Download Lecture 8

PRINCIPLES OF CROP PRODUCTION
ABT-320
(3 CREDIT HOURS)
LECTURE 8
BACKCROSS METHOD
GENETIC BASIS OF INBREEDING
PLANT HYBRIDIZATION
BACKCROSS METHOD
• The crossing of an F1 hybrid with either of its parent is referred to as
backcross. It is used as a method of breeding to improve an otherwise
high yielding and popular variety for a specific character.
• A superior variety which needs improvement for a special character
serves as the recipient or recurrent parent. The donor or non-recurrent
parent acts as a source of gene(s) to be added to the recurrent parent.
Any character with good testability can be transferred in this way.
Oligogenes can easily be transferred by this backcross method. However,
this method can be employed even in the case of polygenic characters.
THE GENETIC BASIS OF INBREEDING
Backcross method involves a type of inbreeding since the F1 is repeatedly
backcrossed with the recipient parent. The F1 contains 50 percent genes
from each of the two parents. Repeated backcrossing of the F1 with the
recurrent parent would systematically increase the proportions of gene
from the recurrent parent at the cost of the genes from the donor
parent.
THE PROCEDURE OF BACKCROSS METHOD
It involves:
1.
2.
3.
Transfer of a single dominant gene
Transfer of a recessive gene
Transfer of a quantitative character
TRANSFER OF A SINGLE DOMINANT GENE
• The recurrent (rr) and donor (RR) parents are crossed to produce the F1
(Rr). The recurrent parent is usually used as the female parent. If a
cytoplasmic character is transferred, the recurrent parent is used as the
male parent.
• In the second season, the F1 plants are backcrossed to the recurrent
parent to produce the first backcross generation (BC1) seeds.
• In the third season, the BC1 plants which consist of Rr and rr in equal
proportions are subjected to screening for the character under transfer.
The desirable plants are selected and crossed with the recurrent parent
to produce the BC2 seeds.
• In the fourth season, the BC2 plants are grown and they will also
segregate in the 1:1 (Rr:rr) ratio. The dominant plants are selected and
backcrossed with the recurrent parent to produce the BC3 seeds.
TRANSFER OF A SINGLE DOMINANT GENE
• In the fifth season, BC3 plants (Rr:rr) are grown and BC4 seeds are
collected as in the case of the previous generation. In the sixth season,
BC4 plants are grown and evaluated. If only four backcrosses are
planned, the desirable BC4 plants are self-pollinated and BC4F2 seeds are
produced.
• In the seventh season, the BC4F2 segregates in the 1:2:1 (RR:Rr:rr) ratio.
The recessive plants are eliminated and others are self-pollinated to
produce BC4F3 seeds.
• In the eighth generation, BC4F3 plants are raised in progeny rows.
Progenies produced by the homozygous (RR) BC4F2 plants will be uniform
for the desired characters, whereas the progenies produced from Rr
plants will show segregation. All the uniform progeny rows with the
desired character are bulked and released as the improved variety.
TRANSFER OF A RECESSIVE GENE
In this case, the donor parent will be the recessive and usually it is taken
as the male parent. In the first season, the dominant x recessive cross is
made and F1 seeds are collected. In the second season, the F1 is grown
and backcrossed with the recurrent (dominant) parent and the BC1 seeds
are collected. In the third season, the BC1 is grown and selfed to produce
BC1F2 seeds. In the fourth season, the BC1F2 is grown, the dominant
plants are eliminated and the other plants are selected and crossed with
the recurrent parent to produce the BC2. In the fifth season, the BC2 is
crossed with the recurrent parent to produce the BC3 generation. In the
sixth season, the BC3 is grown; selfed and BC3F2 seeds are collected. In
the seventh season, the BC3F2 is grown, dominant plants are eliminated
and others are selected. BC4 seeds collected from the selected plants. In
the eighth generation, the BC4 plants are grown and selfed to produce
the BC4F2 seeds. In the ninth season, BC4F2 is grown and recessive plants
are selected, their seeds bulked and the progeny will be the desired
strain of the recurrent parent with the recessive gene from the nonrecurrent parent transferred to it.
TRANSFER OF A QUANTITATIVE
CHARACTER
• Since more number of loci are involved in the control of a quantitative
character and the relative influence of environment in the expression of such
traits is higher, a relatively large population is to be sampled in each backcross
generation. The F1 is raised in the first season by crossing the recurrent parent
and the donor parent. In the second season, BC1 is produced. Sufficient
quantity of crossed seeds is produced so as to facilitate selection in the BC1
itself. In the third season, the BC1 plants are evaluated and those with high
intensity of the character under transfer are selected since they carry the
maximum number of genes for the character. BC1F2 seeds are produced by
selfing.
• In the fourth season, the BC1F2 is raised and is evaluated both for the character
under transfer and the general features of the recurrent parent. Plants
resembling the recurrent parent with the highest intensity of the character
under transfer are selected and harvested individually.
TRANSFER OF A QUANTITATIVE
CHARACTER
In the fifth season, BC1F3 is raised in progeny rows from the above seeds
and evaluated extensively for the character under transfer and general
features of the recurrent parent. The selected F3 plants are crossed with
the recurrent parent to produce BC3 seeds. In the seventh season, the
BC3 plants are grown and selected for the character under transfer and
the general feature of the recurrent parent. At the end, a number of line
resembling the recurrent parent and showing the character under
transfer are bulked to constitute the modified variety of the recurrent
parent.
MERITS & LIMITATIONS OF BACKCROSS
METHOD
This method is very effective to transfer a desired trait to an otherwise
improved variety. Evaluation of the variety can be limited to the extent
of confirming the transfer of the character under consideration. It
provides the ideal solution to utilize the unique properties of an
otherwise unadapted germplasm. The limitation of backcross method is
that the procedure is time consuming and it does not result in the
improvement of other characters.
PLANT HYBRIDIZATION
Genotype peculiarities of two or more different varieties or species of
plants could be brought together only by crossing them. The mating or
crossing of two plants or lines of dissimilar genotype is called
hybridization. In plant hybridization, one of the plants is taken as the
female plant and the other as the male plant. Pollen grains from the
male parent are made to pollinate the stigma of the flowers of the
female parent. The seeds obtained from such a cross are called F1 seeds
and the progeny raised from it is called F1 (First Filial) generation. The F1
is selfed to produce F2 and the subsequent generations like F3, F4 etc are
raised in the same way. These generations are called segregating
generations and they are handled differently based on the scope and
objectives of the breeding program.
OBJECTIVES OF HYBRIDIZATION
The major objectives of hybridization are:
A. Combination Breeding
Combination breeding is the transfer of one or more characters from other
varieties to a particular variety. These characters may be oligogenic or
polygenic. In this way, genes for disease resistance, quality traits etc can be
transferred.
B. Transgressive Breeding
Transgressive breeding is based on transgressive variation of characters in
segregating generations like F2. A cross is made between two strains of plants
and the F2 is screened for transgressive variations.
Transgressive segregation is the segregation of characters beyond the parental
limits, in the segregating generations like F2.
C. Production of Hybrids
Hybrid (F1) plants show higher vigor and yield when compared to parents, in
some cases. This phenomenon is called hybrid vigor. F1 seeds can be raised in
bulk through hybridization and distributed directly for cultivation, especially in
cross-pollinating crops.
TYPES OF HYBRIDIZATION
Based on the genetic difference between parents, hybridization can be
classified into:
1. Inter-varietal Hybridization
2. Distant Hybridization
INTER-VARIETAL HYBRIDIZATION
The cross between the members of the same species (intra-specific) is
called inter-varietal hybridization. In this type of hybridization, different
cross patterns can be used.
1.
Simple Cross
In this case, two parents are used to produce an F1 hybrid.
2.
Complex Crosses
In complex crosses, more than two parents are involved. Such
crosses can be called convergent crosses since they bring genes
from different sources together.
DISTANT HYBRIDIZATION
• Hybridization between the members of different species or hybridization
beyond species level is called distant hybridization. Thus, it may be
interspecific (intra-generic) or inter-generic. When conventional methods
of hybridization fails, para-sexual methods are used in such cases.
• Para-sexual hybridization is the technique of fusing somatic protoplasts
when reproductive cells fail to fuse or fertilize.
THE PROCESS OF HYBRIDIZATION
The major steps involved in the process of hybridization are:
1.
2.
3.
4.
5.
6.
7.
Selection of Parents
Emasculation
Bagging
Tagging
Pollination
Harvesting F1 Seeds
Further handling of the plants
SELECTION OF PARENTS
The choice of the parents depends on the objective of the cross. In
combination breeding, the genetic diversity of the parents is not
important. In the case of transgressive breeding, genetically diverse
plants are selected as parents. If the characteristics of the parents are
not completely known, they are evaluated for the agronomic features.
EMASCULATION
In the case of crops with bisexual flowers, stamens of the flowers of the
female parents are removed or the pollen grains are killed. This process
is called emasculation. Mechanical, physiological or genetic methods of
emasculation are used, depending upon the nature of the crop and the
cross.
MECHANICAL METHODS OF
EMASCULATION
Here, the anthers are removed from the flowers of the female parents.
Hand emasculation and suction method are generally used. For hand
emasculation, the flower buds are opened carefully before anthesis (First
opening of the flower) and the anthers are removed with the help of
forceps. Care should be taken so that the gynoecium of the flowers in
not damaged. In suction method, the petals are removed from the
flowers before anthesis, with the help of forceps. Then, a thin rubber or
glass tube attached to a suction hose is used to suck the anthers from
the flowers.
PHYSIOLOGICAL METHODS OF
EMASCULATION
Here, the anthers are killed with the help of heat treatment, cold
treatment or alcohol treatment.
GENETIC EMASCULATION
Genetic or cytoplasmic male sterility factors are introduced into the
female parents to make them sterile.
BAGGING
The emasculated inflorescences of female plants are covered using
butter paper bags or cloth bags. However, in the case of cross-pollinated
crops, male plants may also be bagged if desired, so as to avoid pollen
mixture. The bags are removed 2-3 days after pollination.
TAGGING
Emasculated flowers are tagged properly after bagging. Circular or
rectangular tags may be used. Details of the cross, date of emasculation,
date of pollination and the number of flowers emasculated must be
noted on the tag. Carbon pencil or permanent ink may be used for
tagging.
POLLINATION
Mature, fertile and viable pollen grains are collected from the male
parent and dusted on the stigma of the female parent. Care should be
taken to see that the pollen grains are dusted at the optimum stage of
viability.
HARVESTING F1 SEEDS
Crossed seeds are harvested carefully and stored to raise the F1
generation.
FURTHER HANDLING OF THE PLANTS
Further handling of the hybrids depends on the objective of the cross. In
the case of hybrid seed production, the F1 seeds are directly released to
farmers. In the case of combination breeding and transgressive breeding,
F2 is raised and the most appropriate solution program is used.
THE END