1. PLANT BIOTECHNOLOGY & GENETIC ENGINEERING (3 CREDIT HOURS)
LECTURE 6
MARKER ASSISTED SELECTION
MORPHOLOGICAL MARKERS
PROTEIN MARKERS
DNA MARKERS
RESTRICTION FRAGMENT LENGTH POLYMORPHISM
RANDOMLY AMPLIFIED POLYMORPHIC DNA
SINGLE SEQUENCE REPEATS
MOLECULAR GENETIC MAPS
THEIR APPLICATIONS IN PLANT BREEDING
ESTs
NEAR ISOGENIC LINES

2. MARKER ASSISTED SELECTION
Beneficial characters of crop plants can be located and selected with the help of some marker genes. This technique is called marker assisted selection. This is more frequent in the case of quantitative characters.
Three types of markers are usually used for selection:
Morphological Markers
Protein Markers
DNA Markers

3. MORPHOLOGICAL MARKERS
Some mutant characters of plants have been found to be linked with other easily observable morphological characters and this property has been utilized in constructing conventional linkage maps. Any character showing association with such morphological markers are supposed to be controlled by genes situated near their loci. The major limitation of morphological markers is their limited number.

4. PROTEIN MARKERS
Proteins are products of gene action. The product of a gene can be used as a marker for the presence of a gene. Different alleles of a gene may produce different proteins. Sometimes, different forms of a protein with same catalytic activity but with different molecular weight and electrophoretic properties may be produced by different alleles. Such enzymes are called isozymes. The difference in enzyme mobility is caused by point mutations resulting in amino acid substitution. The differences in banding patterns observed on electrophoresis can be used for comparison and selection. Isozyme marker alleles can be associated with other characters and selection is practiced. Easily assayable isozymes have been widely used for the characterization of germplasm. However, the availability of useful protein markers is a limitation.

5. DNA MARKERS
DNA is the genetic material. Each chromosome has about base pairs but only 10% of the genome is actively engaged in translation. The rest of the genetic material remains unnoticed in terms of character expression. The assessment of variation at the DNA level provides a chance to map the genome. Specific regions on the DNA molecule both in the coding as well as non-coding regions can be identified as markers. The assessment of DNA sequences polymorphism is the most attractive application of molecular biology for human welfare. DNA markers are such polymorphic sequences seen in different individuals.
A considerable part of the DNA shows highly repetitive sequences in the non-coding regions of DNA. The comparison of these regions can also provide valuable clues of genetic and evolutionary relationships. The DNA sequences can be cut with the help of restriction endonucleases, analyzed with the help of DNA probes and electrophoretic procedures. Accordingly, different DNA-based markers like RFLP, RAPD, SCAR and SSR have been developed.

6. RESTRICTION FRAGMENT LENGTH POLYMORPHISM
This is the technique of comparing the polymorphism of restriction fragments of DNA. A group of enzymes, known as restriction enzymes, is used in RFLP. Each restriction enzyme is capable of identifying a specific site of DNA, usually 4-8 bp in length, at which it cuts both the strands of DNA. The restriction site of one particular restriction enzyme is present at several regions in the genome of an organism. As a result, using an enzyme, the genome DNA can be cut into a number of fragments known as restriction fragments. The length of these fragments depends upon the difference between two adjacent restriction sites. Probing and electrophoresis of these fragments can give an idea of the variation at the level of restriction fragments. Each genotype has a fixed pattern of distribution of fragments for a given enzyme and probe. RFLP refers to this variation in length of the restriction fragments. More than one enzyme and probe can be used so that the variation in genotypes is assessed perfectly. Unique sequence probes are used more frequently so that only restriction fragments complimentary to them are identified and represented in the map. However, the technical complexity of this technique has led to the development of better techniques that are mostly PCR based.

7. RANDOMLY AMPLIFIED POLYMORPHIC DNA
RAPD is a PCR-based technique in which a single short oligoprimer is used to amplify random sequences from a complex DNA template such as a plant genome. The products are then separated by electrophoresis and visualized by UV illumination of ethidium bromide stained gels. Polymorphism of amplified sequences corresponds to genetic differences.

8. SIMPLE SEQUENCE REPEATS
Different types of repeated sequences of nitrogen bases have been observed in organisms. Such regions of redundant DNA are called satellites. Tandem repeats of about bp are called minisatellites or variable number of tandem repeats (VNTR). Repeats with 1-6 bp are called microsatellites or simple sequence repeats (SSR). DNA sequences containing SSRs can be amplified by PCR and SSR variants can be detected by gel electrophoresis. These can be used as molecular markers.

9. MOLECULAR GENETIC MAPS AND THEIR APPLICATIONS IN PLANT BREEDING
Molecular markers serve as landmarks to identify the phylogenetic and parental relationships of crop varieties. Genetic maps can be constructed based on the nature and location of DNA markers. The extent of recombination between the markers can be analyzed so that map distances can be calculated. A complete genetic map requires the coverage of all regions of all the chromosomes in the genome. This is efficiently carried out with the help of computer softwares like MAPMAKER.

10. MOLECULAR GENETIC MAPS AND THEIR APPLICATIONS IN PLANT BREEDING
Molecular markers are useful in constructing high density genetic maps of crops and also for the location of genes in relation to the markers used. Parallelism in gene order (gene synteny) and evolutionary relationships can be analyzed. Marker-assisted early generation selection of transgressive segregants can increase the speed of developing new varieties. Selective transfer of desirable genes from wild germplasm can be made easy, thus making introgression of genes easy. Mapping of quantitative trait loci (QTL) becomes easy with the help of molecular markers. Gene pyramiding (assembly of the polygenes responsible for a character) becomes easy with the help of molecular markers. DNA fingerprinting and characterization of crop varieties and germplasm can be used as a very effective tool to analyze the genetic variability of the crop genetic resources available.

11. EXPRESSED SEQUENCE TAGS
The reverse transcription of mRNAs present in any tissue sample yields a library of cDNAs that correspond to the genes expressed in the sample.
Sequencing a short segment (≈ 100 bases) at one or both ends of each cDNA gives enough information to identify the gene.
These short sequences (tags) are called Expressed Sequence Tags.

12. NEAR ISOGENIC LINES
These are used to study gene-environment interactions.
NILs are created by crossing a donor parent (e.g., wild parent possessing a specific trait of interest) to a recurrent parent (e.g., an elite cultivar).
The F1 hybrid is then backcrossed to the recurrent parent to produce a first backcross generation (BC1).
The BC1 is then repeatedly backcrossed to the recurrent parent for a number of generations (e.g., 7).
The final BC7 will contain practically all of the recurrent parent genome except for the small chromosomal region containing a gene or QTL of interest.
Homozygous F2 lines can be obtained by selfing the BC7 plant.
It should be noted that in order to produce a NIL containing a target gene, the gene has to be selected for during each round of backcrossing.

13. MAPS
Linkage = Genetic
LINKAGE MAPS
PHYSICAL MAPS
SEQUENCE MAPS

14. THE END