Breeding Methods for Asexually Propagated Crops and Clonal Selection Apomixes
Authors: Patel Jinkal K. and Jadhav Shrinivas Shripatrao


Asexual reproduction

• Multiplication of plants without the fusion of male and female gametes is known asexual reproduction.
• Asexual reproduction can occur either
• By vegetative plant parts or
• By vegetative embryos which develop without sexual fusion.
• Asexual reproduction is of two types,
• Vegetative reproduction.
• Apomixis.

Vegetative Reproduction

• Vegetative reproduction refers to multiplication of plants by means of various vegetative plant parts.
• Vegetative reproduction is again of two types :
1. Natural vegetative reproduction.
2. Artificial vegetative reproduction.

Natural vegetative reproduction - In nature, multiplication of certain plants occurs by underground stems, sub aerial stems, root and bulbils.

Artificial vegetative reproduction -Multiplication of plants by vegetative parts through artificial method is known as artificial vegetative reproduction.

Stem cuttings- Sugarcane (Saccharum sp.), Grapes (Vitis vinifera), Roses, etc.

Root cutting- Sweet potato, Citrus, Lemon, etc. Layering, grafting are used in fruit and ornamental crops.

Apomixis- Development of seed or embryo without fertilization.
• Obligate Apomixis- A plant which reproduces only by asexual reproduction.
• Facultative Apomixis- A plant which has the potential to reproduce either sexually or asexually. Both process may occur simultaneously or one may be predominant.

Gametophytic Apomixis- Embryos develop without fertilization from the egg cell.
• Diplospory- Embryo sac is produced from megaspore, which may be haploid or more generally diploid. The embryo in such embryo sac may usually arise by Parthenogenesis.

• Parthenogenesis- Development of embryo from egg cell without fertilization. Example - many grasses like Taraxacum etc.

• Apospory- First diploid cell of ovule laying outside the embryo sac develops into another embryo sac without reduction. Then develops directly from the diploid egg cell without fertilization.

Adventive Embryony: Embryos develop directly from vegetative cells of the ovule, such as nucellus.

Apogamy: Development of embryo either from synergids or antipodal cells.

Advantages of Apomixis
• Obligate apomixis permits fixation of heterosis in the hybrids. Therefore, farmers can resow the seeds produced by apomictic hybrids generation after generation.
• The new hybrid variety could be multiplied from few hybrid seeds in the same manner as pure lines. This greatly simplifies hybrid seed production.

Problems in Utilization of Apomixis
• Estimation of the level of facultative apomixis, is tedious and time consuming.
• In case of facultative apomicts, the proportion of sexual progeny is affected by environmental factors like day-length and temperature.
• In the absence of morphological markers linked with apomictic development, maintenance of apomictic stock becomes difficult.
• The genetic basis of apomixis is not clear in most cases.

Clonal Selection
Clone- Progeny of a single plant obtained by asexual reproduction.

Clonal Selection- A procedure of selection superior clones from the mixed population of asexually propagated crops such as sugarcane, potato etc..

Merits
• Variety evolved by this method retains all the characters of the parental clones for several years.
• Varieties are highly uniform like pure lines. They are highly stable because there is no risk of deterioration due to segregation and recombination.
• Effective method for genetic improvement of asexually propagated crops.
• Useful in isolation the best genotype from a mixed population of asexually propagated crops.
• The selection scheme is useful for maintaining the purity of clone.

Demerits
• This selection method utilizes the variability already present in the population, and it has not been devised to generate variability.
• Genetic makeup cannot be improved by this method without hybridization.
• Varieties developed by clonal selection are highly prone to new of a disease.

Conclusion
• Asexual reproduction reduces the difficulty in hybrid production
• Helps in fixation of heterosis by preventing further segregation

About Author / Additional Info:
am M.Sc (Agri) in Genetics and Plant Breeding, Studding in C. P. Collage of Agriculture, Sardarkrushinagr Dantiwada Ahricultural University