Phylogeny and phylogeography of the circumpolar genus Fraxinus (Oleaceae) based on internal transcribed spacer sequences of nuclear ribosomal DNA.

The DNA sequences of the nuclear internal transcribed spacers (1 and 2) of nuclear ribosomal DNA were determined for 27 taxa of Fraxinus in an attempt to reconstruct the phylogeny of the genus and interpret its biogeographic history in a phylogenetic context. Minimal intra-individual and intraspecific polymorphisms were observed and infrageneric taxa were all recovered in phylogenetic analyses. The tree topologies estimated from parsimony and neighbor-joining analyses of one- and two-parameter substitution rates were congruent and supported by high bootstrap estimates. Indels were also found to be phylogenetically informative. The phylogenetic trees obtained were in quite good agreement with the current taxonomy of the genus but homoplasy was observed in the evolution of floral characters. Polyploidy also appeared to have evolved several times in the genus. Fraxinus nigra was more closely related to European and Asian taxa than to the other American ashes. The Asian F. platypoda had a closer relationship to Asian and European ashes in section Bumelioides than to its presumed North American allies of section Melioides. With respect to the position of F. quadrangulata as sister group to the remainder to the remaining species, the subgenus Fraxinus appeared paraphyletic. Phylogeographical analysis indicated that Fraxinus likely originated in North America, with two subsequent events of intercontinental migration from North America to Asia.