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GENETIC DIVERSITY IN RICE

Agriculture relies heavily on the genetic diversity of crop plants. Ever since the very beginning of agriculture (more than 10 000years ago), during the process of domestication and cultivation of crop plants, a wealth of genetic diversity has been utilized and partly preserved. It is estimated that not even 15 percent of the potential diversity has been utilized. Thousands of valuable allelic variations of traits of economic significance remain unutilized in nearly all crop plants. These can be discovered and effectively used to meet the existing and emerging challenges that threaten world food security. Sadly, this genetic wealth is being eroded due to neglect and over-exploitation. Developmental activities and exploitive land-use planning are destroying natural habitats, and modern varieties are replacing native species and landraces, resulting in a reduction of varietal diversity. Major crop species (rice, wheat and millet) suffered the most during the green revolution. In order to successfully meet future food requirements, it is necessary to manage the continuing genetic erosion and address the issues of genetic conservation and optimum utilization of what remains of the genetic diversity of important crop plants.

Asia

The domestication of rice dates back to antiquity, although the precise time and place of its domestication may never be known. The general consensus, however, is that domestication took place independently in China, India and Indonesia, giving rise to Asia’s three varietal groups: japonica, indica and javanica. There is archaeological evidence that rice was cultivated in India between 1500 and 1000 B.C. With its long history of cultivation and selection under diverse environments, rice acquired wide adaptability enabling it to grow in a range of environments, from deep water to swamps, irrigated and wetland conditions, as well as on dry hill slopes. Probably far more than any other crop, rice can grow under diverse geographical, climatic and cultural conditions.

The quality preferences of rice consumers have resulted in a wide diversity of varieties specific to different localities. Although the exact diversity cannot be gauged, it is estimated to be around 140 000 different genotypes. The IRRI gene bank preserves nearly 100 000 accessions. India alone has 86 330 accessions, of which 42 004 are in the national gene bank (Rai, 1999), which is enriched by further explorations, collections and conservation. Sociocultural traditions have increased the diversity of Indian rices in terms of morphological and quality traits, especially grain size, shape and colour, as well as aroma and endosperm properties. Ancient Ayurvedic literature (Indian Materia Media) from the fifteenth and sixteenth centuries A.D. describes different rices, particularly scented varieties with medicinal and curative properties. As far back as 400 B.C., Susruta, the great Indian pioneer in medicine, described the medicinal properties of rice.

Indian gene sources provided worldwide gains in production and productivity in both tropical and temperate rices before high-yielding varieties were even introduced. The popular varieties of Indonesia (Intan, Peta and Mas) are derivatives of a cross between the Indian variety Latisail and the Chinese variety Cina. Peta is one of the parents of IR 8, the variety which revolutionized rice production in tropical Asia. IR 8 is the most widely-used parent in several crosses in tropical Asia. More than 80 percent of the semi-dwarf varieties grown in tropical Asia have Latisail as one of their ancestors. Similarly, 35 varieties released by IRRI and grown in several tropical Asian countries have one or another Indian variety or wild species in their ancestry. GEB-24, another famous variety from southern India and known for its grain quality, is in the ancestry of 31 varieties developed and released by IRRI. Other Indian varieties, such as Ptb 18, Ptb 21 and CO-18, known for possessing sources of resistance to planthopper, leafhopper, gall midge and rice tungro virus, are ancestors of between 20 and 25 varieties released from IRRI. An Indian accession of the wild species, O. nivara (the only source of resistance for grassy stunt virus), occurs in the parentage of several improved varieties. Thus the rice germplasm collection from primary and secondary centres of diversity - namely: northeastern hills (ARC collection), Koraput region of Orissa, Raipur region of Chattisgarh and peninsular region of India - continues to provide useful genes for rice improvement (Rai, 1999).

In tropical Asia, of several major rice-growing countries, it was India which saw the release of the greatest number (643) of varieties over the last 50 years. India may therefore be used as a case study to examine the genetic diversity of cultivated varieties. Rice breeding in India started at the beginning of the twentieth century with the establishment of rice research stations in Dacca (now in Bangladesh) and Coimbatore. The rice breeding programme was strengthened during the 1920s and 1930s. Until 1960, there were 69 research stations in the country working on rice breeding. These research stations had developed 430 improved varieties by 1960, 27 of them through hybridization; the rest were from pure line selections in different regions. High yield was the most important objective in all breeding programmes. Additional objectives were strong straw, early maturity and resistance to pests. Some of the outstanding varieties developed during this period are: MTU-1, MTU-15 and HR-19 in Andhra Pradesh; Chinsurah-7 in West Bengal; Kodamba strains in Bombay; GEB 24, CO 2, CO 25, CO 26 and ASD-1 in Tamil Nadu; T 141 and SR 26 B in Orissa; Basmati 170 in Punjab; and T-136 in Uttar Pradesh. The variety, GEB 24, was obtained as a spontaneous mutant in the traditional variety, Konmani. It proved very popular and spread to various parts of southern India. During the 1960s, the semi-dwarf varieties, IR 8 and Jaya, were released, ushering in the era of the green revolution in India. Subsequently, breeding efforts focused on: improvement of grain quality; incorporation of resistance to diseases and insect pests; and reduction of the maturity period. Recently, efforts have been intensified to develop hybrids in rice. Fifteen promising hybrids have already been released for commercial cultivation.

In the last three decades, 632 varieties were developed and released for commercial cultivation in India by central and state variety release committees for different ecosystems. Of the 632 varieties, 374 (59%) were released for the irrigated ecosystem, 123 (19.4%) for rainfed shallow lowlands, 87 (13.7%) for rainfed uplands, 30 (4.7%) for rainfed semi-deep water, 14 (2.2%) for deep-water conditions and 33 (5.2%) for hill ecologies. All together, high-yielding varieties occupy 77 percent of the total area in the country.

By examining the donors utilized in the development of the high-yielding varieties, an indication is obtained of the genetic diversity. Through an FAO-sponsored indica/japonica project launched in the 1950s, ADT-27 and Mahsuri (developed in Malaysia) became popular in India. Subsequently, tropical japonica varieties from Taiwan, such as Taichung 65, Taichung Native-1 and Tainan-3, proved to be good donors for developing high-yielding and fertilizer-responsive genotypes. The development of short-duration varieties from the spontaneous dwarf mutant DGWG with Sd1 genes is a landmark in the history of rice breeding. Using largely IR 8, TN-1 and Jaya as donors of dwarf stature and high yield potential, combined with many of the local selected varieties having adaptability and quality traits, several varieties were developed and released in India.

The parents most often used in recombination breeding in India are listed in Table 17. IRRI’s elite germplasm has also been used extensively as a donor in Indian breeding programmes. Details of the germplasm used in breeding for resistance to biotic and abiotic stresses are given in Table 18, while Table 19 lists the varieties developed in the Indian breeding programme for: resistance to insect pests and diseases and to saline/alkaline soils; cold tolerance; and drought resistance.

TABLE 17
Parents most often used in India’s recombination rice breeding programme

S. No.

Parenta

No. derived varieties

Genes

Attributesb

Irrigated ecology

1

IR 8

58

sd1

Photoperiod-insensitive, semi-dwarf plant stature, high yield potential, medium maturity duration, MR to BL and GLH

2

TN (1)

24

sd1

First semi-dwarf and photoperiod-insensitive variety

3

Jaya

14

sd1

Photoperiod-insensitive, semi-dwarf plant stature, high yield potential, medium maturity duration

4

IR 36

15


Early to mid-early, suitable for intercropping, R to GM

5

Basmati 370

10


Aromatic, export quality

6

TKM 6

10


R to SB

7

Mahsuri

8


Indica/japonica derivative, stable, high-yielding, suitable for shallow lowlands and irrigated areas, late duration

8

Sona

8


Fine grain, quality rice, MR to RTV, SB, leafhopper

9

Zinnia 31

7


Quality grain

10

T 90

6


Quality grain

11

IR 50

5


Early duration, high-yield potential, suitable for multiple cropping system

12

PTB 10

5

Gm4

Good for soil problems, R to GM

13

PTB 33

5


R to BPH, WBPH

14

Vikram

5

Gm2

R to GM, medium duration

15

ADT 27

4


Suitable for Kuravai, early monsoon, indica/japonica cross

16

IR 24

4


High yield, early, R to GM, good for saline soils

17

IR 28

4


Earliness, multiple resistance

18

MO 6

4


BPH resistance

19

PR 106

4


Medium maturity, high-yield potential, export type non-Basmati rice

20

Rasi

4


Indica/japonica derivative, stable, high-yielding, suitable for shallow lowlands and irrigated areas, late duration

21

Triveni

4


MR to BL; suitable for direct seeding

22

W 1263

4

Gm1

R to GM, SB

23

W 12708

4

Gm2

R to GM, SB

Rainfed ecology

1

Pankaj

24


Late duration, suited for rainfed lowlands, MR to BL, RTV

2

IR 8

21


Photoperiod-insensitive, semi-dwarf plant stature, high yield potential; MR to BL and GLH

3

Mahasuri

19


Indica/japonica derivative, stable, high-yielding, suitable for shallow lowlands and irrigated areas, late duration

4

TN (1)

16


First semi-dwarf and photoperiod-insensitive variety

5

IR 20

8


High seasonal stability, grain quality, R to RTV

6

Sona

8


Fine grain, quality rice

7

IR 36

7


Early to mid-early, suitable for intercropping, R to GM

8

N 22

7


Drought tolerant

9

CR 1014

6


Late-maturing, stable widely adaptable, quality rice

10

Jaganath

6


Mutant of T141, photosensitive, lowland variety good for delayed rain

11

Jaya

6


Photoperiod-insensitive, semi-dwarf plant stature, high yield potential

12

Fine Gora

5


Drought tolerant

13

Patnai 23

5


High yield potential, lowland variety

1

Bulk H 9

4


Late, photosensitivity

2

M 63 - 83

4


Drought tolerant

3

Rasi

4


Good for irrigated, rainfed uplands; cropping systems because of early maturity

4

RP 5-32

4


Late-maturing, high yield potential

5

Vijaya

4


High seasonal stability, R to leafhopper, tol. to SB

a N22 = selection from Rajbhog.
b T141 = selection from Saruchinamali; R = resistant; MR = moderately resistant; tol. = tolerant; BL= blast; BPH = brown planthopper; GLH = green leafhopper; GM = gall midge; RTV = rice tungro virus; SB = stem borer; WBPH = white-backed planthopper.

TABLE 18
IRRI’s elite germplasm used as donors in Indian breeding programme

Trait

Donor germplasm lines

Resistance to biotic stresses

Stem borer

IR 20, IR 1820-52-2, IR 15795-151-2, IR 580-E420-1-1, IR9828-23-1, IR 4227-28-3-2

Brown planthopper

IR 1539-823-1-4, IR 9-60, IR 1154-243, IR 4819-77-3-2, IR 38699-49-3-1-2, IR 19660-46-1-3-2, IR 13146-45-2-3, IR 13427-45-2, IR 13525-43-2, IR 13564-95-1

White-backed planthopper

IR 2035-117-3, IR 54742-117-3, IR 19661-150-2-2-1

Green leafhopper

IR 38699-49-3-1-2, IR 1364-37-3, IR 262-43-8, IR 1544-238-2-3, IR 13146-45-2-3, IR 759-54-2-2

Gall midge

IR 13429-150-3-2-1-2, IR 36

Blast

IR 14632-2-3, IR 662-1-1-2, IR 3880-29, IR 1544-238-2-3, IR 1416-128-5-8, IR 5533-PP854-1, IR 1905-PP11-29-4-61

Bacterial blight

IR 54, IR 3154, IR 13564-95-1, IR 305-3-1, IR 579-97-2, IR 2797-105-2-2-3, IR 4445-63-1-2, IR 19661-15-2-2-1

Tungro virus

IR 4683-54-2-2, IR 8608-298-3-1, IR 4744-295-2-3, IR 661-98-2-2, IR 9209-48-3-2, IR 4215-405-2-2, IR 4445-63-1-2-2, IR 1721-11-6-8

Multiple resistance

IR 36, IR 30, IR 32, IR 29, IR 28, IR 26, IR 2035-290-2, IR 2070-24, IR 2071-88, IR 1514-AE666, IR 9884-54-3, IR 13168-143-1, IR 13240-81-1, IR 2153-26-3-5, IR 2053, IR 9264-321-3, IR 2863-35-3, IR 3351-38-3-1, IR 4219-35-3-3, IR 1330-3-2, IR 4422-983-6-1, IR 4432-52-6-4, IR 4570-83-3-3-2, IR 2058-78-1

Tolerance to abiotic stresses

Submergence

IR 661-98-2-2, IR 442-2-58, IR 1188-BB-65-1, IR 7691-4-2

Drought

IR 1529-430-3, IR 841-99-1, IR 305-3-1, IR 1721-14-5, IR 2035-117-3, IR 5853-118-5, IR 8072-231-2, IR 665-7-2, IR 5178-1-1-4, IR 21313-36-2-2, IR 10068-11-1

Cold

IR 1846-300-1, IR 3941-14-2-2-3, IR 2637-39-2-21, IR 9224-K1, IR 2298-PLPB-3-2-18, IR 41985-111-3-3-2, IR 31851-63-2-3-2, IR 32429-122-3-1-2, IR 9758K2, IR 39385-124-3-3-2-3, IR 39422-19-3-3-3, IR 9129-K1, IR 42015-83-3-2-2, IR 19774-23-2-2-2, IR 5867-8-2-1, IR 8866-30-3-1-4-2, IR 9202-21-1

Salinity

IR 4432-103-6, IR 2153-26-3-5-2, IR 2061-522-6-2, IR 1820-210-2, IR 4563-52-1, IR 17494-32-3-1-1-3, IR 4515-4-1-15, IR 19799-17-3-1-1, IR 4515-4-1-15, IR 19799-17-3-1-1, IR 5657-33-2, IR 20925-33-3-1-1-28, IR 9764-45-2-2, IR 9852-19-2, IR 4422-98-3-6-1, IR 32307-107-3-2, IR 19735-5-2, IR 4630-22-2-17, IR 1148-19-2-3, IR 29725-21-1-3-2, IR 2863-35-3, IR 9217-58-2, IR 9752-71-3, IR 1529, IR 9884-54-3, IR 10206-29-2, IR 3426-19-2, IR 2053

Alkalinity

IR 2153-918-2, IR 4227-28-3-2, IR 129-209-2-2-2-1, IR 13540-56-3-2-1, IR 13525-43-2, IR 5853-110-5, IR 9217-58-2, IR 9764-45-2-2, IR 1820-210-2

Plant type

IR 8, IR 24, IR 127-80-1, IR 262-43-8, IR 400-28-4-5, IR 497-84-3, IR 498-1-88, IR5, IR 661-98-2-2, IR 20, IR 665-7-2, IR 878B2-112-1, IR 2823-39-9-5, IR 2053, IR 24632-34-2, IR 305-3-1, IR 22, IR 4219-35-3-3, IR 6023-10-1-1, IR 9264-321-3

Grain quality

IR 1529-680-3, IR 480-5-9, IR 2153-338-3, IR 841-85, IR 2863-35-3-3, IR 1550-16-2, IR 4215-405-2-2, IR 4227-28-3-2, IR 12-178-2-3, IR 3351-38-3-1

Earliness

IR 579-48-1, IR 532, IR 10154-23-3-3, IR 10176-24-6-2, IR 10179-2-3-1, IR 28, IR 13168-143-1, IR 8608-298-3-1, IR 747B2-6, IR 1561-228-3-3, IR 36, IR 9202-25-1-3, IR 50, IR 58, IR 72

TABLE 19
Varieties resistance to various stresses in India

Stress

Varieties

Stem borer

Ratna, Sasyasree, Vikas, Saket 4

Brown planthopper

CR1002, Gauri, Jwahar, Kanakam, Kartika, Jyothi, Bharatidasan, HKR120, IR 36

White-backed planthopper

ADT38, Bharatidasan, HKR 120, PR109, Sarasa, Udaya

Green leafhopper

ADT38, IR 24, IR 50, Vikramarya, Sakti, Samruthi

Gall midge

ASD18, CO44, Gauri, Lalat, MDU3, Kakatiya, Asha, Oragallu, Phalguna, Pothana, Rajendradhan-202, Ratnagiri 1, Ratnagiri 2, Samlei, Surekha, Sarathi, Sagarsamba, ADT 39\8, IR 36.

Blast

Rasi, Rasmi, Pinakini, Tikkana, Rajarajan, VL 8

Bacterial blight

Karjat 1, PR 4141, PR 110, PR 111, Ramakrishna, HKR 120, IR 36, Narendra 2, Ajaya, CR 1002, ADT 39´

Rice tungro virus

ASD 15, Ratna, Saket 4, Srinivas, IR 34, IR 50

Sheath blight

ASD 18, ADT 39, Asha, Bhanja, Ratnagiri 13, TKM9, Salivahana

Multiple resistance

Suraksha, Shaktiman, Lalat, Ananga, Kshira, Asha, ASD 18, ADT 38, Narendra 2, Pantdhan 10, IR 36

Saline/alkaline

Bhavani, CSR 6, CSR 10, CST-1, Narendra 1, Panvel-1, Panvel-2, SLR 51214, Vikas, Vytilla 2, Vytilla 4

Cold

Himdhan, Himalaya 741, VL Dhan 221, Barkat, Tawi Himalaya 1, VL Dhan 163, VL Dhan 16, VL Dhan 39

Drought

Sabari, Govind, Gauri, GR2, GR3, Narendra 80, GR5, Aditya, Annada, Ashwini, Heera, Kshira, Poorva, Rasi, Ravi, Ratnagiri 71-1, Ratnagiri 73-1-1, Sakoli 6, Sarjoo 49, Sarjoo 50

Genetic diversity among 42 elite Indian rice varieties was evaluated by Davierwala et al. (2000) using three different types of DNA markers and parentage analysis. The average genetic similarity coefficient across all 861 cultivars was 0.70 and the average coefficient of parentage was 0.10. A set of 18 accessions from the Indian scented rice collection was subjected to random amplified polymorphic DNA (RAPD) analysis. A dendrogram revealed genetic similarity to be in the range of 25 to 77.5 percent (Raghunathachari et al., 2000).

Examination of the pedigree of 29 rice varieties developed through recombination breeding and released in Kerala State between 1966 and 1995 revealed a narrow genetic base, with only 37 ancestors. Either directly or indirectly, of the 37 ancestors, ten contributed 74.14 percent of the genetic base. Similarly, the cytoplasmic base was also limited, as 41.38 percent of varieties could be traced back maternally to the same ancestor (Ptb-10). All 29 varieties (with the exception of Kayanakulam-1) were interrelated with an average coefficient of parentage of 0.137 for 406 combinations of 29 varieties (Shivkumar et al., 1998). The extent of genetic uniformity of rice in selected Asian countries is given in Table 20.

TABLE 20
Extent of genetic similarity in cultivated varieties of rice in selected Asian countries

Country

Extent of uniformity

Tropical Asia

95% of HYV based on a single dwarfing gene Sd1

China

95% of the hybrids based on single source CMS (WA)

Bangladesh

62% descended from common stock

Indonesia

74% descended from common stock and > 50% of rice area under three varieties

Sri Lanka

75% descended from common stock

Myanmar

> 75% rice area under three varieties

Malaysia

> 75 area under one variety (MR 84)

Japan

> 70% area under three varieties

Taiwan

81% descended from common stock and 82% rice area under three varieties

Thailand

50% area under two varieties

Africa

In the African continent, rice varieties released in Nigeria had varying genetic contributions from across the globe (Maji and Fagade, 2002). Most of the varieties bred in Nigeria since 1986 have parents originating from IRRI: 67 percent of the released varieties in Nigeria originate directly from IRRI materials. FARO 15, a highly adaptable and high-yielding variety, received its high-yielding stiff-straw characteristic from its IR 8 parent. The other parent of FARO 15 is BG 79, used because of its wide adaptability to the Nigerian ecosystem. This combination resulted in the development and release of FARO 30, 31 and 32 as early-maturing, high-yielding varieties for irrigated cropping systems.

However, genetic uniformity is quite common in the upland rice-growing zones of Nigeria where farmers had stuck to growing only one variety (FARO 11) before the introduction of early-maturing FARO 46 only a few years ago. Genetic uniformity is also found in the Bende irrigation scheme area, where FARO 12 and 23 are the only common varieties. The main sources of rice genetic diversity in Nigeria can be classified into three basic categories: O. sativa, O. glaberrima and wild species such as O. bathii (Maji and Fagade, 2002). O. glaberrima, which originated in and is endemic to the subregion, is suited to the soils and harsh climatic conditions of the area and has resistance to biotic stresses, such as drought, rice yellow mottle virus (RYMV), weed competitiveness and acidity. On the other hand, O. glaberrima lines are characterized by very low yield potential, grain shattering before full maturity, grain characters which do not appeal to agronomists and consumers, and weak culm that predisposes them to high lodging susceptibility. Other undesirable traits include long awn, black husk at maturity and red seed coat. These limitations are, however, variable, and there is a wide range of materials possessing the positive side of these characters and offering great potential for genetic improvement because of their wide adaptation to various rice-growing ecologies, from uplands to deep water.

Human intervention and the domestication of rice in Africa led to the adoption, selection, development and maintenance of O. glaberrima varieties, in contrast with the O. sativa domestication in Asia and the Far East. These endemic/indigenous materials were able to compete with major weeds in tropical Africa thanks to their early vegetative growth and vigour. Furthermore, these varieties were flood-tolerant (due to their high elongation ability) and resistant or moderately resistant to various abiotic and biotic stresses. Until about 50 years ago, more than half the rice area in Africa was under O. glaberrima varieties; however, O. sativa or the Asian rice varieties have now replaced much of the native cultivated rice. The more successful and popular cultivars of O. glaberrima during the 1960s included: Badande and Jatau (irrigated/floating); and Dan Zaria, Godongaji and Katsina Ala Shendam (upland). About 2 200 accessions of O. glaberrima collected from 22 countries are conserved in the IITAgene bank, with a duplicate set in the IRRI gene bank; the WARDAgene bank has around 300 accessions of O. glaberrima collected between 1985 and 1990. Reasons for the replacement of African native rice varieties with Asian cultivars include the high-shattering, short to medium red grain types, as consumers prefer the higher quality materials available. Nevertheless, the O. glaberrima accessions could be useful donors for specific traits in future rice breeding programmes. The crossing/segregation behaviour of sativa x glaberrima crosses requires further study as there are problems of sterility and persistent segregation with eight or more filial generations; this is also an indication of the existence of multi-allelic loci, probably due to cryptic evolutionary changes in the cultivated Oryza species.

The white-grained O. sativa cultivars (believed to have been introduced more than 2 000 years ago in Africa) have almost completely replaced the native O. glaberrima varieties in recent years. These materials owe their origin to the International Network on Genetic Evaluation in Rice (INGER) under the auspices of FAO. Of the International Agricultural Research Centres, it is the collections of IRRI, IITA and WARDA which have contributed to varietal selection and adaptation, while the countries in Asia which are known to have provided cultivars which have proved successful in Africa are India, Thailand, Sri Lanka, Philippines, Indonesia, Taiwan, Bangladesh and Malaysia. Prominent donor O. sativa cultivars include: Vikram, Mahsuri, Ramadja, ADT-31, Pelitai and Mayang, contributed to INGER by various national programmes. Besides providing high yield and white grain, these and other donor O. sativa varieties and lines have contributed towards a number of economic and ecostability traits, for example, good quality and resistance or endurance to various biotic and abiotic stresses (including salinity, acidity, iron toxicity, drought, poor soil, low nutrients, blast and lodging).

The diverse range of O. sativa materials introduced in Africa are supplemented with prebreeding, in particular with O. glaberrima x O. sativa crosses. This has already been carried out at WARDAusing the backcross method together with anther culture. The new set of germplasm - likely to have good weed competitiveness, drought tolerance and high yield under the low input conditions of resource-poor farmers - should be further developed and promoted through liberal funding and a set of conservation priorities for sustainable use as per the relevant action points under the Global Plan of Action (GPA).

Latin America

In Latin America, there has been a similar narrowing down of the native genetic diversity in the prevailing rice varieties and breeding materials. Asmall number of indigenous and introduced varieties contributed as much as 70 percent to breeding programmes, including Dee-Geo-Woo-Gen, China, Lati Sail, I Geo Tze, Monge Chim Vang A, Belle Patna and Tetap. A systematic and sustainable approach must be adopted in breeding programmes in or aimed at Latin American countries.

Guimaracs (2002) presents a critical analysis of the Latin American region in general and Brazil in particular. The limited use made of the genetic diversity available worldwide has been a concern in Latin America since the late 1980s. Atotal of 143 commercial varieties released in the region from 1971 to 1989 were analysed: it was found that 101 different landraces were involved in the crosses that produced the varieties, but only 14 ancient cultivars contributed 70 percent of the genes.

Brazil took advantage of the green revolution with the introduction and commercial release of semi-dwarf varieties. The substitution of traditional tall varieties in the state of Rio Grande do Sul and Santa Catarina, the main rice-growing region in Brazil, produced yield increases of 30 and 66 percent, respectively. While a great number of crosses continued to be performed every year, following the yield jump of the 1970s, the subsequent two decades saw only limited genetic gain. In general, changes were introduced in terms of shortened growth duration, increased disease resistance and improved quality, while yield potential hardly changed, if at all.

The genetic base of the main varieties sown under lowland irrigated conditions in Brazil was analysed and it was concluded that only seven ancestral varieties were responsible for more than 70 percent of the background of these new varieties. In Rio Grande do Sul, this contribution was as high as 86 percent. Under upland conditions, studies reveal a narrow genetic base for the most commonly cultivated varieties. Six native varieties make up the base for the upland varieties released up to 1992. Forty ancestors were involved in crossing to develop varieties, but only 11 of them accounted for 81 percent of the genes for the varieties released between 1971 and 1993.


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