Breeding of Fruit and Plantation Crops

Chimeras

• A chimera is an individual with one genotype in some of its parts and another genotype in the others. Somatic mutation may often lead to chimeras. When propagated asexually these chimeras may become perpetual. Certain types of Pelargoniums and potatoes are of such chimeras.


• When growth is encouraged from the concealed tissues the real nature of these chimeras is revealed. Somatic mutations either at the terminal or axillary buds in germinating seeds, seedlings or in mature plants can be produced by irradiation or chemical treatment. Artificial creations of such somatic mutations open possibilities of production of new horticultural and agricultural plants.


• Treatment of seeds and vegetative propagules commonly produces chimeras.Shoot tip meristem usually has two functional layers; the outer layer, giving rise to epidermis and a part of leaf mesophyll, and the inner layer producing the rest of the plant tissues including reproductive organs.

Chimeras are of three kinds
Periclinal chimera

• When the entire outer or inner layer is affected, the chimera is known as ‘periclinal chimera’ (inner periclinal or outer periclinal depending upon the layer affected)

Sectorial chimera

• Only a part of the inner or the outer layer is affected (inner sectorial chimera only a part of the inner or the outer layer is affected (inner sectorial and outer sectorial respectively).

Mericlinal chimera

• In mericlinal chimeras, the combination is similar to the periclinal except that the cells carrying the mutant genes occupy only a part of the outer cell layer.
A: Sectorial chimera
B: Periclinal chimera
C: Mericlinal chimera



• In sexually reproducing species, only the inner chimeras (periclinal or sectorial) will be transmitted to the next generation. Outer chimeras will not be recovered since this layer does not contribute to the production of gametes. In clonal crops, however, both outer and inner chimeras can be utilized either as periclinal chimeras (outer or inner) or by producing homogeneous individuals through sexual reproduction (only if the inner layer is affected), tissue culture or other horticultural manipulations, e.g., wounding etc., which induce production of adventitious shoot buds (utilizing both inner and outer chimeras). Sectorial chimeras are unstable in clonal crops and have to be made periclinal through successive clonal propagation and selection for stability.


• Mutant alleles are generally recessive, but some dominant mutations may also occur. In case of sexually reproducing crops, mutation breeding utilizes both recessive and dominant mutations. In dominant mutations, the phenotype can be recognized as a somatic mutation arising from the mutated cell, for example, a colour mutation in an epidermal cell from ‘aa’ (colourless) to ‘Aa’. How ever, recessive mutations are much more numerous than dominant ones. Recessive mutation can occur in the homozygous dominant type as AA – Aa or in the heterozygote as Aa – aa. In the former one, the selfed progeny normally segregate with 25 per cent recessive mutatnt ‘aa’ types.


• Mutation breeding in clonally propagated crops primarily depends on dominant mutation. Recessive mutation may also be utilized provided the clone used for mutagen treatment was heterozygous; for example, if recessive mutant allele is to be useful in a clonal crop, the clone has to have the genotype Aa. Such situations are however, rare. More frequently, the mutants useful in the improvement of clonal crops are dominant mutations.