Two rice species are important cereals for human nutrition: Oryza sativa, grown worldwide, and O. glaberrima, grown in parts of West Africa. These two cultigens–species known only by cultivated plants–belong to a genus that includes about 25 other species, although the taxonomy is still a matter of research and debate.
Oryza is thought to have originated about 14 million years ago in Malesia. Since then, it has evolved, diversified, and dispersed, and wild Oryza species are now distributed throughout the tropics. Their genomes can be classified into 11 groups labeled AA to LL, and most of the species can be grouped into four complexes of closely related species in two major sections of the genus. Just two species, both diploids, have no close relatives and are placed in their own sections of the genus: O. australiensis and O. brachyantha.
Species of the O. meyeriana complex are genetically most different from the cultigens; they are diploid perennials found in dry hillside forests. Species of the O. ridleyi complex are tetraploids inhabiting lowland swamp forests. These two complexes, together with the tetraploid species O. schlechteri and O. coarctata, form the most primitive section of the genus, with a geographical distribution ranging from South Asia through Malesia to New Caledonia.
The O. officinalis complex consists of diploid and tetraploid species found throughout the tropics. All the species in this complex are perennials found in seasonal wetlands; some are rhizomatous and others form runners. They also differ in the habitats where they are found. Some occur in full sun in grasslands, others in partial to full shade in forests. Variation exists within these species as shown by the responses of different populations to pests and diseases.
The O. sativa complex consists of the wild and weedy relatives of the two rice cultigens as well as the cultigens themselves. All are diploids and are found throughout the tropics. The wild relatives of O. glaberrima in Africa consist of the perennial rhizomatous species O. longistaminata, which grows throughout sub-Saharan Africa and Madagascar, and the annual O. barthii, which extends from West Africa to East and southern Central Africa. The annual and weedy relatives of O. glaberrima are found primarily in West Africa.
Among the wild relatives of O. sativa, the perennial O. rufipogon is widely distributed over South and Southeast Asia, southeastern China, and Oceania; the morphologically similar O. glumaepatula is found in South America, usually in deepwater swamps. A closely related annual wild form, O. nivara, is found in the Deccan Plateau and Indo-Gangetic Plains of India and in many parts of Southeast Asia. The habitats of O. nivara are ditches, water holes, and edges of ponds. Morphologically similar to (and sometimes indistinguishable from) O. nivara are the very widely distributed weedy forms of O. sativa, which represent numerous different hybrids between O. sativa and its two wild relatives. Throughout South and Southeast Asia, these spontaneous hybrids 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. O. 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 directly sown by either 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 self-perpetuating weed. In wetland fields planted to O. sativa, O. glaberrima has become a weed.
Ecological diversification in O. sativa, which involved hybridization-differentiation-selection 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 or variety groups 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. Recent DNA studies have identified five subgroups within these two major groups. Indica is divided into indica proper, and aus, a group of diverse varieties from northeastern India and Bangladesh, named for the aus growing season, and which have been found to contain a number of stress tolerance genes that are absent from other variety groups. The Basmati or aromatic group of varieties, mainly from northwestern India and Pakistan, is an offshoot of the japonica variety group, which is further subdivided into temperate and tropical japonica.
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, color, and endosperm properties. The complex groups of cultivars now known are categorized on the basis of hydrologic-edaphic-cultural-seasonal regimes as well as genetic differentiation. 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.
- Natural hybrids between the cultigen and wild relatives, and primitive cultivars of the cultigen in areas of rice diversity.
- Commercial types, obsolete varieties, minor varieties, and special-purpose types in the centers of cultivation.
- Pure-line or inbred selections of farmers’ varieties, elite varieties of hybrid origin, F1 hybrids, breeding materials, mutants, polyploids, aneuploids, intergeneric and interspecific hybrids, composites, and cytoplasmic sources from breeding programs.
The diversity of Asian, African, and wild rices has given breeders a wealth of genetic material to draw on for breeding improved cultivars.