Conventional approaches to rice improvement

Full text

Chapter 2
CONVENTIONAL
APPROACHES TO RICE
IMPROVEMENT
Amir B. Wani and Amjad M. Husaini*
Genome Engineering and Societal Biotechnology Lab, Division of
Plant Biotechnology, Sher-e-Kashmir University of Agricultural
Sciences and Technology of Kashmir, Shalimar, J&K, India
ABSTRACT
Current global population of 7.5 billion is expected to reach 9.7
billion by 2050 AD, most of which resides in the rice growing regions of
Asia and Africa. Ninety percent of rice is produced and consumed in
Asia. Large-scale food shortages were experienced in India and in several
neighboring countries in Asia during the 50s and early 60s. During this
grim scenario, the semi-dwarf, fertilizer responsive, high yielding
genotypes of rice were introduced, which lead to a phenomenal increase
in production and productivity of these crops. Rice breeding programs
based on introducing semi-dwarf and non-lodging plant types, Taichung
Native I and the subsequent variety IR8, revolutionized the rice yields.
Varieties like Jaya and Padma were released in India that marked the
ushering of Green Revolution. The conventional breeding methodologies
* Corresponding Author’s E-mail: amjadhusaini@skuastkashmir.ac.in.

40 Amir B. Wani and Amjad M. Husaini
led to self-sufficiency by mid-eighties in the region. The chapter
discusses the various conventional approaches used for rice improvement.
Keywords: breeding, QTL, hybrid, heterosis, recombination, cytoplasmic
male sterility
INTRODUCTION
The current global population of 7.5 billion is expected to reach 9.7
billion by 2050 AD. Maximum population increase will occur in
developing countries of Asia and Africa, where rice is the staple food.
Globally rice is cultivated now on 167 m ha with an annual production of
around 782 million tonnes and average productivity of 4.7 tonnes per ha.
More than 90% rice is produced and consumed in Asia. The other
continents in which rice is grown are Africa (17.78% of the global area)
and South America (6.4%) (Table 1). Yield improvement, recorded in the
linear growth of the global rice yield at 51 kg per hectare per year from
1961 to 2017 in the FAOSTAT database (FAO 2018), is undisputed
evidence of a major achievement in modern rice science and technology.
Table 1. Area, production and productivity
of selected rice growing countries, 2018
Country Area (000’
ha)
Production
(000’mt)
Productivity
(t/ha)
India 44500 172580 3.87
China 30461 214079 7.02
Indonesia 15995 83037 5.19
Japan 1470 9728 6.61
Republic of Korea 738 5195 7.03
Egypt 555 4900 8.82
USA 1180 10170 8.61
Australia 61 635 10.41
Source: FAO Year Book, 2018.

Conventional Approaches to Rice Improvement 41
Large-scale food shortages were experienced in India and several
neighboring countries in Asia during the 50s and early 60s frequently,
there were dire warnings of impending widespread famines. During this
grim scenario, the semi-dwarf, fertilizer responsive, high yielding
genotypes of rice were introduced, which lead to a phenomenal increase in
production and productivity of these crops. Rice breeding programs in
India were accelerated by introducing semi-dwarf and non-lodging plant
types, Taichung Native I, developed in Taiwan from a cross between semi-
dwarf mutant Dee-geo-woo-gen and Tsai Yuang-Chung. The incorporation
of a dwarfing gene Sd1 from Dee-geo-woo-gen conferred short stature,
fertilizer responsiveness, erect leaves to utilize maximum solar energy to
achieve much higher yields 4-5 t/ha than the traditional rice cultivars. A
single gene mutation brought out by the dwarfing gene Sd1 transformed
the tall plant type to semi-dwarf due to non-responsiveness to endogenous
gibberellins. Subsequently, a miracle variety IR8 from Peta and Dee-geo-
woo-gen in 1966 from IRRI, revolutionized the rice yields. More or less at
the same time, varieties of Jaya and Padma were released in India that
marked the ushering of the Green Revolution, which transformed the
country to a state of self-sufficiency by mid-eighties, arresting rice imports
and beginning an era of exporting the surplus rice earning high foreign
exchange for the country by early nineties.
The thrust areas for rice crop improvement are:
1. Genetic enhancement of yield and grain quality.
2. Identification of diverse CMS sources and male fertility restorers.
3. Identification
of
additional genetically diverse sources of
resistance to biotic and abiotic stresses.
4. Genetic enhancement of yield of quality and specialty rices.
5. Development of ideal new plant types/hybrids for different
ecosystems.
6. Genetic enhancement of biotic stress tolerance through MAS to
generate gene pyramid for durable multiple pest resistance.
7. Identification of mapping genes/QTLs using molecular markers for
desirable traits.

42 Amir B. Wani and Amjad M. Husaini
8. Enhancement of nutritional quality through biofortification with
pre-vitamin A, iron, and zinc content.
9. Breeding for design to suit diverse rice ecologies and prevailing
biotic and abiotic stresses targeting multiple genes through MAS.
The solution to these challenges in conventional breeding lies in hybrid
rice technology and the exploitation of novel genes from wile sources.
1. HYBRID RICE TECHNOLOGY
After harnessing the rice green revolution, the seeds of another rice
revolution were sown in the same decade by Prof. Yuan Long Ping
acknowledged as the father of hybrid rice. It was a cultural revolution in
China, and little information reached the outside world. A decade of
persistent, silent, and arduous efforts of Prof. Yuan and his associates
resulted in developing and identifying heterotic rice hybrids. Hybrid rice
was released for large-scale cultivation and commercialization in China in
1976. The development of successful hybrids in self-pollinated cereal
crops, which was considered an impossibility, was shown to be a reality.
Initial efforts of Dr. Virmani at IRRI during the period 1979-82 were
disappointing. China was not sharing its parental line material then but
agreed to share the same subsequently with IRRI. Dr. Virmani at IRRI
reviewed the research on hybrid rice with the help of Chinese CMS lines.
Reciprocally China has developed several promising hybrid utilizing indica
restorers introduced from IRRI. Since the Chinese CMS lines were
unadapted and highly susceptible to diseases and pests under tropical
conditions, Dr. Virmani developed several new CMS lines adapted to
tropics utilizing WA cytoplasm from Chinese CMS lines V20A and
Zenshan 97A (Virmani, 1994).
Generally, male sterility classification and its exploitation is crop
plants is on a genotypic basis, which includes:
Cytoplasmic Male Sterility (CMS);
Cytoplasmic Genetic Male Sterility (CGMS);

Conventional Approaches to Rice Improvement 43
Genetic Male Sterility (GMS);
Environmental sensitive genic male sterility (EGMS);
Genetically Engineered Male Sterility (GEMS);
Chemically Induced Male Sterility (CIMS).
Now, China covers 53% of its rice area and about 58% of production
under hybrid rice. China is developing Super Hybrid rice targeting a yield
level of 15t/ha (Roy and Dunna 2013). More than 95 percent of the hybrids
developed and cultivated in China and elsewhere are based on only one
CMS source, i.e., the WA (wild abortive) CMS source. Although there is
no dearth of sterility inducing cytoplasms in cultivated and wild rices, it is
often difficult to find the effective restorers resulting in heterotic hybrids
for these diversified sources of cytoplasms. Fortunately, so far, WA
cytoplasm has not shown vulnerability to any major pest or disease. The
frequency of restorer lines for WA cytoplasmic and other CMS systems is
fairly high among the improved lines and varieties of indica rices. The
frequency of maintainer lines is very high in japonica rices, but the
restorers are very rare.
In 1982, the Japanese government initiated super high-yielding rice
breeding program. The target was to achieve the brown rice output yield to
7. 5– 9. 8 t ha-1 in the medium and low yield areas, and over 10.0 t ha-1 in
the high yield areas, and to increase rice yield by 50% in 15 years. In
1989International Rice Research Institute (IRRI) planned the breeding of
‘new plant type rice’ (also called ‘super rice’) with a yield potential of 13 -
15 t ha-1 and an increase of 20-30% over control. In 1996, China too
launched super rice project and bred more than 20 super rice varieties,
including Liangyoupeijiu and Xieyou 9308. Their yields were higher than
10.5 t ha-1. The new hybrid rice combinations were called ‘super hybrid
rice’ (Chen et al. 2000; 2007).

44 Amir B. Wani and Amjad M. Husaini
1.1. Heterosis Breeding Program in Rice
1.1.1. Three-Line Approach or CMS System
In this approach, three lines, cytoplasmic male sterile line or A-line,
the maintainer line or B line, and the restorer line or R-line, are used. The
identification/development of parental lines and their subsequent use to
produce hybrids is critical. The parental lines should possess good general
combing ability, desirable floral traits which promote out pollination, wide
adaptability, resistance to various biotic stresses, and acceptable grain
quality besides a good agronomic base and acceptable plant type. Besides
this, the restorer must possess the characters which promote outcrossing.
These include strong and stable restoration over seasons and locations,
vigorous plant growth habit with a large number of spikelets containing
large plump anthers with a greater number of pollen grains, the
synchronous blooming of the spikelets, complete dehiscence of anthers,
and a large amount of residual pollen. It is possible to enlarge the genetic
base of R lines by accumulating complementary traits from various sources
to meet the breeding objectives. Several new R lines have been developed
in China using this approach.
The most commonly used method of crossing are RxR; AxR; partial
restorer x restorer or partial maintainer x restorer, etc., for RxR crosses,
promising restorers having complementary traits can be used so that a very
high frequency of restorers having many desirable traits can be selected in
segregating generations. At IRRI, a diversified source in the background of
Oryza perennis (IR 66707A) has been developed. Similarly at DRR (India)
six new CMS lines have been developed in the cytoplasmic background of
Oryza rufipogon and Oryza nivara. Diversified CMS sources presently
available are Dissi, Dian, Indonesia paddy, Chin surah Boro II, Hong Lian,
Gambiaca, ARC, Oryza perennis etc. But WA source is by far the most
popular and widely used CMS source as it is quite stable and is easily
restored.

Conventional Approaches to Rice Improvement 45
1.1.1.1. Genetic Improvement of Restorers and Maintainers in Rice
The profitability of hybrid rice technology depends on yield advantage,
market price as determined by grain quality, and hybrid seed cost as
influenced by seed yield in hybrid seed production plots. Hybrid
commercial grown in the country gives 15-20 percent yield advantage over
the best varieties. The moderate level of heterosis realized in the farmers'
field is not economically very attractive for large scale adoption of the
technology in the country.
There is an urgent need to further increase heterosis (20-30%) besides
improving grain quality and other desirable traits like outcrossing ability,
disease and insect pest resistance, etc., through directed breeding.
1.1.1.2. Recombination Breeding
In recombination breeding, desirable traits from different parents are
combined. The steps involved in recombination breeding are development
of segregating generations of single, double or multiple crosses, exercising
selections for desirable segregants in F2 onwards and handling the material
by pedigree method or going in for single seed descent (SSD) method and
exercising selection in F4/F5 stage, test cross evaluation and combining
ability evaluation to know the magnitude of heterosis.
Different recombination breeding methods that can be used for genetic
improvement of restorers and maintainers are: Pedigree, Backcross,
Incomplete backcross, single seed decent, Modified backcross methods,
Multiple convergent improvement, Modified pedigree method and
production of doubled haploid lines through another culture.
1.1.1.3. Iso-Cytoplasmic Restorer Development
Restorer lines that have sterility inducing cytoplasm (S) are referred to
as iso-cytoplasmic restorers because they have cytoplasm similar to CMS
line. Iso-cytoplasmic restorers can be developed by A x R, A x (Rl x R2) or
A x (R x PR) or A x (PR x PR) crosses followed by a selection of
segregants with normal spikelet fertility, i.e., (> 85 - 90%). One advantage
of this system is that presence of restorer genes is indicated by normal
spikelet fertility of the line. However, some times the sterility inducing

46 Amir B. Wani and Amjad M. Husaini
cytoplasm may have some undesirable effects on the per se performance of
the line. In order to utilize such lines more effectively for producing
heterotic hybrids, care has to be taken to use them in combination with
genetically diverse A lines of different genetic makeup. This method has
been used to develop a large number of restorer lines in China and India.
1.1.1.4. Allo-Cytoplasmic Restorer Development
Restorer lines that have normal cytoplasm are called allo-cytoplasmic
restorers. The main objectives of allo-cytoplasmic restorer development
are increasing the magnitude of heterosis through broadening the genetic
base, combining the desirable traits, and increasing the frequency of
restorers. In order to widen the genetic base, outstanding partial restorers
can also be used in breeding programs. The desirable traits of restorer line
are: (1) Tall stature (10-15 cm taller than A line); (2) Synchronous tillering
and long panicles, (3) High pollen production ability and high amount of
residual pollen, (4) Better grain quality and resistance to insect pests and
diseases, (5) Better combining ability for yield traits, (6) complete and
wider spectrum of fertility restoration, (7) Good plant type and sturdy
culm, (8) Complete anther dehiscence.
1.1.1.5. Choice of Parents, Population Size, and Selection Criteria
The choice of parents depends on the objective of the breeding
program. IRRI has adopted specific procedures in its extensive crossing
program. Crosses are made throughout the year; during peak periods more
than 300 pollinations a day may be made. Maintaining parents in pots
would be impossible; instead, they are grown in the field in a hybridization
block (HB), which is planted from four to six times each season at 2-week
intervals. The HB is organized in groups according to the objectives of the
program:
Group I (General) – IRRI varieties and improved lines;
Group II (International) – varieties and improved lines from
national programs;
Group III Agronomic characteristics and grain quality;

Conventional Approaches to Rice Improvement 47
Group IV Disease and insect resistance;
Group V Protein content;
Group VI Drought resistance;
Group VII Adverse soils tolerance;
Group VIII Deepwater and flood tolerance;
Group IX Temperature tolerance;
Group X Miscellaneous.
Genetically diverse donors for various traits like better plant type
(tropical japonica parents possessing wide compatibility genes), better
restoration ability, high pollen production, relatively tall stature, better
grain quality as per the local requirements, multiple disease and insect pest
resistance, good GCA and SCA etc. should be used in the crossing
program. Single, three-way and multiple crosses among restorers and
partial restorers may be used to develop large segregating generations.
Depending on the purpose, the type of crosses may be Rl x R2, Rl x (R2 x
R3), R1 x PR, Rl x (PR x R2), Rl x (PR1 x PR2), PR x (Rl x R2), PR1 x (Rl x
PR2), (Rl x R2) x (R3 x R4), (R1 x PR1) x (R2 x PR2) and so on.
The size of the population should be as large as possible, and it should
not be less than 2000-3000 plants. Care has to be taken to plant only one
seedling per hill. As per the objective of the program and availability of
good segregants with desirable traits, the desired number of plants should
be selected and proceed to further generations. In addition to normal visual
selection based on phenotype, selection for specific measured traits should
be taken into consideration, and a wide range of genetic diversity among
the selected individuals should be maintained.
1.1.1.6. Test Crossing and Combining Ability Evaluation
Relatively uniform recombinant lines should be crossed with one or
two CMS lines, and test cross evaluation has to be carried out to know the
presence of restorer genes. Identified restorers must be crossed with many
genetically diverse CMS lines in Line x Tester design, and hybrids should
be evaluated in 2-3 locations to know the combing ability and heterotic
potential of the newly developed restorers. Hybrids involving selected

48 Amir B. Wani and Amjad M. Husaini
restorer and CMS combinations should be subjected to detailed multi-
locational, and multi-year replicated yield trials, so that good hybrids of
commercial value could be identified. The experience on parental line
improvement in China, IRRI, India, and elsewhere clearly indicates the
usefulness of these methods in developing a large number of restorers
possessing many desirable traits and increased magnitude of heterosis.
1.1.1.7. Maintainer Line Improvement
One of the major challenges in a self-pollinated crop like rice is how to
maximize seed yield in seed production plots. The outcrossing ability of
the parental lines plays an important role in enhancing the seed yield. The
frequency of maintainers among the elite breeding lines is rather low, and
even among them, all are not suitable because of one or the other defect.
Hence, there is a need to combine desirable traits through recombination
breeding. Outstanding partial maintainers with many desirable traits can be
used in the breeding program. Care has to be taken to improve the
combining ability related traits and other traits that are needed for
commercial seed production. The desirable traits of the maintainer lines
are: (1) Relatively dwarf/semi-dwarf stature, (2) Good and synchronous
tillering (3) High stigma exsertion and outcrossing potential, (4) Good
plant type and sturdy culm, (5) Better grain quality and resistance to insect
pests and diseases, (6) Better combining ability for yield contributing traits,
(7) Complete and stable maintenance of sterility, (8) Floral traits for higher
outcrossing like the wider angle and longer duration of glume opening
As indicated in restorer line improvement, i.e., single, double, and
multiple crosses among maintainers and outstanding partial maintainers
may be used to develop large segregating generations. The population size
in F2 should be sufficiently large (> 2000-3000 plants), and careful
selection has to be exercised for outcrossing and combining ability related
traits by retaining sufficient genetic diversity of the segregants. Selection
for plant type, grain type, stigma exsertion, and other easily observable
traits can be made in early segregating generations, and combining ability
of the desirable fixed lines will be tested at F5/F6 stage after confirming the
presence of stable maintainer genes, through test crossing.

Conventional Approaches to Rice Improvement 49
1.1.1.8. Development of CMS Lines
Promising maintainers with many desirable traits can be converted into
a new CMS line through the recurrent backcrossing. In the initial stages, it
is better to maintain at least three paired cross progenies. Each pair of back
cross progeny can be grown in 5-10 rows of 10-15 plants each, along with
corresponding male parents as borders on both sides. Critically evaluate
BC progenies for panicle exsertion, stigma exsertion, complete male
sterility and similarity to the recurrent parent. Only good progenies will be
retained and backcrossed with corresponding male parents. When the
material reaches BC5 stage, it can be multiplied in a large area and tested
for combining ability to find out its suitability in the development of highly
heterotic hybrids.
1.1.2. Two-Line Approach in Hybrid Rice Breeding/Environment-
Sensitive Genic Male Sterility (EGMS) System
Greater dependence on a single source of cytoplasmic male sterility by
using the WA system and the most difficult and laborious process of seed
production and parental line development warrants the development of
alternative methodologies to exploit hybrid vigor in rice (Shukla and
Pandey, 2004). Two-line breeding based on environment-sensitive male
sterility is one such possibility. This system arises due to the response of a
particular variety to changing photoperiods and temperatures, hence two
new kinds of genetic tools viz. photo-sensitive genic male sterility (PGMS)
and thermosensitive genic male sterility (TGMS). Following the chance
discovery in 1973 of a male sterile plant called Nongken 58S, the former
system emerged in the japonica variety Nongken 58 (Shi, 1981). Using the
PGMS system, Yuan (1987) put forth a new strategy of hybrid rice
breeding, which did not involve a maintainer, as it is taken care of by the
shorter period (L13 hours); hence it was called a two-line method, i.e.,
PGMS and non-PGMS.
The EGMS comprises of following three types:
Photoperiod sensitive genic male sterility (PGMS): The line is
sterile when the photoperiod (day length) exceeds 14 hrs, and the

50 Amir B. Wani and Amjad M. Husaini
same line become fertile when subjected to a photoperiod of <13
hrs.
Temperature-sensitive genic male sterility (TGMS): It is sterile
when the temperature exceeds 32oC/24oC (day/night) and becomes
fertile when the temperature is below 24oC/18oC (day/night).
Photo-thermo-sensitive genic male sterility (PTGMS): The
interaction of photoperiod and temperature controls this line. Most
of the PGMS lines earlier discovered, such as the classical
Nongken 58S, were later reported to fall in this category.
TGMS lines are classified into two types based on fertility-sterility
transformation behavior in response to temperature as follows:
1.1.2.1. High-Temperature Sterility – Low Temperature Fertility
Based on critical sterility point (CSP), the temperature at which
complete sterility is induced and critical fertility point (CFP), the
temperature at which maximum fertility is achieved, it can be classified
into four types:
Type 1: High CSP (>32oC) – low CFP (<24oC): This type is
recognized by Chinese as ideal, as it is safe for both hybrid seed
production and multiplication. Spontaneous mutant lines SM3 and
SM5 fall under this category.
Type 2: High CSP (>32oC): Chinese have reported several of the
EGMS lines identified by them to fall under this category. It is not
suited to Chinese conditions, as it would introduce risk in hybrid
seed production.
Type 3: Low CSP (<32oC) – low CFP (<24oC): By virtue of stable
sterility duration over a large region in China, it can be used in
hybrid seed production without any problem. In this type of
EGMS, self-seed multiplication becomes difficult and hence limits
its wide utilization in China. For sub-tropical countries like India,
this type of TGMS is not suited, as only sterile phase is required to

Conventional Approaches to Rice Improvement 51
be more stable in such situations, lines like SA-2 (F43), UPRI, 95-
140 TGMS, UPRI-95-167 TGMS fall under this group.
Type 4: Low CSP (<32oC) – high CFP (>24oC): under this
particular category lines are yet to be identified.
1.1.2.2. High-Temperature Fertility – Low Temperature Sterility
These reverse TGMS lines are sterile under low temperature (22-24oC)
and fertile under high temperature (27oC). JP38S, a spontaneous mutant
isolated in the farmer’s field at Katrain in HP (India) shows complete
pollen and spikelet fertility under high temperatures (30.5oC) and remain
sterile at low temperature (24oC). Hybrid seed production can be done only
at high altitude, low-temperature regions, while self-seed multiplication in
the plains, having high-temperature regimes.
1.1.2.3. Advantages of EGMS
1. It simplifies the hybrid seed production procedures and decreases
the cost of hybrid rice seed, and improvement in seed purity.
2. The chance of the development of heterotic hybrids are greatly
increased as any non-EGMS genotype can be used as a pollen
parent.
3. It avoids the negative effects associated with sterility inducing
cytoplasm.
4. The trait can be easily transferred to high yielding good combiner
genotype as male sterility is controlled by few recessive genes.
TGMS trait is controlled by a single recessive gene and three
independent genes, tms1, tms2, and tms3. Reports indicate that tms1 gene
is located on chromosome 8, tms 2 on chromosome 7, tms3 on
chromosome 6 and tms 4 on chromosome 9 with the help of molecular
markers. Two of the major genes tms-5 and tms-6 in UPRI 95-140 TGMS
were located on chromosome 3 and 7, respectively and are non-allelic to
known genes (Rangbai et al., 2005). Both the genes are mapped with

52 Amir B. Wani and Amjad M. Husaini
STMS markers (Sharma et al., 2006). Hence they can be easily transferred
through backcrossing to known elite, good combining varieties.
1.1.3. Chemical Hybridizing Agent (CHA’s)
CHA is a chemical that can selectively sterilize the male gamete
without affecting the ovular fertility on spraying at a specific dose at the
sensitive stage. Various chemicals tried broadly include ethylene releasing
compounds, highly carcinogenic arsenic compounds, and growth
hormones. China is perhaps the only country where the gametocytes are
used in commercial hybrid seed production. Chemically emasculated rice
hybrids tested along with 3 line bred hybrids are reported to give
consistently comparable and often higher yields. As for seed production,
the hybrid seed yield has been increased from 0.4 t/ha with 40-60 percent
purity to 1.5 t/ha with 80-90 percent seed purity over the years. CHAs must
be able to induce total male sterility selectively. They are highly stage-
specific, i.e., stamen and pistil primordial formation stage (stage –IV).
They are highly genotype-specific, i.e., gametocidal effect varies from one
variety to another. For the oxanilates, stage IV (stamen and pistil
primordial formation stage) to VI (meiotic stage) of rice developmental
stages are most efficient and variety Pusa 150 is more effectively sterilised
by the gamaticidal spray as compared to other varieties indicating genotype
specificity.
1.2. Indica/japonica Hybridization
Rice breeders in tropical ASIA impressed with the relatively higher
yielding ability of japonica varieties, attributable to semi-tall stature and
hence less proneness to lodging and responsiveness to higher doses of
fertilizers felt that recombining of such yield promoting features of
japonicas with wide adaptability and good grain quality of indicas would
prove the best strategy for raising the yield level of tropical rices; which
have been stagnating for long at low yield levels. Convinced of this idea,
the Food and Agriculture Organization launched the ambitious indica

Conventional Approaches to Rice Improvement 53
japonica Hybridization Program in 1952 for improving the yield of tropical
rice. India, along with other countries in the region, was actively involved
in the ten-year-long breeding exercise. However, this project proved
practically a failure except for the evolution of four varieties in all viz.
Mahsuri and Malinja in Malaysia, Circna in Australia, and ADT 27 in
India during 1965. Incidentally, Mahsuri introduced later in India became
one of the most popular varieties ideally suited to wet seasons in Tamil
Nadu and Andhra Pradesh as well as rainfed shallow lowland ecologies of
eastern India. Of several, the major reason for the project's failure was
persistent spikelet semi-sterility in the segregating generations, very low
frequency of desired recombinants due to low percentage crossover, and
slow pace of fixation of homozygous lines for agronomic characters.
Measures like repeated backcrossing to indica parents and double haploid
breeding highly helped overcome the sterility problem.
In Japan, during the early 1950’s an exclusive indica/japonica
hybridization efforts made for transferring blast resistance from indica to
japonica rices resulted in the release of a few commercial cultivars
combining resistance to blast. Similarly, in Korea, an intensive
indica/japonica hybridization programme was launched. Meanwhile, the
tropical rice world witnessed a major yield breakthrough through the
development of dwarf plant type based high yielding varieties. The DGWG
dwarfing gene-based high yielding varieties like IR 8 and TN-1 were
crossed to popular japonica varieties of Korea like Yukara. To overcome
the problem of spikelet sterility, repeated backcrossing was resorted to
using the semi-dwarf indica varieties as recurrent parents. The strategy of
backcrossing or top crossing with indica parent as against the earlier
Japanese practice of backcrossing with japonica parent was found more
rewarding in overcoming the problem of semisterility (Siddiq and
Viraktamath, 2000). Critical evaluation of the progeny of these crosses
especially of the crosses IR667 (IR 8/Yukara/T(N)1) led to the
identification of several lines widely referred to as Suweon series well
adapted to Korean conditions with their yields exceeding 50% over the
best adapted local japonica varieties. The line Suweon 213 named as
Tongil was released in 1971. This development encouraged Korean rice

54 Amir B. Wani and Amjad M. Husaini
breeders, who once had reservations on the strategy of inter subspecific
hybridization for the improvement of japonicas, to exploit this
breeding/selection experience. Unlike in many Asian countries, where
despite all efforts, improvement of tropical rice through indica japonica
hybridization resulted in limited success, it was fairly successful in the
United States. The success was mainly because the local varieties of the
United States were relatively more compatible with both indica and
japonica germplasm.
1.2.1. Constraints Encountered in Indica/Japonica Hybridization
Major problems encountered in the exploitation of indica/japonica
heterosis are hybrid semisterility, poor grain filling, poor grain quality,
excessive plant height and maturity duration, linkage drag and non-
sychrornzed time of flowering (Ikehashi et al., 1992). Hybrid sterility is,
however, the main obstacle in the inter-subspecific heterosis breeding. The
extent of semi-sterility reported to be in the same order as that of heterosis
level in different inter subspecific hybrids, i.e., indica/japonica >
indica/javanica > japonica/javanica is more a reflection of the genetic
relatedness of the parents involved. Poor grain filling characteristic to
indica/japonica hybrids is reported to be caused by early leaf and root
senescence, source-sink imbalance, and poor translocation/partitioning of
assimilates to the grain. Inter sub-specific cross combinations do not
exhibit as high grain yield heterosis as vegetative heterosis due to
constrained translocation; as a result of which, 15-20% of the
photosynthate remains in the flag leaf itself. Indica/japonica hybrids
generally are of poor grain quality as the quality indices of the parents
vastly differ, and F1 produce is F2 grains that segregate for quality traits.
Change in the breeding strategy viz opting for indica/javanica hybrids
rather than indica/japonica hybrids in predominantly indica rice-growing
regions and japonica/javonica hybrids in japonica rice-growing regions
would help overcome the problem of poor grain quality encountered in
inter subspecific hybrids. This strategy has been successful in developing
several inter subspecific hybrids of very high yield potential (100
kg/ha/day) and acceptable quality in China.

Conventional Approaches to Rice Improvement 55
Plant height in indica/japonica hybrids invariably exceeds that of their
parents, resulting in their proneness to lodging. The excessive height is
caused by interaction, possibly between non-allelic dwarfing genes coming
together in Fl from two diverse parents. The problem of extra tall plant
stature in such inter sub-specific crosses can be solved using semi dwarf
parents having allelic dwarfing genes. Also, the growth duration of most of
the indica/japonica crosses is longer than their parents. Parental choice
restricted to photo insensitive early or medium growth duration would help
overcome this problem. Most of the indica/japonica hybrids exhibit
significant segregation distortion in the segregating generations. This
phenomenon poses difficulties in introgressing those characters/genes that
are located in the genomic regions associated with segregation distortion.
Desirable and undesirable characters going together due to tight linkage is
not uncommon in inter sub-specific crosses. This phenomenon referred to
as linkage drag greatly limits the success of inter subspecific crosses.
Generally, indica and japonica parents flower at different times, indica
being earlier by one hour than japonica. This difference in flowering time
make hybrid rice seed production between the two subspecies difficult. In
China, this problem was solved by selecting the japonica mutant 02428
Xuan, which flowers the same time as indica (Ikehashi et al. 1992).
1.2.2. Genetic Basis of Hybrid Semi-Sterility
Hybrid sterility has been the subject of great interest and controversy
for a long among breeders and geneticists. The genetic basis of hybrid
sterility in indica/japonica has been explained through chromosomal and
genic models. Whereas the chromosomal model attributes the hybrid
sterility to cryptic structural differences, the genic model attributes it to an
imbalance of gene groups between the subspecies as the cause of hybrid
sterility. In the absence of convincing cytological evidence favoring the
chromosomal model, different hypotheses have been advanced at the genic
level. Prof. Oka and his coworkers proposed a duplicate recessive lethal
gamete model. They postulated that a series of duplicate genes, which they
called gametic development genes in their double recessive combinations
(S1S2) interrupted the post-meiotic development of gametes carrying them,

56 Amir B. Wani and Amjad M. Husaini
thus causing hybrid semisterility. As per this hypothesis, both male and
female gametes are affected by the same set of gametic lethal genes. The
wide compatibility gene loci (WCGL) model of Ikehashi and Araki (1986)
also referred to as hybrid sterility gene loci (HSGL) is basically a ‘single
locus allelic interaction model’. The model based on their study of the
wide compatibility gene, S-5 located between the loci for chromogen
colour (C) and waxy endosperm (wx) on chromosome 6 assumes the locus
to consist of three alleles viz. S-Xi, S-Xj and S-Xn representing indica,
japonica and neutral alleles. It explains inter sub-specific hybrid
sterility/fertility through allelic interaction, wherein the heterozygote S-
Xi/S-Xj representing indica/japonica cross shows hybrid sterility due to
partial female gamete abortion, while the homozygotes (S-Xi/SXi and S-
Xj/S-Xj) representing indica and japonica parents respectively and
heterozygotes S-Si/Sxn and S-Xj/S-Xn representing indica/WCV and
japonica/WCV are fertile. To overcome semi-sterility in inter subspecific
hybrids, one of the parents must necessarily possess the WC gene S-Xn.
Identification of such loci is possible by a study of various mapping
populations like F2s of single crosses, backcrosses and 3-way crosses.
Generally, a 3-way cross (A/B//C) is made after confirming that a hybrid
(A/C) is semisterile, while other hybrids (A/B and B/C) are fertile. The
progeny of A/B//Cwould segregate in 1:1 ratio for semisterility and fertility
as expected from A/C and B/C single crosses. Similarly, in the backcross
A/C//C, the progeny would segregate into semi-sterile and fertile as
expected from A/C and C/C crosses, respectively. Genetic markers co-
segregating with semi-sterility or fertility are then surveyed to identify the
locus for semisterility.
1.2.3. Discovery of Wide Compatibility Genes
Prof. Ikehashi and his coworkers demonstrated for the first time the
existence of simply inherited sterility neutralizing gene(s) in rice
germplasm, and through its use, the problem of indica/japonica sterility
could be overcome (Ikehashi, 1982). He referred to these varieties that
carried the sterility neutralizing alleles (wide compatibility gene) as wide
compatible varieties (WCV), which produce normal fertile hybrids in

Conventional Approaches to Rice Improvement 57
crosses with both indica and japonica varieties (Ikehashi and Araki, 1988).
Following the pioneering report of Ikehashi (1982) on successfully
overcoming hybrid sterility in indica/japonica crosses using wide
compatible varieties (WCVs) like Ketan Nangka and Dular there was
interest and special effort to identify and study more such WCVs.
However, spectrum and level of compatibility vary among the WCVs. In
several studies, the variety ‘Dular’ has been found to produce highly fertile
hybrids in majority of crosses, and the number of fertile crosses and
percentage spikelet fertility in each of those crosses are much higher than
crosses involving other WCVs such as 02428 and Ketan Nangka, both of
which have been reported to carry neutral allele at S-5 locus (Ikehashi and
Araki 1986). Soon after understanding the genetic basis and breeding
behaviour of hybrid semi sterility wide compatibility, efforts have been
made to apply the same for exploiting yield heterosis in indica/japonica
crosses. To start with, the wide compatibility gene S-5 was incorporated
into many popular indica and japonica varieties for the development of
sterility-free heterotic inter-subspecific hybrids. Though many F1s were
fertile as expected with an expected level of yield advantage, some of them
were found to exhibit still some sterility despite the presence of S-5 gene
(Ikehashi and Araki, 1987).
1.2.3.1. Use of Wide Compatibility Gene in Breeding for Higher Yields
Early efforts to develop high yielding genotypes combining desirable
traits from Indica and Japonica sub-species were not entirely successful
due to poor understanding of the nature, genetics, and breeding behavior of
hybrid semi-sterility in inter-subspecific crosses. With the present
understanding of the phenomena of hybrid semisterility and the ways to
overcome it through the deployment of WCG, it should now be possible to
combine with ease traits of interest from indica and Japonica sub-species
towards the development of more productive varieties and hybrids that will
be discussed in another section.

58 Amir B. Wani and Amjad M. Husaini
1.2.3.2. Ideotype Breeding for Super Rice and Super Hybrid Rice
Breeding work for developing new plant type (NPT) started as early as
1989 (Khush, 1995). Bulu varieties or javanicas from Indonesia have low
tillering, large panicles, and sturdy stems. The germplasm is now referred
to as the tropical Japonicas (NPT-TJ). Around 2000 bulu varieties were
assessed in the field, and donors for developing NPT-TJ were identified.
Few examples are Gendjah Wangkal (for low tillering trait), Ketan Gubat
(for large panicles), Senkeu (for thick stem), and Shen Nung (for short
stature) (Peng et al., 1994). In addition to bulus from Indonesia, many
tropical japonica donors were identified in the germplasm from Malaysia,
Thailand, Myanmar, Vietnam, and Philippines (Virk and Khush, 2003).
Hybridization was undertaken in 1990. Since most of the bulu varieties
were tall, these were crossed with a semi-dwarf breeding line, Sheng Nung
89-366 from China. The selected donors were crossed, and breeding lines
with the proposed ideotype were selected. The first batch of NPT-TJ lines
had large panicles, few unproductive tillers, thick stems and large and dark
green flag leaves. Grain yield, however, was not encouraging. This was
attributed to low biomass production and poor grain filling. Later the
problem was overcome by including only parents that produced a high
percentage of filled grains in subsequent hybridization program. As a
result, the yield performance of the later NPT-TJ line was higher than that
of the previous NPT-TJ, and the Indica check variety, e.g., the best NPT-
TJ line cut yielded IR-72 by 9.5 percent. China released three NPT-TJ
varieties in 2003 and 2004, namely Dianchao 1, Dianchao 3, and Diancho
2. A few second-generation NPT lines produced a significantly higher
yield than the indica check variety, IR72, in several seasons (Peng et al.,
2004). Many second-generation NPT lines out-yielded the first-generation
NPT lines and indica check varieties (Peng et al., 2008)
As the fine tuning process continued, in 1995, modern high yielding
indica varieties/elite lines were included in the hybridization program.
With this, the development of improved NPT lines began. Since these lines
are derivatives from crosses between indica and japonica germplasm, they
came to be known as NPT-IJs. This was necessary, i.e., increasing
biomass, incorporating genes for resistance to tropical diseases and insects,

Conventional Approaches to Rice Improvement 59
and changing grain appearance and quality. During 2004, several NPT-IJ
lines out yielded the best Indian check variety by up to 30 percent. In 2002
dry and wet seasons, several NPT-IJ lines significantly out yielded check
variety IR-72. NPT-IJ line approached the 10 t/ha yield barrier.
In 1996, the Chinese Ministry of Agriculture initiated the super hybrid
rice project to increase rice grain production in China (Table 2). This
project aimed to produce elite rice hybrids by combining ideal plant
architecture with heterosis through hybridizing indica and japonica
subspecies. The objective of the super hybrid rice project to obtain yields
of 12 t/ha on a large scale was achieved in 2004; the next objective is to
obtain yields of 13.5 t/ha.
Table 2. Progress and target of rice yield improvement
in China through inter-sub-specific super hybrid breeding
Hybrid type Year Target (t/ha)
Intervarietial 1996 8.25
Super hybrid phase-I 2000 10.50
Super hybrid phase-II 2004 12.00
Super hybrid phase-III 2010 13.5
Super hybrid phase-IV 2011 14.8
Super hybrid phase-V 2015 16
Until 2018, about 131 rice cultivars were officially approved as super
hybrids by the Ministry of Agriculture in China (http://www.ricedata.cn/
variety/superice.htm). Liang-you-pei-jiu (LYP9) is one among the
representatives, which was developed using a two-line crossing between
PA64S and 93-11. The high yield, fine grain quality, and strong resistance
against bacterial leaf blight and blast diseases are attributed to its
intersubspecific heterosis (Cheng et al., 2007). The additional 15-20%
increase was achieved by the use of heterosis (Yuan, 2003) combined with
the IRRI’s design of a new plant type (NPT) into super hybrid rice (Peng et
al., 2008). Several pioneer super hybrids have a yield advantage of around
20 percent over the current three-line hybrid on a commercial scale. The
area under super rice hybrids has increased to 1.4 m ha with an average
yield of 9.1 t/ha in 2002. In addition, a two-line super hybrid P645/E32 and

60 Amir B. Wani and Amjad M. Husaini
a three-line super hybrid II-32A/Ming 86 created a record yield of 18 t/ha
in 2001. New efforts are on to create second-generation super hybrid rices
with a yield target of 12 t/ha on a large scale. Therefore, super hybrid rice
shows a promising future.
1.2.3.3. Introgression of Genes from Wild Species/Alien Introgression
The main objectives of this alien introgression/wide crossing is (i) to
widen the gene pool of rice and hybrid rice by transferring useful genes for
resistance to major diseases and insect pests and tolerance to abiotic
stresses (ii) to enhance the grain yield of rice through introgression of
useful alleles of wild relatives, (iii) to precisely determine the mechanism
of alien gene transfer. Some of the steps followed for transferring genes
from wild species are:
Identification of useful genetic variability in wild species
germplasm.
Production of hybrids between elite breeding lines of rice and wild
species through direct crosses/or through embryo rescue.
Continued backcrossing with recurrent parents.
Evaluation of advanced fertile backcross progenies for transfer of
target traits from a wild species.
Characterization of alien introgression using molecular workers.
Chromosome location of introgressed gene(s) using mono-somic
alien addition lines (MAALs).
Tagging of introgressed alien genes with molecular markers for
use in marker-assisted selection (MAS).
The genus Oryza has 22 wild species (2n = 24 and 2n = 48)
representing AA, BB, CC, BBCC, CCDD, EE, FF, GG, HHJJ and HHKK
genomes (Ge et al., 1999). Wild relatives of rice, e.g., O. nivara and O.
rufipogon (2n = 24, AA genome) constitute an essential gene pool for rice
improvement. The two species are the closest wild relatives and progenitor
of O. sativa and cross easily with rice cultivars. The diversity in these two
A-genome species is enormous and they have contributed to rice

Conventional Approaches to Rice Improvement 61
improvement programs. O. rufipogon is the source of WA (wild abortive)
cytoplasmic male sterility used widely in hybrid rice production and O.
nivara provided a rare major dominant gene for resistance to grassy stunt
virus. In addition, these two species are widely recognized as sources for
resistance against bacterial blight, hoppers and abiotic stresses such as
acidity. Thus, the diversity available in them has been largely used to
introduce simply inherited traits such as resistance to some important pests
and abiotic stresses. But their use has been largely ignored in yield
improvement programs. This is because, the yield being a complex trait,
breeders traditionally depended on crosses involving phenotypically
promising genotypes to obtain higher yielding genotypes quickly. But
recent evidence from molecular mapping studies suggests that despite their
poor phenotype, wild species can contribute genes for improving complex
traits like yield (Xiao et al., 1998). This has led to a major shift from
looking at phenotype to looking at genotype for traits that enhance yield
(Tanksley and McCouch, 1997).
Inter-specific crosses usually result in progenies with sterility
problems, disruption of favourable linkage blocks and gene combinations,
and most importantly, linkage-drag related problems making it difficult to
select and use superior phenotypes. The advanced backcross method and
the use of molecular markers help identify and transfer agronomically
useful QTL (quantitative trait loci) from wild relatives.
In the advanced backcross method so useful for QTL discovery from
the wild species, introgression, identification, and mapping proceed
simultaneously (Tanksley and Nelson, 1996). By the time mapping of
QTLs for yield or yield, components are completed, introgressed lines of
the variety are also ready. Only those with yield-enhancing QTLs and other
desirable traits such as disease and pest resistance intact can be used in
crossing or evaluated as such for release. It is possible for 1/3 of the
chromosomes to pass through one cycle of meiosis without recombination.
The rapid recovery of recurrent phenotypes in BC2 and BC3 generations
from wide crosses has been reported earlier (Brar and Khush, 1997) and
observed in the studies at DRR (Reddy et al. 2002). A Malaysian accession
of O. rufipogon (IRGC 105491) has been reported to possess 2 major yield

62 Amir B. Wani and Amjad M. Husaini
QTLs which helped increase the yield of the Chinese indica hybrid
V20A/Ce 64 by 17-29%. Although the wild accession was inferior for all
12 traits studied, transgressive segregation was observed for all traits, and
51% of the QTL detected had beneficial alleles from the wild accession
(Xiao et al., 1998).
A second study using an advanced backcross population between the
same O. rufipogon accession and the upland japonica rice cultivar Caiapo
identified beneficial QTL alleles from O. rufipogon for 56% of the trait
enhancing QTL detected. Such yield-enhancing wild QTLs are valuable if
they remain stable over locations, seasons, and different genetic
backgrounds.
A third study using an advanced backcross population between the
same O. rufipogon accession and the long grain tropical japonica cultivar
Jefferson also showed that the O. rufipogon allele was favorable for 53%
of the yield and yield component QTLs.
The advanced backcross method has been used to map yield-enhancing
QTLs introgressed from O. nivara. Heterosis for the number of
panicles/plant and 1000 grain weight was found in hybrids between TKl
and O. nivara. It was concluded that O. nivara has potential for improving
cultivated rice yield components by the recurrent back cross method. The
mean spikelet fertility and 1000 grain weight of BC2F2 and BC3F2 from
TK-1 x O. nivara were better than the recurrent parent TK1.
O. glumaepatula, a diploid wild relative of cultivated rice native to
Central and South America, has also been used in studies on yield-related
QTL. Marker regions accounting for 14 to 73% of the phenotypic variation
in a trait were identified in 9 of the 12 rice chromosomes. About 16% of
the detected QTL had positive alleles contributed by O. glumaepatula.
These were in chromosomal regions associated with tiller number and
panicle number.
O. glaberrima has been used in crosses with O.sativa to improve
mainly the tropical japonicas. NERICA (New Rices from Africa) lines
have been developed from O. sativa x O. glaberrima derivatives in Africa.
Introgressions from O. glaberrima, are reported to be very promising for
upland rice improvement.

Conventional Approaches to Rice Improvement 63
1.3 Introgression from AA Genome of Wild Species,
Which Is Being Used Directly or Indirectly in Hybrid Rice
Breeding Program
1. The first examples of transfer of a useful gene from wild species
are grassy stunt virus resistance from O. nivara to cultivated rice
varieties (Khush 1977), which is presently incorporated in many of
the high-yielding rice varieties
2. The transfer of a cytoplasmic male-sterile (CMS) source from O.
sativa f. spontanea, to develop CMS lines for commercial hybrid
rice production. At present, in most of the hybrid rice program this
CMS source is being used. Interestingly, this is the only
cytoplasmic source which is easily restored and widely used in the
hybrid rice development program.
3. The tungro resistant genes transferred from O. rufipogon (Ram et
al., 2005) into accessions 106423 and 105908.
4. Bacterial Blight (BB) resistance gene Xa 21 transferred from O.
longistaminata into IR 24, IR 64, PR 106, and new plant type.
5. BB resistant gene Xa 23 (t) transferred into Jiagang 30 (Zhang et
al., 1996).
6. A new CMS source from O. perennis transferred into indica rice
(Dalmacio et al., 1995) designated as IR 66707 A.
7. Another CMS line (IR 69700 A) having the cytoplasm of O.
glumaepatula in the background of IR 64 is developed (Dalmacio
et al., 1995).
8. AT DRR itself, 6 CMS lines have been developed using cytoplasm
of O. nivara and O. rufipogon, which is different from previously
used cytoplasm (W A) for hybrid rice breeding programs.
9. The genes/QTLs for iron toxicity, aluminum toxicity, and acid
sulphate tolerance have been transferred from O. rufipogon into
indica back ground (IR 64) and variety IR 73278 6-9-B has been
released in Vietnam (Brar et al., 2005).
10. BPH resistant genes have been transferred from O.rufipogon into
elite breeding lines of indica (Ram et al., 2005).

64 Amir B. Wani and Amjad M. Husaini
11. Highly resistant genes of African gall midge have been identified
and transferred in indica variety from O. longistaminata and O.
glaberrima.
12. Identified and introgressed yield-enhancing loci/QTLs (yld 1.1 and
yld 2.1) from O. rufipogon.
13. Transgressive segregants for yield and yield components resulting
in yield improvement in the introgressed population of O.
longistaminata and O. rufipogon crossed with elite indica varieties
were reported by Brar and Khush (1997).
14. Variety Dhanrasi released in India with genes introgressed from O.
rufipogon for blast resistance, yield traits (No. of grains/panicle)
moderate resistance to rice tungro virus, sheath rot, and stem borer
(Ram et al., 2005).
15. Variety Jarava was released in India with genes introgressed from
O. rufipogon for blast, BPH resistance, and salinity tolerance (Ram
et al., 2005).
16. Yield QTLs from O. rufipogon and O. nivara successfully
introgressed into the CMS line IR 58025A and the restorer KMR-3
at DRR.
17. Two accessions (104423 and 80671) that were resistant to all the
Bacterial Blast strains of the Philippines and India have become
important resources as donors to introduce Bacterial Blast
resistance genes into rice (Ram et al., 2011).
18. A bacterial blight resistance gene Xa38, introgressed from O.
nivara has been mapped to a region of 38.4 kb of chromosome 4.
PCR based site sequence-tagged marker has been developed for
Xa38 to facilitate its use in MAS. Comparison of near-isogenic
lines (NILs) of Basmati rice variety PB 1121 carrying Xa38 with
PB1121 NILs carrying xa13 + Xa21, has shown that besides Xoo
races 1, 2, 3, and 6, Xa 38 shows resistance to Xoo race 5 too (to
which xa13 + Xa21 was susceptible) (Ellur et al., 2016).
19. From O. sativa X O. nivara population, two new QTL’s yld9.1 for
yield and nfg 9.1 for a number of filled grains have been reported
(Neelamraju 2014).

Conventional Approaches to Rice Improvement 65
Table 3. Introgression of genes of wild species into rice (O. sativa)
Trait Donor species Introgr
ession
lines
Wild species
Acce
ssion
No.
Gene
G
e
n
Grassy
stunt virus
resistance
O. nivara 101508 GS AA Several
varieties
released
O.
longista
mi- nata
? Xa21 Varieties
released
Bacterial
blight
resistanc
e
O.
rupogon
RBB16 Xa23 AA Lines in
yield
trial
O.
logistamina
Xa 21 AA NSIC Rc 112
O. nivara Xa38 AA Lines in
yield
trial
O.
ocinalis
Xa 29(t) CC Lines in
yield
trial
O. minuta Xa27
BBC
C
Lines in
yield
trial
O. latifolia Unknown
CCD
D
Lines in
yield
trial
O.
australiensi
Unknown EE Lines in
yield
trial
O.
brachyanth
Unknown FF Lines in
yield
trial
Blast O.
rupogon
Coll-4 ? AA Variety
released
O.
glaberrima
AA Yun Dao
O. nivara Unknown AA Lines in
yield
trial
O.
glumaepatu
Unknown AA Lines in
yield
trial
O. barthi Unknown AA Lines in
yield
trial
O. minuta Pi9
BBC
C
Lines in
yield
trial
O.
australiensi
Pi40 EE Lines in
yield
trial
Tungro tolerance O.
rupogon
105908 ? Lines
under
yield
trial
Brown plant
hopper
tolerance
O.
rupogon
Coll-4 ? Variety
released
O. nivara Bph33(t) AA Lines in
yield
trial
O.
ocinalis
bph11,
bph12,
Bph14,
Bph15,
CC Lines in yield
trial
O.
eichingeri
Bph13 CC Lines in
yield
trial
O. minuta Bph20,
Bph21
BBC
C
Lines in
yield
trial
O. latifolia Unknown
CCD
D
Lines in
yield
trial
O.
australiensi
Bph10,
Bph18
EE Lines in
yield
trial
Cytoplasmi
c male
sterility
O.
sativa
f.
- WA AA Wildly
used in
hybrid
O. perennis 104823 ? AA Line
available
O. nivara - ? Line
available
O.
rupogon
- ? AA Line
available
O.
glumaepatu
100969 ? AA Line
available
Salinity
tolerance
O.
rupogon
Coll-4 ? Variety
released
Tolerance to
acidity
O.
glaberrima
Many ? -
Iron and
aluminium
toxicity
O.
rupogon
106412 ? Lines
are
under
evaluatio
Yellow
stem
borer
tolerance
O.
longista
mi- nata
- ? Lines
are
under
evaluatio
Source: Brar et al., 2005, Khush and Brar, 2017.

66 Amir B. Wani and Amjad M. Husaini
Many varieties have genes introgressed from wild species that have
been released in several countries (Brar et al., 2005, Khush and Brar,
2017). The alien genes of agronomically important traits have been
introgressed from wild species are given in Table 3. Still, there are
prospects of alien gene introgression for increasing productivity.
CONCLUSION
During the 50s and early 60s large-scale food shortages were in India
and several Asian countries. However, green revolution based fertilizer
responsive high yielding semi-dwarf and non-lodging plant types,
Taichung Native I and the subsequent variety IR8, addressed the problems
of food shortage. However, of late, the gains of green revolution
technologies are plateauing, causing great concern and creating doubt to
ensure food security in the decades ahead. In view of the rapidly increasing
population and decreasing and deteriorating resource base, ensuring food
security in the decades ahead is challenging.
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