three-fold: (1) to examine the structure of trait-based variation and local adaptation in both native and novel populations, (2) to compare this to the ecological genetics of these populations, and (3) to contrast newly invaded high elevation sites to an elevation gradient in the native range.

Results/Conclusions

A common garden experiment found no difference in overall means between native and invasive populations for morphological, phenological, and reproductive traits.  However, among-population variance components were much larger for native populations, indicating strong trait-based differentiation compared to invasive populations.  For native populations, variation in leaf length, flowering time, seed production, and seeds per spikelet was strongly explained by local elevation and/or climatic variables, indicating local adaptation.  In contrast, invasive populations largely did not show evidence for local adaptation with the exception of leaf length.

Microsatellite analysis of seven loci revealed similar values of within-population diversity across native and invasive populations.  However, genetic diversity tended to decrease at higher elevation in the invaded range, with some of the highest diversity overall found in low-elevation invasive populations.  This pattern might represent a selective bottleneck as plants move into more stressful environments or potentially the leading edge of invasion where only a subset of individuals are invading.  In contrast, there was a trend toward increased diversity at high elevation for native populations, which may indicate the lack of all-purpose genotypes in harsh and variable environments.