Course Teacher: Dr. Rupendra Kumar Jhade, JNKVV, College of Horticulture,Chhindwara(M.P.) B.Sc.(Hons) Horticulture- 1st Year, II Semester 2019-20 Course: Plant Propagation and Nursery Management Lecture No. 13 Topic: Chimera and Bud Sport

Chimeras: When a mutation occurs within a single cell of a clone, it initially produces an island of mutant cells and the plant becomes a mixture of two different genotypes and if the mutated tissues grow successfully, it results in variegation. Thus, the development of plants with two or more genetically different tissues growing separately but adjacent to each other, usually called as chimera. The term chimera was historically given to a certain unique clonal variation now known to have originated as graft chimera. Most chimeras usually originate from mutations either from spontaneous or induced in one of the cells of the shoot apex. However, chimeras may also result from hybridization. These tend to be unstable but their stability depends on their structure and genotype of the plant. Chances of Variability within a Clone The chimera may also result in the production of variegated foliage in the plants. Citrus, Vitis, Hydrangea, Dahlia, Coleus, Pelargonium, Sansevieria etc. are some examples of chimeras. In all these plants, plastids in some portions of leaves is unable to produce chlorophyll but is able to do so in other parts, resulting in the formation of variegated leaves. Further, mixture of sour and sweet type apple, fuzzy and smooth peaches and thornless blackberries are some other examples of chimeras. Similarly, occurrence of red colour in outer skin of apple and potato with mutated inner layers is also an example of chimeras. Depending on the distribution pattern of the mutated cells, the chimeras have been divided into the following four types: 1. 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, 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. In this chimera, a tissue with one genotype ocour comparatively as a thin layer around genetically different core (non-mutated tissue). This is a relatively stable form of chimera if propagated through stem or grafting. However, it is not stable if propagated by other methods, such as, root cuttings. 2. 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. This type is unstable and tends to change into periclinal chimeras, revert to the non- mutated form, or continue to produce mericlinal shoots. It occurs more frequently because it is the resultant of mutation of a single or a few cells. It is not a stable but with continued vegetative propagation it may either convert to periclinal chimera or to non-mutated tissue or a non-chimeral shoot For example, a lateral bud arising from a mericlinal chimera would become a periclinal chimera (shoot) and from a normal part to develop into a normal shoot Further, if propagation is done from the terminal growing point or from lateral shoots at the tip of mutated portion, the resultant plant will be mutated one. 3. 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. Sectorial chimeras may sometimes result in the development of variegated plants. The variegation in plants is caused by mutations on plasma genes, which affect the chlorophyll development. Due to this, some parts of the leaves produce green-white (albino) colour patterns. However, variegation may sometimes be caused by the attack of viruses. For example, colour breaks in some plants (tulips) are caused by viruses rather than by mutation. Likewise, some variations are caused by "pattern" genes that continue to reproduce that pattern.

4. Graft chimeras: Most chimeras occur naturally, however, these can be induced artificially too by grafting. 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.

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Course Teacher: Dr. Rupendra Kumar Jhade, JNKVV, College of Horticulture,Chhindwara(M.P.) Bud- sports or bud mutations

“When mutation occur and suddenly appear as a chance in the branch of a plant, is called bud sport or bud mutations”, because they appear to have originated within a single bud.  Detection of a new mutant within a clone may require a series of vegetative propagated generations and multiple propagations from many buds of the same plant.  Many ‘sports’ have become commercial cultivars. Mutations may affect fruit (colour, shape, time of maturity), tree structure (spur type), time of bloom, and as a host of other traits.  Sometimes these mutants have highly useful horticultural traits and have given rise to important new cultivars (‘Ruby Red’ grapefruit, red coloured sports of apple and pear.)  On the other hand, mutations may be undesirable and give rise to misshapen fruits, low production and susceptibility to diseases.

 Mutation: “Mutation is a single-step genetic change or sudden heritable change within cells of a clone. In general, these changes take place spontaneously, in the plants regular but rare intervals.”

Source:

1. Sharma RR & Manish Srivastav.2004. Plant Propagation and Nursery Management (First Edition 2004).International Book Distributing Co. Lucknow 226 004 U.P.(INDIA). 2. http://ecoursesonline.iasri.res.in/course/view.php?id=133