The genus Cassiope is comprised of 17 species of dwarf alpine shrubs that exhibit a variety of leaf morphologies, yet have very similar flowers and habits. The genus is circumboreally distributed, with species diversity concentrated in northwestern North America and in eastern Asia. This clade has been previously shown to be strongly supported as sister to the subfamily Ericoideae, but relationships within the genus have never been the subject of any phylogenetic study. The goal of this study was to use three molecular markers from the chloroplast and nuclear genomes to generate a phylogeny of Cassiope. Our initial analyses suggested that three species negatively impacted phylogeny reconstruction by introducing conflict between the nuclear and chloroplast genomes. Reanalysis with those three taxa removed generated a very strongly supported phylogeny, suggesting that these three taxa may be of hybrid origin. Some evolutionary trends in morphology and anatomy are apparent based upon analyses using the restricted taxon sampling. Characters with discrete character states, such as leaf form and stomatal distribution, are tightly correlated but appear to be homoplasious within the genus. Continuously varying characters such as habitat elevation and aspects of wood anatomy show compelling phylogenetic trends that may be related to the fact that these species evolved in alpine and subalpine habitats. Cassiope appears to have a complex biogeographical history in the northern hemisphere, which cannot be completely resolved with the current dataset. However, it is likely that the genus has undergone multiple speciation events in the Himalayas with one or more dispersals into northwestern North America, or alternately with origins in the American Northwest followed by dispersal and subsequent divergence in the Himalayas.
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Vol. 115 • No. 963