Although some polyploids in the genus Polystichum are well studied and have well-resolved evolutionary histories, the origin of the circumboreally distributed allotetraploid Polystichum braunii remains obscure. We use the chloroplast markers rbcL, rps4-trnS, and trnL-F as well as the nuclear markers pgiC and gapCp to demonstrate that P. braunii is a single allotetraploid with a minimum of two origins. The two variants isolated from the nucleus resolve with divergent clades, one eastern Asian and one North American. However, they do not have near allies among morphologically appropriate taxa in our sample; the North American progenitor appears to be extinct. A divergence-time analysis based on the cpDNA markers yielded evidence of an older time of origin for P. braunii than for an array of well-known allotetraploids in the eupolypod ferns. Niche modeling in the light of geological and paleontological evidence leads to the conclusion that the two origins were in Beringia. Since P. braunii is genetically undifferentiated but widely distributed, we argue that it has expanded to its circumboreal range in the recent past, though it has a relatively ancient origin.
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Vol. 42 • No. 1