Aside from the two cultivated species of rice - Oryza sativa and Oryza glaberrima - the Oryza genus includes about another 20 other species, although the taxonomy and classification into species is still a matter of research. These other Oryza species should not be confused with another plant which is also sometimes called “wild rice”, which is actually from the genus Zizania.
Compared with other major crops such as corn or wheat, rice has an extaordinarily diverse genetic resource base. Across all the rice species there are 11 different genome groups, which represent the differences in genetic diversity across the Oryza genus. These genome groups are used to both group and separate species. In some cases there is not always scientific agreement about how the species should be classified.
The AA genome includes the cultivated rice and their closest relatives and ancestors. Species that share the AA genome are the most useful for plant-breeding programs, because they are more easily crossed with the cultivated species because of their genetic similarity.
There are several rice species with AA genomes:
- O. nivara (AA genome), ‘Indian wild rice’, (considered by some to be an annual ecotype of O. rufipogon) grows in swampy areas, such as ditches, water holes, and the edges of ponds; it occurs in tropical Asia—in the Deccan Plateau and Indo-Gangetic Plains of India and in many parts of Southeast Asia; it is considered the main ancestor of O. sativa.
Throughout South and Southeast Asia, weedy forms of O. sativa occur that are look similar to, and sometimes indistinguishable from, O. nivara; these represent numerous hybrids between O. sativa and its two wild relatives (O. nivara and O. rufipogon) – they are found in canals and ponds adjacent to rice fields and in the rice fields themselves.
- O. rufipogon (AA), ‘brownbeard rice’ or ‘red rice’, is a perennial species that occurs in swamps, marshes, open ditches, ponds, riverbanks, lakesides, and at the margins of rice fields from tropical Asia (South and Southeast Asia and southeastern China) to northern Australia; it has also been introduced to the United States, where it is considered a weed.
- O. meridionalis (AA) occurs on New Guinea and in northern Australia at the edges of freshwater lagoons, rivers, temporary pools, and swamps.
- O. barthii (AA) occupies a diversity of habitats in Africa from mopane woodland to savannah to floodplains; it is considered the main ancestor of African cultivated rice (O. glaberrima) and is being used in the upland rice breeding program of the Africa Rice Center (AfricaRice).
- O. longistaminata (AA), ‘longstamen rice’ or ‘red rice’, is a perennial species that occurs in swampy areas, at the edges of lakes and ponds, stream- and riverbanks, and in irrigation canals throughout tropical Africa and in Madagascar, and has been introduced into the United States; it is often found as a weed around rice fields; it is the source of blast-resistance gene Xi21.
- O. glumaepatula (AA) is a perennial species primarily of deepwater swamps and marshes, open ditches and pools, beside river, and near cultivated rice fields; it occurs in tropical South America.
Perennial species of seasonal wetlands; the ‘O. officinalis group’:
- O. minuta (BBCC) occurs beside streams and riverbanks in lowlands in the Philippines and Papua New Guinea
- O. schweinfurthiana (BBCC) is considered by some to be a tetraploid race of O. punctata; it occurs in Africa
- O. officinalis (CC) is found in a variety of habitats; distributed from tropical Asia to Papua New Guinea and northern Australia
- O. rhizomatis (CC) occurs in Sri Lanka
- O. eichingeri (CC) is a forest species that occurs in West, Central, and East Africa, and Sri Lanka; it is the only wild species found in both Africa and Asia
- O. malampuzhaensis (CCDD) is considered by some to be a tetraploid race of O. officinalis; it occurs in southwest India
- O. alta (CCDD) is a deepwater species found in savanna and woodland in Central and South America
- O. grandiglumis (CCDD) is found in savanna and woodland in South America
- O. latifolia (CCDD)is found in a variety of habitats in Central and South America and the southern Caribbean
- O. punctata (BB) occurs in tropical Africa, including Madagascar, in open and semi-open habitats
- O. australiensis (EE) occurs in woodland in northern Australia; it considered by some to be part of the O. officinalis group; it was the source of the Pi40 blast-resistance gene now being used in various breeding programs supported by the International Rice Research Institute (IRRI)
- O. brachyantha (FF) occurs in Africa from Senegal to the African Great Lakes; it is sometimes perennial
- O. schlechteri (HHKK), a forest species, occurs on New Guinea island (Indonesia and Papua New Guinea)
- O. coarctata (KKLL) is found from South Asia to Myanmar
Species of lowland swamp forests:
- O. longiglumis (HHJJ) occurs on New Guinea island (Indonesia and Papua New Guinea)
- O. ridleyi (HHJJ) is found from Southeast Asia to Papua New Guinea
Perennial species of seasonally dry hillside forests:
- O. granulata (GG) occurs in South and Southeast Asia
- O. meyeriana (GG) occurs in Southeast Asia
- O. neocaledonica (GG) is found in New Caledonia