Presented by: Hitesh V. JasaniPh.D. (Plant Molecular Biology and Biotechnology),

Principles of Plant Breeding

Asexual reproduction

Multiplication of plants without the fusion of male and female gametes is known asexual reproduction.

Asexual reproduction can occur eitherBy vegetative plant parts or By vegetative embryos which develop without sexual

fusion.

Asexual reproduction is of two types,1. Vegetative reproduction.

2. 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, subaerial stems, root and bulbils.

Underground stems:

The underground modifications of stem generally serve as strong organs and contain many buds. These buds develop into shoots and produce plant after rooting. Rhizome : Turmeric, Ginger Tuber : Potato Corm : Arvi, Bunda Bulb : Garlic, Onion

Rhizome : Turmeric (Curcuma domestica), Ginger(Zingiber officinale)

Bulb : Garlic (Allium sativum), Onion (A. cepa)

Corm :Bunda (Colocasia antiquorum), Arvi (Colocasia esculenta)

Tuber : Potato (Solanum tuberosum)

Sub-aerial Stems:

These modifications include runner, stolon, sucker etc.. Sub-aerial stems are used for the propagation of mint (Mentha sp.), date palm, strawberry, banana etc.. Runner of Mint

Sucker of Banana Stolon of Strawberry

Bulbils:o Bulbils are modified forms

of flowers that develop into plants directly without formation of seeds.

o These are vegetative bodies as their development does not involve fertilization and seed formation.

o They develop into plants when fall on the ground. Bulbils are found in garlic, lily etc..

Bulbils of garlic

Bulbils in tiger lily

Leaves:o This is not very common and It

is seen in plants such as Bryophyllum.

o It has fleshy leaves and adventitious buds are present at the margins of the leaves.

o These buds fall off and grow into new plants.

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.

Cutting:

Layering:

Grafting:

Significance

Vegetative reproduction has several advantages.

It leads to continuity of same genotype with great precision, because all the progeny have similar genotype and phenotype.

Useful in obtain large number of genetically identical individuals.

Promising genotype can be maintained.

It makes use of desirable bud mutations. Mutants can be directly released as varieties.

Apomixis

• Apomixis: Development of seed or embryo without fertilization (sexual fusion).

• 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.

Parthenogenesis: Development of embryo from egg cell without fertilization.o Haploid : Embryo develops from haploid egg cell.

Example - Nicotiana, Maize etc..o Diploid : Embryo develops from diploid egg cell.

Example - many grasses like Taraxacum etc..

Apogamy: Development of embryo either from synergids or antipodal cells.o Haploid : Embryo develops from haploid synergids or

antipodal cells.o Diploid : Embryo develops from diploid synergids or

antipodal cells. Example – Allium etc..

Apospory: First diploid cell of ovule laying outside the embryosac develops into another embryosac without reduction.

Then develops directly from the diploid egg cell without fertilization.

Examples- Parthenium, Crepis etc..

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

Adventive embryony occurs in Mango, Citrus etc..

The Ideal Apomictic System

1. All the progeny of plants should be apomictic so that progeny have the same genotype as the maternal parent.

2. The apomictic genotype should preferably be fully male fertile and self-incompatible, and reproduce via pseudogamy.

3. In case of diplospory, chromosomes should not pair or recombine during first meiotic division. which may give rise to variation among the progeny.

4. Apomixis should be dominant over sexual reproduction. Usually, apomixis is governed by two or more genes.

5. Expression of apomixis should be little affected by the environment.

Development of Apomictic Lines

Apomictic lines can be developed by the following three different approaches:

1. Gene Transfer from wild species:

Genes controlling apomixis can be transferred into a crop species from a related wild species, e.g., from Tripsacum dactyloides into maize, from Pennisetum orientate into pearlmillet.

2. Induced Mutations:

This approach aims at developing apomictic forms in normally sexually reproducing species by utilising induced or even spontaneous mutations.

These efforts have focused primarily on sorghum, where two mutant lines showing facultative apospory have been isolated.

3. Isolation of Apomictic Recombinants from Interspecific Crosses:

Sometimes apomictic recombinants can be recovered from segregating generations of crosses between two sexually reproducing species. For example, seed formation has been reported in the intergeneric hybrids between T. aestivum and Avena sativa, H. vulgare and T. aestivitm etc…

Role in plant breeding

Rapid production of pure lines

Apomixis is an effective means for rapid production of pureline.

Maintenance of superior genotypes

Apomixis is useful in maintaining the characteristics of mother plant from generation to generation.

Conservation of heterosis

In some cases, hybrid vigour may be conserved for many generation by using recurrent apomixis.

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 purelines. This greatly simplifies hybrid seed production.

Even such parents that flower at different times may be crossed in a greenhouse to obtain few hybrid seeds, which can be used to establish the new hybrid variety.

The nucleus seed of hybrid varieties can be conveniently maintained as hybrid varieties.

Problems in Utilization of Apomixis

• Apomixis is a very complicated phenomenon.

• 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..

Main feature of clones

1. Homologous constitution

The progeny of clone is genetically identical and have same genetic constitution. Thus clones are homologous. No genetic variation within a clone.

2. Heterozygosity

Asexually propagated crops are heterozygous so clone is also heterozygous. Progeny looks similar phenotypically but is heterozygous.

3. Wider adaptation

Clones are more adaptable to environmental variation due to high level of heterozygosity than pure line.

4. Vigorous growth

Clones have hybrid vigour which is conserved due to sexual reproduction. Most of the varieties of sugarcane and potato are hybrids

5. Source of variation

There are three sources of variation in a clone. Viz. bud mutations, mechanical mixture, and occasional sexual reproduction.

6. Segregation in F1When hybridization is done between different clones, segregation occurs in F1 generation. Each F1 plant is potentially a new variety, therefore, selection is practised in F1.

Procedure of Clonal selection

Few to several hundred to thousand superior

plants selected.

Mixture of clones

First Year

Second YearClones from selected Plants

i. Clones from the selected plants grown separately

ii. Inferior clones are rejected

Third YearPreliminary yield trial

i. Preliminary yield trial with standard checks.

ii. Selection for quality, disease resistance, etc..

Disease nurseries may be planted

iii. Few outstanding clones selected

Fourth - Sixth Year

Multilocation yield trials

i. Mutilocation yield trials with standard checks

ii. Best clone Identified for release as a new variety

Seventh Year

Seed multiplication

i. The best clone is released as a new variety

ii. Seed multiplication for distribution begins

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.

Achievements

In India, this method successfully used for developing new varieties in potato, sugarcane, banana, citrus and grapes.

Kufri Red and Kufri safed in potato.Ko 11, ko 22 and Neelam in mango.Co 541, CoS 510 etc.. In sugarcane.Pride monthan and High gate in banana.

References

• Essentials of Plant Breeding By Phundan Singh. 3rd Ed. pp: 52-55, 128-131.

• Plant Breeding By B. D. Singh. 6th Ed. pp: 48-50, 371-373.

• Plant Breeding By B. D. Singh. 9th Ed.pp: 55-57, 377-382.

• leavingbio.net/vegetativepropagation.htm• Vegetative Propagation | Tutorvista.com• Wikipedia, the free encyclopedia• Breeding methods of clonally propagated crops By

Maria Andrade et al., July 2009