Breeding of Vegetable, Spice and Tuber crops (2+1)

Difficulties in hybridization

• Ordinarily intervarietal hybridization presents little problem, if any, while distant hybridization is often beset by many difficulties.
• In many crops, however, some varieties, particularly when used as the female parent, show a considerably lower seed set than other varieties.
• This problem is not a serious one, and can be easily resolved by pollinating a proportionately larger number of flower buds.
• A more serious problem of hybrid (F1) mortality due to lethal genes is encountered in several cross combinations of some crops.
• An excellent example is furnished by wheat (Triticum aestivum) where F1 plants from many cross combinations show hybrid necrosis.
• Leave the affected areas of leaves first turn darker green, then greyish brown as the chlorophyll degenerates and finally the leaf tissue dies.
• The hybrid necrosis in wheat is believed to be the result of two dominant complementary genes Ne1 and Ne2 located on the chromosome 5BL and 2BS, respectively. According to an estimate, about 18% of wheat varieties possess Ne1, while more than 43% have Ne_2.
• There is a considerable variation in the severity of hybrid necrosis in the different cross combinations of wheat.
• In severe cases, necrosis may develop in the first or second leaf stage and the plants may die by the 3-6 leaf stag. In mild cases of necrosis, only leaf tips may be affected and the plants may grow and reproduce normally.
• All the grades between these two extremes are known to occur and a 0-8 scale has often been used for scoring the intensity of necrosis is presumably due to multiple alleles of both Ne₁(Ne₁^w, Ne₁^m and Ne₁^S) and Ne₂^w, Ne₂^m, Ne₂^ms and Ne₂^S) genes, which generate weak (w), moderate (m), moderately strong (ms) or strong (s) necrotic effects.

 

The problem of hybrid necrosis (or other lethal genes) may be overcome by

1. Keeping necrosis crosses out of the breeding programme;
2. Using one parent having recessive alleles of both the Ne genes;
3. Inducing mutation for the recessive allele of the concerned Ne gene, while allele of the concerned Ne gene, where available;
4. Utilizing spontaneous mutants having the recessive allele of the concerned gene, such as praline;
5. Using a chemical treatment, e.g. spray of an amino acid, where available;
6. Growing the f1 plants in an environment if which will prevent the expression of the lethal gene(s), e.g. temperatures over 220C in the case of Ne genes of wheat.