Chimera (plant)
Encyclopedia
Chimeras in botany
are usually single organisms composed of two genetically different types of tissue. They occur in plant
s, on the same general basis as with animal chimeras
. However, unlike animal chimeras, both types of tissues may have originated from the same zygote
, and the difference is often due to mutation
during ordinary cell division
.
of ancient Greek mythology
, a legendary beast made up of parts of several different animals.
The term “chimera” is applied to plant genetic mosaics where mosaic
ism is persistent and cells with two or more different genotypes coexist in a meristem
. However, when describing mosaics in which cells with different genotypes are distributed all around the plant, the term can also be used. Chimeras form a large and heterogenous group of plants, and it is convenient to classify these in three different ways.
, by grafting
, by the layering of mixed populations of cells within callus tissue cultures, and by somatic hybridization through protoplast fusion
.
or genera
, which involve tissues from two genetically different parents, different cultivar
s or different species (which may belong to different genera). The tissues may be partially fused together following grafting
to form a single growing organism that preserves both types of tissue in a single shoot. Just as the constituent species are likely to differ in a wide range of features, so the behavior of their periclinal chimeras is like to be highly variable.
The first such known chimera was probably the Bizzaria
which is a confusion of the Florentine citron
and the sour orange. Perhaps the best-known example of a graft-chimera is Laburnocytisus 'Adamii'
, caused by a fusion of a Laburnum
and a broom
.
constitution. Occasionally chimeras arise from loss or gain of individual chromosomes or chromosome fragments owing to misdivision
. More commonly cytochimeras have simple multiple of the normal chromosome complement in the changed layer. There are various effects on cell size and growth characteristics.
. The majority of variegated-leaf chimeras are of this kind.
All plastid gene- and some nuclear gene-differential chimeras affect the color of the plasmids within the leaves, and these are grouped together as chlorophyll
chimeras, or preferably as variegated leaf chimeras. For most variegation, the mutation involved is the loss of the chloroplast
s in the mutated tissue, so that part of the plant tissue has no green pigment and no photosynthetic
ability. This mutated tissue is unable to survive on its own but is kept alive by its partnership with normal photosynthetic tissue. Sometimes chimeras are also found with layers differing in respect of both their nuclear and their plastid genes.
, these will not be true to type. All propagation
that preserves the variation has to be by cuttings or division. Some types of cuttings, such as root cuttings, will produce entirely new growing points, usually from the inner one of the two types of tissue, so that these cannot be used either.
is widely used to alter the traits of certain plants. Generally speaking, it involves recombinant DNA integration, either through traditional or non-traditional way, and transgenic plant recovery. Agrobacterium mediated transformation
involves callus generation, co-cultivation with agrobacterium and later on followed by plant regeneration. Tissued transformed with exogenous genes
could contain both transformed cells and non-transformed cells. In many plant transformation systems, a selection marker such as the herbicide
or antibiotics resistance genes
is utilized to distinguish the transformed ones with untransformed ones. Even so there is still high chance that transgenic plant became a chimera.
(1)The process of shoot organogenesis
starts form the multicellular origin.
(2)The endogenous tolerance leads to the ineffectiveness of the weak selective agents.
(3)A self-protection mechanisim (cross protection). Transformed cells serve as guards to protect the untransformed ones.
(4)The observable characteristic of transgenic cells may be a transient expression of the marker gene. Or it may due to the presence of agrobacterium cells.
and Green Fluorescent Protein
(GFP) are utilized in combination with plant selective markers (herbicide, antibody etc.) However, GUS expression depends on the plant development stage and GFP may be influenced by the green tissue autofluorescence. QRT-PCR
could be an alternative method for chimera detection.
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
are usually single organisms composed of two genetically different types of tissue. They occur in plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, on the same general basis as with animal chimeras
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...
. However, unlike animal chimeras, both types of tissues may have originated from the same zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
, and the difference is often due to mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
during ordinary cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
.
Derivation
The word chimera in these senses is a reference to the monstrous chimeraChimera (mythology)
The Chimera or Chimaera was, according to Greek mythology, a monstrous fire-breathing female creature of Lycia in Asia Minor, composed of the parts of multiple animals: upon the body of a lioness with a tail that ended in a snake's head, the head of a goat arose on her back at the center of her...
of ancient Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
, a legendary beast made up of parts of several different animals.
Types of plant chimera
Mosaic
Mosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral...
ism is persistent and cells with two or more different genotypes coexist in a meristem
Meristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....
. However, when describing mosaics in which cells with different genotypes are distributed all around the plant, the term can also be used. Chimeras form a large and heterogenous group of plants, and it is convenient to classify these in three different ways.
According to their origin
There are often authentic accounts of the origin of chimeras by spontaneous or induced mutations, by the sorting-out from variegated seedlings after plastid mutationMutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
, by grafting
Grafting
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together. This vascular joining is called inosculation...
, by the layering of mixed populations of cells within callus tissue cultures, and by somatic hybridization through protoplast fusion
Somatic fusion
Somatic fusion, also called protoplast fusion, is a type of genetic modification in plants by which two distinct species of plants are fused together to form a new hybrid plant with the characteristics of both, a somatic hybrid. Hybrids have been produced either between the different varieties of...
.
According to their structure
The distinction between sectorial, mericlinal and periclinal chimeras are widely used.According to their behavior
A distinction can be made between four kinds of chimeras owing to the magnitude of the differences between them and what might be expected from their behavior:Species of graft chimeras
These are produced by grafting together different speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
or genera
Genera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
, which involve tissues from two genetically different parents, different cultivar
Cultivar
A cultivar'Cultivar has two meanings as explained under Formal definition. When used in reference to a taxon, the word does not apply to an individual plant but to all those plants sharing the unique characteristics that define the cultivar. is a plant or group of plants selected for desirable...
s or different species (which may belong to different genera). The tissues may be partially fused together following grafting
Grafting
Grafting is a horticultural technique whereby tissues from one plant are inserted into those of another so that the two sets of vascular tissues may join together. This vascular joining is called inosculation...
to form a single growing organism that preserves both types of tissue in a single shoot. Just as the constituent species are likely to differ in a wide range of features, so the behavior of their periclinal chimeras is like to be highly variable.
The first such known chimera was probably the Bizzaria
Bizzaria
The Bizzaria of Florence, which is probably the first plant chimera obtained, is a graft between the florentine citron and sour orange. It produces branches of regular Florentine Citron including such leaves, and from the other side branches of sour orange. The most interesting part of it is the...
which is a confusion of the Florentine citron
Florentine citron
The citron of Florence is a very fragrant citrus fruit, which is named after its most known origin of cultivation.-History:It was first carefully described by Johann Christoph Volkamer in his Hesperides as well as by others....
and the sour orange. Perhaps the best-known example of a graft-chimera is Laburnocytisus 'Adamii'
+Laburnocytisus 'Adamii'
+Laburnocytisus 'Adamii' is a horticultural curiosity; a small tree which is a graft-chimaera between two species, a laburnum, Laburnum anagyroides, and a broom, Chamaecytisus purpureus +Laburnocytisus 'Adamii' (also known as Adam's laburnum or broom laburnum) is a horticultural curiosity; a...
, caused by a fusion of a Laburnum
Laburnum
Laburnum is a genus of two species of small trees in the subfamily Faboideae of the pea family Fabaceae, Laburnum anagyroides and L. alpinum . They are native to the mountains of southern Europe from France to the Balkan Peninsula...
and a broom
Broom (shrub)
Brooms are a group of evergreen, semi-evergreen, and deciduous shrubs in the subfamily Faboideae of the legume family Fabaceae, mainly in the three genera Chamaecytisus, Cytisus and Genista, but also in many other small genera . All genera in this group are from the tribe Genisteae...
.
Chromosomal chimeras
These are chimeras in which the layers differ in their chromosomeChromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
constitution. Occasionally chimeras arise from loss or gain of individual chromosomes or chromosome fragments owing to misdivision
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
. More commonly cytochimeras have simple multiple of the normal chromosome complement in the changed layer. There are various effects on cell size and growth characteristics.
Nuclear gene-differential chimeras
These chimeras arise by spontaneous or induced mutation of a nuclear gene to a dominant or recessive allele. As a rule one character is affected at a time in the leaf, flower, fruit, or other parts.Plastid gene-differential chimeras
These chimeras arise by spontaneous or induced mutation of a plastid gene, followed by the sorting-out of two kinds of plastid during vegetative growth. Alternatively, after selfing or nucleic acid thermodynamics, plastids may sort-out from a mixed egg or mixed zygote respectively. This type of chimera is recognized at the time of origin by the sorting-out pattern in the leaves. After sorting-out is complete, periclinal chimeras are distinguished from similar looking nuclear gene-differential chimeras by their non-mendelian inheritanceMendelian inheritance
Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...
. The majority of variegated-leaf chimeras are of this kind.
All plastid gene- and some nuclear gene-differential chimeras affect the color of the plasmids within the leaves, and these are grouped together as chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
chimeras, or preferably as variegated leaf chimeras. For most variegation, the mutation involved is the loss of the chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s in the mutated tissue, so that part of the plant tissue has no green pigment and no photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
ability. This mutated tissue is unable to survive on its own but is kept alive by its partnership with normal photosynthetic tissue. Sometimes chimeras are also found with layers differing in respect of both their nuclear and their plastid genes.
Propagation
Because chimeras have more than one type of genetic material, while they may produce viable offspring from seedSeed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
, these will not be true to type. All propagation
Plant propagation
Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants.-Sexual propagation :...
that preserves the variation has to be by cuttings or division. Some types of cuttings, such as root cuttings, will produce entirely new growing points, usually from the inner one of the two types of tissue, so that these cannot be used either.
Chimeras in plant transformation
Genetic engineeringGenetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...
is widely used to alter the traits of certain plants. Generally speaking, it involves recombinant DNA integration, either through traditional or non-traditional way, and transgenic plant recovery. Agrobacterium mediated transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
involves callus generation, co-cultivation with agrobacterium and later on followed by plant regeneration. Tissued transformed with exogenous genes
Transgenesis
thumb|300px|right|A diagram comparing the genetic changes achieved through conventional plant breeding, transgenesis and cisgenesisTransgenesis is the process of introducing an exogenous gene – called a transgene – into a living organism so that the organism will exhibit a new property and transmit...
could contain both transformed cells and non-transformed cells. In many plant transformation systems, a selection marker such as the herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
or antibiotics resistance genes
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...
is utilized to distinguish the transformed ones with untransformed ones. Even so there is still high chance that transgenic plant became a chimera.
Reasons
There are multiple reasons to explain the occurrence of plant chimera during plant recovery stage.(1)The process of shoot organogenesis
Organogenesis
In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero...
starts form the multicellular origin.
(2)The endogenous tolerance leads to the ineffectiveness of the weak selective agents.
(3)A self-protection mechanisim (cross protection). Transformed cells serve as guards to protect the untransformed ones.
(4)The observable characteristic of transgenic cells may be a transient expression of the marker gene. Or it may due to the presence of agrobacterium cells.
Detection
Untransformed cells should be easy to detect and remove to avoid chimeras. Because it’s extremely important to maintain the stable ability of the transgenic plants across different generations. Reporter genes such as GUSGUS reporter system
The GUS reporter system is a reporter gene system, particularly useful in plant molecular biology. Several kinds of GUS reporter gene assay are actually available, depending on the substrate used...
and Green Fluorescent Protein
Green fluorescent protein
The green fluorescent protein is a protein composed of 238 amino acid residues that exhibits bright green fluorescence when exposed to blue light. Although many other marine organisms have similar green fluorescent proteins, GFP traditionally refers to the protein first isolated from the...
(GFP) are utilized in combination with plant selective markers (herbicide, antibody etc.) However, GUS expression depends on the plant development stage and GFP may be influenced by the green tissue autofluorescence. QRT-PCR
Real-time polymerase chain reaction
In molecular biology, real-time polymerase chain reaction, also called quantitative real time polymerase chain reaction or kinetic polymerase chain reaction , is a laboratory technique based on the PCR, which is used to amplify and simultaneously quantify a targeted DNA molecule...
could be an alternative method for chimera detection.