Editing Dioscoreales (section)

==Taxonomy==

=== Pre-Darwinian ===
For the early history from [[John Lindley|Lindley]] (1853){{sfn|Lindley|1853}} onwards, see Caddick ''et al.'' (2000) Table 1,{{sfn|Caddick et al|2000}} Caddick et al. (2002a) Table 1{{sfn|Caddick et al|2002a}} and Table 2 in Bouman (1995).{{sfn|Bouman|1995}} The [[Taxonomy (biology)|taxonomic]] classification of Dioscoreales has been complicated by the presence of a number of [[morphology (biology)|morphological]] features reminiscent of the [[dicotyledons]], leading some authors to place the order as intermediate between the monocotyledons and the dicotyledons.{{sfn|Bouman|1995}}

[[File:Dioscorea - Hooker.jpg|thumb|Male ''Dioscorea batatas'' ([[Dioscorea polystachya|''D. polystachya'']]) in Hooker's ''A General System of Botany'' 1873 ]]
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)<ref name=Lindleydictyogens/> The [[botanical authority]] is given to [[von Martius]] (1835) by APG for his description of the family Dioscoreae or ''Ordo'',<ref name=martius/> while other sources<ref name=taxonomicon/> cite Hooker (Dioscoreales [[Hook.f.]]) for his use of the term "Dioscorales" in 1873<ref name=hooker/> with a single family, Dioscoreae.<ref name=hookerdioscoreae/> However, in his more definitive work, the ''Genera plantara'' (1883), he simply placed Dioscoraceae in the Epigynae "Series".<ref name=bhdiosc/>

===Post-Darwinian===
Although [[Charles Darwin]]'s [[Origin of Species]] (1859) preceded Bentham and Hooker's publication, the latter project was commenced much earlier and [[George Bentham]] was initially sceptical of [[Darwinism]].<ref name=Stuessy/> The new [[phyletic]] approach changed the way that taxonomists considered plant classification, incorporating [[evolutionary]] information into their schemata, but this did little to further define the [[circumscription (taxonomy)|circumscription]] of Dioscoreaceae. The major works in the late nineteenth and early twentieth century employing this approach were in the [[German language|German]] literature. Authors such as [[August Eichler|Eichler]],<ref name=Eichler/> [[Adolf Engler|Engler]]<ref name=Engler/> and [[Richard Wettstein|Wettstein]]<ref name=Wettstein/>  placed this family in the [[Liliiflorae]], a major subdivision of monocotyledons. it remained to [[John Hutchinson (botanist)|Hutchinson]] (1926){{sfn|Hutchinson|1959}} 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]] and [[Roxburghiaceae]]. Of these only Trichopodaceae was included in the [[Angiosperm Phylogeny Group]] (APG) classification (see below), but was subsumed into Dioscoraceae. Stenomeridaceae, as ''[[Stenomeris]]'' was also included in Dioscoreaceae as subfamily Stenomeridoideae, the remaining genera being grouped in [[subfamily]] Dioscoreoideae.{{sfn|Bouman|1995}} Roxburghiaceae on the other hand was segregated in the [[sister group|sister order]] [[Pandanales]] as [[Stemonaceae]]. Most taxonomists in the twentieth century (the exception was the 1981 [[Cronquist system]] which  placed most such plants in order [[Liliales]], subclass [[Liliidae]], class [[Liliopsida]]=monocotyledons, division [[Magnoliophyta]]=angiosperms) recognised Dioscoreales as a distinct order, but demonstrated wide variations in its composition.{{sfn|Caddick et al|2002a}}{{sfn|Bouman|1995}}

[[Rolf Dahlgren|Dahlgren]], in the second version of his taxonomic classification (1982){{sfn|Dahlgren|Clifford|1982}} raised the Liliiflorae to a [[superorder]] and placed Dioscoreales as an order within it. In [[Dahlgren system|his system]], Dioscoreales contained only three families, Dioscoreaceae, [[Stemonaceae]] (''i.e.'' Hutchinson's Roxburghiaceae) and [[Trilliaceae]]. The latter two families had been treated as a separate order ([[Stemonales]], or Roxburghiales) by other authors, such as [[Herbert Huber (botanist)|Huber]] (1969).{{sfn|Huber|1969}} The APG would later assign these to Pandanales and [[Liliales]] 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 the [[Asparagales]]. 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 the [[monogeneric]] Taccaceae, then in its own order, Taccales. Similar considerations were discussed with respect to two Asparagales families, Smilacaceae and Petermanniaceae.{{sfn|Dahlgren|Clifford|1982}}

In Dahlgren's third and final version (1985){{sfn|Dahlgren|Clifford|Yeo|1985}} that broader circumscription of Dioscoreales was created within the superorder [[Lilianae]], subclass [[Liliidae]] (monocotyledons), class [[Magnoliopsida]] (angiosperms) and comprised the seven families [[Dioscoreaceae]], [[Petermanniaceae]], [[Smilacaceae]], [[Stemonaceae]], [[Taccaceae]], [[Trichopodaceae]] and [[Trilliaceae]]. [[Thismiaceae]] has either been treated as a separate family closely related to [[Burmanniaceae]] or as a [[tribe (biology)|tribe]] (Thismieae) within a more broadly defined Burmanniaceae, forming a separate order, [[Burmanniales]], in the Dahlgren system.{{sfn|Merckx|Smets|2014}} The related [[Nartheciaceae]] were treated as tribe Narthecieae within the [[Melanthiaceae]] in a third order, the [[Melanthiales]], by Dahlgren.{{sfn|Dahlgren|Clifford|Yeo|1985}} Dahlgren considered the Dioscoreales to most strongly resemble the ancestral monocotyledons, and hence sharing "dicotyledonous" characteristics, making it the most central monocotyledon order.{{sfn|Bouman|1995}} 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.{{sfn|Bouman|1995}}

=== Molecular phylogenetics and the Angiosperm Phylogeny Group ===
The increasing availability of [[molecular phylogenetics]] methods in addition to morphological characteristics in the 1990s led to major reconsiderations of the relationships within the monocotyledons.{{sfn|Chase et al|1993}} In that large multi-institutional examination of the [[seed plants]] using the [[plastid]] [[gene]] [[Rubisco|''rbc''L]] the authors used [[Dahlgren's system]] as their basis, but followed [[Robert F Thorne|Thorne]] (1992){{sfn|Thorne|1992}} in altering the [[suffixes]] of the superorders from "''-iflorae''" to "''-anae''".{{efn|name=suffix}} This demonstrated that the Lilianae comprised three lineages corresponding to Dahlgren's orders Dioscoreales, Liliales, and Asparagaless.

Under the [[Angiosperm Phylogeny Group]] [[APG system|system]] of 1998,{{sfn|APG I|1998}} which took Dahlgren's system as a basis, the order was placed in the [[monocot]] [[clade]]  and comprised the five families [[Burmanniaceae]], Dioscoreaceae, Taccaceae, [[Thismiaceae]] and Trichopodaceae.

In [[APG II]] (2003),{{sfn|APG II|2003}} a number of changes were made to Dioscoreales, as a result of an extensive study by Caddick and colleagues (2002),{{sfn|Caddick et al|2002a}}{{sfn|Caddick et al|2002b}} using an analysis of three [[genes]], [[RuBisCO|''rbc''L]], ''atp''B and [[18S rDNA]], in addition to [[morphology (biology)|morphology]]. These studies resulted in a re-examination of the relationships between most of the genera within the order. Thismiaceae was shown to be a [[sister group]] to Burmanniaceae, and so was included in it. The [[monotypic]] families Taccaceae and Trichopodaceae were included in Dioscoreaceae, while [[Nartheciaceae]] could also be grouped within Dioscoreales. [[APG III system|APG III]] (2009){{sfn|APG III|2009}} 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,{{sfn|Merckx et al|2009}}{{sfn|Merckx et al|2010}}{{sfn|Merckx|Smets|2014}} the [[APG 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 between ''[[Thismia]]'' and Burmanniaceae,{{sfn|Hertweck et al|2015}} and hence whether Thismiaceae should be subsumed in the latter, or reinstated.{{sfn|APG IV|2016}}

=== Phylogeny ===
[[Molecular phylogenetics]] in Dioscoreales poses special problems due to the absence of [[plastid]] [[genes]] in mycoheterotrophs.{{sfn|Merckx et al|2009}} Dioscoreales is [[monophyletic]] and is placed as a [[sister group|sister order]] to [[Pandanales]], as shown in Cladogram I.{{sfn|Hertweck et al|2015}}{{sfn|APG IV|2016}}

{{cladogram
| title={{anchor|Clad2}}Cladogram I: The [[phylogenetics|phylogenetic]] composition of the monocots.{{sfn|APG IV|2016}}
| align=center
| cladogram={{clade|style=font-size:92%;line-height:100%;;width:500px;

        | label1=monocots
        | 1={{clade
           |1={{clade
            |1=[[Acorales]]
            |2={{clade
               |1=[[Alismatales]]
               |2={{clade
                 |1=[[Petrosaviales]]
                 |2={{clade
                   |1={{clade
                     |1='''Dioscoreales'''
                     |2=[[Pandanales]]
                     }}
                  |2={{clade
                   |1=[[Liliales]]
                   |2={{clade
                     |1=[[Asparagales]]
                     |label2= [[commelinids]]
                     |2= {{clade
                        |1=[[Arecales]]
                        |2=[[Poales]]
                        |3={{clade
                           |1=[[Zingiberales]]
                           |2=[[Commelinales]]
                           }}
                        }}
                      }}
                    }}
                   }}
                 }}
               }}
             }}
            }}

         }}
     }}

=== Evolution ===
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 clade [[Pandanales]] split up around 121 million years ago during Early Cretaceous when the [[stem group]] was formed. Then it took 3 to 6 million years for the [[crown group]] to differentiate in Mid [[Cretaceous]].

=== Subdivision ===
The three families of Dioscreales constitutes about 22 genera and about 849 species{{sfn|Christenhusz|Byng|2016}} making it one of the smaller monocot orders.{{sfn|Merckx et al|2010}} Of these, the largest group is ''[[Dioscorea]]'' (yams) with about 450 species. By contrast the second largest genus is ''[[Burmannia (plant)|Burmannia]]'' with about 60 species, and most have only one or two.{{sfn|Merckx et al|2010}}

Some authors,{{sfn|Merckx|Smets|2014}} preferring the original APG (1998)families, continue to treat Thismiaceae separately from Burmanniaceae and Taccaceae from Dioscoreaceae.{{sfn|Merckx et al|2010}} 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.{{sfn|Hertweck et al|2015}}

{{cladogram
| title={{anchor|Clad2}}Cladogram II: Relationship of Dioscoreales families{{sfn|Hertweck et al|2015}} (number of genera){{sfn|Christenhusz|Byng|2016}}
| align=center
| cladogram={{clade|style=font-size:92%;line-height:100%;;width:500px;

| label1=Dioscoreales
        |1={{clade
         |1={{clade
             |1= [[Dioscoreaceae]] (9)
             |2= [[Burmanniaceae]] (8)
              }}
         |2= [[Nartheciaceae]] (5)
               }}
             }}
            }}

=== Etymology ===
Named after the [[type genus]] ''[[Dioscorea]]'', which in turn was named by [[Carl Linnaeus|Linnaeus]] in 1753 to honour the Greek physician and botanist [[Dioscorides]].{{sfn|Bouman|1995}}