Phylogeny of Elms (Ulmus, Ulmaceae): Molecular Evidence for a Sectional Classification

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Abstract
The approximately 45 woody species of Ulmus (Ulmaceae) have been placed in five to nine sections on the basis of morphological characters. Cladistic analyses of chloroplast DNA restriction site variation were employed to examine phylogenetic relationships among 29 Ulmus accessions, including representatives from all proposed sections and subsections, and Zelkova serrata. Sufficient variation was detected to construct cladograms with branches both well-resolved and supported. The cpDNA results are largely congruent with those based on nuclear ribosomal DNA. Inclusion of 18 morphological/chemical characters further resolved relationships within the genus. Intrageneric relationships implied by the molecular and combined cladograms differ from previous classifications in a number of respects. Three species, U. crassifolia, U. serotina, and U. thomasii, which have been placed in two or three sections, were found to form a well differentiated monophyletic group (sect. Trichoptelea). The maintenance of sections Anisoptelea and Trichocarpus and the recognition of subsections Foliaceae and Glabrae within section Ulmus are not supported. The inclusion of U. mexicana, sometimes treated as the distinct genus Chaetoptelea, within Ulmus is supported. The molecular evidence supports the distinctiveness of U. rubra and the recognition of two subgenera: Oreoptelea (sects. Blepharocarpus, Chaetoptelea, and Trichoptelea s. 1.) and Ulmus (sects. Lanceifolia, Microptelea, and Ulmus).

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