Nilaparvata lugens Stal

12 Pages·2014·0.26 MB·English

Nepal Journal of Science and Technology Vol. 15, No.1 (2014) 145-156
Conventional and Molecular Studies of Brown Planthopper
(Nilaparvata lugens Stal) Resistance Genes in Rice: A Basis for
Future Study of Natural Insect Resistance Genes Using Molecular
Markers in Nepal
Prem N. Sharma1, Naoki Mori2, Shigeo Takumi2 and Chiharu Nakamura2
1Entomology Division, Nepal Agricultural Research Council, Lalitpur, Nepal
2Faculty of Agriculture, Kobe University, Kobe, Japan
e-mail: [email protected]
Abstract
Rice productivity is greatly affected by various biotic and abiotic stresses. Insect-pests are one of the major biotic
constraints to cause significant losses in rice production. Brown planthopper (BPH), Nilaparvata lugens Stål, is
the most serious insect-pest of rice in Asia where most of the world rice is produced. Controlling insects using
chemicals is already proven detrimental not only to environment but also to human health. Integrated Pest
Management (IPM) is the best approach to control insect pests. Host plant resistance is the principal component
of IPM along with biological, cultural and physical methods. Use of varietal resistance is the best option to control
BPH. Many BPH resistant rice varieties with natural BPH resistance have been developed and widely used against
BPH. However, frequent breakdown of monogenic resistance by new BPH biotypes has been a serious threat to
control BPH. To overcome such difficulty in the use of monogenic resistance, development of durable resistance
is needed as the sustainable means to control BPH. To develop durable resistance, pyramiding of BPH resistance
genes and quantitative trait loci (QTLs), through marker-assisted method, is needed. For this, many BPH resistance
genes and QTLs have already been identified and mapped on rice chromosomes. This article reviews identification,
mapping and pyramiding toward successful cloning of BPH resistance genes/QTLs and provides the basis/
guidelines to work on natural insect resistance genes using molecular markers in Nepal.
Key words: rice, brown planthopper (BPH), Nilaparvata lugens, resistance genes, QTLs, mapping, pyramiding
Introduction phloem sap as a result rice plants show severe damage
Rice is the most important staple food crop of the world, symptoms commonly referred to hopper-burn. BPH
and Asia is the main region of its cultivation. Rice also damages rice plants indirectly by acting as a vector
production, however, has been threatened by insect- in transmitting viral diseases such as grassy stunt and
pests as one of the major biotic constraints. The brown ragged stunt. Both direct and indirect damages by BPH
plant hopper (BPH), Nilaparvata lugens Stal cause considerable yield loss of rice in Asia (Rivera et
(Homoptera: Delphacidae) is one of the most al. 1966, Ling et al. 1978, Sogawa & Cheng 1979).
destructive insect-pests of rice in Asia and is
distributed worldwide where the rice is cultivated. The availability of broad-spectrum insecticides
Frequent outbreaks of BPH have been reported together with chemical fertilizers and improved crop
particularly in tropical, subtropical and temperate varieties, after the Green Revolution, greatly led the
regions in South, Southeast and East Asia and Oceania. farmers to depend on agro-chemicals for controlling
BPH damages rice plants directly by feeding the insects including the BPH. However, excessive reliance
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Nepal Journal of Science and Technology Vol. 15, No.1 (2014) 145-156
on agro-chemicals including insecticides has resulted were screened and large numbers of resistance sources
in adverse side effects such as development of identified as well as many BPH resistant varieties were
pesticide resistance in insect pests, pest resurgence, bred and cultivated in the Philippines, Japan and other
elimination of beneficial insects including natural Asian countries since 1970s. Unfortunately, during
enemies and serious environmental pollution along 1970s, continuous and intensive cultivation of resistant
with human health hazards (Georghiou 1986, Chelliah rice varieties in some tropical countries of Asia created
& Heinrich 1980, Heinrichs et al. 1982). Many BPH a serious problem in BPH control due to the frequent
and white-backed planthopper (WBPH) outbreaks appearance of new and virulent BPH biotypes. The
were reported in farmers’ fields to be induced by first BPH resistant cultivar IR26 with Bph1 gene was
insecticides (Heinrichs et al. 1982, Salim & Heinrichs released in the Philippines in 1973 and in Indonesia
1987). To overcome such negative impacts in the use and Vietnam in 1974; and it was widely grown in these
of chemical pesticides, scientists developed an countries. But its genetic resistance was broken down
ecological approach for pest control, which is now in the Philippines in 1977 and in Indonesia and Vietnam
popularly known as integrated pest management (IPM) in 1978 due to the occurrence of new BPH biotypes
(Huffaker & Smith 1980). IPM utilizes host plant (Feuer 1976, Anonymous 1975, Stapley 1975). Another
resistance as a principle component along with cultural BPH resistant cultivar IR36 bred to carry bph2 gene
and biological methods together with judicious use of was also broken down by more virulent BPH biotype
agro-chemicals. The host plant resistance i.e. the (IRRI 1982, Murata et al. 1998). Thus, breakdown of
natural resistance that exists in host plant species and BPH resistance one after another due to the frequently
their relatives has been challenged for long enough developed BPH biotypes created serious difficulty in
time during evolution should provide promising and breeding for host plant resistance against BPH. It was
more readily acceptable means to control insect pests. therefore necessary to look for additional genetic
To effectively utilize such natural resistance systems sources for widening the genetic base to develop the
in IPM, the resistance mechanism should be fully durable BPH control strategy. The occurrence of new
understood at molecular level. For this, natural insect BPH biotypes compelled the scientists to search for
resistance genes have been cloned and a number of additional sources of resistance to cope with such
studies have been conducted for elucidation of their frequently developed virulent BPH biotypes.
structure, expression and function (Sharma et al.
2003a). Genetic analysis and identification of BPH
Here in this article, towards map-based cloning of resistance genes in rice
BPH resistance genes as a model case, The work on rice resistance against BPH was
identification, mapping, cloning and pyramiding of undertaken only after the establishment of IRRI in the
BPH resistance genes/QTLs are reviewed; and Philippines in 1960 which initiated the studies on
future strategy to develop rice varieties with durable varietal resistance to this pest in 1966. The breeding
resistance against virulent BPH biotypes has been programs to develop BPH-resistant rice varieties were
discussed as well. In addition, future prospect and initiated in 1968. Athwal et al. (1971) initially crossed
guidelines for studying varietal resistance against four resistant varieties, Mudgo, C022 and MTU15 (all
BPH using molecular markers in Nepal are Bph1 gene donors) and ASD7 (a bph2 gene donor)
illustrated. with a susceptible TN1, and identified two loci for
resistance, Bph1 and bph2. Using the biotype 1 as the
Use of varietal resistance against BPH and test insect, Lakshmi narayana & Khush (1977) analyzed
resistance breakdown by new BPH biotypes 28 varieties and identified two new genes, Bph3 and
in Asia bph4. The two Sri Lankan varieties, Rathu Heenati and
Development of efficient mass-screening techniques Babawee were the donors for Bph3 and bph4
such as bulked seedling test and mass-rearing methods respectively. Another new gene bph5 was identified
of BPH (Athwal et al. 1971, Kaneda & Kisimoto 1979, when the crosses were made between ARC10550 and
Choi et al. 1979) accelerated the work on varietal TN1 by using the test insect biotype 4. ARC10550 was
resistance against BPH in Asian countries especially resistant to biotype 4 in South Asia but it is susceptible
at the International Rice Research Institute (IRRI) and to all the three biotypes, biotype 1, 2 and 3 in Southeast
Japan. Thousands of world germ plasm collections Asia (Khush et al. 1985). Kabir & Khush (1988)
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Prem N. Sharma et al./Conventional and Molecular Studies .......
analyzed additional 17 rice varieties resistant to to these populations. This study detected seven
biotype 4 but susceptible to biotype 1, 2, and 3 using significant QTLs that were scattered over on six
biotype 4 as the test insects, and identified two new different rice chromosomes (Khush 1989, Cohen
genes, Bph6 and bph7. The genes Bph6 and bph7 1997, Sharma et al. 2003a). Other two major BPH
were carried by Swarnalata and T12 respectively. resistance QTLs, Qbp1 and Qbp2, were identified
Nemoto et al. (1989) identified a new gene bph8 in by Chinese researchers from the crosses between
three varieties, TC5 (Thai collection 5), TC11, and B5, a BPH resistant breeding line with introgressed
Chin Saba. This gene was allelic in these varieties resistance genes from O. officinalis, and Minghui
and conferred resistance to biotypes 1, 2, and 3. 63, a susceptible indica cultivar, using the BPH
Ikeda (1985) identified a new gene Bph9 in three biotypes 1 and 2 (Huang et al. 2001, Sharma et al.
varieties, Kaharamana, Balamawee and Pokkali. This 2003a). Xu et al. (2002) further identified seven main
gene was also allelic in these varieties but was effect QTLs and many epistatic QTLs for
different from Bph1 and Bph3. Bph10 was the first quantitative resistance to BPH using the
gene identified in a wild relative (Oryza recombinant inbred lines (RILs) derived from a
australiensis) of rice (Jena & Khush 1990) which Lemont and Teqing cross. Lemont is moderately
was later introgressed into an indica breeding line resistant and Teqing is highly susceptible to BPH.
(Ishii et al. 1994). Other two BPH resistance genes The BPH biotypes were thus found always useful
identified in a wild species (Oryza officinalis) of in resistance breeding against BPH in rice. By now,
rice were bph11(t) and bph12(t) (Hirabayashi et al. at least 24 major genes and several major/minor
1998, 1999). Besides these major genes, several QTLs against BPH have been identified in rice
major/minor QTLs of BPH resistance have been cultivars and its wild relatives. Out of the 24 major
identified in rice and its wild relatives. QTLs for BPH resistance genes, at least 19 genes have already been
resistance were first identified by Alam et al. (1998) mapped on rice chromosomes (Sharma et al. 2003a,
using the doubled haploid lines (DHLs) derived from Jena et al. 2006, Chen et al. 2006, Jairin et al. 2007,
a cross between an indica variety IR64 and a Rahman et al. 2009, Brar et al. 2009, Myint et al.
japonica variety Azucena using the BPH 2012). Twelve major genes including the major/minor
populations collected from IRRI farm (IRRI colony) QTLs of BPH resistance initially identified and
and Central Luzon Island (CL colony) in the mentioned in this section have been shown in
Philippines. These colonies were fully adapted to Table 1.
Bph1. IR64 showed moderate resistance response
Table 1. List of initially identified and/or mapped brown planthopper resistance genes/QTLs
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Nepal Journal of Science and Technology Vol. 15, No.1 (2014) 145-156
Chromosomal assignment, mapping, in a moderately resistant variety IR64; the QTLs were
pyramiding toward cloning of major genes/ located on 6 of the 12 rice chromosomes (Khush 1989,
QTLs of BPH resistance by molecular Cohen et al. 1997). Two major BPH resistance QTLs,
Qbp1 and Qbp2, were further identified by Chinese
marker assisted method
researchers in a BPH resistant breeding line B5
Of the 12 major BPH resistance genes mentioned
possessing resistance genes from O. officinalis (Huang
above, at least seven of them have already been
et al. 2001). Five and six RFLP markers were detected
assigned to and mapped on individual rice
from the contiguous regions of chromosome 3 and 4,
chromosomes (Table 1). The chromosomal assignment
respectively, using the bulk DNA from resistant and
and determination of precise position of BPH
susceptible individuals through bulked segregant
resistance genes on rice chromosomes have accurately
analysis. A linkage analysis then mapped the two
been achieved only when the molecular marker based
QTLs, Qbp1 and Qbp2 on the long arm of chromosome
studies started since 1990s. Initially, Bph1 and bph2
3 and the short arm of chromosome 4, respectively.
were assigned to chromosome 4; and Bph3 and bph4
Since the same resistance source i.e. O. officinalis was
to chromosome 10 by classic trisomic analysis and
used in the above studies, the two major genes bph11(t)
linkage analysis using morphological markers (Ikeda
and bph12(t) respectively identified on chromosome
& Kaneda 1983, Ikeda 1985). However, using marker-
3 and 4 in the previous studies (Hirabayashi et al.
assisted selection (MAS) method, chromosomal
1998, Hirabayashi et al. 1999) might represent the two
assignment of these genes was later corrected. Bph1
major QTLs, Qbp1 and Qbp2, respectively (Sharma et
was mapped in a breeding line TKM6 on rice
al. 2003a). A further study (Xu et al. 2002) identified
chromosome 12 by RFLP (restriction fragment length
seven main effect QTLs and many epistatic QTLs for
polymorphism)-marker based analysis (Hirabayashi &
quantitative resistance to BPH in a moderately
Ogawa 1995) and this study was later confirmed by
resistant indica variety Teqing. These QTLs were
RAPD (random amplified polymorphic DNA) and
found distributed over all the 12 rice chromosomes.
RFLP-markers based mapping studies using different
mapping populations (Murata et al. 1997, Murata et
al. 1998 and Jeon et al. 1999). The gene bph2, which
was previously considered recessive in indica
background was confirmed to be dominant in japonica
background and mapped on the long arm of
chromosome 12 close to the dominant gene Bph1
based on RFLP and AFLP (amplified fragment length
polymorphism) analyses using large mapping
populations (Murai et al. 2001, Sharma et al. 2003b).
Based on bulked segregant and linkage analyses using
RFLP markers, the bph4 gene from the Sri Lankan indica
variety Babawee was reassigned to the distal region
of the short arm of chromosome 6 instead of
chromosome 10 (Michelmore et al. 1991, Kawaguchi
et al. 2001). Based on RFLP marker analysis, a dominant
gene Bph10 in an indica line, introgressed from O.
australiensis, was mapped on the long arm of
chromosome 12 and another dominant gene Bph9 from Fig. 1. A bulked segregant analysis profile of an AFLP
a Sri Lankan variety Pokkali was also mapped on the marker on a polyacrylamide gel used to detect the
long arm of chromosome 12 (Ishii et al. 1994, Murata Bph1 linked AFLP markers. em5814N is an AFLP
et al. 2001). Besides mapping of several major genes marker linked to the Bph1 gene. RP: DNA from
of BPH resistance, many major/minor QTLs of BPH resistant parent. RB: bulked DNA from resistant
resistance have also been mapped on rice
Fs. SP: DNA from susceptible parent. SB: bulked
chromosomes. Alam et al. (1998) were the first who 2
DNA from susceptible Fs. M: DNA size marker
detected seven significant QTLs for BPH resistance 2
(Sharma et al. 2003b)
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Prem N. Sharma et al./Conventional and Molecular Studies .......
Towards pyramiding of BPH resistance genes and marker, KAM4, showed a complete co-segregation with
map-based cloning, construction of fine linkage maps Bph2 and, the Bph2 was mapped between two AFLP
for BPH resistance genes were required. Accordingly, markers, KAM2 and KAM3, within a 1.1-cM region
as a first attempt, high resolution linkage maps for (Murai et al. 2001). These AFLP markers (six markers)
Bph1 and Bph2 were constructed and tightly linked were successfully converted into PCR-based STS (five
molecular markers were identified for these genes. markers named KPMs) and CAPS (one marker named
Pyramiding of these genes was also successfully KPM2) markers (Murai et al. 2001, Sharma et al. 2004).
achieved by marker-assisted method using large These converted PCR-based STS (sequence tagged
mapping populations and tightly linked molecular sites) and CAPS (codominant cleaved amplified
markers. To construct high-resolution linkage maps polymorphism sequence) markers could predict that
for Bph1 and Bph2, respectively, Sharma et al. (2003b) Bph1 and Bph2 located at 10-cM apart on the long arm
and Murai et al. (2001) adopted a high efficiency gene of chromosome 12 (Sharma et al. 2004). After
scanning system (Mano et al. 2001) that enabled accomplishing the construction of fine linkage maps
efficient AFLP marker selection and mapping using a for Bph1 and Bph2 and confirming that they were non-
large number of segregating progenies. To fine-map allelic, pyramiding of these genes was successfully
Bph1, BPH bioassay and DNA analyses were achieved by Sharma et al. (2004) by marker-assisted
performed for F and F families derived from a cross method using the identified tightly linked markers.
2 3
between Tsukushibare, a susceptible japonica variety Briefly, for marker-assisted pyramiding of Bph1 and
as a maternal parent, and Norin-PL3, a japonica Bph1- Bph2 genes, crosses were made between homozygous
introgression line as a paternal parent. Polymorphic carrier parents for Bph1 and Bph2 genes that were
AFLP markers associated to Bph1 were detected derived from Tsukushibare/Norin-PL3 and
through the bulked segregant analysis (Fig. 1) and Tsukushibare/Norin-PL4 crosses, respectively. Tightly
were employed in the linkage analysis (Fig. 2) using linked molecular markers of Bph1, one co-dominant
the DNA of F segregating individuals. The Bph1 AFLP marker (em24G) and two resistant (R)-associated
2
locus was mapped within the region of 5.8 cM between dominant AFLP markers (em5814N and em32G) were
two molecular markers, em5814N (AFLP marker) and used to identify the genotypes of Bph1 carrier lines.
R2708 (RFLP marker). The two closest AFLP markers, Similarly, tightly linked markers of Bph2, one co-
em5814N and em2802N associated to Bph1 locus were dominant marker (KPM2) and two R-associated
located at 2.7 cM proximal to the Bph1 locus on the dominant markers (KAM4 and KPM3) were used to
long arm of chromosome 12 (Sharma et al. 2003b). For identify the genotypes of Bph2 carrier lines. Using these
constructing a high resolution linkage map for Bph2, molecular markers, the DNA of F, F and F progenies
2 3 4
F -F families derived from a cross between were analyzed for searching the useful recombinants
4 5
Tsukushibare and Norin-PL4 (a japonica Bph2- (pyramid lines) that possessed both the genes in the
introgression line) were subjected to BPH bioassay homozygous condition. Two recombinant plants (rice
followed by bulked segregant and linkage analyses. line no. 64-4 and 64-6) were obtained in F progenies,
3
The Bph2 locus was delimited with eight AFLP markers both of which were reconfirmed to be the pyramided
within the region of 4.2 cM on the long arm of plants in F progenies by marker assisted selection
4
chromosome 12 (Murai et al 2001). A closest AFLP (Sharma et al. 2004) (Fig. 3).
Fig. 2. A linkage analysis profile using an AFLP marker ‘em5814N’ associated to Bph1 on a polyacrylamide gel. R & S:
resistant and susceptible lines of rice, a & b: resistant and susceptible ckecks respectively. m: DNA size marker
(Sharma et al 2003b)
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Nepal Journal of Science and Technology Vol. 15, No.1 (2014) 145-156
As above, a considerable progress has so far been 2004). These genes, hopefully, will be cloned in future
achieved in identification, determination of carrier through map based cloning approach.
chromosomes and linkage mapping of BPH resistance
genes/QTLs. However, no BPH resistance genes/ Recently, many additional major genes and QTLs of
QTLs have been successfully cloned yet. Towards BPH resistance have been identified and mapped on
map based cloning of BPH resistance genes, rice chromosomes by marker-assisted method (Table
construction of fine linkage maps at least for Bph1 2 and 3). For more recent progress and detailed
and Bph2 has already been accomplished (Sharma et information on these BPH resistance genes/QTLs, see
al. 2003b, Murai et al. 2001), and these closely linked Jena et al. 2006, Chen et al. 2006, Jairin et al. 2007,
genes have already been pyramided (Sharma et al. Rahman et al. 2009, Brar et al. 2009, Myint et al. 2012.
Table 2. Recently identified and/or mapped additional major brown planthopper resistant genes
Table 3. List of brown planthopper resistance QTLs mapped on rice chromosomes
QTLs Chromosome Reference
Qbp1 3L Huang et al. 2001
Qbp1 (Bph14t) 3 Ren et al. 2004
Qbp2 (Bph15t) 4 Ren et al. 2004
Qbph1 (Bph14) 3L Yang et al. 2004
Qbph2 (Bph15) 4S Yang et al. 2004
Qbph3 3 Sun et al. 2005
Qbph4 4 Sun et al. 2005
Qbph4 4S Liu et al. 2009
Qbph6 6S Sun et al. 2007
Qbph7 7 Liu et al. 2009
Qbph9 Liu et al. 2009
Qbph10 10 Sun et al. 2005
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Prem N. Sharma et al./Conventional and Molecular Studies .......
The pyramiding and cloning of BPH resistance genes/QTLs and introducing them into rice
resistance genes/QTLs should provide the cultivars/varieties through transformation that greatly
facilitates and speeds up the breeding programs to
sustainable means to slow down the
develop durable resistance against such new BPH
development of new BPH biotypes and
biotypes. Several cases have been reported for
consequently resolving the problem of BPH
pyramiding of insect resistance genes. Pyramiding of
resistance breakdown in rice in Asia.
BPH resistance genes, Bph1 and Bph2, has been
The breakdown of monogenic resistance by new BPH
successfully achieved by marker-assisted method
biotypes has been a serious threat in BPH management
(Sharma et al. 2004) (Fig. 3). An estimated resistance
in rice production in Asia. For solving such a problem
level of the pyramided line was equivalent to Bph1
of resistance breakdown by the occurrence of new
but higher than bph2. Two other BPH resistance genes,
insect biotypes, rice varieties should be developed/
Bph14 and Bph15, have been reported to be pyramided,
identified with durable resistance. Pyramiding of BPH
such lines showed a higher resistance than the single-
resistance genes/QTLs should provide a sustainable
gene lines (Li et al. 2006, Brar et al. 2009). Pyramiding
means for developing the durable resistance against
of green rice leafhopper (GRH) resistance genes, Grh2
such frequently occurring new BPH biotypes (Alam
and Grh4, also resulted in a higher level of resistance
& Cohen 1998, Huang et al. 2001, Xu et al. 2002). Two
in the pyramided line than in the line carrying one of
approaches can be utilized for pyramiding of BPH
these two genes (Fujita et al. 2006, Myint et al. 2012).
resistance genes and QTLs. One approach is through
In the latest case, the BPH resistance genes, BPH25
resistance breeding by crossing among the BPH
resistance genes/QTLs carrier rice lines/germplasms/ and BPH26, have been pyramided, which have shown
varieties and selecting pyramided lines by marker- a higher level of resistance (Myint et al. 2012).
assisted selection. Another is cloning of BPH
Fig. 3. Marker assisted pyramiding of BPH resistance genes Bph1 and Bph2 in rice. Both Bph1 and Bph2 associated markers
are present in the pyramided plants. a & b: resistant & susceptible checks, respectively. m: DNA size marker (Sharma
et al 2004)
Towards pyramiding of BPH resistance genes/QTLs of the quantitative resistance to BPH can be further
and map-based cloning and ultimately developing the increased by pyramiding many responsible QTLs
durable resistant varieties against new BPH biotypes, through marker-assisted selection (Sharma et al. 2003a,
a variety of major genes/QTLs of BPH resistance Xu et al. 2002). As an urgent need, fine linkage maps
should be identified and mapped on rice chromosomes. for every major genes and major QTLs of BPH
Studies on BPH resistance QTLs have in fact revealed resistance have to be constructed to find the tightly
that the number of resistance QTLs in the rice linked molecular markers that can be utilized in
germplasms is very large, and suggested that the level pyramiding and map-based cloning of BPH resistance
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genes and QTLs. As described above, until now, fine season (Manandhar 1999). In Chitwan district, the early
linkage maps at least for Bph1 and Bph2 have already season rice is often damaged with peak infestation in
been achieved. These genes, however, cannot be used the later part of May (Manandhar 1999, Pokhrel et al.
in resistance breeding because of their broken 1998-1999). The rice variety CH-45 was planted mostly
resistance by virulent BPH biotypes in most of the as the early season rice in Chitwan and the total area
rice growing areas in Asia. Despite this fact, successful damaged was estimated to be 1,568 hectares with an
cloning of these genes is still necessary as a first step estimated grain yield loss of 4,262 mt (Pokhrel et al.
to understand the structure, function and expression 1998-1999). Recently, the rice varieties Makawanpur-1
of the natural BPH resistance genes, which should and Mansuli have been reported to be moderately
provide a fundamental knowledge for the evaluation tolerant to BPH (Paneru & Giri 2011).
of putative paralogues/alleles of the resistance genes
that, in turn, should provide an insight into solving BPH has recently become more widespread in the
the serious problem of resistance breakdown by new eastern to western parts of Terai region and also in
BPH biotypes. In a cloning strategy, the PCR-based river basin areas of hilly region in Nepal. Severe form
markers already obtained in these studies should be of outbreaks causing extensive damage to rice crops
useful to screen a rice BAC (bacterial artificial might occur at any time in future. Work on varietal
chromosome) library that is constructed using the resistance as a fundamental component of IPM needs
pyramid lines of Bph1 and Bph2 (Sharma et al. 2003a, to be initiated soon to prevent any future possible
Sharma et al. 2004). A contig within the introgressed severe outbreaks of BPH. As an urgent need, the
region of the chromosome can be constructed in future released varieties and promising lines of rice that are
as the next step towards the map-based cloning of being regularly developed every year in the rice
these two major BPH resistance genes. commodity program and rice breeding program in
NARC (Nepal Agricultural Research Council) have to
be screened for their resistance response to BPH. Both
Outbreaks of BPH and future strategy/
laboratory and field screening methods can be adopted
prospect of BPH resistance study in Nepal
to identify BPH resistant and susceptible rice lines.
In Nepal, BPH was formerly a secondary pest but
Identified promising BPH resistant varieties of rice can
nowadays it has become a major pest of rice. Farmers
be released and directly recommended to farmers to
generally use broad-spectrum insecticides as a quick,
grow wherever and whenever the BPH outbreaks can
cheap and easy means of insect control. However,
be predicted. To search for sources of resistance, large
excessive use of chemicals has caused negative effects
numbers of rice cultivars/germplasms/varieties should
such as insect resurgence, destruction of natural
be collected from inside and outside the country/IRRI
enemies/non-targeted pests and development of
and they have to be screened for sources of BPH
pesticide resistance in insect populations including
resistance which can be utilized in breeding programs
BPH. As a result, serious and frequent outbreaks of
to develop the BPH resistant varieties. To perform
BPH have been reported in the past years in different
effective varietal resistance evaluation, adequate
parts of Nepal. The serious outbreaks of BPH occurred
facilities need to be established for BPH bioassay such
in Chandradangi, Jhapa in 1977 and 1978, in Sunsari
as mass rearing of BPH and screening for its resistance.
and Makawanpur in 1988, in Kumroj and Kathar of
There should also be the provisions of well-equipped
Chitwan in 1996, and in various regions of Dhanusha,
laboratory and glass house facilities to strengthen the
Mahottari, Bara, Rupandehi, Kapilvastu, Parsa,
programs. Studies of BPH biotypes and differential
Rautahat, Sunsari, Morang, Udayapur, and Siraha
lines of rice need to be carried on simultaneously as
districts in 1998. Damages caused by BPH outbreaks
the prerequisites to further progress the resistance
were more serious particularly in Dhanusha, Bara and
programs. Differential lines of rice in response to BPH
Kapilvastu (Manandhar 1999, Pokhrel et al. 1998-1999).
biotypes need to be identified and maintained. BPH
The peak infestation in most of these regions occurs
biotypes in relation to BPH resistance genes should
in September and October causing most serious
also be studied, characterized and maintained. After
damage. The rice variety Basmati was found more
the basic works and studies on varietal resistance are
susceptible whereas a local variety Valsari was found
achieved, the varietal resistance programs can be
comparatively less susceptible to BPH in the main
further advanced with molecular marker technology.
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Prem N. Sharma et al./Conventional and Molecular Studies .......
National and international collaborations are required BPH is increasingly becoming a serious threat in rice
to integrate molecular marker technology in the BPH production in Nepal as well. Varietal resistance works
resistance works. The following are the general areas/ need to be initiated soon to identify and develop the
guidelines/long term strategy to study BPH BPH resistant rice varieties. As an urgent need, the
resistance in rice by molecular marker assisted released varieties and promising lines of rice that are
method: (i) BPH resistant lines have to be being regularly developed every year in NARC have
differentiated for whether they are controlled by to be screened for their resistance/susceptibility
monogenic or QTL gene/s. (ii) Monogenic resistant response to BPH. Identified promising BPH resistant
lines should be distinguished for whether a new rice varieties can be directly recommended to farmers
gene or an already identified gene is conferring as an immediate solution to prevent any future possible
resistance on them. (iii) Chromosomal assignment outbreaks of BPH in Nepal, if the outbreaks can be
and linkage mapping have to be performed for the predicted. Simultaneously, the molecular marker
newly identified genes. (iv) Fine linkage maps need technology need to be integrated in the BPH resistance
to be constructed for major genes to identify the works to further progress and advance the varietal
tightly linked molecular markers associated to these resistance works in Nepal.
genes. (v) Pyramiding of major genes can be
References
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markers. (vi) Map-based cloning can be proceeded Anonymous. 1975. Brown planthopper infestation in various
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