Author: SANDEEP KUMAR BANGARWA
- Chimera ( in genetics) is an organism or tissue that contains at least two different sets of DNA, most often originating from the fusion of as many different zygotes (fertilized eggs).
- A plant or plant part that is a mixture of two or more genetically different types of cells.
- A chimera may be a “graft hybrid,” a bud that in plant grafting appears at the junction of the scion and stock and contains tissues of both plants.
- In a chimera the components maintain their identity but are arranged in a definite pattern at the growing point.
- Chimeras arise when a cell undergoes mutation. This mutation may be spontaneous or it may be induced by irradiation or treatment with chemical mutagens. If the cell which mutates is located near the crest of the apical dome, then all other cells which are produced by division from it will also be the mutated type. The result will be cells of different genotypes growing adjacent in a plant tissue, the definition of a chimera.
- Chimeral plants may originate by grafting, spontaneous mutation, induced mutation, sorting-out from variegated seedlings, mixed callus cultures, or protoplast fusion.
- Colchicine has been widely used to induce cytochimeras of fruiting plants.
- A plant is said to be a chimera when cells of more than one genotype (genetic makeup) are found growing adjacent in the tissues of that plant.
- Plant chimaeras comprise cells of distinct genomes, whereas in genetic mosaics, cells of different genotypes derived from the same zygote.
- Variegated plants are perhaps the most common types of chimeras. The cells in a variegated leaf all originated in the apical meristem of the shoot, but some cells are characterized by the inability to synthesize chlorophyll. These appear white rather than green even though they are components of the same tissue system.
- Cytochimeras:- Different ploidy levels coexisting in shoot apical meristem. Cells with higher ploidy levels are larger.
- The pattern of cell division, frequency of cell division, and layered organization of the cells in the apex interact in determining the type of chimera which is produced and the stability of the pattern which results.
- Variegation:- Presence of distinct markings or different colors on a portion of the plant or the entire plant. Manifested as streaks, stripes, blotches, or differences between the leaf or petal margins and the leaf or petal mid-region .
- Variegated Chimeras:- Cells of one color are clonally related. Cell division planes regulate the patterns. Rate and duration of cell division determines the size and shape of the streak, stripe, or blotch.
- Chimeral plants can be categorized on the basis of the location and relative proportion of mutated to non-mutated cells in the apical meristem.
2. Mericlinal Chimeras
3. Sectorial Chimeras
1. Periclinal Chimeras
- One or more apical layers is genetically distinct from another apical layer.
- Periclinal chimeras are the most stable and can be vegetatively propagated.
- A mutation produces a periclinal chimera if the affected cell is positioned near the apical dome so that the cells produced by subsequent divisions form an entire layer of the mutated type.
- The resulting meristem contains one layer which is genetically different from the remainder of the meristem.
- Stability dependent on tunica-corpus arrangement.
- Green and white –most common.
- A segment of one or more layers is genetically different.
- Mericlinal chimeras are produced when the derivatives of the mutated cell do not entirely cover the apical dome.
- A mutated cell layer may be maintained on only one portion of the meristem giving rise to chimeral shoots or leaves which develop in that portion while those that differentiate on all other portions of the meristem are normal, nonchimeral shoots.
- Many mericlinal chimeras involve such a limited number of cells that only a small portion of one leaf may be affected.
- As was the case with periclinal chimeras, mericlinal chimeras are generally restricted to one cell layer.
- Revert to periclinal or non –chimeral.
- Often appear as sectorial.
- Size of segments varies.
- A segment of all apical cell layers is genetically different.
- Sectorial chimeras result from mutations which affect sections of the apical meristem, the altered genotype extending through all the cell layers.
- This chimeral type is unstable and can give rise to shoots and leaves which are not chimeras.
- Revert to mericlinal or periclinal.
- Occur at early embryonic stages.
- Both normal types and mutated types can be produced, depending upon the point on the apex from which the shoots differentiate.
Dermen, H. 1955. A 2-4-2 chimera of McIntosh apple. J. Wash. Acad. of Sci. 45:324-327.
2. Dermen, H. 1960. Nature of Plant Sports. The Horticultural Magazine, July 1960, pp. 123 173.
3. Esau, K. 1965. Plant Anatomy. Second Edition, John Wiley & Sons, Inc., New York, pp. 89 112.
4. Stewart, R.N. and H. Dermen. 1979. Ontogeny in monocotyledons as revealed by studies of the developmental anatomy of periclinal chloroplast chimeras. Amer. J. Bot. 66, pp. 47-58.
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