2. CONTENT
1) Introduction
2) History of Distant Hybridization
3) Objective of Distant Hybridization
4) Types of Distant Hybridization
5) Features
6) Barriers associated with Distant Hybridization
7) Techniques to overcome Barriers of Distant Hybrid
8) Applications of Wide crossing in crop improvement
9) Limitations of Distant Hybridization
10) Achievements
3. HYBRIDIZATION
Hybridization:- Mating between two different strains.
Distant Hybridization:- Hybridization between individuals from different species,
belonging to the same genus or to different genera is termed as distant
hybridization and such crosses are known as distant crosses or wide crosses.
TYPES -
• Interspecific Hybridization:- When the individuals from two distinct species of the
genus are crossed, it is known as interspecific hybridization.
e.g. Oryza sativa x Oryza perennis.
• Intergeneric Hybridization:- When the individuals being crossed belong to species
from two different genera, it is referred as intergeneric hybridization.
e.g. Triticum spp. X Rye (Secale cereale).
4.
5. OBJECTIVE OF DISTANT
HYBRIDIZATION
Wide crossing or distant hybridization has been used in
the genetic improvement of some crop plants. It is an
effective means of transferring desirable genes into
cultivated plants from related species and genera
6. INTER- SPECIFIC HYBRIDIZATION
Examples:-
• Nerica, an upland rice for Africa
• Oryza sativa (Asian upland rice) : non-shattering , resistant to lodging,
high yield potential
• Oryza glaberrima (African rice) : drought tolerant, disease resistant,
weed suppressing
• Nerica rice combines the best of both species.
7.
8.
9. INTERGENERIC CROSSES
• Triticale, cereal created in the lab.
• Triticale, a cross(intergeneric cross) between wheat and rye , was
produced by embryo rescue of the product of fertilization and a
chemically induced doubling of the chromosomes.
• Embryo rescue become necessary when fertile offspring is never
produced by an interspecific cross.
12. Interspecific hybridization gives rise
to three types of crosses
a) Fully fertile
b) Partially fertile
c) Fully sterile in different crop species
13. FULLY FERTILE CROSSES
Interspecific crosses are fully fertile between those species that have
complete chromosomal homology. Chromosome in such hybrids have normal
pairing at meiosis and the resulting F1 plants are fully fertile.
Example- Cotton, wheat, oat and soybean.
Cotton:
There are four cultivated species of cotton Viz.,
• G. hirsutum and G. barbadense (Tetraploid, 2n=52)
• G. arboreum and G. herbaceum (Diploid, 2n=26)
14. PARTIALLY FERTILE CROSSES
• Interspecific crosses are partially fertile between those species which differ
in chromosome number but have some chromosome in common.
• In such situation , F1 plants are partially fertile and partially sterile.
• Example- Wheat, Cotton, Tobacco.
15. FULLY STERILE CROSSES
Interspecific crosses are fully sterile between those species which do not
have chromosomal homology. Such hybrids can be made self fertile by
doubling the chromosomes through colchicine treatment e.g. Tobacco,
wheat, cotton, brassica and vigna species.
17. MAIN FEATURES OF INTERSPECIFIC
OR INTERGENERIC HYBRIDIZATION
It is used when the desirable character is not found within the species of a crop.
It is an effective method of transferring desirable gene into cultivated plants
from their related cultivated or wild species.
It is more successful in vegetatively propagated species like sugarcane and
potato than in seed propagated species.
It gives rise to three types of crosses viz. a) fully fertile, b) Partially fertile and
c) Fully sterile in different crop species.
It leads to introgression which refer to transfer of some genes from one species
into genome of another species.
F1 hybrid between two genus are always sterile. The fertility has to be restored
by doubling of chromosome through colchicine treatment
18. BARRIERS ASSOCIATED WITH WIDE
CROSSES
The major problems associated with wide crosses are:
1. Failure of zygote formation / Cross incompatibility
2. Failure of zygote development / Hybrid inviability
3. Failure of F1 seedling development / Hybrid sterility
4. Hybrid breakdown.
19. FAILURE OF ZYGOTE FORMATION /
CROSS INCOMPATIBILITY.
Inability of the functional pollens of one species or genera to effect
fertilization of the female gametes of another species or genera is
referred to as cross incompatibility.
• It may be due to –
1. Failure of fertilization, because the pollen may not germinate.
2. Pollen tube is unable to reach to embryo sac and hence sperms are
not available for fertilization
3. Pollen tube may burst in the style of another species Eg: Datura.
4. The style of the female parent may be longer than the usual length
of the pollen tube growth therefore the pollen does not reach the
embryo sac. Eg: Zea mays and Tripsacum sp.
20. 5.Pollen tubes of polyploidy species are usually thicker than those of
diploid species.
6.When a diploid is used as female and a polyploidy as male, the
polyploidy pollen tube grows at a slower rate in the diploid style than it
would be in a polyploid style.
These barriers are known as pre-fertilization barriers
21. FAILURE OF ZYGOTE DEVELOPMENT /
HYBRID INVIABILITY
The inability of a hybrid zygote to grow into a normal embryo under the
usual conditions of development is referred to as hybrid inviability.
• This may be due to :
1. Lethal genes
2. Genetic disharmony between the two parental genomes
3. Chromosome elimination
4. Incompatible cytoplasm
5. Endosperm Abortion
22. 1. Lethal genes –
Some species carry a lethal gene, which causes death of the interspecific hybrid
zygote during early embryonic development. The lethal gene generally does not
have any effect on the species carrying it but affects only interspecific hybrid
embryos.
Eg: Aegilops umbellulata carries a lethal gene with 3 alleles :
Le for early lethality
Ll for late lethality
l for non lethality
This lethal gene is active against the zygote produced by crossing Aegilops
umbellulata with diploid wheat .
2. Genetic Disharmony between the two parental genomes –
The genetic imbalance between the two parental species may cause the death of
embryos.
Eg. Cotton spp.
Brassica spp.
23. 3. Chromosome elimination -
In some cases of distant hybridization, chromosomes are gradually
eliminated from the zygote. This generally does not prevent embryo
development, but the resulting embryo and the F1 plants obtained
from such crosses are not true interspecific hybrids since they do not
have the two parental genomes in full. Generally chromosomes from
one are successively eliminated due to mitotic irregularities
Examples:
1. Hordeum bulbosum x H. vulgare
2. Hordeum bulbosum x Triticum aestivum
3. Triticum aestivum / durum x Zea mays
24.
25. 4. Incompatible cytoplasm –
Embryo development may be blocked by an incompatibility between cytoplasm of the
species used as female and the genome of the species used as male. Such an interaction,
more generally, leads to hybrid weakness and male sterility in the hybrids or may some
times leads to failure of embryo developments.
5. Endosperm Abortion –
Seeds from a large number of distant crosses are not fully developed and are shrunken due
to poorly developed endosperm. Such seeds show poor germination and may often fail
to germinate. When the endosperm development is poor or is blocked, the condition is
generally known as endosperm abortion.
Examples:-
Triticum x secale – Triticale . In this case the endosperm aborts at a much later stage so
that a small frequency of viable seed is obtained.
Hordium bulbosum x H. vulgare – the endosperm aborts at an early stage so that viable
seeds are not produced.
• In case of endosperm abortion - embryo rescue culture is practiced.
26. FAILURE OF HYBRID SEEDLING DEVELOPMENT /
HYBRID STERILITY
Hybrid sterility :
Hybrid sterility refers to the inability of a hybrid to produce viable off spring.
The main cause of hybrid sterility is lack of structural homology between the
chromosomes of two species.
Some distant hybrids die during seedling development or even after initiation
of flowering. The mechanisms involved in the failure of seedling development
most likely involve complementary lethal genes.
Examples:-
1. In cotton-certain inter specific hybrids appear normal, but die in various
stages of seedling growth; some plants die at flowering
2. Inter specific and inter-generic F1 hybrids of wheat show both chlorosis
and necrosis
27. HYBRID BREAKDOWN
• Hybrid breakdown is a major problem in interspecific crosses.
• When F1 hybrid plants of an interspecific crosses are vigorous and fertile
but there F2 progeny is weak and sterile it is known as hybrid breakdown.
• This may be due to the structural difference of chromosomes or
problems in gene combinations.
28.
29.
30. STERILITY IN DISTANT HYBRIDS :
• Distant Hybrids show variable sterility ranging from complete fertility
to complete sterility
• Eg. L.esculentum x L.pimpinellifolium hybrid in completely fertile
while sugarcane maize hybrid is completely sterile.
• The sterility of distant hybrids may be caused by
Cytogenetic
Genetic or
Cytoplasmic factors
31. CYTOGENETIC BASIS OF STERILITY :
• Most of the interspecific hybrids show reduced chromosome pairing and in
extreme cases all the chromosomes may be present as univalents.
• The distribution of chromosome in such cases is irregular and it leads to
the formation of unbalanced gametes resulting in partial to complete
sterility.
• Inter specific crosses also show rings and chains at metaphase-I (indicating
translocations).
• Bridges and fragments at anaphase-I (indicating inversions)
• Loops at pachytene (indicating duplications or deletions).
These cytological aberrations also reduce fertility. Fertility in such hybrids
is improved by doubling their chromosome number, that is by producing
amphidiploids from them.
32. GENETIC BASIS OF STERILITY :
Chromosome pairing in some interspecific hybrid is regular, but they
show variable sterility which is due to genes.
Eg. The F1 hybrid between foxtail millet, setaria italica and its wild
relative S.viridis showed normal pairing and regular formation of
bivalents. But pollen and ovule sterilities were 70 and 50% respectively.
CYTOPLASMIC BASIS OF STERILITY
In some interspecific hybrids, sterility is produced by the cytoplasm. In
such cases, the reciprocal crosses produce fertile hybrids.
Eg: Oenothera.
33. APPLICATIONS IN CROP
IMPROVEMENT
1. Alien addition lines:
Carries one chromosome pair from a different species in addition to
somatic chromosome complement. The purpose of the alien additions
generally is the transfer of disease resistance from related species.
Eg. transfer of disease resistance from Nicotiana glutinosa to N.
tabacum
2. Alien substitution lines :
It has one chromosome pair from different species in place of the
chromosome pair of the recipient parent.
34. 3 . Introgression of Genes –
Transfer of small chromosome segments with desirable genes.
(A) Disease and insect resistance
35.
36. 4. Development of New crop species :
Eg. Triticale
5. Utilization as New hybrid varieties :
Eg. F1 hybrids in cotton (G.hirsutum x G. barbadense)
Sugarcane : All the present day commercial varieties are complex
inter-specific hybrids involving S. officinarum & S. spontanium
37. LIMITATIONS OF DISTANT
HYBRIDIZATION
1) Incompatible crosses
2) F1 sterility
3) Problems in creating new species
4) Lack of homoeology between chromosome of the parental species
5) Undesirable linkages
6) Problems in the transfer of recessive oligo genes and quantitative traits
7) Lack of flowering in F1
8) Problems in using improved varieties in distant hybridization
9) Dormancy
38. ACHIEVEMENTS
Hybrid varieties:-
• Upland cotton – MCU-2, MCU-5, Khandwa1, Khandwa2 etc are
derivatives of interspecific hybridization.
• Hybrid between Pearl millet x Napier grass - Hybrid Napier which is
very popular for its high fodder yield and fodder quality e.g. Jaywant
and Yashwant
• Interspecific hybrids in cotton- Varlaxmi, Savitri, DCH-32, NHB-12, DH-
7, DH-9 etc.
• Prabhani Kranti variety of bhindi
