Taxonomy of Allium
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).
The difficulties arise from the fact that the genus displays considerable polymorphism and has adapted to a wide variety of habitats.
Furthermore, traditional classications had been based on homoplasious characteristics (what turn out to be independently evolved similar features in species from different lineages).
[1] Allium includes a number of taxonomic groupings previously considered separate genera (Caloscordum Herb., Milula Prain and Nectaroscordum Lindl.)
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.
[1] The main centre of diversity is the Old World with species rich areas in Central Asia as well as the Mediterranean Basin.
[9] In 1763, Michel Adanson, who proposed the concept of families of plants, included Allium and related genera as a grouping within 'Liliaceae'[10] as Section IV, Les Oignons (Onions), or Cepae in Latin.
In this system Allium was one of fourteen genera in Ordo VI, Asphodeles (Asphodeli), of the third class (Stamina epigyna) of Monocots.
[13] He placed Allium in an unnamed monotypic section of Asphodeli defined as Fleurs en ombelle, racine bulbeuse.
[19] Despite recent advances the precise taxonomy of Allium remains still poorly understood with incorrect descriptions being widespread.
[26] In 2006 Friesen, Fritsch, and Blattner[19] described a new classification with 15 subgenera, 56 sections, and about 780 species based on the nuclear ribosomal gene internal transcribed spacers.
[2] Banfi and colleagues (2011) have suggested that the phylogenetic trichotomy of this genus Allium sensu lato is sufficiently distinct as to warrant splitting it into three separate genera.
[33] The three evolutionary lineages and 15 subgenera here represent the classification schemes of Friesen et al. (2006)[19] and Li (2010),[2] and subsequent additional species[34][31][35][30] and revisions.
[2] Disjunct distribution, involving the western Mediterranean (type species) and southwest Asia East Asia This large monophyletic subgenus is extremely diverse, both morphologically and ecologically and is characterised by leaves with one row of vascular bundles, absence of palisade parenchyma and a subepidermal position of laticifers, with a predominant base chromosome number x=7.
[5] Within N America, the genus covers most of the area south of the 53rd parallel, including the oak hillsides of California and Oregon, deserts of Nevada and Texas, alpine meadows of Utah and Idaho, prairies of Nebraska and Manitoba, and forest glades of Missouri and Arkansas.
[2] Of the five subgenera, the large Melanocrommymum and the oligo- or monotypic Caloscordum, Vvedenskya and Porphyroprason are bulbous and the remaining small subgenus Anguinum is rhizomatous.
East Asia (A. prattii, A. ovalifolium) This Eurasian subgenus, the second largest, is complex and has had a confusing taxonomic history and is extremely diverse, morphologically.
On the other hand, a number of monophyletic subgroups were recognised, with about 40 clades, although their exact relationships remained not fully resolved.
[1][28][40] Description: The subgenus is characterised by true tunicated bulbs, annual roots, leaves that are mostly broad and flat with subterranean sheath parts that are barely visible above the ground, scapes that are strong and most often strictly upright and of varying length, and large, fasciculate to globular inflorescences.
[28][2] History: Early (prior to 1950) classifications of Allium included many of the members of this subgenus within the bulbous section, Mollium based on morphological characteristics.
Molecular methods in the 1990s confirmed the identity of Melanocrommyum as a distinct monophyletic group, together with the presence of several subgroups, but the deeper relationships remained inconsistent.
[42] Kamelin (1973) provided an alternative arrangement of sections,[43] which was supplanted by the Gatersleben Allium Group classification (1992) which used a broad range of variables.
[24] The use of molecular markers to develop phylogeny began in the 1990s and showed that the subgenus was a well separated taxon with a number of subgeneric groupings.
While Melanocrommyum itself appeared monophyletic, most of the sections were either para- or polyphyletic, favouring the formation of a larger number of smaller subgroups.
The remaining sections are either oligogotypic with 2–8 species (Compactoprason, Pseudoprason, Miniprason, Brevicaule, Thaumasioprason, Verticillata) or monotypic (Acaule, Aroidea, Popovia).
The latter usage is associated with the presence of cysteine sulphoxides and also radical scavenger activity, although many members of the subgenus possess less of these compounds and lack the distinctive taste and smell of garlic and onion, their properties appear to be associated with dithiodipyrroles and sulfur-pyridins.
These substances also occur in the ornamental species, that were introduced into European and North American gardens in the early 19th century, and now are represented by an increasing number of named cultivars and hybrids.
5 sections Subgenus Allium, the youngest of the subgenera, is predominantly Mediterranean but its distribution extends east towards Central Asia.
[32] Section Codonoprason is strongly monophyletic[19][45] and has its centre of diversity in the Mediterranean region, particularly Greece and Turkey, but extends to other areas of Europe, North Africa and the Middle East.
