eMonocot Dioscoreaceae

Dioscoreaceae form the major part of a small but systematically and economically significant order of monocotyledons. The most diverse and important element of Dioscoreales is the yam genus, Dioscorea L., with over 600 accepted species names worldwide. Many yam species have edible tubers (Coursey 1967). There are three major cultigens, the winged yam (D. alata L.), the guinea yams (D. cayenensis Lam./ D. rotundata Poir.) and the lesser or Asiatic yam D. esculenta (Lour.) Burkill.

Dioscoreales (see below) are found in temperate and tropical regions worldwide, with the highest diversity being in the seasonally dry tropics of Central South America (Knuth 1924, Kirizawa et al. 2010), Mexico (Sosa et al. 1987, Tellez & Schubert 1994), the Caribbean (Raz 2004), South Africa (Knuth 1924), Madagascar (Burkill & Perrier 1950, Randriamboavonjy et al. in prep) and Indochina (Prain & Burkill 1936, 1938, Wilkin & Thapyai 2009) where Dioscorea species numbers are highest. The yam allies are either plants of the wet tropics, where the other genera of Dioscoreaceae (Burkill 1951, 1960, Burkill & Perrier 1950) and Burmanniaceae (Maas-van der Kamer 1998) occur, or plants of temperate and montane tropical habitats Nartheciaceae, Tamura 1998).

The papers by Caddick et al. (2002 a, b) showed that Dioscoreaceae is characterised by simultaneous microsporogenesis, a climbing habit, leaves with reticulate secondary veins and petioles with a basal and apical swelling (pulvinus). It comprises four genera, Dioscorea, Tacca J.R.Forst. & G.Forst., Trichopus Gaertn. and Stenomeris Planch., which can be differentiated using the characters in Table 1.

Many species of the genus Dioscorea are or have been sources of food and medicine. The tubers of at least 50 species are sources of dietary starch in both a subsistence and an economic context (Coursey 1967). They vary widely in their ease of collection, palatability and degree of chemical protection from herbivory. Some are only used in times of famine, such as D. dumetorum (Kunth) Pax in Subsaharan Africa (Wilkin 2001, 2009). Its tubers are near the soil surface, and thus easy to collect, but need to have toxic alkaloids removed before they can be eaten safely. Others are protected by being more deeply buried, for example many of the endemic species of Madagascar or the candle yam in Thailand (D. brevipetiolata Prain & Burkill). Spiny roots protect the tuber in some forms of D. esculenta. In many parts of the tropics wild yams are a vital food source, especially when staple grains run out. Perhaps the best example of this is Madagascar (Jeannoda et al. 2003, 2007; Randriamboavonjy et al., in prep.). Of the ca. 40 species in Madagascar, 32 are endemic, of which at least 24 are edible and 12 or more are eaten regularly in different parts of the country when rice stores have been exhausted. There are introduced, cultivated taxa but their use remains small-scale in gardens and the endemic wild yams are preferred. Similar edible taxa are encountered in the Caribbean in the radiation formerly separated as the genus Rajania L (Raz 2004). As Wilkin et al. (2005) showed, edible taxa are concentrated in the later branching clades of Dioscorea.

The picture is rather different in, for example, West Africa and New Guinea, where there is extensive field cultivation of the D. cayenensis/D. rotundata complex and D. alata and D. esculenta respectively as staple starch sources (Coursey 1967). However, yams are also taken from the forest in these areas (e.g. Hladik & Dounias 1996, Dounias 2001). In Africa this is often species such as D. praehensilis Benth., D. abyssinica Hochst ex Kunth., D. sagittifolia De Wild. or D. baya De Wild., supposed wild relatives of D. cayenensis/D. rotundata. People also transplant wild forest yams into their gardens (Hildebrand et al. 2002). In New Guinea D. nummularia Lam. appears to be a semi-domesticate (Malapa et al. 2005). Thus there is a continuum among food yams from taxa where human selection has been limited to those which have been extensively modified by it (e.g. Dumont et al. 2006).

In economic terms, medicines have perhaps been an even more important product of Dioscorea than food. Many species are rich in steroidal saponins. In particular, the New World species D. mexicana Schiedw., D. composita Hemsl. and D. floribunda M. Martens & Galeotti were used in the 1950s and 60s as the source plants for the contraceptive pill (Coursey 1967, Marks 2001).  Other taxa rich in steroidal compounds include D. deltoidea Wall. ex Griseb., D. sylvatica Eckl. and its relatives (Wilkin et al. 2010) and the genus Tacca. As well as synthetic human hormones, yam steroids have also been used to make corticosteroid drugs such as cortisone. The range of medicinal uses of steroids is vast (Sparg et al. 2004). However, such compounds are today usually synthesised de novo rather than precursors being sourced from plant material. It is the earlier branches of Dioscorea which are the richest in steroidal saponins (Wilkin et al. (2005).

Preliminary data from Thailand (Wilkin & Thapyai 209) suggest that 17 of 43 (ca. 40%) Dioscoreaceae species should be given IUCN Red List categories of Near Threatened, Vulnerable, Endangered or Critically Endangered (IUCN 2001). In Madagascar and the Comoros (Randriamboavonjy et al., in prep), this figure is 18/42 (ca. 43%). Thus there is a significant need for targeted species conservation. D. bako Wilkin. (Wilkin et al. 2008) is a good example, a large yam which remarkably was not discovered by the many botanists of the 19^th and 20^th centuries who worked in Madagacar. It is edible, and preferred by people living in the small area of central Western Madagascar to which it is endemic. They are already reporting that it is harder to find than it used to be. Burmanniaceae and Nartheciaceae also contain significant numbers of threatened taxa. The research on yam conservation, ethnobotany, domestication and sustainable use necessitated by this conservation challenge needs to be underpinned by a stable and universal taxonomy. There have been numerous floristic studies in last 30 years, especially of Dioscorea, but no global treatment of Dioscoreaceae since Knuth (1924). Thus there is a need for a reliable guide to accepted names, descriptions, identification tools and rich supporting information. It is hoped that delivering a global treatment of the family via this scratchpad will help to facilitate closer collaboration between applied yam researchers and systematists.

The affinities of Dioscoreales, the taxa which comprise it and their relationships to each other were uncertain for many years, but significant progress has been made since 1995. It has been shown to be sister to Pandanales (e.g. The Angiosperm Phylogeny Group 2009) and to comprise three families, Burmanniaceae, Dioscoreaceae and Nartheciaceae (Caddick et al. 2002a, b). Until the 1990’s, Burmanniaceae were linked to Orchidaceae and Nartheciaceae to Liliaceae respectively. Dioscoreaceae was thought to belong to a group of plant families with reticulate-veined leaves, which were the most closely-related monocots to the dicots.

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