Revista Científica UDO Agrícola Volumen 6. Número 1. Año 2006. Páginas: 1-10
Recent advances in understanding genetic basis of heterosis
in rice (Oriza sativa L.)
Avances recientes en el
conocimiento de la base genética de la heterosis en
arroz (Oriza sativa L.)
Sofi Parvez
|
Received: 07/26/2006 |
Reviewing
ending: 09/07/2006 |
Review
received: 11/14/2006 |
Accepted: 11/28/2006 |
ABSTRACT
Heterosis is perhaps one of
the greatest practical achievements of the science of plant breeding and has
been extensively used in crop improvement. Therefore, an understanding of its
potential genetic basis is imperative. Extensive studies in crop plants
including rice have been made to elucidate the genetic factors underlying heterosis. Various research groups have proposed dominance,
overdominance and epistasis as major genetic basis of
heterosis and recent advances in molecular biology
have helped to validate these findings in various crop species. Despite,
tremendous advances in molecular marker techniques, QTL analysis and genomics,
conclusive evidence in support of either of these theories is still elusive, as
all of these factors seem to be mutually non-exclusive. Nowadays, focus is
increasingly shifting to study heterosis at genomic
level to identify the genomic regions that evoke heterotic
effect and introgress such regions into elite rice
lines to develop high yielding hybrids. Advances have also been made in expression
profiling and relate differences in transposon and repeat content in parental
lines to heterotic effect.
Key words: Rice, heterosis,
genetic basis, molecular markers
RESUMEN
Heterosis es quizás uno de los
mayores logros prácticos de la ciencia del mejoramiento de plantas y ha sido
extensivamente usada en el mejoramiento de los cultivos. Por lo tanto, un
conocimiento de su base genética potencial es imperativo. Se han realizado
extensivos estudios en plantas cultivadas incluyendo el arroz para elucidar los
factores genéticos que causan la heterosis. Varios
grupos de investigación han propuesto la dominancia, la sobredominancia
y la epistasis como principales bases genéticas de la
heterosis y avances recientes en biología molecular
han ayudado a validar estos descubrimientos en varias especies cultivadas. A
pesar de los avances tremendos en las técnicas de marcadores moleculares,
análisis de QTLs y análisis genómico, una evidencia
conclusiva en soportar una de estas teorías todavía no se ha definido, como
todos estos factores parecen ser mutualmente no exclusivos. En la actualidad,
el enfoque está moviéndose rápidamente hacia el estudio de la heterosis a nivel genómico para identificar las regiones
genómicas que induzcan el efecto heterótico e
introducir tales regiones dentro de líneas elites de arroz para desarrollar
híbridos con altos rendimientos. Se han realizado también avances en el perfil
de expresión y relacionar diferencias en
el contenido repetitivo y del transposon en líneas
parentales para efecto heterótico.
Palabras clave: Arroz, heterosis,
bases genéticas, marcadores moleculares
INTRODUCTION
The phenomenon of superiority of F1
over its parents is heterosis (Syn. hybrid vigour). The term heterosis was
coined by Shull (1908) for quantitative measure of superiority of F1
over its parents. The phenomenon of heterosis has
been a powerful force in the evolution of plants and has been exploited
extensively in crop production (Birchler et al. 2003). The successful development
of hybrid maize in 1930 gave great impetus to breeders of other crops including
rice to utilize the principle of hybrid production by exploiting heterosis. In fact the exploitation of heterosis
has been the greatest practical achievement of the science of genetics and
plant breeding (Alam et al. 2004). The impact of this phenomenon can be judged by the
fact that rice in its wild state produces only a few hundred spikelets whereas, the improved inbred varieties produce
about 40,000 filled spikelets and rice hybrids about
52,000 filled spikelets per square meter (Mir, 2002).
Heterosis
is a widely documented phenomenon in diploid organisms that undergo sexual
reproduction. Although rice is a naturally self pollinated
crop, strong heterosis is observed in their F1
hybrids. Though heterosis has been observed for
various morphological, physiological and biochemical characters, in an applied
breeding programme, the concern primarily with the
economic yield potential (Ahmad. 1996). In practical breeding programmes, usually the standard heterosis
is considered, which is defined as superiority of F1 hybrid as
compared to highest yielding check, and is estimated as:
In rice, heterosis was first reported by Jones (1926) who observed
that some F1 hybrids had more culms and yield than their parents.
Between 1962 and
Genetic basis of heterosis in rice
The genetic basis of heterosis has been a topic of contentious debate for almost
a century now and is still shrouded in mystery. The earlier workers put forth
their suppositions based on quantitative genetic models but with the
advancements in molecular genetics, we have been able to study this phenomenon
in a more refined way. In fact, the recent studies in maize and rice to attempt
an interpretation of heterosis have been greatly
facilitated by molecular markers. The marker data offers an impeccable profile
of genomic regions involved in trait expression and are expected to unravel the
unexplained basis of heterosis (Robin, 2001).
Earlier studies put forth two
possible mechanisms of heterosis: (i) Dominance
hypothesis and (ii) over-dominance hypothesis. Theoretically, the two concepts
are based on two different genetic phenomenon but in most of the situations,
both lead to similar expectations (Mukherjee, 1995). In either case, inbreeding
leads to a decline in vigour while out-breeding leads
to increased vigour. In case of both dominance and
over-dominance concepts, the decline in vigour is
proportional to decrease in heterozygosity
irrespective of the number of dominant and recessive alleles and degree of
dominance. The difficulty of precise demarcation of either of two basic
assumptions arises due to a number of factors.
i)
Distinction between true over-dominance and
pseudo-over dominance. Linkage disequilibrium often causes bias in estimation of
genetic components (non-additive), and as such heterosis
may arise from repulsion phase linkage or complementary epistasis as well.
ii)
Effect of pseudo-alleles which cannot be classified as
dominance or over-dominance.
iii)
Presence or absence of selection pressure may lead to heterosis due to two different genetic mechanisms.
iv)
Over-simplification of genetic models may lead to
wrong interpretations.
Xu (2003)
stated that as a complex character involving yield and yield components, heterosis should be genetically controlled by many genes.
Although genetic study of quantitative traits has identified a limited number
of QTL, each explaining a relatively large proportion of genetic variation,
much more QTLs could be found when multiple populations are considered. For a
specific hybrid, heterosis is more likely genetically
controlled by a relatively small number of genes; for explanation of heterosis involved in all hybrids derived from a species, a
large number of QTLs will be needed. Heterozygosity
and its related gene interactions are the primary genetic basis for explanation
of heterosis because the hybrid is heterozygous
across all genetic loci that differ between the parents. Thus, the degree of heterosis depends on which loci are heterozygous and how
within locus alleles and inter-locus alleles interact with each other.
Interaction of within-locus alleles results in dominance, partial dominance, or
overdominance, with a theoretical range of dominance
degree from zero (no dominance) to larger than 1 (overdominance).
Interaction of inter-locus alleles results in epistasis. Genetic mapping
results have indicated that most QTLs involved in heterosis
and other quantitative traits had a dominance effect. As statistical methods
that can estimate epistasis more efficiently became available, epistasis has
been found more frequently and proven to be a common phenomenon in the genetic
control of quantitative traits including heterosis.
With so many genetic loci involved, it is unlikely that there is no interaction
at all between any pair of them.
Syed
and Chen (2005) indicated that Arabidopsis
thaliana Col and Ler ecotypes share similar
genetic backgrounds and, indeed, the performance of RILs for most of the traits
examined remained within mean values of the two parents (Col and Ler) ruling out dominance complementation for the majority
of traits. However, a large amount of variation was observed in the F1
(or backcross) hybrids derived from each of the RILs and its parent, Col Col Col
Swanson-Wagner et al. (2006) state among other mechanisms, one attractive
hypothesis for the existence of underdominant and overdominant gene action invokes the action of small
interfering RNAs (siRNAs). siRNAs
are typically derived from transposons and repeats, although some genes and
other sequences can generate siRNAs. siRNAs can regulate gene expression by cleaving target
mRNAs and via transcriptional silencing. Maize inbreds
differ radically in transposon and repeat content. Hence, inbreds
are likely to differ in their complement of siRNAs.
If siRNAs from one inbred do not match genes from the
other inbred, the resulting hybrid could exhibit novel patterns of gene
expression, including overdominance or underdominance. Overall, the results are consistent with
the hypothesis that multiple molecular mechanisms contribute to heterosis.
Guo et al. (2004) found that the allelic
expression variation occurred frequently in maize hybrids. The differential
expression between the alleles could potentially result in hybrids surpassing
the inbred parents in expression in different dimensions, such as (1) expression
level, (2) expression timing/duration, and (3) response to developmental and
environmental cues. The data suggest that the two parental alleles in maize
hybrids may be regulated differentially during plant development and in
response to environmental signals. Although only a small number of genes were
analyzed using one each of the hybrid, distinct allelic expression patterns
were found between a modern and an old hybrid. This work demonstrates that the
maize hybrid is an excellent system to study allele expression variation
because alleles are compared within the same genotype of a hybrid and equally
affected by genetic background or environmental factors.
Auger et al. (2005) concluded from their data that nonadditive gene expression is quite prevalent in hybrids.
The question arises as to whether and how these nonadditive
expression levels contribute to heterosis. The
triploid data indicate that allelic dosage affects the nonadditivity
and therefore gene regulatory interactions are involved. Further work will be
required to determine what spectrum of gene expression, if any, is correlated
with heterosis.
Dominance as major genetic basis of heterosis
The dominance hypothesis was
promulgated by
Recent advancements in molecular
genetics have made it possible to detect and individually analyze the loci
underlying heterosis (Xiao et al. 1995). Molecular linkage maps coupled with quantitative
genetic analysis help in getting a better perspective of genetic basis of heterosis. Stuber et al. (1992) were first to use QTL
analysis for detecting genomic regions (QTL’s) contributing to heterosis. In rice Xiao et
al. (1995) used F1 of an indica
variety (9024) and a Japonica variety (LH422) and developed Recombinant Inbred
Lines (RIL’s) and back-cross Inbred lines (BC1 F7 and BC2
F7; Table 1). All the traits studied were subjected to QTL analysis
by single point basis and interval mapping. Using QTL data from all these
combined populations, they estimated the differences in phenotypic means of
heterozygotes and homozygotes over all portions of genome. From the overall
results of their study they found that:
1)
Most of the QTL’s (73 %) were detected in only one of
two backcross generations. In 82% of these cases heterozygotes had higher
phenotype (F1 plants have a higher value of each phenotypic trait
measured in comparison to either parent) as compared to the respective
homozygotes.
2)
23 % of QTLs were detected in both backcross
populations and each pair was mapped to same chromosomal location. In all these
cases heterozygotes fell between two homozygotes. This finding suggested that
complementation of dominant (or partially dominant) alleles at different loci
in F1 was major contributor to F1 heterosis
for different traits.
|
Table 1. Correlation
Coefficients between Genome heterozygosity and
trait value (Xiao et al., 1995). |
||
|
Trait |
BC1 |
BC2 |
|
Plant height |
0.204
** |
0.081 |
|
Days to heading |
-0.004 |
0.021 |
|
Days to maturity |
-0.027 |
0.026 |
|
Panicle length |
0.143* |
-0.021 |
|
Panicles per plant |
-0.082 |
-0.048 |
|
Spikelets per panicle |
0.062 |
-0.013 |
|
Grains per panicle |
0.069 |
-0.026 |
|
Percent seed set |
0.028 |
-0.016 |
|
1000-grain weight |
0.068 |
0.099 |
|
Spikeletes per plant |
0.026 |
-0.041 |
|
Grains per plant |
0.037 |
-0.057 |
|
Grain yield |
0.091 |
0.017 |
|
* p < 0.05 and ** p <
0.01 |
||
This conclusion
is supported by two important findings.
The correlation coefficient between
genome heterozygosity and trait values by regressing
the trait value of each BC1 F7 family on its percentage
of genome heterozygosity should reflect the
importance of heterozygosity per se to the expression of a particular trait. The values of the
correlation coefficient (r) for most of the traits was very low and non significant. Even some of the heterozygotes had lower
phenotypes than respective homozygotes. Thus heterozygosity
is not an essential feature of heterosis as proposed
in over-dominance theory.
The table reveals that except for plant
height and panicle length correlation coefficients for all traits are
non-significant which implies that heterozygosity is
not essential for heterosis. All other traits for
both populations and plant height and panicle length for BC/LH422 showed no
relationship between the genome heterozygosity and
trait performance, indicating thereby that overall genome heterozygosity
alone had little effect on trait expression.
One of the important assumption of
dominance hypothesis is that we should be able to isolate, from segregating
populations, a true breeding individual which is as vigorous as F1
(because in dominance hypothesis AA = Aa). In their
experiment Xiao et al. (1995)
observed two recombinant inbred lines whose phenotype exceeded that of F1,
and true breeding individuals as vigorous as F1 were observed for
all traits including grain yield.
Digenic
interactions between markers associated with significant QTLs and all other
markers were not significant. Thus epistasis cannot be attributed as the cause
of F1 heterosis. However, due to inherent
inefficiencies and low resolution of marker based QTL studies in detecting
epistasis (Tanksley. 1993), the possibility of
occurrence of some level of epistasis cannot be totally excluded.
The analysis of QTL x E interactions
revealed that gene action of a QTL did not change from dominance to recessiveness or partial dominance to over-dominance from
one environment to other.
These lines of evidence reinforce
the conclusion that dominance is the major genetic basis of heterosis
in rice. Although the same results do not come out with QTL analysis in maize
even though both rice and maize belong to Gramineae,
share many orthologous genes and have evolved from a common ancestor. Stuber et al
(1992) concluded that over-dominance is major genetic basis of heterosis in maize. The possible explanations of this contrast are:
a)
Maize possess a large number of genes for which
alleles interact in a truly over-dominant manner whereas rice does not.
b)
The observed over-dominant gene action may be due to
pseudo-over-dominance or occurrence of dominant and recessive alleles in coupling
phase linkage (Crow, 1952).
c)
QTL mapping at present is a low resolution process.
The
evidence for dominance as a major genetic basis of heterosis
was also provided by Hua et al (2002) who performed a QTL analysis using F2
populations. They found that correlation between genotype heterozygosity
and trait performance was very low, implying thereby that heterozygotes are not
always advantageous for performance. They also concluded that dominance is a
major genetic basis of heterosis in rice. Singh et al. (2004) studied the components of heterosis in rice and concluded that dominance is the chief
cause of heterosis.
Over-dominance as major genetic basis of heterosis
The
hypothesis advocating over-dominance as major genetic basis of heterosis was first proposed by Shull (1910) and East
(1908). The same concept was later advocated by Gustafsson
(1938), Stadler (1939) and
Difficulties
in discriminating true over-dominance from pseudo-overdominance
are major opposition to this hypothesis. Jones (1917) was first to propose that
linkage causes great problems in identification of overdominance
and in fact pseudo-overdominance arising out of
repulsion phase linkage may often be misinterpreted as true overdominance.
In such a situation the pair of linked loci would mimic a single overdominant locus thereby skewing the measure of true overdominance (Budak et al. 2002).
Brewbaker (1964)
described four theories to explain over-dominance:
1. Supplementary allelic action
2. Alternative pathways
3. Optimal amount
4. Hybrid substance
Jinks (1983) was a strong opponent of over-dominance as genetic basis of heterosis in crops like rice where according to him great
improvements have been made in performance
of inbred lines by alternating cycles of hybridization and reextraction
(pedigree selection). However it is difficult to exclude role of overdominance in heterosis in
both autogamous and allogamous
crops.
Several recent studies on genetic
basis of heterosis in rice have came
up with strong evidences in support of over-dominance.
Li et al. (2001) studied the genetic basis of heterosis
and inbreeding depression in rice by using five interrelated mapping
populations comprising a Lemont (japonica)/Teqing (indica) RIL, two BC and two test cross populations using Zhong 413 and IR64 as testers. The non-additive gene action accounted for 62
% of trait variation while additive gene action accounted for 28.1 % of trait
variation of F1 mean values. They found that most of the QTL’s (~
90%) contributing to heterosis were over-dominant
especially for grain yield, biomass, panicles per plant and grains per panicle.
One of the important findings of the study was that there was no evidence of
pseudo-over-dominance from repulsion phase linkage of completely or partially
dominant QTL’s for yield components as proposed by Crow (1952). Similar results
were reported by Luo et al. (2001) using similar set of mapping populations. They
concluded that over-dominant loci are the major genetic basis of inbreeding
depression and heterosis in rice, especially for
panicle per plant and grains per panicle. They stated that pronounced
over-dominance resulting from epistasis by multi-locus genotypes appears to
explain the longstanding dilemma of how inbreeding could arise from
over-dominant genes. Hua et al. (2003) detected many heterotic
loci in RIL’s from a cross between parents of Shanyou
63 and found high degree of over-dominance in many heterotic
loci. Suresh et al. (2004) studied
molecular marker heterozygosity and heterosis using a set of SSR and RAPD markers and found
significant positive correlation between marker heterozygosity
and heterosis in relation to traits such as
productive tillers/plant, biomass yield and grain yield per plant.
Epistatis as a major genetic basis of heterosis
Dominance
and over-dominance (both proposed in 1808) remained the major genetic
understandings of the cause of heterosis even though
both faced contradictions. The advent of
molecular marker systems such as isozymes, RFLP, AFLP and high density molecular linkage
maps made it possible to dissect the
loci causing heterosis, in terms of effects and
dominance relationships, with more precision and reliability.
Both dominance and over-dominance
concepts are based on single-locus model. But Wright (1968) proposed that most
of the quantitative traits are conditioned by many loci and as such each gene
replacement may have effects on many characters because genes invariably do
interact with each other. He visualized
a “net-like” structure of population genotypes such that the variations of most
characters are affected by many loci such that each gene replacement may have
effects on many characters. Based on such a perspective, epistasis should be
one of the major genetic components in case of quantitative traits. Hallauer and Miranda (1988) also proposed that epistasis
should contribute significantly to heterosis.
A classical study in rice by Yu et al. 1997 using F3
population derived from bagged F2 plants from a cross between Zhenshan 97 and Minghui 63
(Parents of Shanyou 63, the best hybrid in China
accounting for 25 % of hybrid rice acreage) (Tables 2 and 3), the most striking
finding of the study was the prevalence of epistasis in rice, with three
pronounced features.
|
Table 2. Summary of the
significant (p < 0.01) interactions identified in 1994 and 1995 by
searching all possible two locus interactions. |
||||
|
Trait |
Interaction (traits) |
1994 |
1995 |
Common |
|
Yield |
AA |
60 |
91 |
9 |
|
|
AD/DA |
51 |
73 |
3 |
|
|
DD |
4 |
18 |
0 |
|
Tillers/plant |
AA |
79 |
105 |
17 |
|
|
AD/DA |
28 |
42 |
1 |
|
|
DD |
10 |
6 |
0 |
|
Grains/panicle |
AA |
52 |
80 |
9 |
|
|
AD/DA |
56 |
74 |
10 |
|
|
DD |
4 |
16 |
0 |
|
Grain weight |
AA |
84 |
102 |
27 |
|
|
AD/DA |
47 |
71 |
19 |
|
|
DD |
15 |
16 |
9 |
|
Number of Tests |
|
7585 |
7681 |
|
|
Source
: Yu et al. 1997 |
||||
|
Table 3. Two locus
interactions for grain per panicle simultaneously detected by two-way
analysis of variance at P < |
|||
|
Locus 1 |
Locus 2 |
Type
(1994) |
Type
(1995) |
|
RG532 (1) |
RM 4 (11) |
AA |
AA |
|
RG173 (1) |
RM203 (3) |
AA |
AA |
|
C547x (1) |
RG 634 (2) |
AA |
AA |
|
RG236 (1) |
R1440 (7) |
AD DA |
AD DA |
|
C112 (1) |
G389a (11) |
AA |
AA |
|
MX 7b (2) |
Waxy (6) |
DA |
DA |
|
C1447 (5) |
C677 (10) |
AA DA |
AA --- |
|
C1447 (5) |
G389 a (11) |
AA |
AA |
|
G1458 x (5) |
G342 (6) |
AA |
AA |
|
G193 x (5) |
G 342 (6) |
AA |
AA |
|
RG360 (5) |
RG653 (6) |
AD DA |
AD DA |
|
RG360 (5) |
G343 (6) |
AD |
AD |
|
R830 (5) |
RZ404 (9) |
AA DD |
AA DD |
|
C1023 (7) |
C794 (11) |
DA |
DA |
|
Numbers
in parenthesis represent chromosomal
locations |
|||
|
Source : Yu et al. 1997 |
|||
1) Two-locus analysis resolved
larger number of loci contributing to trait expression. For grains per panicle only,
counting interactions simultaneously, the significant two-locus interactions
detected 25 QTL’s on 9 of 12 rice chromosomes compared with 5 and 7 QTL’s
detected in two years for this trait.
2) All the three types of interactions i.e. A x
A, A x D and D x D occurred among various two-locus combinations.
3) Multiple
interaction terms were found in a considerable proportion of interacting
two-locus combinations in all traits.
Lack of correlation between genotype
heterozygosity and trait expression was also observed
in this study, which implies that, collectively, the effect of dominance and/or
overdominance made only limited contributions to the heterosis. Dominant interactions (DD) were most relevant to
F1 data but AA was more commonly detected than AD and DA types.
The
study also suggested possibility of higher-order interactions at least for most
complex trait (grain yield). There are some lines of evidence implying
existence of higher-order interactions.
1)
Fewer QTL’s were detected for yield than other traits
and smaller amount of phenotypic variation was accounted for by them.
2)
At the two-locus level, the numbers of interactions
detected for yield were less than component traits. This suggests involvement of genetic
components not resolved by either single locus or two- locus analysis.
3) Significant
two-locus interactions revealed “Chain-like” relationship among interacting
two-locus combinations such that locus 1 interacted with locus 2, which in turn
interacted with locus 3 and so on and so
forth (Table 2). This implies higher-order
multi-locus interactions.
Luo et al (2001) also found many epistatic QTL pairs for yield and yield components. Most
epistasis occurred between complementary loci, suggesting that grain yield
components were associated more with multi-locus genotypes than with specific
alleles at the individual loci.
More
recently, Hua et
al. (2003) studied “immortalized F2” population produced by randomly intermitting
RIL’s derived from Zhenshan 97/Minghui
63 which are the parents of Shanyou 63, which is the
best hybrid in China. They
observed significant two-locus interactions by two-way ANOVA across entire
genome, DD interaction occurred at predominantly high frequency, followed by
AD/DA, with AA being the least frequent.
CONCLUSION
The understanding
of the phenomenon of heterosis in terms of its
genetic basis is far from adequate even after molecular dissection of the
process and factors contributing to it. The majority of the earlier studies
speculated dominance and over-dominance as the genetic mechanism of heterosis but the recent studies have revealed that linkage
and epistasis may also have a role to play (Budak et al. 2002). However, one common
observation in all the studies has been that no single hypothesis holds true
for all the experiments and crops. It is, thus likely that the heterosis is crop dependant and
population dependant. This seems to resolve the
conflicting reports from experiments designed to study the genetic basis of heterosis. Different studies which focused on understanding
genetic basis of heterosis have came
up with conclusions regarding different genetic elements such as dominance,
over-dominance and epistasis as possible genetic mechanisms responsible for heterosis. The challenge now is how to put the pieces
together to frame a comprehensive picture (Hua et al. 2003). In case of rice there was
a strong case for dominance as depicted by Xiao et al. (1995) because there were many points regarding
over-dominance such as pseudo over-dominance or repulsion phase linkage of
dominant alleles. However, recent study of Yu et al. (1997) provided strong evidence for two-locus and
multi-locus interactions (epistasis) especially for traits such as grain yield,
which are complex in nature. They found that heterosis
is not controlled by single locus alone, whether the locus behaves in dominant
or over-dominant fashion, linkage and epistasis has a major role. Even net like
gene interaction is prevalent for most of traits including even seemingly
simple traits like days to heading.
Thus, the effects of dominance,
over-dominance and epistasis of various forms are not mutually exclusive in the
genetic basis of heterosis, as opposed to what was
previously debated in favour of different hypothesis
(Allard, 1960). All of these components
have a role to play depending upon the
genetic architecture of the population (Hua et al. 2003) i. e. single-locus heterotic effects (caused by partial, full-and
over-dominance), all three forms of digenic interactions (AA/AD/DA and DD) and probably
multi-locus interactions. Thus, these results may help reconcile the century
long debate on the role of dominance, over-dominance and epistasis as genetic
basis of heterosis.
Two different types of allele
interaction, both within-locus and inter-locus, each should play an important
role in the genetic control of heterosis.
Contribution of a specific locus to heterosis could
be due to any single type of these interactions. When multiple loci are
involved that were not taken into account in the early 1900s, various
combinations of within-locus and inter-locus interactions (especially dominance
x dominance interaction) could contribute to the genetic control of heterosis. A full understanding of heterosis
will depend on cloning and functional analysis of all genes that are related to
heterosis. This process would be very similar to that
for understanding disease resistance genes that functionally appear much
simpler than heterosis (Xu,
2003).
The current research on molecular
breeding with heterosis aims at identification of
specific genomic regions in crop plants
like rice (heterotic chromosome blocks) wherein specific genomic regions conditioning heterotic expression are to be identified in diverse lines
in parents which can be used for
development of superior hybrids. Already
in maize, the pioneer Hi-Bred International Inc. is approaching the dissection
of heterosis in maize using a “Gene Calling”
technology. This approach uses molecular biology and bioinformatics to dissect
expressed DNA sequences responsible for hybrid vigour.
Advances in rice genomics and molecular markers will help devise similar
systems for dissection of heterosis at DNA level to
precisely understand its genetic basis for practical application in hybrid rice
development.
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