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First published online August 13, 2009; doi:10.3732/ajb.0800400 American Journal of Botany 96: 1627-1635 (2009) © 2009 Botanical Society of America, Inc.

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Molecular evidence of reticulate evolution in the subgenus Plantago (Plantaginaceae)¹

² National Institute for Basic Biology, National Institutes of Natural Science, Nishigonaka 38, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan ³ Yamagata University, Faculty of Science, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan ⁴ Graduate School of Science, University of Tokyo, Science Building #2, 7-3-1 Hongo, Tokyo 113-0033, Japan

ABSTRACT

Polyploidization is a frequent evolutionary event in plants that has a large influence on speciation and evolution of the genome. Molecular phylogenetic analyses of the taxonomically complex subgenus Plantago were conducted to elucidate intrasubgeneric phylogenetic relationships. A nuclear-encoding single-copy gene, SUC1 (1.0–1.8 kb), was sequenced in 24 taxa representing all five sections of the subgenus Plantago and two taxa from subgenus Coronopus as the outgroup. Fifteen known polyploids and one putative polyploid were sampled to examine polyploid origins and occurrence of reticulate evolution by cloning and sequence analysis of SUC1. Phylogenetic relationships were estimated using maximum parsimony, neighbor-joining, and Bayesian analyses. For the first time, our analysis provides a highly resolved phylogenetic tree. Subgenus Plantago formed a well-supported monophyletic clade. In contrast, alleles from polyploid species were scattered across the whole SUC1 phylogenetic tree, and some independent allopolyploids originated from hybridization between distant lineages. One reason for this taxonomic complexity can be attributed to reticulate evolution within the subgenus Plantago. Our results also suggest the possibility of two independent long-distance dispersals between the northern and southern hemispheres.

Key Words: allopolyploid • molecular phylogenetics • Plantaginaceae • Plantago • polyploids • reticulate evolution • SUC1

Received for publication 2 December 2008. Accepted for publication 15 April 2009.

FOOTNOTES

¹ The authors thank the Royal Botanic Gardens, Kew, UK, DNA Bank for providing DNA specimens; Dr. K. K. Shimizu (University of Zurich) for providing P. alpina; Dr. Y. Omori (Yokosuka City Museum) for providing P. major var. japonica; Dr. Hiroshi Ikeda (University of Tokyo) for determining the chromosome numbers of P. major, P. major var. japonica, and P. formosana; and Dr. K. I. Christensen (University of Copenhagen), who kindly helped the authors examine specimens in the Botanic Garden and provided seeds of P. uniglumis and P. raoulii. This study was supported by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan, the Sumitomo Foundation, and the Interdisciplinary Research Institute of Environmental Sciences.

⁵ Author for correspondence (e-mail: tsukaya{at}biol.s.u-tokyo.ac.jp)

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