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Table 1. Taxa in the genus Oryza: the complexes and
genome groups to which they belong.
Complex/taxon Genome group Distribution
O. schlechteri Tetraploid Papua New
Guinea
O. brachyantha FF Africa
O. ridleyi complex
O. longiglumis Tetraploid Papua New
Guinea
O. ridleyi Tetraploid Southeast Asia
O. meyeriana complex
O. granulata Diploid South and
Southeast Asia
O. meyeriana Diploid Southeast Asia
O. officinalis complex
O. officinalis CC Tropical Asia
to Papua New
Guinea
O. eichingeri CC East and West
Africa
O. rhizomatis CC Sri Lanka
O. minuta BBCC Philippines,
Papua New
Guinea
O. punctata BB, BBCC Africa
O. latifolia CCDD Central and
South America
O. alta CCDD Central and
South America
O. grandiglumis CCDD South America
O. australiensis EE Australia
O. sativa complex
O. glaberrima AA West Africa
(cultigen)
O. barthii AA Africa
O. longistaminata AA Africa
O. sativa AA Worldwide
(cultigen)
O. nivara AA Tropical Asia
O. rufipogon AA Tropical Asia
O. meridionalis AA Tropical
Australia
O. glumaepatula AA South America
distinguishable from) O. nivara are the very
widely distributed weedy forms of O. sativa (O.
fatua), which represent numerous different
hybrids between O. sativa and its two wild
relatives. Throughout South and Southeast Asia,
these spontaneous forms are found in canals and
ponds adjacent to rice fields and in the rice fields
themselves.
The primary center of diversity for O.
glaberrima is in the swampy basin of the upper
Niger River. Two secondary centers are to the
southwest near the Guinean coast.
Oryza glaberrima varieties can be divided
into two ecotypes: deepwater and upland. In
West Africa, O. glaberrima is a dominant crop
grown in the flooded areas of the Niger and
Sokoto River basins. It is broadcast on hoed
fields. On shallowly flooded land, a rainfed
wetland crop is either directly sown by
broadcasting or dibbling, or transplanted. About
45% of the land planted to rice in Africa belongs
to the upland (dryland) culture, largely under
bush fallow or after the ground has been hoed.
Some African farmers still use axes, hoes, and
bush knives in land preparation. In hydromorphic
soils, O. glaberrima behaves like a selfperpetuating
weed. In wetland fields planted to
O. sativa, O. glaberrima has become a weed.
Ecological diversification in O. sativa,
which involved hybridization-differentiationselection
cycles, was enhanced when ancestral
forms of the cultigen were carried by farmers
and traders to higher latitudes, higher elevations,
dryland sites, seasonal deepwater areas, and tidal
swamps. Within broad geographic regions, two
major ecogeographic races were differentiated as
a result of isolation and selection: (1) indica,
adapted to the tropics; and (2) japonica, adapted
to the temperate regions and tropical uplands.
The combined forces of natural and human
selection; diverse climates, seasons, and soils;
and varied cultural practices (dryland preparation
and direct seeding vs puddling of the soil and
transplanting) led to the tremendous ecological
diversity now found in Asian cultivars.
Selections made to suit cultural preferences and
socioreligious traditions added diversity to
morphological features, especially grain size,
shape, and color, and endosperm properties.
The complex groups of cultivars now known
are categorized on the basis of hydrologicedaphic-cultural-seasonal
regimes as well as
genetic differentiation (Fig. 1).
Within the last 2,000 years, dispersal and
cultivation of the cultivars in new habitats have
further accelerated the diversification process.
Today, thousands of rice varieties are grown in
more than 100 countries.
The full spectrum of germplasm in the genus
Oryza consists of the following:
• Wild Oryza species, which occur
throughout the tropics, and related
genera, which occur worldwide in both
temperate and tropical regions.
Importance of rice 3