Genetic analysis of Bromus tectorum in the eastern and western Mediterranean regions

<p class="MsoBodyText">Genetic diversity within <span>and</span> <span>among</span> 42 native populations of <i>Bromus <span>tectorum</span> </i><b>(</b>cheatgrass)<i> </i>was characterized within<i> </i>two <span>regions, </span>the<span> eastern</span> <span>Mediterranean</span> <span>and</span> the<span> western</span> <span>Mediterranean.</span> Two <span>hypotheses </span> were tested for the genetic diversity of these populations: 1) populations <span>from</span> the<span> eastern</span> <span>Mediterranean</span> are more genetically diverse <span>compared</span> with populations to the west, a <span>potential consequence </span>of the species' westward <span>dispersal</span> with the <span>spread</span> of agriculture<span> and 2) </span>populations across the Mediterranean contain<span> comparable </span><span>genetic </span>diversity but display high genetic differentiation, a potential consequence of <span>both</span> <span>regions</span> having <span>served</span> <span>as</span> refugia during <span>glacial</span> <span>advances</span> in the <span>late</span> Quaternary <span>Period.</span>  Populations in the eastern <span>Mediterranean possess</span> 16 <span>polymorphic</span> <span>loci</span> and 37 multilocus <span>genotypes</span>.  In <span>contrast,</span> populations <span>from</span> the <span>western</span> <span>Mediterranean</span> include a subset of these polymorphic <span>loci</span> (9)<span> and</span> fewer multilocus <span>genotypes</span> (19), <span>consistent</span> with the <span>dispersal</span> of <i>B. tectorum </i>with the<span> east-west Holocene spread</span> <span>of</span><span> agriculture.  </span>Among the 19 multilocus <span>genotypes</span> <span>identified</span> in populations from the western <span>Mediterranean</span><span>,</span> 13 are <span>undetected</span> among <span>eastern</span> <span>Mediterranean</span> populations.  <span>Average</span> <span>genetic</span> diversity within populations <span>from</span><span> the eastern Mediterranean </span>is nonetheless comparable to the <span>genetic </span>diversity in populations <span>from</span> the Iberian Peninsula, whereas diversity is lowest in the populations from southern France<span>.</span>  <span>Our results </span><span>suggest</span><span> a prominent role for agriculture in the grass's western spread, although </span><span>glacial</span> history <span>and</span> <span>environmental</span> heterogeneity<span> also could </span><span>have</span> <span>influenced</span> the<span> grass's </span><span>genetic diversity</span>.  The exceptionally <span>high</span> <span>level</span> of self-pollination (&gt; <span>99%)</span> in <i>B. <span>tectorum</span> </i>has contributed to <span>preserving</span> the genetic<span> signature</span> <span>associated</span> with the species' <span>biogeographical</span> history<span> across the Mediterranean Region</span><span>.</span>  </p> <p> </p>