Sunday, April 23, 2017

[Botany • 2017] Dioscorea irodensis • A New Species of Critically Endangered Edible Yam (Dioscoreaceae) Endemic to northern Madagascar and Its Conservation

 Dioscorea irodensis  Wilkin, Rajaonah & Randriamb.

Morphological character data are used to show that a distinct morphotype of Dioscorea L. from the Irodo valley (East of Sadjoavato) in Antsiranana Province of Madagascar is an undescribed species, differing in its leaf arrangement, pubescence form and male inflorescence structure from all other taxa. It is described as Dioscorea irodensis Wilkin, Rajaonah & Randriamb., illustrated and a distribution map and ecological information provided. It is known from three sites, but is likely to have been eradicated from one of them. The population that has been studied in the field contains a very low number of adult plants. Tubers have been extracted for use as food at a level that appears to be unsustainable. Thus its provisional IUCN conservation status assessment is that it is critically endangered (CR). Its vernacular name in Irodo is Bemandry.

Key Words: conservation, critically endangered, distribution, edible, ex situin situ, Madagascar, morphology, new species, yam 

Fig. 1 Vegetative and reproductive organ morphology in Dioscorea irodensis.
 A habit of fruiting plant, showing leaves borne in clusters of up to 7 on short herbaceous branches; B detail of coarse, erect to spreading indumentum; C dehisced capsule; D seed and seed wing; E habit of male flowering plant with leaves in early development; F part of a male inflorescence showing condensed cymules of flowers; G a single cymule showing the cymule bract and inflorescence axis indumentum; H side view of a male flower showing pubescent tepal external surfaces; J half male flower showing three stamens, torus morphology and pistillode; K flower viewed from above showing inner and outer tepal whorls and anther presentation; L tuber drawn following its usual orientation in the soil and curved apex that subtends a shoot.
Scale bars: A, E = 2.5 cm; B = 5 mm; C, D = 2 cm; F = 4 mm; G = 2 mm; H – K = 1.5 mm; L = 6 cm.
 drawn by Lucy Smith. 
 DOI: 10.1007/s12225-017-9677-6  

Fig. 2 Photographs of Dioscorea irodensis showing its vegetative morphology in fruit and underground organs.
A leaves and an infructescence reduced to a solitary submature capsule; B node with a cluster of leaves on a short lateral shoot showing an infructescence at dehiscence; C two tuber apices (current (marked a) and previous year (b)) and separated body of current year’s tuber (c) with stem and leaves disentangled from surrounding vegetation. The size and curved apex of the tuber (indicated by arrows) linked to its horizontal habit is shown. 

Dioscorea irodensis Wilkin, Rajaonah & Randriamb., sp. nov. 

Type: Madagascar, Antsiranana: Diana, Antsiranana II, Anivorano, Irodo, ala fady S of village towards Irodo river estuary, 12°39'6.3"S 49°31'38.2"E, ♀ fr. 8 Feb. 2015, P. Wilkin, J. A. Kennerley, F. Rakotonasolo, M. Hamido & M. Tsaratiana 1675 (holotype TAN!; isotype K!).

Recognition. Tuber horizontal in soil with a curved apex (derived from digging up two plants and inferring a similar position in others from the shape of extraction holes and the Malagasy vernacular name). Leaves in clusters of up to 7 on short herbaceous branches, clustered particularly towards bases of main vegetative stems (Fig. 1A, 2B), blades thin in texture, densely pubescent below and when immature but coarse and never forming a tomentum as in Dioscorea ovinala. Plants at the locality near Irodo represented by Wilkin et al. 1674 and 1675 often have white spotting on their leaf blades which has the appearance of pathogenic infection (see Fig. 2A). Male inflorescences (Fig. 1 E, F) with irregularly spaced, spirally arranged dense cymules of 2 – 8 pedicellate flowers or rarely solitary, vs flowers (sub)sessile, or rarely with a pedicel to 0.5 mm long and solitary or in groups of 2 – 4 (D. ovinala). Fruit not inflated and fleshy during development but thin-lobed and capsular throughout, single layered at dehiscence. Restricted to a small area East of Sadjoavato in Antsiranana Province.

Distribution and habitat. Endemic to the Irodo river Valley and Sahafary forest in Antsiranana Province (Map 1) at altitudes from 30 – 230 m. It is possible that the three collections from South of the village of Irodo represent two subpopulations on either side of the river as opposed to two distinct populations (the term population is used here in an ecological sense, except in the Conservation Status section), but the specimen from Sahafary forest is spatially isolated. At Irodo, it is found in semi-deciduous forest with a canopy to 10 m and a clear shrub layer, the principal canopy trees being Colvillea and Tamarindus, with Pachypodium in the shrub layer. Soil a brown sandy loam, possibly alluvial, or red sand over limestone bedrock. The Sahafary Forest specimen states that it was from scrub forest on red sand.

Etymology. Named for the village of Irodo and the Irodo River valley where this species is found.

Vernacular name. Bemandry in the village of Irodo. This name appears to be applied particularly to species with horizontal tubers, for example the element of Dioscorea soso Jum. & H. Perrier sensu lato with the same tuber habit.

Uses. Tuber edible cooked. Rather watery following cooking and thus not fully satisfying the appetite. Dioscorea maciba, known as batatala in Irodo, is the more sought after species of the two that are found in the Irodo valley and surrounding areas. Not believed to be sold in markets but consumed usually in March/April.

Paul Wilkin, James A. Kennerley, Mamy Tiana Rajaonah, Geodain Meva Huckël, Feno Rakotoarison, Tianjanahary Randriamboavonjy and Stuart Cable. 2017. A New Species of Critically Endangered Edible Yam Endemic to northern Madagascar, Dioscorea irodensis (Dioscoreaceae) and Its Conservation. Kew Bulletin. 72; 15.  DOI: 10.1007/s12225-017-9677-6

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