YAMS OF MADAGASCAR:
SYSTEMATICS
INTRODUCTION
The Dioscoreaceae (yam family) is, like
many others, very rich in Madagascar
in both numbers of species and unique
forms. A knowledge of the systematics
of the family is vital if we are to sustainably
utilise and conserve its species. Work
on the systematics of Malagasy yams is
being carried out by Dr. Paul Wilkin
at RBG, Kew in partnership
with scientists at the Parc
Botanique et Zoologique de Tsimbazaza
in Antananarivo and the Département
de Biologie et Ecologie Végétales,
University of Antananarivo.
Yams are members of the genus Dioscorea,
and are important to Malagasy people,
especially on a local scale, as food
providers or as medicines derived from
the forest or from small-scale cultivation.
The tubers are used as a starch source.
They vary in their degree of edibility;
some can be eaten raw (e.g. Dioscorea
soso, Dioscorea fandra);
others are simply boiled or baked (e.g Dioscorea
nako), while some need extensive
preparation such as immersion in running
water for 1-3 days or drying in the sun
(e.g. Dioscorea antaly). Traditional
medicinal uses are a feature of Dioscorea,
since the genus is rich in steroidal
saponins. The most frequently encountered
medicinal use of yams in Madagascar is
the treatment of burns, ulcers and other
skin complaints with the bulbils of Dioscorea
bulbifera.
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The fruits of a fascinating yam relative found only in eastern
Madagascar, Trichopus sempervirens (formerly Avetra
sempervirens). This species has male and female parts in
one flower (like most plants). The true yams in Dioscorea have
male and female flowers on separate plants. In addition, the
fruit has a single large ruminate seed and is indehiscent, while
Dioscorea capsules are dehiscent and usually contain six winged
seeds.
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INTRODUCTION
Comparative morphology
is the primary method for research
on Malagasy yam systematics.
A specimen database has been
produced in order to simplify
the mechanics of production of
the taxonomic account.
Malagasy yams are also being used in studies of micromorphology
and molecular systematics which aim to discover the phylogeny of
the genus Dioscorea as a whole.
Studies of ethnobotany, ecology and biochemistry are being planned
with colleagues at University of Antananarivo.
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A female plant
of Dioscorea fandra from
near Isalo in SW Madagascar
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A bulbil of Dioscorea
bulbifera, or Hofika in Merina
RESULTS
The previous classification
in the Flore de Madagascar contains 32
species of the genus Dioscorea; 27 are
endemics (not found elsewhere), three
are cultigens and two are widely dispersed
African taxa. This research has already
yielded a checklist of 42 species from
Madagascar. The 10 additional taxa are
all endemics. Two have already been described
in Kew Bulletin - D. karatana from
Mt. Vatovavy in eastern Madagascar and D.
namorokensis from Namoroka in the
North-West. A complete treatment will
be produced by 2005. Comparative studies
have revealed that all of the endemic
Malgasy yams form a single monophyletic
clade except D. antaly from
the West and North-West. This species
has a particularly unusual branching
tuber. The endemic genus Avetra was shown
to belong in the same genus as Trichopus
zeylanicus, an Asian taxon.
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A male plant of Dioscorea
ovinala from Namoroka in
NW Madagascar
A female plant
of Dioscorea ovinala (Angaroka
in Sakalava) from NW Madagascar
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FUTURE WORK
1) A full systematic account will be completed by 2005, including
keys to male and female plant material, descriptions and illustrations
2) Fieldwork in the North of Madagascar is needed to study a number
of important edible endemic species.
3) A book on the yams of Madagascar, combining systematics with
applied research, is being planned.
4) The specimen database will be used in GIS-based studies
of biogeography.
5) A phylogeny of Malagasy yams will be produced in order to study
character evolution and biogeography; it will be based on ITS or
ncpGS sequence data.
The tuber of Dioscorea ovinala (Angaroka in
Sakalava or Oviala in
Merina)
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Digging for tubers
of Dioscorea soso (Bemandry in
Sakalava) at Antsanitia near Mahajanga,
NW Madagascar
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CONTACT
Paul Wilkin, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
Email
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