Interspecific relationships of Betula remain elusive, making it difficult to examine morphological diversification and historical biogeography of the genus. In this study, sequences of the internal transcribed spacer region of nuclear ribosomal DNA were used to estimate relationships within Betula. Subgenera and sections or subsections, as traditionally circumscribed, are not monophyletic. Morphological characters traditionally used to categorize major groupings of Betula may have independently evolved more than once, including the shrubby habit and racemose female catkin arrangement. Betula maximowicziana is not united with other species of section Betulaster, which is unique in having a racemose female catkin arrangement and poorly developed side lobes of bracts. However, Betula nigra of section Eubetula falls within section Betulaster. Three biogeographic disjunctions are recognized in Betula, two between eastern Asia and eastern North America (B. costata–B. alleghaniensis and B. alnoides–B. nigra) and one between central Asia and eastern North America (B. medwediewi–B. lenta). Our results support the closer affinity of B. uber, which is an endangered species in eastern North America, to B. lenta than to B. alleghaniensis. Nonetheless, interspecific relationships of Betula are generally weakly supported due to the small number of informative characters of the DNA region, and more data are needed to further test the hypotheses.
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Vol. 107 • No. 929