The most commonly cultivated rice species, Oryza sativa (Asian or paddy rice), is grown primarily in the humid tropics and subtropics, with some culti- vation on flooded upland sites, such as central California. Another less important rice, Oryza glaberrima(African rice), is grown in East Africa, but is being replaced by O. sativa.
The cultivated rices and their ancestors are considered to be diploid (2n = 24), although their high chromosome number indicates that they could be ancient, diploidized polyploids (Moringa, 1965). Ten genomes have been identified among the various sections of Oryza, based on chro- mosome pairing relationships (Vaughan, 1994), molecular markers (Wang et al., 1992; Aggarwal et al., 1999; Ge et al., 2001) and sequencing (Ge et al., 1999). The cultivated species and their closest relatives carry the A genome and form what is referred to as the sativacomplex. The A genome is further divided with superscripts to denote small pairing aberrations and partial sterility among the various diploid species (Table 8.3). The two cultivated species are morphologically very similar, but they show some chromosomal divergence and their hybrids are sterile (Oka, 1974, 1975).
A wide array of cytological, morphological and molecular markers have confirmed that the wild progenitor of O. sativa was Oryza rufipogan, and O. glaberrimadiverged from Oryza breviligulata(Oka, 1988; Joshi et al., 2000;
Bautista et al., 2001; Ishii et al., 2001), but, beyond this point, it is difficult to
develop a consistent phylogeny. Reproductive isolating barriers are not com- plete between most species, and their morphological differences are often obscured due to introgression (Harlan, 1969; Morishima and Oka, 1970).
There is even debate over whether perennial and annual types were the prog- enitors of the cultivated forms or some intermediate annual/perennial popula- tion (Chang, 1995; Joshi et al., 2000). The progenitors of cultivated rice may only be conceptual taxa that do not now exist because of introgression between wild and cultivated taxa.
The origins of Asian rice cultivation are also clouded. Many authorities consider India to be its cradle, but strong cases have been made for much ear- lier origins in central China and South-East Asia (Higham, 1984; Maloneyet al., 1989; Normile, 1997). From its beginnings somewhere in Central Asia around 10,000 BP, rice cultivation probably moved into Korea and Japan by 3000 BP. The cultivation of African rice probably began in the Niger delta about 3500 years ago and spread gradually across tropical East Africa. Asian rices arrived in Africa about 2000BP(Chang, 1975). Rice found its way to the New World in 1647, when its cultivation was begun in the Carolinas.
O. sativa underwent substantial differentiation as its cultivation spread across the world. Three main races are now recognized based on ecocultural criteria that include both temperate (japonica) and tropical areas (indica and javanica, Table 8.4). Javanica may have arisen through hybridization between Table 8.3. Genomes and distribution of the species of Oryzain section Oryza(based on Vaughn, 1994; Chang, 1995: Ge et al., 2001).
Chromosome Genome
Species and complex number (2n) designation Geographical distribution
O. sativacomplex 24
O. sativaL. 24 AA Worldwide (cultivated)
O. glaberrimaSteud. 24 AgAg West Africa (cultivated)
O. barthiiA. Chev. 24 AgAg Africa
O. glumaipatula 24 AgpAgp South and Central America
O. longistaminata 24 AlAl Africa
O. meridionalis 24 A?A? Australia
O. nivara 24 AA Tropical and subtropical Asia
O. rufipoganGriff. 24 AA Tropical and subtropical Asia O. officinaliscomplex 24
O. punctataKotschy 24, 48 BB, BBCC Africa
O. eichingeriA. Peter 24 CC Africa
O. officinalisWall. 24 CC Tropical and subtropical Asia
O. rhizomatis 24 CC Sri Lanka
O. minutaJ. S. Presl. 48 BBCC South-East Asia
O. altaSwallen 48 CCDD South and Central America
O. grandiglumisProd. 48 CCDD South and Central America
O. latifoliaDesv. 48 CCDD South and Central America
O. australiensisDomin. 24 EE Australia
indica and japonica (Second, 1982, 1985; Glaszmann et al., 1984).
Tremendous diversity exists within all these groups, due to isolation and selec- tion under diverse conditions, and sufficient divergence has occurred between races of the Japonica group to cause hybrid sterility (Engle et al., 1969).
Rye
There are three clearly defined species of rye (Spencer and Hawkes, 1980):
(i) Secale cereale L., the cultivated species, which also exists as a highly diverse annual weed in farms in Iran, Afghanistan and Transcaspia;
(ii) Secale montanum Gussh., an outbreeding, widely distributed assem- blage of perennial races located from Morocco east through the Mediterranean countries to Iraq and Iran; and (iii) Secale sylvestre Host., an annual inbreeder, which is widely distributed from Hungary to the steppes of southern Russia. One additional taxon, Secale vaviloviiGrossh., may be sufficiently unique to warrant species status (Spencer and Hawkes, 1980; Zohary and Hopf, 1993).
Most authorities believe that S. cereale evolved from S. montanum Gussh. (Riley, 1955; Khush and Stebbins, 1961). These two species are sim- ilar cytologically, but vary by two reciprocal translocations involving three pairs of chromosomes. Stutz (1972) proposed that the translocations were accumulated gradually through two steps involving interspecies hybridiza- tion; however, the recent molecular information indicates that S. cereale is Table 8.4. Major characteristics distinguishing races of O. sativa(based on Nayar, 1973).
Character Japonica Javanica Indica
Primary area of cultivation Japan and Taiwan Indonesia South-East Asia
Grain shape Short Large Narrow
Length of second leaf blade Short Long Long
Angle between second leaf and stem Small Small Large
Texture of plant parts Hard Hard Soft
Angle between flag leaf and stem Medium Large Small
Flag leaf Short, narrow Long, wide Long, narrow
Tiller number Large Small Large
Tiller habit Erect Erect Spreading
Leaf pubescence None Little More
Glume pubescence Dense Dense Sparse
Awns Usually absent Usually present Usually absent
Shattering Difficult Difficult Easy
Panicle length Short Long Medium
Panicle branching Few Many Intermediate
Panicle density High Moderate Moderate
Panicle weight Heavy Heavy Light
Plant height Short Taller Tall
probably a direct derivative of S. montanum(Vences et al., 1987; Murai et al., 1989). It is possible that S. cerealearose by chance in an isolated popu- lation via parapatric speciation.
As with oats, rye developed as a secondary crop. It was probably picked up as a weed when the wheat–barley assemblage arrived in western Asia, where the native species are widely distributed. Like the other grain species, agronomic traits, such as rachis fragility, ear branching and growth habit, are determined by only a few genes. The precise origin of rye domestication is unknown, but it was being cultivated at several locations in the general area of Turkey, north-western Iran and Armenia by 6000BP (Hillman, 1978;
Evans, 1995). Rye arrived in Europe as a cultivated crop by 4000 BP (Khush, 1962). Because of its tough constitution, it may have performed better than wheat and barley in the cooler, nutrient-poor northern climates and therefore attracted human attention. In modern times, tetraploid and hexaploid wheats have been artificially hybridized with rye to form the new crop called Triticale(Larter, 1995).