Plant Propogation and Nursery Management

Chimeras

• It is the most important kind of genetic variant within clones typified by various kinds of variation. The name chimera was historically given to certain unique clonal variants now known to have arisen as graft chimeras.
• Chimeras develop because of the unique architecture of the apical meristem and the strategic location of the mutation in a dividing cell near the apex of the apical meristem.
• The three important types of chimeras are based upon the distribution patterns of mutated and non-mutated cells (Fig.4.4).

Fig.4.4: Formation of different chimeras

• Buds arising at different positions on a sectorial chimera may produce shoots consisting of mutated and non-mutated cells, entirely of mutated cells or entirely of non- mutated cells or sectorial (rarely), mericlinal or periclinal chimeras as described herewith.
• Periclinal chimera: The mutated tissues occupy layers of cells that completely surround an inner core of non mutated tissue. For examples in many red colored fruit cultivars in apple (Plate 4.2), the red pigment is located only in the epidermal layers; whereas the cells of the inner tissue have alleles for green or yellow colour. Similarly, some black berry (Rubus spp.) cultivars are thorn less; because the cells making up the epidermis do not have this allele. Periclinal chimeras are relatively stable if propagated stem or grafting.

 

Plate 4.2 Periclinal chimera Plate 4.3 Mericlinal chimera

• Mericlinal chimera: This combination is similar to the periclinal expect that the cells carrying the mutant gene occupy only a part of the outer cell layer. In case of a red mutant on a yellow delicious fruit, the surface of the fruit may have longitudinal streaks or sectors of red on an otherwise yellow surface (Plate 4.3). This type is unstable and tends to change into periclinal chimeras, revert to the non-mutated form, or continue to produce mericlinal shoots.
• Sectorial chimera: The mutated cells in this combination occupy an entire sector of the stem including all layers of the shoot apex. Sectorial chimeras appear if the mutation occurs in roots and very early stages of embryos where the cells of the growing point do not occur in layers. In general, this type is unstable and tends to revert to mericlinal and periclinal chimeras.
• Graft chimera: In horticulture, a graft-chimera may arise in grafting at the point of contact between rootstock and scion and will have properties intermediate to those of its parents. A graft-chimera is not a true hybrid but a mixture of cells, each with the genotype of one of its parents is a chimera. Hence, the once widely used term "graft-hybrid" is not descriptive; it is now frowned upon. In practice graft-chimaeras are not noted for their stability and may easily revert back to one of the parents.