WIDE HYBRIDIZATION
Authors: Satish Kumar, Vikas Gupta and Chandra Nath Mishra
ICAR-Indian Institute of Wheat and Barley Research, Karnal-132001 Haryana

Hybridization between individuals from different species, belonging to the same genus or two different genera, is termed as distant or wide hybridization. When individuals being crossed belong to species from two different genera, it is referred as intergeneric hybridization. When individuals from two distinct species of the same genes are crossed it is known as interspecific hybridization.

BARRIERS FOR CROSSING

Several wild species are not crossable with the commercial cultivars due to various isolation barriers. The isolation barrier may be pre-zygotic that prevents fertilization and zygote formation or postzygotic in which fertilization takes place, hybrid zygotes are formed but they are inviable or give rise to weak or sterile hybrids.

Pre-zygotic barriers

1. Failure of pollen germination
2. Slow growth of the pollen tube
3. Inability of the pollen tube to reach the ovary
4. Arrest of pollen tube in the style, ovary and ovule.
These are due to genic differences or differences in ploidy between species.

Postâ€"Zygotic barriers

1. Hybrid inviability and weakness leading to chromosome elimination, lethality and embryo abortion.
2. Hybrid sterility
3. Hybrid breakdown with weak or sterile individuals in F2 owing to recombination of the gene complements of the parental species.

Techniques to overcome isolation barriers

Pre zygotic barriers can be overcome by the following techniques.

i. Mechanical removal of style followed by pollination of the exposed stylar end
ii. Bud pollination.
iii. Use of growth hormones such as GA3, IAA, NAA etc.,
iv. Invitro fertilization
v. Protoplast fusion
vi. Chromosome doubling before hybridization.
vii. Adopting bridging species technique

Post zygotic barriers can be overcome by

i. Chromosome doubling (Amphidiploidy)
ii. Back crossing
iii. Embryo rescue
iv. Tissue culture techniques.


Breeding procedure for wide hybridization

1. Backcross breeding

When interspecific crosses between two species of varying ploidy level are made invariably the hybrids are sterile. By chromosome doubling with application of Colchicine, amphidiploids can be produced. Such amphidiploids are fertile.

Cultivated tobacco, Nicotiana tabacum (2n=24) which crossed to N. glutinosa (2n=12) produced sterile F1 and by chromosome doubling an amphidiploid N. digluta (2n=36) was produced. This was reasonably fertilize with N. tabacum. By repeated backcrossing, a mosaic resistant line with 2n=24 was developed.

2. Amphidiploidy

The manmade cereal Triticale is an intergenic allopolyploid combining Triticum aestivum (Wheat 2n=42) and Secale cereale (rye â€"2n-14).

Rapanobrassica was synthesised by crossing Raphanus sativus, radish (2n-18) and Brassica olereaceae cabbage (2n-20).

3. Bridging species Technique

When direct crosses between two species are difficult, a third species is used in such crosses. Hexaploid wheat, Triticum aestivum (2n-42) does not cross with diploid species. When T. dicoccoides (2n=28) is crossed to Aegilops umbellulata (2n=14) and an amphidiploid was produced it crossed with T. aestivum (2n=42). Nicotiana sylvestrin (2n=24) is the bridging species to transfer nematode resitance from N. repanda (2n-48) to N. tacbaccum (2n=48).

4. Alien-addition and Alien substitution lines

By crossing two unrelated species of different ploidy level and doubling the chromosome number of the sterile F1, fertile amphidiploids are obtained. The amphidiploid in backcrossed to the cultivated species repeatedly twice or thrice and them selfed. In the selfed progeny, plants with one chromozome pair from the donor species in addition to the normal diploid chromosome of the parent species may be present and the y are called alien â€"addition lines. In certain other plants, one chromosome pair of the donor species may substitute one chromosome pair of the parent species when they are called alien substitution lines. By adopting the above methods, mosaic resistance from Nicotiana glutinosa (2n=24) was transferred to N. tabaccum (2n-48) by alien addition (2n=48+2) and alien substitution (2n=48-2+2).


About Author / Additional Info:
I am working as a Scientist in ICAR- Indian Institute of Wheat and Barley Karnal, Haryana under ICAR, New Delhi.