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Jorgensen, Stacy A.

Plant Biology

2012

MS

Allopolyploids incorporate genomes from different progenitors, usually different species; they often arise vIa a hybridization event followed by restoration of fertility through chromosome doubling. Allopolyploidy as a mechanism of speciation is frequent in the ferns, and polyploids comprise as much as 44% ofthe genus Polystichum. Polystichum braunii, a fertile allotetraploid with unidentified progenitors, is circumboreally distributed in wet forests on calcareous talus. The chloroplast sequences rbcL, rps4-trnS, and trnL-F resolve P. braunii in two places: with species of Section Metapolystichum and in an unresolved group with the northwestern North American polyploids P. setigerum and P. andersonii. The single-copy nuclear markers gapCp and pgiC each yield two variants, on resolving with Sect. Metapolystichum and the other in a poorly resolved group with North American and Austral taxa. Polystichum braunii lacks close relatives among morphologically appropriate candidate progenitors present in the sample. It shares a chloroplast genome with tetraploid P. andersonii and hexaploid P. setigerum, consistent with the hypothesis that P. braunii is one of the progenitors of P. setigerum. These results suggest that P. braunii is an old tetraploid with absent progenitors.