The precise taxonomy of the genus Allium is still poorly understood with incorrect descriptions being widespread. With over 850 species distributed over the Northern Hemisphere Allium is the sole genus in the Allieae, one of four tribes of subfamily Allioideae (Amaryllidaceae). New species continue to be described and Allium is both highly variable and one of the largest monocotyledonous genera, but the precise taxonomy of Allium is poorly understood, with incorrect descriptions being widespread. The difficulties arise from the fact that the genus displays considerable polymorphism and has adapted to a wide variety of habitats. Furthermore, traditional classifications had been based on homoplasious characteristics (what turn out to be independently evolved similar features in species from different lineages). However, the genus has been shown to be monophyletic, containing three major clades, although some proposed subgenera are not. Some progress is being made using molecular phylogenetic methods, and the internal transcribed spacer (ITS) region, including the 5.8S rDNA and the two spacers ITS1 and ITS2, is one of the more commonly used markers in the study of the differentiation of the Allium species. Allium includes a number of taxonomic groupings previously considered separate genera (Caloscordum Herb., Milula Prain and Nectaroscordum Lindl.) Allium spicatum had been treated by many authors as Milula spicata, the only species in the monospecific genus Milula. In 2000, it was shown to be embedded in Allium.
Description The genus Allium are herbaceous geophytes is characterised by bulbs enclosed in membranous tunics, that may become fibrous and may be carried on rhizomes, with tepals that are free or almost free, and a subgynobasic style. The majority of species produce cysteine sulphoxides that are the source of their distinctive garlic and onion odor and taste. About twenty species are grown as edible crops, such as onions, garlic and leeks, while others are foraged from the wild, such as ramps. Many species are xerophytic and the over 850 species are found almost exclusively in the Northern Hemisphere, being particularly diverse in the warm dry summers and cool wet winters of the Mediterranean. The main centre of diversity is the Old World with species rich areas in Central Asia as well as the Mediterranean Basin. A second centre, in the New World, is western North America.
History
Descriptions of Allium taxonomy date back at least as far as Carolus Clusius' Rariarum plantarum historia (1601). When Linnaeus formerly described the genus Allium in his Species Plantarum (1753), there were thirty species with this name. He placed Allium in a grouping he referred to as Hexandria monogynia (i.e. six stamens and one pistil) containing 51 genera in all. In 1763, Michel Adanson, who proposed the concept of families of plants, included Allium and related genera as a grouping within 'Liliaceae' as Section IV, Les Oignons (Onions), or Cepae in Latin. De Jussieu is officially recognised as the first formal establishment of the suprageneric grouping into families (Ordo) in 1789. In this system Allium was one of fourteen genera in Ordo VI, Asphodeles (Asphodeli), of the third class (Stamina epigyna) of Monocots. Jean Henri Jaume Saint-Hilaire (1805), who developed the concept of Amaryllidaceae, continued Jussieu's treatment of Allium under Asphodeli (which he considered synonymous with Adanson's Liliaceae and Jussieu's Asphodeli). He placed Allium in an unnamed monotypic section of Asphodeli defined as Fleurs en ombelle, racine bulbeuse. Calice à six parties egales (umbellate flowers, bulbous, calyx of six equal parts). Subsequently, de Candolle reverted the family name back to Liliaceae from Asphodeli. He divided the Liliaceae into a series of Ordres, and the second ordre was named Asphodèles, based on Jussieus' family of that name, in which he placed Allium. The term 'Alliaceae' then reappeared in its subfamilial form, Allieae, in Dumortier's Florula Belgica (1827), with six genera. The 'Alliaceae' have been treated as Allieae within the family Liliaceae (or Aspholecaceae, a partial synonym) by most authorities since. Regel produced a major monograph of the genus in 1875, and this remained the major reference work for over 100 years till the molecularly based study of Friesen and colleagues in 2006. Despite recent advances the precise taxonomy of Allium remains still poorly understood with incorrect descriptions being widespread.
Phylogeny
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