Molecular systematics of Cercocarpus H.B.K. (Rosaceae)
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Cercocarpus H.B.K. (Rosaceae) is a New-World genus comprised of montane xerophytic shrubs and trees found in deserts, chaparral, and montane regions of western North America. Cercocarpus has been shown to be infected by Frankia, a nitrogen-fixing soil actinomycete. Frankia strains provide their hosts with a source of fixed nitrogen, a nutrient that often controls plant growth. This dissertation set out to create molecular phylogenies for Cercocarpus for later studies of coevolutionary relationships between Cercocarpus and Frankia. Only by knowing the evolutionary histories of both lineages can an in-depth study of coevolution proceed. Cercocarpus lacks a consensus classification or phylogeny. Past confusion about the number and circumscription of species within Cercocarpus arose from a lack of clear morphological discontinuities between taxa. Variations in leaf morphology show a continuum within and between taxa, making delineation of species difficult. Due to the variation in morphology, extensive sampling was conducted of all described taxa of Cercocarpus. The external transcribed spacer (ETS) of the nuclear ribosomal DNA was developed in Cercocarpus for the purpose of phylogenetic reconstruction. We document unexpected, deep coalescence of paralogous ETS types in Cercocarpus. This is the first record of the maintainence of ETS paralogues that were not produced by recent hybridization and lack of concerted evolution or pseudogene formation. We produced three phylogenies for Cercocarpus taxa, one based on ETS sequence data, another on AFLP presence/absence data, and a third based on chloroplast intergenic spacer sequences. Areas of congruence between the three trees supports clear taxonomic conclusions. Most notably, C. montanus is distinct from C. betuloides, the two newly described species of Cercocarpus have affinities to C. fothergilloides, and the endangered C. traskiae is more closely related to C. betuloides than C. fothergilloides, as previously thought. Taxon sampling had a great impact on tree topology, highlighting the need to do thorough sampling for closely related plant groups. We also discovered all three independent phylogenies were significantly incongruent, suggesting a complex, reticulating evolutionary history.