Origin of African rice from Asian rice
N.M. Nayar*
Department of Botany, University of Kerala, Kariavattom 695 581, Trivandrum, Kerala, India.
Abstract
The usual assumption is that African cultivated rice (Oryza glaberrima Steud.) originated about 3500 years ago
from the annual wild rice, O. barthii Cheval, and Asian cultivated rice (O. sativa L.) was introduced into west
Africa in the late 15th century CE by Europeans. Evidence from several social and biological science disciplines
have been collated and/or re-interpreted to draw the following inferences. (1) There are several accounts of rice
culture in northern and western Africa from early centuries of the Common Era. (2) The Senegambian
monoliths, which have been linked to the time of origin of African rice 3500 years ago, have been carbon-dated
to the 7th century CE. (3) Neolithic culture had hardly reached tropical west Africa ca 3500 years ago. (4) The
characteristics of O. barthii — bigger and bolder grains, higher yields, easy crossability with the two rices,
intermediate features of African and Asian rice, and weediness — suggest that it is a hybrid. (4) The rarity of O.
glaberrima even at the time of its initial identification, the acceptance by local farmers of having both cultivated
rices growing together, repeated discovery of O. glaberrima-like forms in non-African rice regions, and the
prevalence of close colinearity between the two cultivated rice species at the molecular and chromosomal levels
suggest close relationship between them. These data indicate that Asian rice was probably introduced into west
Africa during the early centuries of the Common Era, that African rice arose from the Asian rice only later than
this, and that the wild rice, O. barthii, is a hybrid derivative of the two cultivated rices.
Introduction
Rice belongs to the genus Oryza L., family Poaceae. It is a small genus of 20–25 species with a pan-tropical and
sub-tropical distribution. Two species of the genus are cultivated — Oryza sativa, the universally cultivated
Asian rice, and O. glaberrima, the west African cultivated rice. African rice is now only rarely grown in pure
stands, but it is instead grown in mixture with the Asian rice in various proportions. The extent of even this form
of mixed cultivation is diminishing as it is being replaced with ‘pure’ Asian rice.
Porteres (1945, 1950, 1956, 1962, 1976) is almost the only author to have worked on the origin of African
rice. He had suggested that it originated from the annual wild rice, O. barthii, about 3500 years ago in the Inland
Niger river Delta (Mali) (IND). He also proposed that Asian rice was introduced into west Africa by European
traders and colonialists in the 15th and later centuries. Some of his propositions, including the time of origin,
have since been found to be incorrect (Gray, 1962; Posnansky and McIntosh, 1976). However, his original
views on the origin of African rice continue to persist in the literature.
A fair amount of information on north and west African history and archaeology and also rice molecular
biology has become available since the mid-20th century. In this paper, an attempt has been made to collate and
interpret the available information on the origin of African rice from various sources, including archaeology,
history, ecology, botany, genetics, cytogenetics and molecular biology. Brief accounts on the distribution of rice
species in west Africa, climate and ecology, and history and archaeology are also given, primarily as
background information.
Rice in west Africa
Five wild Oryza species occur in Africa, including in west Africa (Vaughan, 1994; Clayton et al., 2005).
Among them, O. barthii and O. longistaminata Cheval & Roehr. have the widest and densest distributions
(Table 1). Both occur in extensive stands in aquatic and/or seasonally wet situations, and also as weeds in rice
fields. Oryza barthii is an annual self-fertilizing species, while O. longistaminata is a tall, robust, rhizomatous
perennial species. Oryza longistaminata occurs in more stable habitats. It is partially self-incompatible and
cross-fertilizing (Nayar, 1958, 1967). The remaining three wild species have restricted and scattered
distributions (Table 2).
West Africa is about 6 million km2 in area, and rice occupies about 8% of the total crop area, ranking fifth
in area, after millets (subfamily Panicoideae, various genera) (21% area), sorghum (Sorghum bicolor (L.)
Moench) (19%), maize (Zea mays L.) (12%) and cassava (Manihot esculenta Crantz) (9%); and rice is then
followed by yams (Dioscorea spp.) (5%) (FAOSTAT, 2009).
Climate and ecology
In west Africa, the southern and southwestern regions receive more than 2000 mm precipitation annually and
the northern savannahs bordering the Sahara Desert less than 100 mm. The region has several rivers flowing to
the sea. Some of these rivers have coastal floodplains and some others coastal mangrove vegetation; there are
*
Email: [email protected], [email protected].
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
1.18.1
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
Table 1. Characteristics of the three common (AA genome) west African rice species
Character / reference
Habit
Ligule length, shape
Roschevicz (1931)
Chatterjee (1948)
Tateoka (1962, 1963)
Clayton et al. (2005)
Vaughan (1994)
Spikelet length, breadth
Roschevicz (1931)
Chatterjee (1948)
Tateoka (1962, 1963)
Vaughan (1994)
Clayton et al. (2005)
Spikelet hairiness
Roschevicz (1931)
Chatterjee (1948)
Tateoka (1962, 1963)
Vaughan (1994)
Clayton et al. (2005)
Other features
Roschevicz (1931)
Chatterjee (1948)
Tateoka (1962, 1963)
Vaughan (1994)
Clayton et al. (2005)
1.18.2
O. glaberrima
Annual
O. saitva
Annual, but can be perennated
O. barthii
Annual
3–4 mm, orbicular
Up to 6 mm
Mostly less than 6 mm,
sometimes up to 10 mm;
tip always round or truncate
1.5–2 mm long, truncate,
an ecilitate membrance
<13 mm long
15–45 mm, membranous,
acicular; entire, or often 2-lobbed
15–45 mm in lower leaves
15–50 mm in lower leaves, very rarely
somewhat shorter; glabrous, very rarely
scantily hairy
15–30 mm long, entire or lanceolate, acute;
an eciliate membrance
15–45 mm in lower leaves; tip acute
3–4 mm, oblong, entire or somewhat notched,
glabrous, membranous
Up to 6 mm
Mostly less than 6 mm in lower leaves,
sometimes up to 10 mm;
tip always round or truncate
2–6 mm long, trunctate or obtuse; an eciliate
membrance
<13 mm long
7–8 × 2.5–3 mm
6.5–7 × 2 mm
>7 mm long
>7 × 2.9–3.6 mm
7–8 mm long
6.5–7.0 × 2 mm
6.5–7.0 × 2 mm
>7 mm long, sometimes shorter
Usually 4–8.5 × 2–4 mm
8–11 × 2.5–3.5mm
10–11 × 3.0–3.5 mm
10–11 mm long
>7mm long
7.8–11.0 × 2.8–3.4 mm
7–11 × 2.5–3.4 mm
Densely covered with hard rows
of small tubercles;
completely glabrous or
rarely with single, short hairs
Perfectly glabrous
Perfectly, or almost perfectly,
glabrous, some times hispid
Usually almost glabrous;
not hispid
Surface reticulate, glabrous or
hispid
Hard, densely covered with rows of fine
tubercles; with sparse pubscence of hard
thorn-like hairs
Hard, densely covered rows of fine tubercles,
with some pubescence consisting of very
small, hard cilia
Not given
Not given
Not given
Always hispid
Not given
Hispid
Surface reticulate, glabrous
Surface reticulate, glabrous or hispid
Almost unbranched
inflorescence branches in groups
of 1–2
Panicle branches undivided
Usually awnless, when present
with minute bristles or smooth
Spikelets persistent to almost
shattering
Spikelets elliptic or oblong,
laterally compressed, persistent
Inflorescence branching twice or thrice
Inflorescence branching twice, rarely thrice, in
groups of 1–3 in lower parts
Not given
Awns present or absent
Not given
Always awned
Not given
Deciduous
Spikelets persistent, elliptic or oblong,
laterally compressed
Spikelets oblong, laterally compressed, falling
entire
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
Table 2. Spikelet morphology and distribution of three west African wild rice species
Particulars (reference)
O. brachyantha
O. eichingeri
O. punctata
Distribution
Burkina Faso, Cameroon,
Central African
Angola, Benin, Cameroun,
(Vaughan, 1994)
Central Africa, Guinea, Mali, Republic, Côte
CAR, Chad, Congo, Côte
Niger, Senegal, Sierra Leone, d’Ivoire, Kenya,
d’Ivoire, Ethiopia, Ghana,
Sudan, Tanzania, Zaire,
Rwanda, Sri
Kenya, Madagascar, Malawi,
Zambia
Lanka, Tanzania,
Mozambique, Nigeria,
Uganda, Zaire
Sudan, Swaziland, Tanzania,
Uganda, Zaire, Zambia,
Zimbabwe
Genome group
FF
CC
BB, BBCC
(Vaughan, 1994)
Fertile spikelet size
6.5 × 9.25 × 1.25–1.5 mm
4.5–5.8 ×
49–62 × 1.9–2.6 mm
(Clayton et al., 2005)
1.5–1.8
mm
(length × breadth × thickness)
Caryopsis size
(Clayton et al., 2005)
4.0–4.75 mm
Not given
Not given
also some inland swamps. The local inhabitants have taken rice cultivation to high levels of refinement using
only simple tools (Carney, 2001; Linares, 2002; Fields-Black, 2008).
West Africa is broadly divisible into three zones from south to north — forest (humid, ‘semihot’), savannah
(guinea savannah, grass savannah and sudan savannah), and desert. All three zones stretch from west to east and
are each 100–400 km broad. Rice is cultivated here in five situations — rainfed upland, rainfed lowland,
irrigated, inland deepwater and mangrove swamps (Buddenhagen, 1978).
The palaeoclimate of the region has been discussed by several authors (Nicolson, 1976; Hasan, 1997;
Vernet, 2002), who have all highlighted the overwhelming influence of the Sahara Desert on the climate and
human behavior in the region, including their migrations, and food habits. Since agricultural origins took place
in the Holocene epoch (12 000 BP to present), we shall mainly discuss that period (Table 3).
Table 3. Holocene climate of west Africa
Period
Environment / activities
20 000–14 000 BP
Exceptionally dry
12 000–8000 BP
Very humid
8000–2000 BP
Intermittent desiccations and wet periods
8000–4000 BP
Humid period suddenly ends
3500–2600 BP
Beginnings of agropastoral society
3000–2000 BP
Continuous climatic oscillations; movement of northerners to south begins
300 BCE to 300 CE
Very arid environment
300–1100 CE
More stable environment; new farming systems develop, increased sedentism,
evidences for pre-Arab east–west and north–south trade and commerce
From 12th century CE The benign period ends; experiences less predictable, but hotter climate as now
400–800 CE
Much human activity; from 700 CE, active trade with north Africa,
Mediterranean, east Africa, also possibly in Indian Ocean
Source: Nicolson (1976); Hasan (1997); MacEachern (2005); McIntosh (2006).
The very arid environment of 300 BCE to 300 CE was followed by a more stable period up to ca 1100 CE. The
period 300–1100 CE saw increased sedentism (MacEachern, 2005) and the development of new farming systems
using wild and semi-wild plants (including grasses), cattle, and fish (Connah, 1976; Neumann, 1999). There was
much human activity during 400–800 CE, including extensive trade connections with north Africa and the
Mediterranean region, and possibly also across the Indian Ocean (McIntosh, 2006). By 800-1000 CE, west
Africa (savannah and Sahel regions) had established extensive trade contacts with the Arabs (MacEachern,
2005). Various locations in this region have shown evidence of early pre-Arab, trans-Saharan trade in both the
directions, north–south and east–west. This is attested by the presence of several artifacts originating from
external trading systems, the most notable ones being the bronzes, beads and burial chambers obtained from
Igbo-Ukwa, southeast Nigeria (9th century CE) (Insoll and Shaw, 1997). The benign climate period came to an
end in about the 12th century CE (McIntosh, 2006). Since then, west Africa has been experiencing less
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
1.18.3
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
predictable, but hotter climates (Table 3). The climate, ecology and ‘physiology’ of the region has had profound
influence on all aspects of human endeavor, including agriculture and food habits, since prehistoric times.
Agricultural origins
Agriculture originated comparatively late in west Africa. Moreover, herding preceded crop agriculture here
(Phillipson, 2005), unlike in other regions. The earliest archaeobotanical evidence for crop growing comes from
Mauritania, Ghana, Burkina Faso, Mali and Nigeria (after 2000 cal BCE) and consisted of wild pearl millet
(Pennisetum glaucum (L.) R.Br.) impressions on pottery. However, evidence for the transition of wild forms to
domesticated forms have yet to be found. Much less is known about the other indigenous crops such as cowpea
(Vigna unguiculata (L.) Walp.), sorghum, rice and yams (Kahlheber and Neumann, 2007). This can be
attributed to various reasons — the low density of settled people, the regular movement of people from and to
the region, and the relatively plentiful supply of aquatic resources, small game, and several semi-wild and
primitive domesticates of okra (Hibiscus esculentus L.), cowpea, bambara groundnut (Vigna subterranean (L.)
Verdc.), oil palm (Elaeis guineensis Jacq..), wild rice, other wild grasses, and several multipurpose trees.
Rice archaeology of Africa
Archaeological data on the place and timings of rice domestication in Africa are few (Murray, 2005). There are
only three reports of rice from archaeological sites in west Africa, but the taxonomic identity of the material is
unclear.
The McIntoshs’ intensive studies in Jenne Jeno (Mali) have contributed to developing a clear understanding
of the prehistory of the region (McIntosh, 1985, 1994, 2006). The earliest phase of occupation started here
during 250 BCE to 50 CE. The population was heavily reliant on aquatic resources besides a small domesticated
cow (Bos sp.) that had been introduced earlier from northeast Africa. There is no evidence of domesticated
plants. In Phase II (50-500 CE), however, they “recovered a substantial sample of well preserved chaff of O.
glaberrima and of its wild annual form O. barthii”, and the associated charcoal yielded a carbon date of 40-50
CE. However, they did not have the unique inner glume characters in O. glaberrima by which its identity could
be confirmed (Table 1) (Roschevicz, 1931; Terrell et al., 2001). Hence, the specific identity of the rice chaff
needs to be viewed with caution.
The second report is from Kursakata, Nigerian Lake Chad basin (Gajiganna culture, 1000 BCE — to 1000
CE) (Neumann, 2003; Klee et al., 2000). The authors obtained carbonized seeds or fruits of 54 taxa, including
rice. The rice samples consisted of 407 charred and 7 uncharred grains. The caryopses were oblong and laterally
flattened, and measured 5.0–7.0 mm (length, L), 1.0 mm (breadth, B) and 1.5–2.2 mm (thickness, T) — they
identified them as O. longistaminata.
The third report is from Dia (middle IND), Mali (Bedaux et al., 2001; Murray, 2005, 2007). It had four
phases, radiocarbon-dated to 791–413 BCE, 785–411 BCE, 799–403 BCE and mid-first millennium CE. Rice was
one of the most frequently encountered materials. The husked grains measured 4.35 (L), 1.39 (B) and 2.02 mm
(T). Murray (2005, 2007) studied the ancient rice grains by charting L–B and L–T — ratios on two axes, and
prepared scatter diagrams using 134 ancient grains, 91 modern O. glaberrima grains, 68 modern O. barthii
grains, and Katayama’s (1992, 1994) data for O. longistaminata.
Murray (2005, 2007) observed extensive overlap of grain dimensions among the material she studied.
Although she mentioneds the difficulty in determining their species status from the collected grains, she
concludedthat the majority of ancient grains most closely resembled modern African rice. She further remarked
that the ancient grains were consistently smaller than modern material, and showed little change in shape or size
from 800 BCE to the 16th century. The smaller size and lack of change suggested to the author that for more than
2000 years, human selection focused more on ‘other plant traits’ than on increasing grain size, but she does not
specify what these traits were.
There is a difficulty in using length–breadth–thickness ratios for discriminating different species. For
example, the three measurements, 5.00 × 1.25 × 1.50 mm, 8.00 × 2.00 × 2.40 mm, and 10.00 × 2.50 × 3.00 mm,
will give the same L–B and L–T ratios of 4.00 and 3.30, respectively, and they will plot in the same position on
a scatter diagram, even though they may represent three different African species. The seeds that Murray (2005)
recovered could have been of one of the wild Oryza species that occurs in the region, possibly O. brachyantha
A.Chev. & Roehr., the spikelet dimensions of which match those obtained by Murray (Table 2). Further, the
early humans are unlikely to have negatively selected for the second most important economic character in an
agricultural crop (the first may be reduced shattering; see Takahashi, 1955; Nayar, 1958, 1973; Doebley, 2006)
— namely, from bolder grains and more panicles to smaller grains and lower yields. This has been the instinct
of early humans during the domestication processes.
A significant archeological finding of rice has been from the Red Sea port of Berenike. It was one of the
main transshipment ports during the height of the Roman Empire. The trade used to be three way — among
Rome, the African coast, and the west coast of India and beyond. The discovered grains were of Asian rice,
possibly from the Malabar coast of India (inferred from the presence of dried black peppercorns and coconut
1.18.4
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
shell; Van der Veen, 1999; Cappers, 2007). The trade is assumed to have begun in prehistoric times and peaked
during the 5th to 12th centuries CE (Sherif, 1981; McPherson, 1993; Parker, 2008).
History of rice in the region
The first historical account of the introduction of rice into Africa appears to be that of Alexander the Great (4th
century BCE). He introduced rice into Egypt after his invasion of India (Roschevicz, 1931; Lewicki, 1974). But
its cultivation did not establish then. In Egypt, rice cultivation began only in 639 CE (Porteres, 1950). Earlier,
Strabo (63 BCE to 20 CE), the Greek historian and philosopher, observed the growing of rice in Cyrinaica
(Libya) in about 12 CE (Lewicki, 1974). This settlement (Cyrinaica) was on one of the main caravan routes
between north and west Africa.
Madagascar received the first Asian rices probably as early as about 3000 BP, when the first settlers from
the Far East came to the Tuliar region (southwest). The second major migration came from Indonesia in the
early Common Era, and the third one from south India 10–12 centuries later (Lu and Chang, 1980).
Madagascar’s southeast and south Asian connections have been noted by several authors, as we shall see later.
East Africa also obtained many rice varieties from India also during the historical period (Porteres, 1950; Lu and
Chang, 1980).
The Islamic scholar, al-Bakri provided (in 1068 CE) the first account of rice cultivation along the Niger
River in west Africa, (Lewicki, 1974; Carney, 2001). He described the then prevailing cultivation practices. The
well-known Moroccan traveller, Ibn Batuta journeyed along the Niger River (14th century), and noticed the
abundance of rice in the IND region (Gibb, 1929). Similar observations of rice culture in west Africa were made
by several early travellers to the region, long before the first Europeans arrived in the early 15th century
(Lewicki, 1974).
Thus, rice has been cultivated in northern and western Africa for about the last 2000 years. But what cannot
be determined with certainty is the identity of the cultivated rice — African or Asian, or even one of the wild
species, O. barthii and O. longistaminata, that occur widely in the region. The chances are that they were Asian
rice, since these rices appear to have arrived in west Africa much earlier from the north, northeast and/or east
Africa.
African rice, O. glaberrima was described as a new species in 1858 by Steudel. The material that Steudel
used had been collected in 1845–1848 by Jardin, a French naval officer. Steudel distinguished O. glaberrima
from O. sativa on the basis of the glabrousness of spikelets and leaves (Porteres, 1955). Leprieur, a French
pharmacist working in French Guinea, had earlier collected O. glaberrima (in 1826), but he had mislabeled it as
O. sativa. Porteres (1955) re-examined the herbarium sheet of Leprieur (available at the National Natural
History Museum, Paris), which contained two panicles. Porteres identified one as O. sativa and the second as O.
glaberrima. Thus, this becomes the earliest herbarium sheet of O. glaberrima. The next collection of O.
glaberrima was made 51 years later, in 1899, by Chevalier in upper Senegal. Incidentally, these may be
indicative of both the rarity of O. glaberrima even at the time of its first collection (19th century) and the
difficulty in distinguishing O. glaberrima from O. sativa under field conditions.
Other than from the north, some authors have suggested that rice could have come to west Africa from
either the northeast or the east across central Africa (Nayar, 1973; Carpenter, 1978). There has been active
maritime trade and movement of people across the Indian Ocean both from south and southeast Asia, and also
along the coast of east Africa, the Arabian peninsula, the coasts of Iran and Pakistan, Malabar, and Sri Lanka.
This was going on unequivocally from the 5th century CE — several authors believe from about 5th century
BCE. Almost half the present human population of Madagascar is of Indonesian stock, and the Malagasy
language that they speak, their culture, customs and architecture are all of Indonesian origin (Murdoch, 1959;
Mokhtar, 1981; Sherif, 1981; Verin, 1981; McPherson, 1993; Reade, 1996). The writings of Strabo (63 BCE —
to 19 CE), Pliny (23–79 CE) and ‘Periplus’ (50–70 CE, Schoff 1912) also mention these east Africa– — south
Asia ‘encounters’.
A noteworthy finding has been the recovery of banana (Musa sp.) phytoliths from Cameroun (Nkane) and
Uganda (Munsa) dated to 840-370 BCE (Mbida et al., 2006) and 4560-3640 BCE (Lejju et al., 2006),
respectively. The question then arises: if bananas, a south and southeast Asian fruit crop, could arrive in east and
central Africa between the first and fourth millennia BCE, could not Asian rice, the staple food of the people of
this region (southeast Asia) and possessing a far superior transportability and keeping quality than banana, also
have been brought along by the very same people who brought banana (and several other Asiatic plants like
sugarcane, jack fruit and mango) to Africa?
The ancestral species
African rice could only have originated indigenously, as there is no account of its introduction from elsewhere
— from other regions or continents. And it could have originated from only one of the locally growing rice
species.
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
1.18.5
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
The five wild Oryza species that occur in Africa, including west Africa, are O. barthii, O. longistaminata,
O. brachyantha, O. eichingeri Peter and O. punctata Kotschy ex Steud. (Tables 1, 2; Vaughan, 1994). The last
three species are usually ruled out as putative ancestral species of O. glaberrima, because of their different
genomic composition (BB, CC cf. AA of African rice), and differences in gross morphology (smaller spikelets,
smaller plants, etc.), life history, and other traits (Table 2). Asian rice was also not generally reckoned as a
putative ancestral species, because it is widely held as a recent introduction. Of the remaining two species, O.
barthii and O. longistaminata, the latter species too was excluded as the immediate putative ancestor because of
its robust, strongly rhizomatous perennial habit, strong reproductive barriers, and other characters. This leaves
us with O. barthii.
Oryza barthii as possible ancestor
Roschevicz (1931) was the first to propose O. barthii as the progenitor of the African rice. Porteres (1945, 1962,
1976) and Morishima et al. (1963) have supported this proposal.
Oryza barthii is an annual, self-fertilizing species. Along with the strongly perennial and partially selfincompatible species, O. longistaminata, it is widely distributed throughout tropical Africa (Oka and Chang,
1964; Vaughan, 1994; Clayton et al., 2005). Until recently, local people used to harvest the grains of both
species for food, use in rituals, and sale as special food, depending on the region where they were harvested.
They occur in wetlands — e.g. swamps, wetlands, river banks, seasonal water bodies, water channels — and as
weeds in rice fields (Oka and Chang, 1964; Oka et al., 1978). Generally, O. longistaminata occupies more
stable habitats and O. barthii marginal ones. The habitats of O. barthii are usually considered as characteristic of
weedy taxa (Baker and Stebbins, 1965). The seed set in O. barthii is good to normal.
In two extensive field surveys in west Africa, Oka et al. (1978) observed that more than two-thirds of rice
fields in west Africa contained mixtures of 2–4 rice species. Oryza sativa has steadily replaced O. glaberrima in
west Africa since the mid-20th century. Where present, most farmers grow them deliberately in a mixture.
Various reasons are given for this — difficulties in identification in early stages for weeding, both yielding
equally, insurance against unpredictable weather conditions, and so on. The two species also have similar
cooking quality (Oka and Chang, 1964; Oka et al., 1978).
The discriminating morphological characters of the African rice species are given in Tables 1, 2 and 5. The
main differences are in ligule length, panicle branching, spikelet hairiness and dimensions, and life-history
traits. There is no single character by which O. glaberrima can be distinguished from O. sativa. Several authors
have pointed out that both species possess parallel variation. One distinctive feature of O. barthii is its larger
spikelet dimensions vis-à-vis O. glaberrima and O. sativa. Generally, O. barthii is morphologically intermediate
between the two cultivated species (Tables 2 and 5).
Table 4. Pollen fertility and seed setting in different cross combinations
Species
O. sativa
O. glaberrima
O. barthii
O. sativa
†74.7 / 1–99%
–
–
815 / 51%
– / 42%
– / 38%
O. glaberrima
0.4 / 0–3%
95.8 / 43–99%
–
238 / 39%
145 / 62%
– / 59%
O. barthii
0.4 / 0–4%
95.6 / 22–99%
95.8 / 50–99%
68 / 32%
219 / 62%
79 / 58%
O. longistaminata
25.1 / 0–84%
29.3 / 0–49%
11.0 / 0–25%
– / 13%
– / 4%
– / 5%
O. longistaminata
–
–
–
–
–
–
63.3 / 25–85%
– / 3%
Source: Adapted from Chu et al. (1969).
† First row: mean and range of pollen fertility; second row: number of cross combinations tested and seed set; – no data.
Table 5. Characters of O. sativa, O. glaberrima, O. sativa × O. glaberrima and O. barthii
Character
sativa†
glaberrima†
sativa–glaberrima†
Ligule length (mm)
11.5–32.5
3.0–4.5
7.0–20.5
Spikelet (length ×
7.3–8.5 ×
8.1–8.5 ×
8.7–9.2 ×
breadth, mm)
2.8–3.6
3.3–3.6
3.0–3.3
Spikelet hairiness
Long to medium-long,
Trace
Long, dense
dense to moderate
Panicle length (cm)
14.9–21.0
18.1–21.7
14.8–22.4
Plant height (cm)
62.8 (49.9–71.9)
66.8 (57.9–76.5)
71.8 (59.5–80.0)
No. ear-bearing
6.2 (4–9)
8.0 (5–11)
7.0 (5–11)
tillers
barthii‡
2.0–130
7.0–11.0 ×
2.5–3.5
Glabrous or
hispid
Not given
Up to 1 m
Not given
† Source: Nayar (1973).
‡ Range: from Table 1.
1.18.6
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
The hybrids between the two cultivated species and theirs with O. barthii have been studied by several
authors (see Nayar, 1973 for a review). In extensive crossings (869 combinations/8774 pollinations), Chu et al.
(1969a) obtained 51% success in inter-varietal O. sativa crosses, 62% in inter-varietal O. glaberrima crosses,
58% in inter-varietal O. barthii crosses, 40% in O. sativa–O. glaberrima crosses, 35% in O. sativa–O. barthii
crosses, and 60% in O. glaberrima–O. barthii crosses (Table 4). While O. sativa crosses with both O.
glaberrima and O. barthii with moderate success, their F1 pollen fertility is very low.
Meanwhile, hybrids between O. glaberrima and O. barthii are highly fertile and F1 meiosis is largely
normal. The F1 hybrids showed dominance for nine characters of the wild parent, heterosis in four other
characters, and dominance of the O. glaberrima character in just one case (Table 5) (Nayar, 1967). The F2
generation segregated in normal Mendelian fashion (Oka and Chang, 1964). Natural populations of O. barthii
contain many intermediate types and those occurring in disturbed habitats are sometimes designated as O.
stapfiiRosch. The spikelets of O. barthii are bigger and bolder than those of O. glaberrima (Tables 1 and 5), as
is the 100-grain weight. The number of ear-bearing tillers is also greater in the former. These may be a
manifestation of heterosis (Table 5).
Hybrids between the two cultivated species have also been studied by several workers (see Nayar, 1973).
They are characterized by low levels of pollen stainability and nearly complete seed sterility, but they show low
levels of embryo-sac fertility. The most detailed studies of this hybrid have been made by Morinaga and
Kuriyama (1957) and Morishima et al. (1963) (Table 5). In the F1 hybrids, the ligule length is intermediate, and
spikelet hairiness is similar to that of O. sativa. The other characters are intermediate between those of the two
parents. Thus, the morphology and breeding behavior of the hybrid progenies closely resemble those of O.
barthii.
Sano and coworkers have extensively studied the prevalence of hybrid sterility in O. sativa–O. glaberrima
crosses. They are male sterile and female fertile. Four genes are responsible for causing sterility, of which two
are gamete eliminators and the other two are a pollen-killer and a sporophytic sterility gene (Sano, 1986). In rice
fields, hybrids between O. glaberrima and O. barthii are encountered frequently (Oka et al., 1978). However,
natural hybrids between the two cultivated species are extremely rare and are only very seldom observed in the
field (Oka et al., 1978). Artificially produced O. sativa–O. barthii hybrids have also been studied by several
workers (see Nayar, 1973). They are fairly fertile and meiosis is largely normal. The studies conducted by
different authors show that the nature and extent of cytogenetic and genetic anomalies shown by these hybrids
are no more extensive than those shown by inter-racial hybrids — the japonica–indica1 of O. sativa (Morishima
et al., 1963; Chu et al., 1969).
A comparison of the yield components of O. barthii and O. glaberrima also brings out certain interesting
features (Table 5). The spikelet dimensions and 100-grain weight are higher in O. barthii. There are more
spikelets per panicle in O. glaberrima. The number of panicle-bearing tillers (PBT) is variable, but overall, this
too appears to be higher in O. barthii. Thus, O. barthii possesses superior yield components and higher yield
potential than O. glaberrima. The larger, bolder spikelets and the greater plant vigor of O. barthii may be
manifestations of heterosis resulting from the hybridization of the two cultivated rice species. The occurrence of
O. barthii in disturbed habitats may be indicative of its weediness and hybrid character (Stebbins, 1950; Baker
and Stebbins, 1965).
For all these reasons, it is difficult to consider O. barthii as the species ancestral to African rice, O.
glaberrima. It may not even strictly fulfill the requirements of a species distinct from O. glaberrima, as it is
neither morphologically discontinuous nor reproductively isolated. These properties would restrict the
interchange of genes between the two taxa, which is usually considered a prerequisite for species formation
(Stebbins, 1950; Levin, 2000; Coyne and Orr, 2004).
Thus, the morphological, genetic, cytogenetic and ecological characteristics of O. barthii suggest that it is a
hybrid derivative of the two cultivated rice species. The parallelism of O. barthii with the form spontanea of
Asian rice in Asia is obvious in its nature, distribution, ecology and genetic characteristics.
Oryza sativa as progenitor of O. glaberrima
As already noted, there is copious historical and archaeological evidence for the presence of Asian rice in north
and eastern Africa (including Madagascar) from prehistoric, and definitely from early historical times. With
evidence of extensive trans-Saharan and trans-central African movement of traders and goods during this period
(see Table 3), it could be expected that Asian rice was introduced into west Africa much earlier than the usually
assumed timeframe of the 15th century (see Table 3). Accounts of Arab travellers and chroniclers have already
been presented. The extensive influence of external human contacts obtained from the Igbo-Ukwa excavations
(southeast Nigeria, 9th century CE) have also been noted (Insoll and Shaw, 1997).
1
Although ‘indica’ and ‘japonica’ are almost ubiquitously treated as subspecies within O. sativa, Prof. Nayar points out that
there is no formal status for these names, which should therefore not be designated ‘subspecies’ or printed in italics — Ed.
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
1.18.7
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
Nayar (1973) was the first to suggest that O. glaberrima arose from O. sativa. This proposal received
partial support from Second (1986) from isozyme studies. No one else has commented on this.
The frequent, unpredictable and often drastic climate changes taking place in the Sahel–Sudan region since
the beginning of the Common Era (Table 3) could have been inducing genetic mutations in O. sativa or,
alternatively, such regimes may have been favoring the expression of rare alleles, leading to the
formation/display of variant forms in nature. Some of these mutants would have been better adapted to the
constantly changing environment of the region, and they would have been selected and retained by farmers for
further cultivation. These may have included increased resistance to one or more of the biotic and/or abiotic
factors — especially heat, flooding and weeds. This protection provided by geographical isolation would have
given the mutant form the initial barrier to, or protection from, gene flow required to maintain its genetic
integrity. Those plants possessing less distinctive physiological and morphological changes would have
remained unnoticed and unselected in the O. sativa cultivated plots. Farmers in west Africa are known to accept
mixed heterogeneous populations of the two rice species in rice fields. They also often mix seeds of the two
species for planting in their fields (e.g. Oka and Chang, 1964; Oka et al., 1978; Ghesquière et al., 1997; Semon
et al., 2005). The last two authors have attributed this phenomenon to natural crossings between the two rice
species. However, as we have already noted, there are strong crossing barriers between them (Chu et al., 1969a;
Sano, 1986) and their natural hybrids are rarely observed in the field.
A novel form would therefore have developed over time into a neo-species that came to be designated as O.
glaberrima. The evolution of O. glaberrima from O. sativa is therefore an instance of sympatric or parapatric
evolution.
Supporting evidence
There are several leads to support the origin of African rice from Asian rice. The first is the repeated
observations of Porteres (1945, 1950, 1956) and several others that that the two cultivated rices show close
parallel variation, and all the characters considered unique to African rice are found also in Asian rice, including
that of west Africa. The second is the report of an invalidly published species, O. jeyporensis, from the Jeypore
tract of Orissa, India (Govindaswamy and Krishnamurthy, 1958). While these authors stated that this new taxon
most closely resembled O. glaberrima, Sampath (1962) observed that it most resembled O. stapfii (O. barthii).
The third is of Seetharaman and Ghoral (1976) observing several O. glaberrima characters in the Assam rice
collection. Both Assam in northeast India and Jeypore tract in southeast India, are major centers of rice
diversity.
The fourth is the lateness in describing O. glaberrima even at the time of its initial identification (i.e. in the
mid-19th century), its rarity, and its close morphological similarity with O. sativa. When Linnaeus initially
erected the genus Oryza, he described only one cultivated species, O. sativa. It took 32 years after African rice
was first collected in 1826 to describe it as a new species (in 1858), and a further 51 years (in 1909) to make the
next collection of this species (Porteres, 1955).
The fifth evidence relates to the rice germplasm collected by Ghesquire and Second (1985) from western
India. Their O. rufipogon collection (five typical annual forms from Gujarat) most closely resembled O. barthii
in isozyme studies. In electrophorectic studies, Lolo and Second (1988) found that 22 out of 36 presumed loci
were polymorphic. They concluded that their O. rufipogon collection showed characteristics of both O. barthii
and also of O. rufipogon found elsewhere in India. The authors then suggested that in ancient times, O. barthii
might have been introduced into India from Africa along with sorghum and pearl millet. This appears to be an
improbable proposition.
Studies using cytogenetic and molecular techniques too have thrown light on the structure of west African
rice and its close affinity with Asian rice. However, the prevalence of natural hybridization among the Agenome species in west Africa may sometimes confound a direct interpretation of the molecular-marker data.
Ohmido and Fukui (1995) characterized the chromosome complement of O. glaberrima using karyotype
analysis, molecular cytology, and McFish imaging (molecular fluorescence in situ hybridization). The general
features of prometaphase chromosome were similar to those of O. sativa except for some minor variations. The
chromosome length and arm ratios were identical in 9 of the 12 chromosomes. They were slight differences (1%
significance) in the three others — in chromosome 1 for relative length (12.5 ± 0.8 cf. 13.6 ± 1.1) and in
chromosomes 4 and 5 for arm ratios (3.23 ± 0.24 cf. 2.93 ± 0.29 and 1.50 ± 0.13 cf. 1.24 ± 0.13). The FUSC
(faint unstable small condensation) region in chromosomes 6 and 12 of O. sativa was also present in O.
glaberrima. McFish imaging showed that the three heavily condensed regions observed in chromosome 11 and
Sat-chromosome 9 of O. sativa were also present in O. glaberrima. Similar was the case with the 5s locus
rDNA in chromosome 9 and 45s rDNA locus in chromosome 11. These, together with the normal meiosis
observed by numerous earlier authors (Morinaga and Kuriyama, 1957; Hu, 1960; see Nayar, 1973), indicate a
close phylogenetic relationship between the two rice species. This would be difficult to obtain if had they
evolved from two different ancestral species (O. rufipogon and O. barthii), on two different continents (Asia
and Africa), and at two different times (ca 7 and 3 millennia BP), as some authors have suggested.
1.18.8
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
Further evidence is provided by the first interspecific (O. sativa × O. glaberrima) microsatellite-based
linkage map (Lorieux et al., 1999). This map comprised 129 markers representing 112 discrete marker loci
(total map length: 1923 cM). The genome coverage was about the same as that of the two intraspecific
microsatellite maps of O. sativa with which it was compared (of Akagi et al., 1996, and Chen et al., 1997),
except to some extent in three chromosomes: (numbers 4 and 10, and short arm of chromosome 7). The authors
observed very good colineanity between O. sativa and O. glaberrima with only small nonsignificant markers
and inversions.
In evolutionary relationship studies of AA genome species, both Cheng et al. (2002), using SINE insertion
analysis, and Park et al. (2003), using MITE-AFLP, found that O. glaberrima exhibited the least intraspecific
variability. Further, it also showed a close sister/parent–progeny relationship with O. sativa. Similar
observations have been made by numerous other workers.
A study of the recent sequence evolution of indica and japonica rices (Ma and Bennetzen, 2004) has
provided unintended support for this proposal. The authors used O. glaberrima as the reference material. They
used 263 primer pairs and successfully polymerase chain reaction (PCR) amplified 196 (75%) ‘single-band’
products. These were then sequenced and BLAST-searched (Basic Logic Alignment Search Tool). As many as
168 of them were appropriately aligned to the indel regions of indica and japonica, indicating that they were
orthologous. Out of a total of 36 952 bp of common sites studied in O. glaberrima and O. sativa (indica and
japonica), the authors identified 519 (1.4%) substitutions between indica and japonica, and 764 (2.1%)
substitutions between japonica and O. glaberrima. Oryza glaberrima was equally distant from both indica and
japonica. The authors then observed that their data “at the large-scale genomic sequence level strongly support
the previous observations” on the prevalence of sister relationship between the two rice species. They further
observed that the point mutations that differentiated O. glaberrima from O. sativa indica and japonica were not
unevenly distributed, as would be expected if segments from either sativa group had been introduced into the
African rice. These conclusions lend further support to the proposition that O. glaberrima arose by gene
mutations or by expression of rare alleles in O. sativa.
Semon et al. (2005) studied the prevalence of linkage disequilibrium (LD) in 198 accessions of O.
glaberrima using 93 nuclear microsatellite markers. They obtained abundant simple-sequence-repeat (SSR)
diversity (9.4 alleles/locus) with mean polymorphism content (PIC) value of 0.34, a gene-diversity (HC) value
of 0.27, and allele size of 67–388 bp. They used O. sativa as control. High elevated levels of LD were detected
even among distantly located markers. They felt that elevation in LD was not due to physical linkage, but to
other factors as such as population structure. They then grouped the sample into five sub-populations, two of
which clustered with O. sativa, and the other three with specific combinations of phenotypic traits that appeared
to reflect specific ecological adaptations, they said. Of the 198 accessions, 88 (44%) were admixtures with
varying levels of ancestry shared among the five groups. Another 68 (34%) accessions shared at least 1%
ancestry with O. sativa, and 11 accessions had >52% ancestry. They attributed this to widespread natural
hybridization taking place between the two rice species. The observation of high levels of LD in O. glaberrima
was ascribed to the “population structure driven by introgression with O. sativa”.
However, the authors’ explanations/inferences can be viewed in a different perspective. Natural crossing
between the two cultivated species rarely takes place (Oka et al., 1978; Sano, 1986) as we have already noted.
The sub-populations/cultivars of O. glaberrima that showed various combinations of phenotype traits could be
the result of mutations that took place in O. sativa, favoring different ecological niches of cultivation. The
admixture of ‘varying levels of ancestry’ may reflect stages in the evolution of O. glaberrima. Thus, this study
may be supportive of the close relationship between the two species.
From the foregoing account, it becomes obvious that African rice arose from Asian rice through the initial
development of sterility barriers between the two species followed by a progressive accumulation of desired
characters. The initial protection needed for developing isolating mechanisms under sympatric conditions would
have been provided by the farmers selecting the seeds of desirable variant types and cultivating them elsewhere
under isolation. This would have led to the strengthening of sterility barriers leading to the development of what
is now designated as O. glaberrima. Under field conditions, this may happen only rarely, but in this case, the
protection provided by the farmer would have sealed the process of sympatric/peripatric speciation.
Time and place of origin
Porteres’ (1962) proposal that O. glaberrima originated in the IND was based on linguistic and biodiversity
considerations. The Bantus (who originally inhabited this area) were only gatherers, but the Negroes (who
followed them) domesticated African rice, and the Mande people (who followed the Negroes) further improved
it, he suggested. The Sahel region is characterized by numerous wetlands and the region borders the Sahara
Desert. Oryza glaberrima shows much diversity here. Portéres proposed also two secondary centers of
origin/diversity for African rice, the Senegal–Gambia lowlands and the Guinea highlands.
Porteres’ (1962) observations on the time of origin of O. glaberrima (ca 3500 years ago) came in for much
criticism at the Third Conference on African History and Archaeology (Gray, 1962). Tucker observed that it
Second Africa Rice Congress, Bamako, Mali, 22–26 March 2010: Innovation and Partnerships to Realize Africa’s Rice Potential
1.18.9
Theme 1: Rice genetic diversity and improvement
Nayar: Origin of African rice
was erroneous to draw philological inferences without performing comparative philological studies. Clark stated
that even 2000 years ago, it was the hunter–gatherers who were occupying most of sub-Sahara, and Neolithic
people were present in only a small portion of the region — implying that domestication of African rice could
not have taken place prior to this period (2000 BCE). Subsequent studies have shown that the megalithic
structures found along the Gambia River are datable to only 700 CE (Posnansky and McIntosh, 1976; Hill,
1978).
Recent authors (Neumann, 2003; Phillipson, 2005) have confirmed the comparatively late emergence of
Neolithic culture in west Africa. It is in this context and keeping in mind the regular contacts and movement of
humans from southeast and south Asia to east Africa, that Nayar (1973) and Carpenter (1978) had earlier
suggested that O. sativa might have been introduced into west Africa from the east coast of Africa from about
the 8th or 11th centuries CE, or even earlier, through one of the regular caravan routes from either the
Ethiopia/Sudan region across the southern Sahara, or across tropical central Africa from east Africa. There is no
disputing that rice cultivation began in Egypt in the 7th century CE (Porteres, 1950). The recent findings of
banana phytoliths from Cameroon and Uganda (1st to 4th millennia BCE) lend further oblique evidence to the
very early arrival of the principal Asian crop plants in eastern Africa.
Thus, on the basis of various historical and archaeological evidence, we can conclude that Asian rice (O.
sativa) arrived in west Africa, possibly in the early centuries of the Common Era, and definitely long before the
European traders and conquerors took it there in the 15th century CE and later.
The earliest archaeological remains of rice have been obtained from the IND (Mali) and the Nigerian Lake
Chad region (Murray, 2005; McIntosh, 2006; Kahleber and Neumann, 2007). With the limited availability of
archaeological data from the Sahel region, it is difficult to indicate a place of origin for African rice. However,
going by the records of past human movements and migrations from pre- and early historic periods, and
variability patterns, the place of origin appears most likely to be the IND, as suggested initially by Porteres
(1955).
In African rice, unlike certain other crop plants, there is no single character whose initial induction in the
progenitor species could have transformed it into a primeval form of O. glaberrima. However, in rice, as in
other cereals, the acquisition of nonshedding of grains would have been the first step toward domestication
(Takahashi, 1955; Nayar, 1958, 1973; Doebley, 2006). Assuming that African rice arose from Asian rice, the
primitive African rice would have developed by acquiring progressively small mutations contributing various
adaptive features, such as increased capacity to withstand the various biotic and abiotic stresses, as it would
already possess nonshedding grains.
Cultivated rice in west Asia is today a mixture of the two species, both physically and genetically (Oka and
Chang, 1964; Oka et al., 1978; Cheng et al., 2002; Park et al., 2003; Ma and Bennetzen, 2004; Semon et al.,
2005). Further, in this region, both rice species show parallel variation for the complete range of characters
(Porteres, 1956; Oka et al., 1978). The fact that there is no single character in African rice by which it can be
distinguished from Asian rice could explain both the enormous delays in describing African rice as a separate
species, and the rarity of its rediscovery/collection even during the time of its initial description in the late 19th
century.
We may then conclude that African rice was derived from the Asian rice, after the latter species was
introduced from outside the region in the early centuries of the Common Era. This initial development may have
taken place in the IND or even the Lake Chad region. Because of the drastic climatic variations prevalent in the
region, small mutations may have been taking place all the time in rice fields, and they may account for the wide
heterogeneity present in west African rice fields. Some workers have ascribed this to natural hybridization
taking place between the two cultivated species. However, this happens only rarely. It is well documented that
rice cultural practices prevalent in west Africa are much simpler than those practised elsewhere, including in
south and southeast Asia. This would perpetuate the prevalent heterogeneity in the traditional rice fields of the
region.
The hybrid derivatives of the two cultivated species (now known as O. barthii) may have established on the
fringes of the cultivated fields, and after backcrossing continually with either of the parental species, may have
colonized areas beyond the cultivated fields, wherever suitable niche areas were available. Its hybridity may
have aided its spread and establishment throughout the tropics of Africa.
Acknowledgements
I thank the Africa Rice Center for inviting me to attend the Second African Rice Congress (Bamako, Mali
Republic, 22–26 March 2010) and present this paper. I also thank Professor Loren Ellstrand for giving
constructive comments on an early version of this paper.
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Nayar: Origin of African rice
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