The
introduction of Hybrid Crop Varieties has enabled spectacular increase in
productivity owing to hybrid vigor and increased uniformity, to produce hybrid
seeds a pollination Control system is required to prevent unwanted self pollination or sib-pollination of the female line
(Katja Kempi, Mario Gils, 2011). Pollination control systems are
procedures in plant breeding or systems adopted by a plant
breeder in order to obtain a desired
hybrid cultivars. In other words
the process of breeding hybrid “hybridization “ is best achieved through pollination
control systems. These systems render
the pollen of one parent line non-viable
(male sterile or
female line) to
ensure pollination by the chosen parent line. The objective is to
develop hybrid crop species that are resistant to abiotic stresses, pest and
diseases, viruses, and also crops with high agronomic performances and
discard plants with
undesirable characters or traits.
Some of these systems includes:
1.
Emasculation: This is carried through the mechanical
removal of the anthers e.g. in corn, where the male flowers are separated from
the female flowers, the process is called
“detasseling” and involves the removal of the male flower from the plant.
Castration
can also be carried out through mechanically removing the male flower organs or
by spraying chemical hybridizing agents that prevent the development of active
pollen. E.g. according to Mendel’s procedures, to prevent self-fertilization in
“test” flowers, anthers were removed from those chosen to be seed parents
before their pollen-receiving parts (stigma) are fully mature. The stigma of
this test flower was protected against any foreign pollen (via insect or air)
with the help of its covering by a bag. Pollen from the designated pollen
parent was transferred at the appropriate time to the stigma of the seed-parent
flower. To be more precise, the pollen at the dehiscence stage is brought from
the plant to be used as a male parent and by the help of brush is dusted on the
feathery stigma of the emasculated (operated) flower.
At
the time of such cross-pollination, the pollen should be mature and stigma be
receptive. (Mario Gils, Katja Kempe, 2011).
2. Genetic
Method/System: This can be used to generate the derived male sterility in
crops that posses full or “perfect” flower (male and female). The system is
either based on mutated mitochondria DNA or else nuclear encoded genes or to some
other cytoplasmic factors outside the nucleus, resulting in the transformation
of the fertile cytoplasm into a sterile one. According to (Srivastva and
Sarkission, 1969) as quoted by (Katja, 2011) male sterile conditions may arise
by mutation in mitochondrial bodies (mtDNA) of a normal fertile plant. It is
known that mitochondrial are the respiratory sites (power house) within the
cell. Mutation in mitochondium may reduce the respiratory capacity of the cell
and have result in loss of fertility (Rhoade, 1950) as quoted by (Katja, 2011)
with two types of cytoplasm (i.e. sterile and fertile) at the most, only two
kinds of genotypes are possible. One of them is sterile and the other fertile.
E.g. cotton plants that resist pests by incorporation of a bacterial gene
encoding a protein toxic to insect.
3. The
“Split Gene” System: (Katja, 2011) this is a biotechnological concept for
pollination control, in particular it is a novel system suggested for the
industrial production of hybrid wheat. It is based on splitting a
tapetum-expressed barnase gene, which causes male-sterility by barness ablation
into two fragments.
Pollen
oblation into two fragments. A precursor-T- DNA (provector) containing two
complementary tape-tum-expressed barenase gene fragments controlled by a
tapetum-specific promoter is transformed into plants. Site-specific deletions
of the T-DNAs during plant development leads to two alternative derivation,
with each producing only one of the two complementary barnase precusor protein.
crossing those plants that carry the respective complementary loci with each
other leads to progeny that carry the two barnase gene fragments in allelic
positions. These plants are male-sterile and are used as the female crossing
partners for hybrid breeding. The hybrid progeny plants are fertile as the
barnase fragments segregate completed in the F1. For the maintenance
of the female crossing partner, the heterozygous plant can be crossed to a
homozygous line.
4. Male
sterility: Male sterility is a reproduction deficiency of some plants where
male organs in hermaphrodite flowers are rendered defunct (Sharma, 1994). Male
sterility is of great value in the production of hybrid seeds. It is
characterized by non-functional pollen grains, while female gametes functions
normally. There are two types
i.
Genetic male sterility.
This is ordinarily governed by a single recessive gene, ms, A male sterile line
may be maintained by crossing it with heterozygous male fertile plants. Such a
mating produces 1:1 male sterile and male fertile plants. E.g progeny from ms ms x Ms ms crosses one used as
female, and interplanted with homozygous male fertile (ms ms) pollinator. Eg
Cotton, barley, tomato, smflower, cucurbits etc. while
ii.
Cytoplasm male sterility. This is
determined by the cytoplasm. Since the cytoplasm of a zygote comes primarily from
egg cell, the progeny of such male sterile plant would always be male sterile.
The male sterile line is maintained by crossing it with the pollinator strain
used as the recurrent parent in the backcross programme since its genotype is
identical with that of the male sterile line.
Such a male fertile line is known as the maintainer or B as it used to
maintain the male sterile line. It is observed in zea mays, Nicotiana,
tobacum, Helianthus annus, oryza sativa,
sorghum bicolor, tritium aestivum, gossypium hrisutum. May be utilized in
producing hybrid seed in ornamental spp, or spp where a vegetative part is of
economic value.
Use
of chemical hybridizing agent. This
system prevents the unwanted self-
pollination of the female parental line. It is a castration method
carried out in order to prevent the
development of active pollen in
plant crops.
Chopra
(2003) reported that these are
chemicals used for pollen abortion which
make the plants male sterile without
having any side effect on ovule fertility. Such gametocides could be used to produce male
sterile versions of any breeding line, thus obviating the need for
laborious backcrossing as required for the transfer of genic or cytoplasmic
male sterility. For using in F1
hybrid seed production, chemical hybridizing
agent (CHA) should cause only pollen abortion and not affect ouvle fertility, have no mutagenic effects, be easy and economical to
apply and should have no adverse side effects.
According
to (Sneep et al, 1979) as quoted by
(Chopra 2003) in vegetable, positive results were obtained with GA3 and GA4/7 in lettuce and oniou, with maleic hydrazide in tomato and onion, and with
sodium 2,3 - dichloroisobutyrate in
tomato. Hence for efficient application of CHA heterosis breeding CHA need to
be systemic and flowering has to be more determinate.
REFERENCES
1.
Chopra V.L (2003) Plant Breeding, Theory and
Practice, Oxford and IBH
Publishing Co Ltd Newdelhi, P.261-263)
2.
Sharma j.r (1994) Principles and Practice of Plant Breeding Tata McGraw- Hill Publisher Co Ltd , New Delhi P 47- 49
3.
Katja Kempe and Mario Gils (2011) Molecular Breeding (www. Springerlink.com)
4. Cytoplasmic Male Sterility – Wikipedia the
Free encyclopedia
5.
Science Direct. Com - trend in Plant Science vol 7,
Issue 5, May
2002
6. Indian Agricultural Resources -Seeds