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(American Journal of Botany. 2008;95:1030-1039.) doi: 10.3732/ajb.0800017 © 2008 Botanical Society of America, Inc.

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Phylogeny, biogeography, and rates of diversification of New World Astragalus (Leguminosae) with an emphasis on South American radiations¹

² Section of Evolution and Ecology, University of California, Davis, California 95616 USA ³ Molecular Ecology and Evolutionary Studies Laboratory, Department of Chemistry and Biology, Universidad de Santiago, Chile

ABSTRACT

This study uses phylogenetic relationships of New World representatives of the species-rich genus Astragalus (Leguminosae; Papilionoideae) to follow up on recent evidence pointing to rapid and recent plant diversification patterns in the Andes. Bayesian and maximum likelihood phylogenetic analyses were done using nuclear rDNA ITS and chloroplast spacers trnD-trnT and trnfM-trnS1, either separately or in combination. The effect of using partitioned vs. nonpartitioned analyses in a Bayesian approach was evaluated. Highest resolution was obtained when the data were combined in partitioned or nonpartitioned Bayesian analyses. All phylogenies support two clades of South American species nested within the North American species, implying two separate invasions from North to South America. These two clades correspond to the original morphological classification of Johnston (1947 Journal of the Arnold Arboretum 28: 336–409). The mean ages of the South American clades were very recent but still significantly different (1.89 and 0.98 Ma). Upper and lower bounds on rates of diversification varied between 2.01 and 0.65 species/Ma for the older clade and 2.06 and 1.24 species/Ma for the younger clade. Even the lower bounds are still very high, reasserting Neo-Astragalus in the growing list of recent rapid radiations of plants, especially in areas with a high physiographic diversity, such as the Andes.

Key Words: Astragalus • biogeography • chloroplast spacers • internal transcribed spacer • Leguminosae • phylogeny • rates of diversification

Received for publication 15 January 2008. Accepted for publication 3 June 2008.

FOOTNOTES

¹ The authors thank J. G. Burleigh for providing valuable methodological discussions and help with the analyses, E. Spencer for use of his laboratory to do part of the molecular work in Chile, and L. Faundez, N. García, M. F. Wojciechowski, and C. Hughes for providing plant material. They also thank two anonymous reviewers for helpful comments on the manuscript.

⁵ Present address: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721 USA

⁴ Author for correspondence (e-mail: rscherson{at}uchile.cl); present address: Institute of Ecology and Biodiversity, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile

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