populations to be the purest form of the species. A better 

 genetic understanding of "pure" northern C_^ hookerianum is needed 

 to interpret variability in westcentral Montana C^. hookerianum 

 and C^ lonqistylum . Distant samples of C_^ scariosum from Nevada 

 and Oregon are also genetically distinct from westcentral Montana 

 C. scariosum . Furthermore, the relative genetic similarity of C. 

 scariosum and C^. hookerianum from westcentral Montana raises the 

 question of whether introgression has affected all of the species 

 in this region. 



In the PCA analysis of RAPD data, putative hybrid plants are 

 placed near or intermediate between C_^ lonqistylum . C. 

 hookerianum , and C_^ scariosum plants, consistent with the 

 hypothesis that they arose by hybridization. However, until the 

 genetic composition of "pure" C^ lonqistylum , C. hookerianum , and 

 C. scariosum is understood, through rangewide sampling of the 

 species, it will not be possible to distinguish between natural 

 variability in each species and variability induced by 

 hybridization/ introgression. 



Hypothesized Recent Evolutionary History of Cirsivun lonqistyl\im 



The evolutionary history of Cirsium species in Montana can 

 be hypothesized in light of existing genetic, morphologic, and 

 geographic information. Our preliminary DNA analysis suggests 

 that Cirsium lonqistylum , C. hookerianum . and C^ scariosum are 

 well-differentiated entities in the allopatric parts of their 

 ranges, apparently representing separate evolutionary lineages. 

 During glacial times, the northern species of the group, C. 

 hookerianum , was likely forced southward into sympatry with C. 

 lonqistylum in westcentral and southern Montana. Cirsium 

 scariosum also colonized the region, most likely from its 

 principal range to the southwest. Gene flow via wind-dispersed 

 seed and nondiscriminating pollinators probably occurred among 

 many populations in westcentral Montana, resulting in regional 

 introgression of the three species. Cirsium lonqistylum genes 

 have introgressed into C^ hookerianum and C_^ scariosum producing 

 distinct westcentral Montana races of these species. Because 

 genes from all three species are likely widespread throughout 

 westcentral Montana, distinguishing between introgression and 

 natural variation within each species is not possible without 

 understanding genetic patterns and variability outside of 

 westcentral Montana. 



A phylogenetic analysis of all Cirsium species in the region 

 is needed to test the evolutionary hypotheses presented above. 

 Knowledge of phylogenetic relationships and processes in Cirsium 

 would allow managers to judge: the biological significance of C. 

 lonqistylum . whether human activity or natural processes have 

 produced the current biological situation, and whether any 

 further management action is warranted. 



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