Plant Breeding 25 Jun 2026 · 6 views · By Shrikant Sir

PB Ch 29. Clonal Selection

Characteristics of Asexually Propagated Crops

Majority are perennials (sugarcane, fruit trees). Annual vegetatively propagated crops are mostly tuber crops (potato, cassava, sweet potato).
Many show reduced flowering and seed set — cannot easily be improved by sexual hybridization.
They are invariably cross-pollinated — highly heterozygous.
Show SEVERE inbreeding depression upon selfing.
Majority are polyploids: sugarcane (8x-12x), potato (4x=2n=48), sweet potato (6x=2n=90).
Many species are interspecific hybrids: banana (Musa acuminata x M. balbisiana), sugarcane (Saccharum officinarum x S. spontaneum complex).

What is a Clone?

Clone: A group of plants produced from a single plant through asexual reproduction.

Characteristics of a clone:

All individuals within a single clone are genetically identical.
Phenotypic variation within a clone is due to environment only.
Clones are highly heterozygous — unlike pure lines.
Theoretically immortal — can be propagated indefinitely without genetic change.
Clones may degenerate due to viral/bacterial infection and somatic mutations.

Sources of Variation for Clonal Selection

Local varieties — existing material in cultivation
Introduced material
Hybrids (from sexual crosses between clones)
Segregating populations (from sexual crosses)

10.4 Procedure of Clonal Selection — Year by Year

Year 1: From a mixed variable population, a few hundred to a few thousand desirable plants are selected. Rigid selection for simply inherited characters with high heritability. Plants with obvious weaknesses eliminated. Each selected plant becomes the source of one clone.

Year 2: Clones from selected plants grown separately, generally WITHOUT replication (because of limited supply of propagating material for each clone, and large number of clones involved). Characteristics of clones become more distinct than in the previous generation. Inferior clones eliminated by breeder's judgement. 50-100 clones selected on basis of clonal characteristics.

Year 3: Replicated PRELIMINARY YIELD TRIAL with a suitable check included. A few superior clones with desirable characteristics selected for multilocation trials. Quality evaluation and disease resistance tests conducted in separate nurseries.

Years 4-8: REPLICATED YIELD TRIALS at several locations along with suitable checks. Yielding ability, quality, and disease resistance rigidly evaluated. Best clones superior to the check in one or more characteristics identified for release.

Year 9: Superior clones multiplied and RELEASED as varieties.

Bud Selection

A specialised form of clonal selection.
In perennial crops (fruit trees) and in those crops where flowering does not take place easily, somatic (bud) mutations may produce desirable phenotypic changes.
A mutant bud with a superior character (better fruit size, colour, taste, disease resistance) is propagated vegetatively to establish a new clone.
The process of identifying and multiplying such mutant buds is called bud selection.
Requires large numbers of plants to be observed.
Requires trained persons who can detect the mutant buds among millions of normal buds.
Practised in commercial plantations of fruit trees.

Clonal Degeneration

The loss in vigour and productivity of clones with time is known as clonal degeneration.

Causes:

Somatic mutation: Accumulation of spontaneous mutations over successive vegetative generations. Frequency low (10^-5 to 10^-7) but accumulates over time.
Viral diseases: The most important cause of clonal degeneration in practice. Viruses accumulate progressively in vegetatively propagated material over generations. Examples: Potato Virus Y (PVY), Potato Leaf Roll Virus (PLRV) in potato; Sugarcane Mosaic Virus (SMV) in sugarcane; Banana Bunchy Top Virus (BBTV) in banana.
Bacterial diseases: Phytoplasmas and other systemic bacterial pathogens can accumulate in vegetatively propagated material.

Solution for viral degeneration:

Meristem culture for producing virus-free planting material.
Apical meristems are free from systemic viruses (viruses spread through vascular system which has not yet reached the meristem).
Used commercially for potato, banana, sugarcane, cassava, strawberry.

Merits and Demerits of Clonal Selection

Merits	Demerits
Varieties are stable and easy to maintain — no genetic change upon propagation	Utilises only natural variability already present in the population — selection cannot create new variation
Avoids inbreeding depression — clonal crops are highly heterozygous; clonal propagation maintains this	Sexual reproduction is necessary for creation of variability through hybridization — two separate operations (sexual cross + clonal selection)
Combined with hybridization — sexual crosses generate variability; clonal selection fixes desirable genotypes	Applicable only to vegetatively propagated crops
Only method to directly improve vegetatively propagated crops without disrupting their genotype	Clonal degeneration — virus accumulation requires periodic replacement with meristem-derived virus-free material
Hybrid vigour is easily utilised — heterozygous genotype maintained indefinitely through clonal propagation	Large initial number of clones makes early stages labour-intensive
In asexually propagated crops, a heterozygous genotype with high performance can be directly used as a variety	Perennial life cycle and reduced fertility in some crops make sexual hybridization difficult

Comparison: Clone vs Pure Line vs Inbred

Feature	Clone	Pure Line	Inbred Line
Mode of pollination in crop	Cross-pollination	Self-pollination	Cross-pollination
Reproduction method	Asexual (vegetative)	Sexual (self-pollination)	Sexual (artificial self-pollination)
Genetic make-up in natural population	Heterozygous	Homozygous	Heterozygous
How obtained	Asexual reproduction from single plant	Natural self-pollination from single homozygous plant	Artificial selfing + selection for 6-7 generations
How maintained	Asexual reproduction	Natural self-pollination	Artificial selfing / close inbreeding
Plants within entity are genotypically	Identical	Identical	Almost identical (not completely)
Used directly as a variety?	YES — clone is the commercial variety	YES — pure line is the commercial variety	NO — inbreds used to develop hybrids or synthetic varieties
Genetic make-up within variety	Heterozygous (all same heterozygous genotype)	Homozygous	Almost homozygous

Achievements of Clonal Selection

Through clonal selection:
- Potato: Kufri Red (from Darjeeling Red Round), Kufri Safed (from Phulwa).
- Banana: Bombay Green (bud selection from Dwarf Cavendish), Pidi Monthan (from Monthan).

Through hybridization + clonal selection:
- Potato: Kufri Alankar, Kufri Kuber, Kufri Sindhuri, Kufri Kundan, Kufri Chamatkar, Kufri Jyothi (late blight resistant), Kufri Sheetman (frost resistant).
- Sugarcane: Co 1148, Co 1158, CoS 510, Co 975, CoS 109, Co 541.
- Mango: Pedda Neelam, Chinna Suwarnarekha.
- Citrus: Robertson Navel Orange, Yuvaraj Blood Red (sweet orange).
- Turmeric: Kesari, Kasturi.

IFoS 2022 (Q4b, 15M) — Explain the methods for creating variability. How are the types of selection responsible for population improvement in crop plants?

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PB Ch 30. Mutation Breeding →