Principles of Plant Breeding (2+1)

Clonal selection

• Asexual reproduction covers all modes of multiplication of plants where normal gamete formation and fertilization does not take place making these distinctly different from normal seed producing crops. In the absence of sexual reproduction, the genetic composition of plant material being multiplied remains essentially the same as its source of plant.
• It was this unique characteristic of asexual reproduction that helped Thomas Knight to develop of number of cultivars of fruits and vegetables including grapes, apple, pear, and peaches during 1811 to 1838.
• In contrast to sexually producing plants, the F1 crosses in the asexually propagated plants were heterozygotes from which knight selected superior plants and propagated vegetative to develop stable varieties without any deterioration due to segregation of gene combinations.
• Asexually propagated crops are highly heterozygous but identified clone is the best amongst large number of such heterozygote present in naturally occurring or artificially created variation, there may be a presence of unfavorable alleles in homozygous.
  
• A process of inbreeding and selection combined with hybridization can thus lead to production of more productive hybrids. A high degree of inbreeding depression can be expected but new clones developed by crossing of two clones from two different population of selfed ones are likely to produce highly heterozygous and more productive clone, but crossing of inbred lines of same clone carry deleterious allele and clone in homozygous state.