Genetic variation in Bromus tectorum (Poaceae) from the Mediterranean Region: Biogeographical history of native populations

Background/Question/Methods

The genetic consequences of immigration are determined by the amount and distribution of genetic diversity within and among native source populations, founder population size, and the number of independent introduction events. The genetic diversity of native populations is influenced by the species’s characteristics (e.g., reproductive and mating systems) and its evolutionary history, especially large-scale biogeographical events. The exceptionally high level of self-pollination (> 99%) in Bromus tectorum largely preserves the genetic signature associated with the biogeographical history of its native populations - events influenced by past climatic events and anthropogenic activity. Here we characterize genetic diversity within and among 42 native populations of B. tectorum drawn from two regions: the Eastern Mediterranean (Israel, Jordan and Syria) and the Western Mediterranean (Portugal, Spain and France). Two contrasting hypotheses were tested: 1) populations from the Eastern Mediterranean are more genetically diverse compared with Western populations, a consequence of founder effects during the species’s dispersal with the spread of agriculture from the Middle East, and 2) populations from the Eastern and Western Mediterranean are equally diverse because both regions served as refugia during the last Quaternary glacial advances.

Results/Conclusions

Eastern Mediterranean populations possess 16 polymorphic loci and 37 multilocus genotypes, while populations from the Western Mediterranean have a subset of these polymorphic loci (9) and fewer multilocus genotypes (19).  The results indicate that founder effects associated with the East to West dispersal of B. tectorum may have produced lower genetic diversity among Western Mediterranean populations.  However, 12 of the 19 multilocus genotypes indentified in the Western Mediterranean populations were not detected in the Eastern Mediterranean populations; these results suggest the genetic diversity of eastern populations have been reshuffled in western populations, most likely though out-crossing.  On average, genetic diversity within populations from Israel, Syria and Jordan is similar to the genetic diversity in populations from Spain and Portugal; diversity is lowest for populations from France.  Large founder populations or multiple dispersal events, or both, apparently led to sufficiently large propagule pressure to produce Spanish and Portuguese populations that are genetically diverse. The lower genetic diversity of the French populations may reflect glaciation’s influence on dispersal, or boundaries to colonization (e.g. the Pyrenees) or both.  Anthropogenic activity, climatic events in the Quaternary and environmental heterogeneity may have all influenced the genetic structure of these populations.