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| Dioscoreales | |
|---|---|
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| Dioscorea communis | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Monocots |
| Order: | Dioscoreales Mart.[1][2][3] |
| Type species | |
| Dioscorea villosa | |
| Families | |
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| Synonyms | |
TheDioscoreales are anorder ofmonocotyledonousflowering plants, organized under modernclassification systems, such as theAngiosperm Phylogeny Group or theAngiosperm Phylogeny Web. Among monocot plants, Dioscoreales are grouped with thelilioid monocots, wherein they are asister group to thePandanales. In total, the order Dioscoreales comprises three families, 22genera and about 850species.
Dioscoreales contains thefamilyDioscoreaceae, which notably includes theyams (Dioscorea) and several otherbulbous andtuberous plants, some of which are heavily cultivated as staple food sources in certain countries.
Certain species are found solely in arid climates (incl. parts ofSouthern Africa), and have adapted to this harsh environment ascaudex-forming,perennial caudiciformes, includingDioscorea elephantipes, the "elephant's foot" or "elephant-foot yam".
Older systems tended to place all lilioid monocots withreticulate veined leaves (such asSmilacaceae andStemonaceae together with Dioscoraceae) in Dioscoreales; as currentlycircumscribed byphylogenetic analysis, using combinedmorphology andmolecular methods, Dioscreales now contains many reticulate-veinedvines within the Dioscoraceae, as well as themyco-heterotrophicBurmanniaceae and theautotrophicNartheciaceae.
Dioscoreales arevines orherbaceous forest floor plants. They may beachlorophyllous orsaprophytic.Synapomorphies include tuberous roots, glandular hairs,seed coat characteristics and the presence ofcalcium oxalatecrystals.[5] Other characteristics of the order include the presence ofsaponinsteroids, annularvascular bundles that are found in both thestem andleaf. The leaves are often unsheathed at the base, have a distinctivepetiole andreticulate veined lamina. Alternatively they may be small and scale-like with a sheathed base. Theflowers areactinomorphic, and may bebisexual ordioecious, while the flowers orinflorescence bear glandular hairs. Theperianth may be conspicuous or reduced and thestyle is short with well developed style branches. Thetepals persist in the development of thefruit, which is a drycapsule orberry. In the seed, theendotegmen istanniferous and theembryo short.[6]
All of the species except the genera placed inNartheciaceae express simultaneousmicrosporogenesis. Plants inNartheciaceae show successivemicrosporogenesis which is one of the traits indicating that the family is sister to all the other members included in the order.
For the early history fromLindley (1853)[7] onwards, see Caddicket al. (2000) Table 1,[8] Caddick et al. (2002a) Table 1[5] and Table 2 in Bouman (1995).[9] Thetaxonomic classification of Dioscoreales has been complicated by the presence of a number ofmorphological features reminiscent of thedicotyledons, leading some authors to place the order as intermediate between the monocotyledons and the dicotyledons.[9]
While Lindley did not use the term "Dioscoreales", he placed the family Dioscoraceae together with four other families in what he referred to as an Alliance (the equivalent of the modern Order) called Dictyogens. He reflected the uncertainty as to the place of this Alliance by placing it as a class of its own between Endogens (monocots) and Exogens (dicots)[10] Thebotanical authority is given tovon Martius (1835) by APG for his description of the family Dioscoreae orOrdo,[3] while other sources[11] cite Hooker (DioscorealesHook.f.) for his use of the term "Dioscorales" in 1873[12] with a single family, Dioscoreae.[13] However, in his more definitive work, theGenera plantara (1883), he simply placed Dioscoraceae in the Epigynae "Series".[14]
AlthoughCharles Darwin'sOrigin of Species (1859) preceded Bentham and Hooker's publication, the latter project was commenced much earlier andGeorge Bentham was initially sceptical ofDarwinism.[15] The newphyletic approach changed the way that taxonomists considered plant classification, incorporatingevolutionary information into their schemata, but this did little to further define thecircumscription of Dioscoreaceae. The major works in the late nineteenth and early twentieth century employing this approach were in theGerman literature. Authors such asEichler,[16]Engler[17] andWettstein[18] placed this family in theLiliiflorae, a major subdivision of monocotyledons. it remained toHutchinson (1926)[19] to resurrect the Dioscoreales to group Dioscoreaceae and related families together. Hutchinson's circumscription of Dioscoreales included three other families in addition to Dioscoreaceae,Stenomeridaceae,Trichopodaceae andRoxburghiaceae. Of these only Trichopodaceae was included in theAngiosperm Phylogeny Group (APG) classification (see below), but was subsumed into Dioscoraceae. Stenomeridaceae, asStenomeris was also included in Dioscoreaceae as subfamily Stenomeridoideae, the remaining genera being grouped insubfamily Dioscoreoideae.[9] Roxburghiaceae on the other hand was segregated in thesister orderPandanales asStemonaceae. Most taxonomists in the twentieth century (the exception was the 1981Cronquist system which placed most such plants in orderLiliales, subclassLiliidae, classLiliopsida=monocotyledons, divisionMagnoliophyta=angiosperms) recognised Dioscoreales as a distinct order, but demonstrated wide variations in its composition.[5][9]
Dahlgren, in the second version of his taxonomic classification (1982)[20] raised the Liliiflorae to asuperorder and placed Dioscoreales as an order within it. Inhis system, Dioscoreales contained only three families, Dioscoreaceae,Stemonaceae (i.e. Hutchinson's Roxburghiaceae) andTrilliaceae. The latter two families had been treated as a separate order (Stemonales, or Roxburghiales) by other authors, such asHuber (1969).[21] The APG would later assign these to Pandanales andLiliales respectively. Dahlgren's construction of Dioscoreaceae included the Stenomeridaceae and Trichopodaceae, doubting these were distinct, and Croomiaceae in Stemonaceae. Furthermore, he expressed doubts about the order's homogeneity, especially Trilliaceae. The Dioscoreales at that time were marginally distinguishable from theAsparagales. In his examination of Huber's Stemonales, he found that the two constituent families had as close an affinity to Dioscoreaceae as to each other, and hence included them. He also considered closely related families and their relationship to Dioscoreales, such as themonogeneric Taccaceae, then in its own order, Taccales. Similar considerations were discussed with respect to two Asparagales families, Smilacaceae and Petermanniaceae.[20]
In Dahlgren's third and final version (1985)[22] that broader circumscription of Dioscoreales was created within the superorderLilianae, subclassLiliidae (monocotyledons), classMagnoliopsida (angiosperms) and comprised the seven familiesDioscoreaceae,Petermanniaceae,Smilacaceae,Stemonaceae,Taccaceae,Trichopodaceae andTrilliaceae.Thismiaceae has either been treated as a separate family closely related toBurmanniaceae or as atribe (Thismieae) within a more broadly defined Burmanniaceae, forming a separate order,Burmanniales, in the Dahlgren system.[23] The relatedNartheciaceae were treated as tribe Narthecieae within theMelanthiaceae in a third order, theMelanthiales, by Dahlgren.[22] Dahlgren considered the Dioscoreales to most strongly resemble the ancestral monocotyledons, and hence sharing "dicotyledonous" characteristics, making it the most central monocotyledon order.[9] Of these seven families, Bouman considered Dioscoreaceae, Trichopodaceae, Stemonaceae and Taccaceae to represent the "core" families of the order. However, that study also indicated both a clear delineation of the order from other orders particularly Asparagales, and a lack of homogeneity within the order.[9]
The increasing availability ofmolecular phylogenetics methods in addition to morphological characteristics in the 1990s led to major reconsiderations of the relationships within the monocotyledons.[24] In that large multi-institutional examination of theseed plants using theplastidgenerbcL the authors usedDahlgren's system as their basis, but followedThorne (1992)[25] in altering thesuffixes of the superorders from "-iflorae" to "-anae".[a] This demonstrated that the Lilianae comprised three lineages corresponding to Dahlgren's orders Dioscoreales, Liliales, and Asparagaless.
Under theAngiosperm Phylogeny Groupsystem of 1998,[26] which took Dahlgren's system as a basis, the order was placed in themonocotclade and comprised the five familiesBurmanniaceae, Dioscoreaceae, Taccaceae,Thismiaceae and Trichopodaceae.
InAPG II (2003),[27] a number of changes were made to Dioscoreales, as a result of an extensive study by Caddick and colleagues (2002),[5][28] using an analysis of threegenes,rbcL,atpB and18S rDNA, in addition tomorphology. These studies resulted in a re-examination of the relationships between most of the genera within the order. Thismiaceae was shown to be asister group to Burmanniaceae, and so was included in it. Themonotypic families Taccaceae and Trichopodaceae were included in Dioscoreaceae, whileNartheciaceae could also be grouped within Dioscoreales.APG III (2009)[29] did not change this, so the order now comprises three families Burmanniaceae, Dioscoreaceae and Nartheciaceae.
Although further research on the deeper relationships within Dioscoreales continues,[30][31][23] theAPG IV (2016) authors felt it was still premature to propose a restructuring of the order. Specifically these issues involve conflicting information as to the relationship betweenThismia and Burmanniaceae,[32] and hence whether Thismiaceae should be subsumed in the latter, or reinstated.[1]
Molecular phylogenetics in Dioscoreales poses special problems due to the absence ofplastidgenes in mycoheterotrophs.[30] Dioscoreales ismonophyletic and is placed as asister order toPandanales, as shown in Cladogram I.[32][1]
| Cladogram I: Thephylogenetic composition of the monocots.[1] |
The data for the evolution of the order is collected from molecular analyses since there are no such fossils found. It is estimated that Dioscoreales and its sister cladePandanales split up around 121 million years ago during Early Cretaceous when thestem group was formed. Then it took 3 to 6 million years for thecrown group to differentiate in MidCretaceous.
The three families of Dioscreales constitutes about 22 genera and about 849 species[33] making it one of the smaller monocot orders.[31] Of these, the largest group isDioscorea (yams) with about 450 species. By contrast the second largest genus isBurmannia with about 60 species, and most have only one or two.[31]
Some authors,[23] preferring the original APG (1998)families, continue to treat Thismiaceae separately from Burmanniaceae and Taccaceae from Dioscoreaceae.[31] But in the 2015 study of Hertwerk and colleagues, seven genera representing all three families were examined with an eight gene dataset. Dioscoreales was monophyletic and three subclades were represented corresponding to the APG families. Dioscoreaceae and Burmanniaceae were in a sister group relationship.[32]
| Cladogram II: Relationship of Dioscoreales families[32] (number of genera)[33] | |||||||||||||||
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Named after thetype genusDioscorea, which in turn was named byLinnaeus in 1753 to honour the Greek physician and botanistDioscorides.[9]
Species from this order are distributed across all of the continents exceptAntarctica. They are mainlytropical orsubtropical representatives, but some members of familiesDioscoreaceae andNartheciaceae are found in cooler regions ofEurope andNorth America. Order Dioscoreales contains plants that are able to form an underground organ for reservation ofnutritions as many othermonocots. An exception is the familyBurmanniaceae which is entirely myco-heterotrophic and contains species that lackphotosynthetic abilities.
The three families included in order Dioscoreales also represent three different ecological groups of plants.Dioscoreaceae contains mainly vines (Dioscorea) and other crawling species (Epipetrum).Nartheciaceae on the other hand is a family composed of herbaceous plants with a rather lily-like appearance (Aletris) whileBurmanniaceae is entirelymyco-heterotrophic group.
Many members ofDioscoreaceae producetuberousstarchyroots (yams) which formstaple foods in tropical regions. They have also been the source ofsteroids for thepharmaceutical industry, including the production oforalcontraceptives.
