Genome Size And Phylogenetic Inference In Allium Species

Agnès Ricroch1,2, Sophie Nadot1 and Roxana Yockteng1

1 Laboratoire Ecologie, Systématique et Evolution. CNRS UMR 8079. Bâtiment 360. Université Paris-Sud. 91405 Orsay. France
2 Chaire de Génétique évolutive et d’amélioration des plantes. Institut National Agronomique Paris-Grignon. 16. rue Claude Bernard. 75231 Paris Cedex 05. France

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    The genus Allium (Alliaceae) is of major economic importance since it includes vegetables, herbal crops and ornamentals. It exhibits a great diversity in various morphological characters and particularly in life forms (bulbs or rhizomes). The purpose of the present investigation was to examine possible correlations between nuclear DNA and GC contents and life cycle of 25 Allium species, including major vegetable crops and their wild allies, belonging to the three major subgenera Allium, Amerallium, and Rhizirideum. For the first time the GC content of the 25 species and the nuclear DNA content of A. cyaneum, A. montanum and A. vavilovii were investigated. Our results revealed a significant variation in nuclear content (2C and 1C DNA) as well as in the GC content, although no correlation could be found between GC content and nuclear DNA content. On the basis of the nuclear DNA content analysis, we suggest a revision of the taxonomic placement of two species. We propose to place A. montanum in section Rhizirideum instead of section Schoenoprasum, and we propose to place A. roylei in section Cepa rather than in section Oreiprason. A phylogenetic analysis based on ITS sequences offered strong support for the classification using morphological characters. Most subgenera were monophyletic, except Rhizirideum, which appeared paraphyletic due to the inclusion of the subgenus Allium. The subgenus Amerallium, which includes all species growing in the New World, appeared as a well-defined clade. The reconstruction of life form character evolution on the ITS tree showed that rhizomes in the genus Allium evolved independently several times. The bulbous life form was shown to be ancestral within the genus. Moreover, a correlation was found between ploidy and life form, rhizomatous species being mostly tetraploid.