TAXONOMY, ORIGIN AND DISTRIBUTION OF DIOSCOREA SPECIES: A WILD TUBER CROP

Sanjeet Kumar

sanjeet.biotech@gmail.com

Dioscorea species is commonly known as Yam worldwide. It has diverse vernacular names. In Odisha, it is known as Ban Aalu. Most of species are edible but with bitter taste. Some species are non-edible too. Rural and tribal communities consume the edible species after boiling or after through processing. Sometimes they leave the tubers in running water for removing bitterness taste. They cook the tubers with main meal. They have medicinal values. Most of the species are used to check birth control and to cure skin infections by the tribal people of Odisha. Several species of this genus serve as staple crops in many parts of the world (Mabberley, 1997; Martin, 1974). In general, all of the principal yam species are frost-intolerant and vigor is affected at temperatures below 20°C. A temperature range of 25-35°C is common in the majority of the yam producing districts and Coursey (1967) suggests that the rate of growth of Dioscorea increases with an increase in temperature. Coursey (1967) does note that extremely high temperatures coupled with dry conditions are deleterious to the vigor and growth of the plant. The majority of yams, both wild and cultivated, are found in regions of the yam zone that receive anywhere from 1-3 m of rainfall annually (Al-Shehbaz and Schubert 1989, Coursey, 1967). The plant is documented to occur at altitudes between 200-1300 m (Wilkin, 2001). The “civilization of the yam,” or “the yam zone” as it has also been referred to, includes regions of West Africa extending from central Ivory Coast in the west to the Cameroon mountains on the eastern edge of the range and from the forested areas in the north to the more humid savannahs comprising portions of the southern perimeter of the region (Ayensu and Coursey, 1972).

Taxonomy of Dioscorea species

The genus Dioscorea, published in 1753 by Linnaeus (Genera Plantarum), was named after the Greek physician “Pedinios Dioscorides”, who was a medical officer in the Roman army at the time of Nero and authored the most comprehensive tome on herbal medicine of the time, De Materia Medica Libri quinque (Coursey, 1967). Dioscorea is in the family Dioscoreaceae, which is assigned to the order Dioscoreales. Recent molecular evidence suggests that two other families should be included in the order; the Burmanniceae and the Nartheciaceae (Caddick et al. 2002), both of which are represented in North America. The Burmanniaceae genera found in North America are: Apertia, Burmannia and Thismia. Apertia is represented by one species, A. aphylla, Burmannia by three; B. biflora, B. capiata and B. flava, and Thismia by one, T. Americana (Lewis, 2003). Apertia aphylla and the three Burmannia spp. occur in Florida (Wunderlin and Hansen, 2003).

The Nartheciaceae is represented by three genera in North America (Nathecium, Aletris and Lophiola) (Utech, 2003), the latter two of which occur in Florida. There are five Aletris species in the state (A. aurea, A. bracteata, A. obovata, A. lutea and A. farninosa and one Lophiola (L. aurea) (Wunderlin and Hansen, 2003).

Following the circumscription of Caddick et al. (2002), the family Dioscoreaceae includes 4 genera; Dioscorea, Trichopus, Tacca and Stenomeris, although molecular phylogenetic studies by Merckx et al. (2006) place Tacca as a sister to the tribe Thismieae of the Burmanniaceae. In the New World, only Dioscorea and Tacca are found. Tacca is represented by one South American species, T. parkeri. Dioscorea is by far the largest genus in the family, with the number of species estimated to be from 350-400 (Caddick et al. 2002) to 850 (Al-Shehbaz and Schubert 1989).

An early treatment of Dioscorea divided the genus into 4 sub-genera, which were further divided into 60 sections (Knuth, 1924). Using this classification, D. bulbifera was placed in the sub-genus Helmia, in section Opsophyton subsection Euopsophyton. Burkhill (1960) introduced an alternate classification of the Old World yams, but he did not use sub-genera. He recognized 23 sections of Dioscorea, including a redefined Opsophyton in which he placed D. bulbifera. The other invasive yam in Florida, D. alata (winged yam), was placed in the section Enantiophyllum (Knuth, 1924; Burkhill, 1960).

Based on anatomical characters, Ayensu (1972) recognized 30 sections of Dioscorea, including section Opsophyton in which D. bulbifera was placed (under sub-section Euopsophyton). Wilkin et al. (2005) indicated that the genus required a complete taxonomic revision, which should be based on DNA. He tentatively separated species into 8 clades based on sequences of two plastid genes. Dioscorea bulbilfera was placed in the ‘compound leaf’ clade (even though air potato does not have compound leaves), which also included three species from Thailand, two fromMadagascar and one from Malawi.

Wilkin et al. (2005) placed D. alata in the Enantiophyllum, as had previous classifications. Within the continental United States, two native Dioscorea are found; D. floridana and D. villosa, along with four exotic species; D. alata, D. bulbifera, D. polystachya (formerly D. oppositifolia) and D. sansibarensis. The latter species, which was only known from Miami-Dade Co. and one location in Collier Co., may now have been eradicated (Pemberton, pers. comm.). A seventh species, D. quaternata, was reported in the past (Al-Shehbaz and Schubert 1989; USDA, NRCS 2002; Wunderlin and Hansen, 2003) but has recently been synonymized with D. villosa (Raz, 2002).