Sheet n°304 - september 2008
The role of the slave trade in the evolution
of American wild rice species
© IRD / Olivier Barrière
uman domestication
of rice is contemporary with the dawn of
agriculture, less than 10
000 years ago. Cultivation of this cereal began
in Asia and from that
base the practise appears to have spread, to
conquer the rest of the
world, even though its
domestication also took
place independently in
West Africa. The wild
species of rice are now
distributed everywhere the tropics, including the Americas where their existence was
up to now presumed
to date back much further. A study IRD scientists have published recently showed on the
contrary that the appearance of wild rice species in the Neotropics
(from southern Mexico
to northern Argentina)
is linked to the European-based slave trade.
In America, hybridization between several cereals of the Oryza genus
imported from the Old
World would have given
rise to new wild species.
Crossings between wild
rice and cultivated rice
could have occurred
spontaneously in the
rice fields kept in cultivation by the slaves.
This particular feature
could make South American wild rice varieties
potential candidates for
developing a new type
of cultivated rice, both
more resistant and more
productive.
© IRD / Jean-Pierre Montoroi
H
West African rice field just before harvest. Once cut, the rice panicles are carefully arranged in sheaves.
Rice is the world’s foremost cereal
crop as a human food source. Today’s cultivated varieties derive from
the species Oryza sativa and Oryza
glaberrina, domesticated respectively
in Asia and West Africa. Besides these
two domestic taxa, there are around 20
species of wild rice of the genus Oryza,
all located in the tropics. Their common
ancestor appears to have emerged from
Eurasia about 50 million years B.P. For
several decades, scientists have been
using molecular biology techniques to
explore the evolutionary path which has
led to the current diversity of wild rice
species. A study published by an IRD research team has shed light on the origin
of the settlement of wild rice on the American continent.
Three colonization theories for evolution of rice in the Neotropical region
have been confronting each other for
many years. The first one takes continental drift as the basis for explaining
the current distribution of American rice
in the tropical zone, considering that
their ancestors already existed before
Africa separated from South America.
The second postulates the major role
of natural seed dispersion across extremely long distances, with migrating birds
as agents. The third links the arrival of
rice in America to the great exploratory
voyages and to the slave trade run by
Europeans between West Africa and
America. Recent dating methods using
the “molecular clock” were applied to the
species present in the American tropical
zone. This method, founded on the principle that genetic mutations accumulate
in a genome at a rate globally proportional to the time that has elapsed, arrived
at an estimated date for the appearance
of American wild rice of 300 000 years.
However, a relative genetic proximity for
certain fractions of the genome of these
cereals suggests a much more recent
origin by the interaction of crossings
between different species. To overcome
the constraints imposed by molecular study of a hybrid species grouping
together several genomes in one, the
IRD team took advantage of available
historical documentation. Investigation
of the main European herbarium collections compiled by XVIIth and XIXth Century naturalist explorers who travelled
around the American continent revealed
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CONTACT :
GÉRARD SECOND
Unité mixte de recherche
Diversité et Adaptation
des Plantes Cultivées
(Cirad-IRD-Inra-Montpellier
SupAgro-Université
Montpellier 2)
Adress :
IRD
BP 64501,
34394 Montpellier
Cedex 5
Tel : +33 (0)4 67 41 63 09
gerard.second@ird.fr
REFERENCE :
Second G., Rouhan G.,
Human-Mediated Emergence as a Weed and
Invasive Radiation in the
Wild of the CD Genome
Allotetraploid Rice Species (Oryza, Poaceae)
in the Neotropics, PLoS
ONE, 2008, 3(7): e2613.
Doi:10.1371/journal.pone.0002613
that almost all the rice specimens listed
during this period belong to the same
hybrid species. Moreover, contrary to
what the first flora describe concerning
a number of neighbouring species, the
distribution area of American rice is in
fact clearly delimited by the activity of
Europeans. Examination of the historical
literature also shows that the triangular
trade that became established from the
XVIth Century between Europe, West
Africa and America was the decisive
factor in the appearance of the first species of the genus Oryza on the American
continent. At that time, seeds were first
transported inadvertently in the holds
of slave-ships. It was also established
that sailors deliberately took with them
seeds of a range of exotic plant species,
including wild rice, on their many transoceanic voyages. The hybrid rice first
emerged among crops cultivated on the
Caribbean islands, were then transported to the mainland by sea, mixed with
stocks of cultivated rice seed. The presence of the hybrid rice species inland
was for a long time limited to a perimeter
around the first Brazilian gold mines that
had attracted the colonists with their slaves. The location of this mine on the watershed between the River Amazon and
the River Paraguay then enabled new
rice species, born of crosses between
wild rice of the Old World and domes-
tic rice, to be conveyed by the current of
the two rivers and gradually to invade a
large area of South America.
The particularly vigorous American rice
therefore seems to be the fruit of human
migrations across the oceans. They appear to have the special feature of combining in the same genome the genetic
characters of many different wild species
of the Old World and rice that was already being grown there. This arrangement
provided them with a number of qualities,
including abundant seed production,
ability to be pollinated by cultivated rice
varieties, or high resistance to disease.
Confirmation of this theory at molecular level could make these Neotropical
species potential candidates for development of a new type of domestic rice. In
contrast to a hybrid rice currently being
developed by China, the “super-hybrid” rice that could be derived from
domestication of American species
would offer the advantage that ricegrowers could multiply it empirically,
in the same way as for the traditional
varieties they grow already.
Grégory Fléchet - DIC
Translation - Nicholas Flay
KEY WORDS :
rice, migration, genome,
domestication
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© IRD / Germinal Rouhan
Sheet n°304 - september 2008
For further information
Map showing a possible path for the evolution of wild rice species in the tropical
Americas.
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
Tél. : +33(0)4 91 99 94 90 - fax : +33(0)4 91 99 92 28 - fichesactu@ird.fr