Phylogeny and Phylogeography of the Circumpolar GenusFraxinus(OLEACEAE) Based on Internal Transcribed Spacer Sequences of Nuclear Ribosomal DNA

Abstract

The DNA sequences of the nuclear internal transcribed spacers (1 and 2) of nuclear ribosomal DNA were determined for 27 taxa ofFraxinusin an attempt to reconstruct the phylogeny of the genus and interpret its biogeographic history in a phylogenetic context. Minimal intraindividual 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 nigrawas more closely related to European and Asian taxa than to the other American ashes. The AsianF. platypodahad a closer relationship to Asian and European ashes in sectionBumelioidesthan to its presumed North American allies of sectionMelioides.With respect to the position ofF. quadrangulataas sister group to the remainder to the remaining species, the subgenusFraxinusappeared paraphyletic. Phylogeographical analysis indicated thatFraxinuslikely originated in North America, with two subsequent events of intercontinental migration from North America to Asia.