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

(a) Clone and clonal selection: steps involved

A clone is a group of genetically identical individuals derived from a single plant by vegetative propagation (or apomixis), so each member keeps the exact genotype of the parent, including its heterozygosity. Clonal selection is the selection of superior clones from a variable population of vegetatively propagated crops such as potato, sugarcane, banana, citrus and grapes. It exploits both additive and non-additive variance and fixes the genotype at once, so the clonal fidelity of the selected plant is retained.

Steps:

First, a genetically variable base population is assembled, from germplasm, existing varieties, seedlings of sexual crosses or mutants.

Second, individual plants showing superior yield, quality and disease resistance are selected.

Third, each selected plant is propagated vegetatively to give a clone, and clones are compared in a preliminary non-replicated trial.

Fourth, promising clones enter replicated yield trials against a check variety, to separate genetic merit from environment.

Fifth, the best clones are tested at several locations and seasons, and for disease and quality.

Sixth, the superior clone is multiplied rapidly, with virus-free planting material, and released as a variety.

(b) Centre of origin and the law of homologous series of variation

A centre of origin, as defined by N.I. Vavilov, is the geographic region where a crop was first domesticated and where its genetic diversity, including dominant alleles and wild relatives, is greatest. Vavilov identified eight main centres, including the Hindustan centre, with its Indo-Malayan sub-centre, which is the origin of rice, chickpea, pigeonpea, cucumber, mango, brinjal, cotton (Gossypium arboreum) and Saccharum barberi; sugarcane Saccharum officinarum originated in New Guinea. A centre of primary diversity is distinguished from secondary centres, where diversity developed after spread. Such centres guide collection and conservation of germplasm and wild relatives.

The law of homologous series of variation (Vavilov, 1920) states that genetically related species and genera show similar series of heritable variations, so that a trait known in one species can be expected in related ones. Thus awned and awnless spikes, hairy and glabrous glumes, and different grain colours recur across wheat, barley, rye and oat, just as the periodic table lets one predict missing elements. Breeders use it to look for desired traits in related species and genera.

(c) Self-pollination, cross-pollination and often cross-pollination, and their facilitating mechanisms

Self-pollination (autogamy) is the transfer of pollen to the stigma of the same flower or plant, as in wheat, rice, barley, groundnut and chickpea (over 95% selfing). Cross-pollination (allogamy) is the transfer of pollen to a flower on a different plant, as in maize, rye, pearl millet, sunflower and Brassica campestris (over 95% crossing). Often cross-pollinated crops, such as sorghum, cotton, pigeonpea and safflower, show a variable 5-30% outcrossing, depending on environment and insects.

Mechanisms promoting self-pollination: bisexual flowers; cleistogamy, where pollination occurs in the closed flower, as in some wheat and barley and in Viola; homogamy, where anthers and stigma mature together; anthers placed close to or around the stigma, as in rice; and flowers opening after pollination.

Mechanisms promoting cross-pollination: dioecy, with male and female flowers on separate plants (papaya, date palm); monoecy, with separate unisexual flowers on one plant (maize, castor, cucurbits); dichogamy, where maturity of anthers and stigma is separated in time, as protandry (maize, sunflower) and protogyny (pearl millet); herkogamy, a physical barrier between anthers and stigma; heterostyly (Primula); self-incompatibility; male sterility; and floral features suiting insects or wind, such as nectar, showy petals and light abundant pollen.