Transcript crop production, lec 11.

Plant breeding
concepts
ABT-320
Dr. Rabia Amir
Lecture 11
LOGO
AUTOPOLYPLOIDY BREEDING
 Autopolyploidy is the condition in which the same
genome (x) is present in an organism more than two
times.
 Autotriploid (3x) and autotetraploid (4x) plants are
important in plant breeding.
AUTOPOLYPLOIDY BREEDING
•
•
•
Autotriploids possess three
identical sets of chromosomes.
Autotriploidy occurs naturally
in low frequency.
They can be produced by
crossing an autotetraploid (4x)
with a diploid of the same
species (2x).
•
They are usually sterile and
non-seed producing.
•
Examples: Fruit crops like
banana, apple, grape,
watermelon etc.
•
Autotetraploids (4x) possess
four copies of the same genome.
•
They may arise spontaneously
or can be induced by doubling
the chromosomes of diploid
species by colchicine treatment.
•
Examples of autotetraploid
crops are rye, groundnut, potato
and coffee.
ALLOPOLYPLOIDY BREEDING
 Allopolyploids are polyploids in which more one
genome are present. An allotetraploid is otherwise
called amphidiploid because it contains two
genomes twice (X1X1 + X2X2).
 Natural allopolyploid crop plants: They develop in
nature
spontaneously.
Breadwheat
(Triticum
aestivum) (2n = 6x = 42) is an allohexaploid with three
genomes.
PRODUCTION OF ARTIFICIAL ALLOPOLYPLOIDS
 It can be done by interspecific and intergeneric crosses and
subsequent chromosome doubling has been carried out with
different levels of success.
Chromosome
doubling is
usually
affected by
treating the
diploids with a
chemical
known as
colchicine.
•It is applied in
concentrations
ranging from 0.01%
to 0.5%.
•It is applied to
growing tips,
meristematic cells,
seeds and buds in
aqueous solutions.
Duration of
treatment
varies from 24
hours to 96
hours
depending
upon the plant
species.
COLCHIPLOIDY
 Colchicine induced polyploidy is known as colchiploidy.
 It induces polyploidy by inhibiting spindle formation during cell
division.
 Chromosomes do not get segregated at the time of meiosis,
resulting in the production of diploid gametes, which on fusion
give rise to polyploid plants.
Triticum durum (4X)
AABB
x
Secale cereale (2X)
RR
ABR F1(3X): EMBRYO RESCUE
CHROMOSOME DOUBLING
HEXAPLOID TRITICALE (6X)
AABBRR
APPLICATIONS OF ALLOPOLYPLOIDY
BREEDING
 Allopolyploids can be used to produce new crop species, for
interspecific gene transfer and for bridge crosses.
 Many artificial allopolyploids have been synthesized in different
crops.
 Tetraploid species of wheat and cotton have been produced
artificially by interspecific hybridization and induction of
amphiploids.
RAPHANOBRASSICA
 Raphanobrassica
is
the
first
example
of
intergeneric
hybridization in plants.
 This was developed in 1927 by crossing radish (Raphanus sativus,
n = 9) with cabbage (Brassica oleracea, n =9).
 An
amphiploid
was
developed
by
hybridization
and
chromosome doubling.
 He could not combine the agronomical characters of the crops.
 The hybrid had the roots of cabbage and leaves of radish.
 However, this experiment proved the feasibility of intergeneric
hybridization.
TRITICALE
 Another significant example of intergeneric hybridization
followed by polyploidization is the synthesis of the new cereal
triticale.
 Triticale is a man-made cereal produced by crossing wheat with
rye.
 Triticale combines the winter hardiness and high protein content
of rye with the bread making quality of wheat.
 Hexaploid and octoploid triticales have been developed in this
way.
ANEUPLOIDY BREEDING
Aneuploids are organisms that show monosomy, nullisomy,
trisomy, tetrasomy, etc.
They are not directly useful in crop
improvement, but they can be used
indirectly in different ways.
Some of the major uses include locating
genes through monosomic and nullisomic
analyses and interspecific gene transfer.
Other uses include developing alien addition lines and alien substitution lines of crop
and analysis of chromosomal aberrations.
Applications
of aneuploidy
breeding
DISTANT HYBRIDIZATION
 Distant hybridization or wide crossing is the mating between
distantly related individuals.
 Sexual or somatic cells may be involved in this fusion.
 When fusion takes place between somatic cells, it is called
parasexual hybridization.
 Distant hybridization may be of two types:
INTERSPECIFIC
HYBRIDIZATION
INTERGENERIC
HYBRIDIZATION
INTERSPECIFIC HYBRIDIZATION
 Hybridization between two species of the same genus usually
takes place by sexual fusion.
 It is usually practiced to transfer desirable genes from wild
species of plants to cultivated species.
 Interspecific crosses help in introgressive hybridization which is
the transfer of some genes from one species into the genome of
another species.
INTERSPECIFIC HYBRIDIZATION
 Interspecific crosses may be fully fertile, partially fertile or
sterile. E.g., wheat 6X × 4X.
 Fertility level of interspecific crosses depends on the homology
of chromosomes in the parental species.
 In the case of sterile crosses, amphidiploidy is induced with
colchicine and the fertility is restored.
INTERGENERIC HYBRIDIZATION
 This refers to crosses between two different genera of the same
family.
Such
crosses
are
not
commonly
used
in
crop
improvement.
 Intergeneric crosses can be used when the desirable genes are
not present in the same genus, but they are present in allied
genera.
INTERGENERIC HYBRIDIZATION
 F1 hybrids of this type of crosses are always sterile. However,
they can be made fertile by chromosome doubling.
 Intergeneric hybridization has been used successfully in the
development of the synthetic cereal, for example, triticale.
TECHNIQUES TO MAKE WIDE CROSSES
SUCCESSFUL
SELECTION OF PLANTS
The most
compatible
parents
available should
be selected for
the crosses.
RECIPROCAL CROSSES
MANIPULATION OF
PLOIDY
Reciprocal cross
may be
attempted when
one parental
combination
fails.
Diploidization
of solitary
genomes to
make them
paired, to
make the cross
fertile.
BRIDGE CROSSES
When two parents
are incompatible, a
third parent that is
compatible with
both the parents
can be used for
bridge crosses and
thus it becomes
possible to perform
cross between the
original parents.
USE OF POLLEN
MIXTURE
Unfavorable
interaction
between pollen
and pistil in the
case of wide
crosses can be
overcome to
some extent by
using pollen
mixture.
MANIPULATION OF
PISTIL
Decapitation of
the style will
sometimes
prove helpful in
overcoming
incompatibility.
USE OF GROWTH
REGULATORS
Pollen tube
growth can be
accelerated by
using growth
hormones like
IAA, NAA, 2,4-D
and Gibberellic
acid.
PROTOPLAST
FUSION
When fusion
of gametes
fails,
protoplast
fusion of
somatic cells
can be
attempted.
EMBRYO
RESCUE
Hybrid zygotes
formed by wide
crosses may fail
to grow in a
number of cases.
The zygotes are
taken out and
grown in in vitro
medium to
overcome this
problem.
PROBLEMS ASSOCIATED WITH WIDE
CROSSES
 Cross Incompatibility
 Hybrid Inviability
 Hybrid Sterility
 Hybrid breakdown
ROLE OF WIDE CROSSES IN CROP
IMPROVEMENT
 Wide crosses are generally used to improve crop varieties for
disease resistance, pest resistance, stress resistance, quality,
adaptation, yield etc.
 These crosses can even be used to develop new crop species.
 Techniques like alien addition and alien substitution may also be
effective.
ROLE OF WIDE CROSSES IN CROP
IMPROVEMENT
ALIEN
ADDITION
Addition of
chromosomes
of a wild
species (foreign
species) to the
normal
compliments of
a cultivated
species.
ALIEN
SUBSTITUTION
Effective
Techniques
Replacement
of one pair of
chromosomes
of a cultivated
species with
those of a wild
donor species.
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