Lecture No. 3
Rice
Commercial exploitation of heterosis to increase rice yields has been successfully
demonstrated in China. It has a yield advantage of more than 30% over conventional pure
line varieties. Strategies for developing hybrid rice will include the following.

Three line method or CMS system

Two line method or PGMS and TGMS systems

One line method or apomixis system
CMS system is the most effective genetic tool for developing hybrid rice. CMS is
controlled by interaction between cytoplasm and nuclear genes. The first practically
useable CMS system in rice was discovered by Prof. Yuan Long Ping and his team in
Hainan island of southern China in 1970 in Oryza sativa f. spontanea. This system was
designated as wild Abortive (WA) system of male sterility. There are several other male
sterile systems discovered in rice but WA system is the most commonly used (95%)
system within and outside China. It involves A, B and R lines.
A line: It has sterile cytoplasm and recessive nuclear genes.
B line: Normal fertile cytoplasm and the recessive nuclear genes. A and B are said to
be isogenic lines.
R line: It has either sterile (or) normal sterile cytoplasm. However, the nucleus
contains the dominant genes, hence the line is definitely male fertile.
Maintenance of male sterile lines
In the maintenance of male sterile lines (both GMS and CGMS), no matter
whether the propagation of parents or the production of hybrid seeds is involved, they
must all be grown under isolation. In the production chain, there is a needs of three
isolation regions for propagating a male sterile line, a male parental line and producing
hybrid seeds respectively as shown in Fig.
Genetic principles of A, B and R lines
GMS lines are maintained by crossing them with its fertile sib (heterozygous at
one locus). In the progeny, plants segregate for sterile and fertile in 1:1 ratio, in
which fertile plants are rogued out at flowering. Two to three seeds are generally
sown per hill so that full population of male sterile plants can be maintained even
after removing fertile plants. For ensuring at least 10 q of seed yield ha-1, the
population of male sterile plants should be kept at 30,000 ha-1 after rouging
fertile plants.
In contrast, CGMS lines are maintained by crossing them with its isogenic
maintainer lines. In the progeny, all the plants will be sterile. Since there is no
segregation for fertility in hybrid seed production plots, with proper management
practices, the population can be maintained to harvest good quality hybrid seeds.
The restorer lines are maintained by selfing them or by open pollination in
isolation.
Maintenance of parental lines / production of nucleus seed
Parental lines get contaminated at different stages of handling and, it is necessary
to purify them (at least once in three years). Parental lines have to be purified
under the direct supervision of the rice breeder.
Purification process essentially involves four steps : (i) Growing the source
material (source nursery); (ii) Test crossing (Test Cross Nursery); (iii) Evaluating
the test crosses (identification
nursery); and (iv) Multiplication of the lines
(Multiplication Nursery).
Source nursery
(i)
The source material of A-, B- and R-lines is grown side by side for
convenience. B-line has to be sown five days after sowing the A-line and Rline has to be staggered depending on the flowering difference between Aand R-lines.
(ii)
Observe the individual plants carefully in vegetative stage and remove the offtypes.
(iii)
At flowering stage select 200-250 typical CMS plants which are completely
male sterile (to be confirmed by incomplete panicle exertion, 100 per cent
pollen sterility and withered shriveled anthers).
(iv)
Similarly select 200-250 typical plants of maintainer and restorer lines based
on their key characteristics.
Test crossing
(i)
Make 20-250 paired crosses between selected plants of CMS line with those
of maintainer and restorer lines.
(ii)
Use 4-5 panicles of a selected CMS plant for crossing with a selected B-line
plant and another 2-3 panicles of the same plant for crossing with the selected
R-line plant.
(iii)
Produce atleast 100-150 seeds for each pair of A x B cross, while 30-40 seeds
would be enough for the A x R cross.
(iv)
Label all the crossed panicles of A x B as A1 x B1, A2 x B2 ……An x Bn.
Similarly the A x R crosses are labeled as A1 x R1, A2 x R2 …. An x Rn.
The seeds have to be harvested and threshed with due care
Identification nursery
The main purpose of identification survey is to identify those progenies which are
not uniform or true to type, so that these can be eliminated in order to maintain purity.
In case of A x B crosses, we look for higher spikelet fertility and uniformity. The
steps involved are as follows :
(i)
Take out 25-30 seeds from each of the A x B crosses and all the seeds of A x
R crosses and raise the nursery. Keep the remaining seeds of A x B crosses to
be sown 21 days after for multiplication nursery.
(ii)
Grow all paired crosses of A x B and A x R crosses in the main field. No
isolation is required for this.
(iii)
Observe the progenies of A x B crosses for stable male sterility, true to type
and uniformity.
fertile plants etc.
Identify those progenies which show off-types, partially
(iv)
Among the A x R crosses, identify those which exhibit poor restoration and
lack of uniformity.
(v)
The most important decision to be taken in identification nursery is to identify
the crosses which deviate from the normal standard characteristics, so that
these can be rejected in the multiplication nursery.
Multiplication nursery
In the multiplication nursery the deviant progenies along with the corresponding
B-lines are removed before flowering based on the information gathered in the
identification nursery. With regard to the restorers, the restorer lines which produce
defective progenies with poor restoration and lack of uniformity are discarded. The seeds
of the remaining lines are bulked to obtain purified R-lines.
CMS line multiplication
(i)
After sowing a part of the seeds of A x B crosses for the identification
nursery, remaining seed is sown 21 days later along with the corresponding Blines. This material has to be planted in an isolated (>500 m) area. Each paired
cross progeny is flanked by the corresponding B-lines so as to get more seed
set.
(ii)
Based on the observations made in A x B crosses in the identification nursery,
mark those which are found to be defective and remove them along with their
corresponding B-lines before flowering.
(iii)
The remaining A x B pairs are allowed to cross pollinate. It is advisable to
take up supplementary pollination to enhance the seed set.
(iv)
The B-lines are harvested first, threshed separately and bulked. This forms the
source seed for nucleus seed production of B-lines.
(v)
After the harvest of B-lines the plot is observed carefully and the fallen tillers
of B-line if any, should be removed. The A lines are harvested and threshed
separately. This forms are source seed for the nucleus seed production of Aline.
Restorer line multiplication
(i)
The restorer lines used as pollen parents for A x R crosses are grown as plant
to row progenies in a separate block three weeks after transplanting A x R
progenies in identification nursery.
(ii)
The restorer lines which were responsible for the deviant A x R progeny as
observed in the identification nursery are marked and removed before
flowering.
(iii)
After assessing the spikelet fertility of the corresponding A x R progenies, the
seeds of the corresponding R lines which show normal restoration line will be
bulked as source seed for nucleus seed production of R-line.
In India by using WA cytoplasm many hybrids were released. Eg.
APRH 1, APRH 2, CORH 1, KRH 1 – 1994.
KRH 2, DRRH 1
– 1996
CORH 2, ADTRH – 1
– 1998
PUSH RH 10, HR 1 120
- 2001