(a) What is clone ? What do you understand by clonal selection ? Describe the various steps involved in clonal selection. 15 (b)…

Part (a): Clonal Selection

A clone is a group of genetically identical individuals derived from a single parent plant through vegetative propagation. Since they arise from somatic cells, clones preserve the exact genetic constitution of the parent, maintaining heterozygosity if the parent was heterozygous, or homozygosity if the parent was homozygous. Clonal selection is the process of identifying and selecting superior individual plants (clones) from a mixed population of vegetatively propagated crops to improve yield, quality, or disease resistance.

The steps involved are:

1. Selection of Superior Clones: From a field of mixed clones (e.g., in potato or sugarcane), individual plants exhibiting superior traits like high yield, uniformity, or disease resistance are tagged.
2. Clonal Evaluation Trials: The selected clones are grown in replicated trials under controlled conditions to verify their performance against check varieties. This ensures the superiority is genetic and not due to environmental luck.
3. Multiplication and Release: Successful clones are multiplied vegetatively (via cuttings, tubers, or tissue culture) and released as improved varieties. This method is crucial for crops like potato, citrus, and sugarcane where sexual reproduction is difficult or undesirable.

Part (b): Centre of Origin and Law of Homologous Series

The Centre of Origin is the geographic region where a plant species first evolved and developed its distinctive properties. Nikolai Vavilov identified eight such centres, including the Hindustan Centre (India), which is the origin of rice (Oryza sativa), cotton, and Saccharum barberi (jaggery cane). Note that Saccharum officinarum (sugar cane) originated in New Guinea/Indo-Malaya. Understanding these centres helps in conserving indigenous crop diversity and finding wild relatives for breeding.

Vavilov’s Law of Homologous Series of Variation states that related species within a genus tend to exhibit similar variations in parallel. For example, if a trait like early maturity appears in one wheat species, similar variations are likely to exist in other wheat species. This law guides breeders to search for specific desirable traits in wild relatives of the same genus, accelerating crop improvement.

Part (c): Pollination Mechanisms

Self-pollination (Autogamy) occurs when pollen from the anther of a flower falls on the stigma of the same flower. Cross-pollination (Allogamy) involves pollen transfer from the anther of one flower to the stigma of another flower on a different plant. Facultative (Often) Cross-pollination occurs in crops that can self-pollinate but are frequently cross-pollinated by insects or wind, such as maize and sorghum.

Mechanisms Facilitating Self-Pollination:

1. Cleistogamy: Flowers do not open; pollination occurs within the bud (e.g., Lathyrus sativus).
2. Homogamy: Anthers and stigma mature simultaneously within the same flower, ensuring self-fertilization (e.g., rice, wheat).
3. Chasmogamy with Adaptations: In some chasmogamous flowers, anthers are positioned close to the stigma, or the stigma is receptive before anthesis (protogyny), facilitating selfing if no cross-pollinators are present.

Mechanisms Facilitating Cross-Pollination:

1. Dichogamy: Temporal separation of male and female maturity. Protandry (male first, e.g., cotton) or Protogyny (female first, e.g., Cucumis) prevents selfing.
2. Self-Incompatibility: A genetic mechanism where pollen from the same plant fails to fertilize the ovule, enforced by biochemical barriers in the style.
3. Male Sterility: Genetic or cytoplasmic factors render anthers sterile, ensuring the plant can only receive foreign pollen (used in hybrid seed production).
4. Floral Adaptations: Large, showy flowers with nectar attract insects (entomophily), while light, abundant pollen facilitates wind pollination (anemophily) in crops like maize.

Conclusion

Clonal selection, understanding centres of origin, and manipulating pollination mechanisms are foundational to crop improvement. Clonal selection preserves superior genotypes in vegetatively propagated crops. Vavilov’s concepts guide the exploration of genetic diversity in wild relatives. Finally, understanding pollination systems allows breeders to control mating systems, enabling the production of pure lines or hybrids, thereby enhancing agricultural productivity and food security.