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