Barriers to fertilization, methods of overcoming incompatibilities

Barriers to fertilization:-

Introduction:-

>Prevent the fusion of male and female gametes. 

> It limits effective utilization of the hybrids. 

> Self Incompatibility.

> Very frequent in inter-specific and inter- generic hybridization program.

Major Fertilization Barriers:- 

1. Temporal and spatial isolation of species:- Non synchronous flowering of the parental species due to different agro-ecological or geographical background. 

Methods to overcome:- 

i. Early / staggered sowing 

ii. Suitable photoperiodic treatment 

iii. Vernalisation

2. Pre-fertilization Barriers:- Prevent Fertilization.

a. Failure of pollen germination on stigma:-

i. Unilateral incompatibility (UI):-

> Incompatibility operates in one direction, whereas the reciprocal cross is successful.

> The crosses show incompatible when an SI species is used as a female parent (SI x SC). 

> For active recognition of the pollen, Self-incompatibility inhibition is necessary. 

ii. Inhibition on the stigma surface:-

> Result in the arrest of pollen germination or pollen tube entry into the stigma.

Factors:- Lack of effective adhesion, full hydration and pollen germination factors.

Techniques to overcome:-

i. Effective pollination:- Pollen must be transferred to the correct place and at the correct time. Pollen must hydrate properly.

ii. Mentor pollination:- Pollen which is fully compatible with the intended seed parent.

b. Insufficient growth of pollen tube in style:-  

> Causes:-

i. The arrest of pollen tubes in the stigma. 

ii. Long style of flower. 

> Arrested pollen tubes show abnormalities like:

• Thicker tubes. 

• Excessive deposition of callose. 

• Swollen tips. 

• Branching of tubes 

> Growing pollen tubes utilize styler nutrients. Arrested pollen tube growth is the inability of the pollen tubes to utilize styler nutrient.

Techniques to overcome:-

i. Reciprocal crosses.

ii. Mentor pollination.

iii. Use of plant growth regulators.

iv. Pollen may have to be applied in a medium favoring germination. 

v. By pass stigma and style completely and apply pollen directly to the ovule.

c. Failure of male gamete to unite with female gamete [Cross incompatibility]:-

> Inability of the functional pollens of one species to effect fertilization of the female gametes of another species is referred to as cross incompatibility. 

> The failure of male and female gametes to unite to form zygote in interspecific and intergeneric hybrids is known as cross incompatibility. 

> This is major problem in distant hybridization.

3. Post-fertilization Barriers:- Prevent Embryo Development.

a. Failure of zygote development:-

> In some wide crosses, fertilization occurs and zygote formation also take place. But, the zygote does not grow. 

> Inability of a hybrid zygote to grow into a normal embryo under the normal conditions of developement is known as hybrid inviability. 

> Causes:- 

i. Lethal genes. 

ii. Lack of chromosoamal homology. 

iii. Disharmony between nuclear and cytoplasmic gene.

iv. Unfavourable interaction between embryo and endosperm.

b. Hybrid sterility:-

> The inability of a hybrid to produce viable offspring.

> This problem is more severe in intergeneric crosses than in interspecific crosses.  

> Causes:-

i. Lack of structural homology between the chromosomes of two species turn in to non-pairing or reduce pairing. 

ii. Structural chromosomal aberration.

c. Hybrid breakdown in F2 or later generation:-

> In this f1 plants are vigorous and fertile but become sterile or weak in f2 or later generation. 

> Causes:- 

i. Gene combination:- Unfavorable combinations of dominant and recessive genes breakdown due to segregation and recombination 

ii. Structural differences:- Structural differences in the chromosomes, gametes with deletion and duplication

d. Insufficient growth of Hybrid plants:-

> Flower shedding.

> Delayed maturity of fruits.

> Chlorophyll deficient.

> Seed abortion.

> Due to unfavourable interactions between Genes of two species.

Techniques to overcome:-

i. Choice of parents:-

- Which are crossing competible.

- Having chromosomal similarity or homology (so that atleast fertility can be restored).

- Choose the parent from:

i. Primary gene pool 

ii. Secondary gene pool 

iii. Tertiary gene pool

ii. Reciprocal cross:- The interspecific cross of black gram as female and green gram as male was not successfully reported but the reciprocal cross was found successful.

iii. Modification of ploidy levels:-

- Eg: If Parent A having 2n=24 chromosome number & Parent B having 2n=12 cromosome number than first, 

- In some cases haploid is produced. Or In some cases the semisterile f1’s chromosome is doubled for restoration of fertility. Doubling of chromosome

iv. Modification of pistil:- Use short styled species as female If, 

Maize♀ × Tripsacum ♂

- Because of the pollen tube of Tripsacum have not enough growth to reach due to long style of maize.

v. Mentor pollination:-

- The pollen which has been purposefully treated is called mentor pollen. 

- The compatible pollen is treat: 

i. To improve its fertilization ability.

ii. To retain power to stimulate incompatible pollen to accomplish fertilization.

vi. Bridge species:-

- Example: If there was two genotype A and C can not directly cross then genotype B which can cross with both A and C is used as bridge species. 

vii.Chemical treatment of the pistil or pollen:- Use of Growth Regulator.

- Eg. N. tabacum does not hybridize with N. rependa. But this cross is possible when IAA applied to pedicle of flowers in a lanoline paste 

It cause:

i. More rapid pollen tube growth 

ii. Longer pistil viability 

- Vigna radiata x Vigna ambellate. EACA (immunosuppressant) applied before pollination.

- Interspecific crosses in cotton. IAA, NAA and GA3 applied to crossed buds to Prevent shedding and Use of growth regulator.

viii. Embryo Culture:- 

- Isolation & growth of an immature or mature embryo in vitro, with the goal of obtaining a viable plant. 

- The first attempt to grow the embryos of angiosperms was made by Hannig (1904) from two Crucifers Cochleria & Raphanus.

ix. Protoplast fusion or somatic hybridization:- Development of hybrid plants through the fusion of somatic protoplasts of two different plant species/varieties is called somatic hybridization.

x. Grafting:-

- Carried out in horticultural crops. 

- In which stock and scion is used.