Assessing Plant Genetic Diversity by Molecular Tools

This paper is an overview of the diverse, predominantly molecular techniques, used in assessing plant genetic diversity. In recent years, there has been a significant increase in the application of molecular genetic methods for assessing the conservation and use of plant genetic resources. Molecular techniques have been applied in the analysis of specific genes, as well as to increase understanding of gene action, generate genetic maps and assist in the development of gene transfer technologies. Molecular techniques have also had critical roles in studies of phylogeny and species evolution, and have been applied to increase our understanding of the distribution and extent of genetic variation within and between species. These techniques are well established and their advantages as well as limitations have been realized and described in this work. Recently, a new class of advanced techniques has emerged, primarily derived from a combination of earlier, more basic techniques. Advanced marker techniques tend to amalgamate advantageous features of several basic techniques, in order to increase the sensitivity and resolution to detect genetic discontinuity and distinctiveness. Some of the advanced marker techniques utilize newer classes of DNA elements, such as retrotransposons, mitochondrial and chloroplast based microsatellites, thereby revealing genetic variation through increased genome coverage. Techniques such as RAPD and AFLP are also being applied to cDNA-based templates to study patterns of gene expression and uncover the genetic basis of biological responses. The most important and recent advances made in molecular marker techniques are discussed in this review, along with their applications, advantages and limitations applied to plant sciences.


Introduction
Understanding the molecular basis of the essential biological phenomena in plants is crucial for the effective conservation, management, and efficient utilization of plant genetic resources (PGR).In particular, an adequate knowledge of existing genetic diversity, where in plant population it is found and how to best utilize it, is of fundamental interest for basic science and applied aspects like the efficient management of crop genetic resources.The improvement of crop genetic resources is dependent on continuous infusions of wild relatives, traditional varieties and the use of modern breeding techniques.These processes all require an assessment of diversity at some level, to select resistant, highly productive varieties.
The assessment of genetic diversity within and between populations is routinely performed at the molecular level using various laboratory-based techniques such as allozyme or DNA analysis, which measure levels of variation directly.Genetic diversity may be also gauged using morphological, and biochemical characterization and evaluation: (i) Morphological characterization does not require expensive technology but large tracts of land are often required for these experiments, making it possibly more expensive than molecular assessment.These traits are often susceptible to phenotypic plasticity; conversely, this allows assessment of diversity in the presence of environmental variation.(ii) Biochemical analysis is based on the separation of proteins into specific banding patterns.It is a fast method which requires only small amounts of biological material.However, only a limited number of enzymes are available and thus, the resolution of diversity is limited.(iii) Molecular analyses comprise a large variety of DNA molecular markers, which can be employed for analysis of variation.Different markers have different genetic qualities (they can be dominant or co-dominant, can amplify anonymous or characterized loci, can contain expressed or non-expressed sequences, etc.).The concept of genetic markers is not a new one; in the nineteenth century, Gregor Mendel employed phenotype-based genetic markers in his experiments.Later, phenotype-based genetic markers for Drosophila melanogaster led to the founding of the theory of genetic linkage, occurring when particular genetic loci or alleles for genes are inherited jointly.The limitations of phenotype-based genetic markers led to the development of DNA-based markers, i.e., molecular markers.A molecular marker can be defined as a genomic locus, detected through probe or specific starters (primer) which, in virtue of its presence, distinguishes unequivocally the chromosomic trait which it represents as well as the flanking regions at the 3' and 5' extremity [1].
Molecular markers may or may not correlate with phenotypic expression of a genomic trait.They offer numerous advantages over conventional, phenotype-based alternatives as they are stable and detectable in all tissues regardless of growth, differentiation, development, or defense status of the cell.Additionally, they are not confounded by environmental, pleiotropic and epistatic effects.
An ideal molecular marker should possesses the following features: (1) be polymorphic and evenly distributed throughout the genome; (2) provide adequate resolution of genetic differences; (3) generate multiple, independent and reliable markers; (4) be simple, quick and inexpensive; (5) need small amounts of tissue and DNA samples; (6) link to distinct phenotypes; and, (7) require no prior information about the genome of an organism.Nevertheless, no molecular marker presents all the listed advantages.
The different methods of molecular assessment differ from each other with respect to important features such as genomic abundance, level of polymorphism detected, locus specificity, reproducibility, technical requirements and cost.Depending on the need, modifications in the techniques have been made, leading to a second generation of advanced molecular markers.
Genetic or DNA based marker techniques such as Restriction Fragment Length Polymorphism (RFLP), Random Amplified Polymorphic DNA (RAPD), Simple Sequence Repeats (SSR) and Amplified Fragment Length Polymorphism (AFLP) are now in common use for ecological, evolutionary, taxonomical, phylogenic and genetic studies of plant sciences.These techniques are well established and their advantages and limitations have been documented [2][3][4].
Recently, a new class of advanced techniques has emerged, primarily derived from combination of the earlier, more basic techniques.These advanced marker techniques combine advantageous aspects of several basic techniques.In particular, the newer methods incorporate modifications in the basic techniques, thereby increasing the sensitivity and resolution in detecting genetic discontinuity and distinctiveness.The advanced marker techniques also utilize newer classes of DNA elements such as retrotransposons, mitochondrial and chloroplast based microsatellites, allowing increased genome coverage.Techniques such as RAPD and AFLP are also being applied to cDNA-based templates (i.e., sequences of complementary DNA obtained by mRNA retrotranscription) to study patterns of gene expression and uncover the genetic basis of biological responses.The recent development of highthroughput sequencing technology provides the possibility of analysing high numbers of samples over smaller periods of time.The present review details the molecular techniques of genetic variability and their application to plant sciences.

Molecular Assessment of Genetic Diversity
Analyses of genetic diversity are usually based on assessing the diversity of an individual using either allozymes (i.e., variant forms of an enzyme that are coded for by different alleles at the same locus) or molecular markers, which tend to be selectively neutral.It has been argued that the rate of loss of diversity of these neutral markers will be higher that those which are associated with fitness.In order to verify this, Reed and Frankham [5] conducted a meta-analysis of fitness components in three or more populations and in which heterozygosity, and/or heritability, and/or population size were measured.Their findings, based on 34 datasets, concluded that heterozygosity, population size, and quantitative genetic variation, which are all used as indicators of fitness, were all positively correlated significantly with population fitness.
Genetic variability within a population can be assessed through: 1.The number (and percentage) of polymorphic genes in the population.
2. The number of alleles for each polymorphic gene.
Proteic methods, such as allozyme electrophoresis, and molecular methods, such as DNA analysis, directly measure genetic variation, giving a clear indication of the levels of genetic variation present in a species or population [7] without direct interference from environmental factors.However, they have the disadvantage of being relatively expensive, time consuming and require high levels of expertise and materials in analysis.Given below is an overview of the different types of markers used for assessing genetic diversity (adapted from Spooner et al. [8]).

Biochemical Markers
The use of biochemical markers involves the analysis of seed storage proteins and isozymes.This technique utilizes enzymatic functions and is a comparatively inexpensive yet powerful method of measuring allele frequencies for specific genes.
Allozymes, being allelic variants of enzymes, provide an estimate of gene and genotypic frequencies within and between populations.This information can be used to measure population subdivision, genetic diversity, gene flow, genetic structure of species, and comparisons among species out-crossing rates, population structure and population divergence, such as in the case of crop wild relatives [8][9][10][11][12][13].Major advantages of these types of markers consist in assessing co-dominance, absence of epistatic and pleiotrophic effects, ease of use, and low costs.Disadvantages of isozymes include: (i) there are only few isozyme systems per species (no more than 30) with correspondingly few markers; (ii) the number of polymorphic enzymatic systems available is limited and the enzymatic loci represent only a small and not random part of the genome (the expressed part) -therefore, the observed variability may be not representative of the entire genome; (iii) although these markers allow large numbers of samples to be analyzed, comparisons of samples from different species, loci, and laboratories are problematic, since they are affected by extraction methodology, plant tissue, and plant stage.

Molecular Markers
Molecular markers work by highlighting differences (polymorphisms) within a nucleic sequence between different individuals.These differences include insertions, deletions, translocations, duplications and point mutations.They do not, however, encompass the activity of specific genes.
In addition to being relatively impervious to environmental factor, molecular markers have the advantage of: (i) being applicable to any part of the genome (introns, exons and regulation regions); (ii) not possessing pleiotrophic or epistatic effects; (iii) being able to distinguish polymorphisms which not produce phenotypic variation and finally, (iv) being some of them co-dominant.
The different techniques employed are based either on restriction-hybridization of nucleic acids or techniques based on Polymerase Chain Reaction (PCR), or both (see Table 1 for a list of the main molecular analysis techniques).In addition, the different techniques can assess either multi-locus or single-locus markers.Multi-locus markers allow simultaneous analyses of several genomic loci, which are based on the amplification of casual chromosomic traits through oligonucleic primers with arbitrary sequences.These types of markers are also defined as dominant since it is possible to observe the presence or the absence of a band for any locus, but it is not possible to distinguish between heterozygote (a/-) conditions and homozygote for the same allele (a/a).By contrast, single-locus markers employ probes or primers specific to genomic loci, and are able to hybridize or amplify chromosome traits with well-known sequences.They are defined as co-dominant since they allow discrimination between homozygote and heterozygote loci.
Basic marker techniques can be classified into two categories: (1) Key: H = High; M= Medium; L = Low; Y = Yes; N= No

Restriction-Hybridization Techniques
Molecular markers based on restriction-hybridization techniques were employed relatively early in the field of plant studies and combined the use of restriction endonucleases and the hybridization method [14].Restriction endonucleases are bacterial enzymes able to cut DNA, identifying specific palindrome sequences and producing polynucleotidic fragments with variable dimensions.Any changes within sequences (i.e., point mutations), mutations between two sites (i.e., deletions and translocations), or mutations within the enzyme site, can generate variations in the length of restriction fragment obtained after enzymatic digestion.
RFLP and Variable Numbers of Tandem Repeats (VNTRs) markers are examples of molecular markers based on restriction-hybridization techniques.In RFLP, DNA polymorphism is detected by hybridizing a chemically-labelled DNA probe to a Southern blot of DNA digested by restriction endonucleases, resulting in differential DNA fragment profile.The RFLP markers are relatively highly polymorphic, codominantly inherited, highly replicable and allow the simultaneously screening of numerous samples.DNA blots can be analyzed repeatedly by stripping and reprobing (usually eight to ten times) with different RFLP probes.Nevertheless, this technique is not very widely used as it is time-consuming, involves expensive and radioactive/toxic reagents and requires large quantities of high quality genomic DNA.Moreover, the prerequisite of prior sequence information for probe construction contributes to the complexity of the methodology.These limitations led to the development of a new set of less technically complex methods known as PCR-based techniques.

Markers Based on Amplification Techniques (PCR-Derived)
The use of this kind of marker has been exponential, following the development by Mullis et al. [15] of the Polymerase Chain Reaction (PCR).This technique consists in the amplification of several discrete DNA products, deriving from regions of DNA which are flanked by regions of high homology with the primers.These regions must be close enough to one another to permit the elongation phase.
The use of random primers overcame the limitation of prior sequence knowledge for PCR analysis and being applicable to all organisms, facilitated the development of genetic markers for a variety of purposes.PCR-based techniques can further be subdivided into two subcategories: (1) arbitrarily primed PCR-based techniques or sequence non-specific techniques; and, (2) sequence targeted PCR-based techniques.Based on this, two different types of molecular markers have been developed: RAPD and AFLP.

Random Amplified Polymorphic DNA (RAPD)
RAPDs were the first PCR-based molecular markers to be employed in genetic variation analyses [16,17].RAPD markers are generated through the random amplification of genomic DNA using short primers (decamers), separation of the obtained fragments on agarose gel in the presence of ethidium bromide and finally, visualization under ultraviolet light.The use of short primers is necessary to increase the probability that, although the sequences are random, they are able to find homologous sequences suitable for annealing.DNA polymorphisms are then produced by "rearrangements or deletions at or between oligonucleotide primer binding sites in the genome" [17].As this approach requires no prior knowledge of the genome analyzed, it can be employed across species using universal primers.The major drawback of this method is that the profiling is dependent on reaction conditions which can vary between laboratories; even a difference of a degree in temperature is sufficient to produce different patterns.Additionally, as several discrete loci are amplified by each primer, profiles are not able to distinguish heterozygous from homozygous individuals [18].Arbitrarily Primed Polymerase Chain Reaction (AP-PCR) and DNA Amplification Fingerprinting (DAF) are independently developed methodologies, which are variants of RAPD.For AP-PCR [16], a single primer, 10-15 nucleotides long, is used and involves amplification for initial two PCR cycles at low stringency.Thereafter, the remaining cycles are carried out at higher stringency by increasing the annealing temperatures.

Amplified Fragment Length Polymorphism (AFLP)
To overcome the limitation of reproducibility associated with RAPD, AFLP technology was developed by the Dutch company, Keygene [19,20].This method is based on the combination of the main analysis techniques: digestion of DNA through restriction endonuclease enzymes and PCR technology.It can be considered an intermediate between RFLPs and RAPDs methodologies as it combines the power of RFLP with the flexibility of PCR-based technology.
The primer pairs used for AFLP usually produce 50-100 bands per assay.The number of amplicons per AFLP assay is a function of the number selective nucleotides in the AFLP primer combination, the selective nucleotide motif, GC content, and physical genome size and complexity.AFLP generates fingerprints of any DNA regardless of its source, and without any prior knowledge of DNA sequence.Most AFLP fragments correspond to unique positions on the genome and hence can be exploited as landmarks in genetic and physical mapping.The technique can be used to distinguish closely related individuals at the sub-species level [21] and can also map genes The origins of AFLP polymorphisms are multiple and can be due to: (i) mutations of the restriction site which create or delete a restriction site; (ii) mutations of sequences flanking the restriction site, and complementary to the extension of the selective primers, enabling possible primer annealing; (iii) insertions, duplications or deletions inside amplification fragments.These mutations can cause the appearance/disappearance of a fragment or the modification (increase or decrease) of an amplifiedrestricted fragment.

Sequence Specific PCR Based Markers
A different approach to arbitrary PCR amplification consists in the amplification of target regions of a genome through specific primers.With the advent of high-throughput sequencing technology, abundant information on DNA sequences for the genomes of many plant species has been generated [22][23][24].Expressed Sequence Tags (EST) of many crop species have been generated and thousands of sequences have been annotated as putative functional genes using powerful bioinformatics tools.ESTs are single-read sequences produced from partial sequencing of a bulk mRNA pool that has been reverse transcribed into cDNA.EST libraries provide a snapshot of the genes expressed in the tissue at the time of, and under the conditions in which, they were sampled [25].Despite these advantages, however, EST-SSRs are not without their drawbacks.One of the concerns with SSRs in general is the possibility of null alleles, which fail to amplify due to primer site variation, do not produce a visible amplicon.Because the cDNA from which ESTs are derived lack introns, another concern is that unrecognized intron splice sites could disrupt priming sites, resulting in failed amplification.Lastly, as EST-SSRs are located within genes, and thus more conserved across species, they may be less polymorphic than anonymous SSRs.Although the use of EST possesses these limitations, several features of EST sequence libraries make them a valuable resource for conservation and evolutionary genetics.ESTs are an inexpensive source for identifying gene-linked markers with higher levels of polymorphism, which can also be applied to closely related species in many cases [26][27][28].EST libraries are also a good starting point for developing tools to study gene expression such as microarrays or quantitative PCR assays [22].

Microsatellite-Based Marker Technique
Microsatellites or Simple Sequence Repeats (SSR) are sets repeated sequences found within eukaryotic genomes [29][30][31].These consist of sequences of repetitions, comprising basic short motifs generally between 2 and 6 base-pairs long.Polymorphisms associated with a specific locus are due to the variation in length of the microsatellite, which in turn depends on the number of repetitions of the basic motif.Variations in the number of tandemly repeated units are mainly due to strand slippage during DNA replication where the repeats allow matching via excision or addition of repeats [32].As slippage in replication is more likely than point mutations, microsatellite loci tend to be hypervariable.Microsatellite assays show extensive inter-individual length polymorphisms during PCR analysis of unique loci using discriminatory primers sets.
Microsatellites are highly popular genetic markers as they possess: co-dominant inheritance, high abundance, enormous extent of allelic diversity, ease of assessing SSR size variation through PCR with pairs of flanking primers and high reproducibility.However, the development of microsatellites requires extensive knowledge of DNA sequences, and sometimes they underestimate genetic structure measurements, hence they have been developed primarily for agricultural species, rather than wild species.Initial approaches were principally based on hybridization techniques, whilst more recent techniques are based on PCR [33].Major molecular markers based on assessment of variability generated by microsatellites sequences are: STMSs (Sequence Tagged Microsatellite Site), SSLPs (Simple Sequence Length Polymorphism), SNPs (Single Nucleotide Polymorphisms), SCARs (Sequence Characterized Amplified Region) and CAPS (Cleaved Amplified Polymorphic Sequences).

Single Nucleotide Polymorphisms (SNPs)
Single nucleotide variations in genome sequence of individuals of a population are known as SNPs.SNPs are the most abundant molecular markers in the genome.They are widely dispersed throughout genomes with a variable distribution among species.The SNPs are usually more prevalent in the noncoding regions of the genome.Within the coding regions, when an SNP is present, it can generate either non-synonymous mutations that result in an amino acid sequence change [34], or synonymous mutations that not alter the amino acid sequence.Synonymous changes can, however, modify mRNA splicing, resulting in phenotypic differences [35].Improvements in sequencing technology and an increase in the availability of the increasing number of EST sequences have made analysis of genetic variation possible directly at the DNA level.
The majority of SNP genotyping analyses are based on: allele-specific hybridization, oligonucleotide ligation, primer extension or invasive cleavage [36].Genotyping methods, including DNA chips, allele-specific PCR and primer extension approaches based on SNPs, are particularly attractive for their high data throughput and for their suitability for automation.They are used for a wide range of purposes, including rapid identification of crop cultivars and construction of ultra high-density genetic maps.

Markers Based on Other DNA than Genomic DNA
There are also other highly informative approaches used to study genetic variation based on organelle microsatellite sequences detection; in fact, due to their uniparental mode of transmission, chloroplast (cpDNA) and mitochondrial genomes (mtDNA) exhibit different patterns of genetic differentiation compared to nuclear alleles [37,38].Consequently, in addition to nuclear microsatellites, marker techniques based on chloroplast and mitochondrial microsatellites have also been developed.The cpDNA, maternally inherited in most plants, has proved to be a powerful tool for phylogenetic studies.Due to increasing numbers of recent examples of intraspecific variation observed in cpDNA, there is additional potential for within-species genetic variation analysis [39,40].CpDNA has been preserved well within the genome, and consequently has been employed widely for studying plant populations through the use of PCR-RFLP and PCR sequencing approaches [40].They are also employed in the detection of hybridization/introgression [41], in the analysis of genetic diversity [42] and in obtaining the phylogeography of plant populations [43,44].
Mitochondrial DNA in plants, in contrast, has been demonstrated to be an unsuitable tool for studying phylogenesis and genetic diversity, being quantitatively scarce.
At the nuclear level, another type of sequence employed largely for studying genetic diversity is ribosomal RNA (rRNA).Ribosomal RNA genes are placed on the specific chromosomal loci Nor, and organized in tandem repeats which can be repeated up to thousands of times.Since some regions of rRNA are well preserved in eukaryotes, it represents a very useful phylogenetic tool.Conversely, other regions such as the "Internal Transcriber Spacers" (ITS) are so variable that they can be used to analyze polymorphism at the intraspecific level.

Transposable Elements-Based Molecular Markers
Although transposon insertions can have deleterious effects on host genomes, transposons are considered important for adaptative evolution, and can be instrumental in acquiring novel traits [45][46][47][48][49]. Retrotransposons have so far received little attention in the assessment of genetic diversity, despite of their contribution to the genome structure, size, and variation [50].Additionally, their dispersion [51,52], ubiquity [53,54] and prevalence in plant genomes provide an excellent basis for the development of a set of marker systems, to be used alone or in combination with other markers, such as AFLPs and SSRs.Retrotransposon-based molecular analysis relies on amplification using a primer corresponding to the retrotransposon and a primer matching a section of the neighbouring genome.To this type of class of molecular markers belong: Sequence-Specific Amplified Polymorphism (S-SAP), Inter-Retrotransposon Amplified Polymorphism (IRAP), Retrotransposon-Microsatellite Amplified Polymorphism (REMAP), Retrotransposon-Based Amplified Polymorphism (RBIP) and finally, Transposable Display (TD).

RNA-Based Molecular Markers
Studies of mechanisms which control genetic expression are essential to better understand biological responses and developmental programming in organisms.PCR-based marker techniques such as cDNA-SSCP, cDNA-AFLP and RAP-PCR are used for differential RNA studies, using selective amplification of cDNA.

Real-Time PCR
Real-time polymerase chain reaction is a laboratory technique based on the polymerase chain reaction, amplifying and simultaneously quantifying a targeted DNA molecule [55].It enables both detection and quantification (as absolute number of copies or relative amount when normalized to DNA input or additional normalizing genes) of a specific sequence in a DNA sample.
The procedure follows the general principle of polymerase chain reaction; its key feature is that the amplified DNA is quantified as it accumulates in the reaction in real time after each amplification cycle.Two common methods of quantification are: (i) the use of fluorescent dyes that intercalate with double-stranded DNA, and (ii) modified DNA oligonucleotide probes that fluoresce when hybridized with a complementary DNA.The major advantage of this technique consists in its sensitivity and speed due to the system of detection (spectrophotometric respect to ethidium bromide) and the quick changes of temperature.Real-time PCR is, therefore, particularly suitable for molecular markers based on PCR amplifications.In fact, the number of conservation and phylogenetic studies are now increasingly using real-time PCR for assessment of genetic variation [56].

Diversity Arrays Technology (DArT)
DArT is a generic and cost-effective genotyping technology.It was developed to overcome some of the limitations of other molecular marker technologies such as RFLP, AFLP and SSR [57].DArT is an alternative method to time-consuming hybridisation-based techniques, typing simultaneously several thousand loci in a single assay.DArT is particularly suitable for genotyping polyploid species with large genomes, such as wheat.This technology generates whole-genome fingerprints by scoring the presence/absence of DNA fragments in genomic representations generated from samples of genomic DNA.DArT technology consists of several steps: (i) complexity reduction of DNA; (ii) library creation; (iii) the microarray of libraries onto glass slides; (iv) hybridisation of fluoro-labelled DNA onto slides; (v) scanning of slides for hybridisation signal and (vi) data extraction and analysis.DArT acts by reducing the complexity of a DNA sample to obtain a "representation" of that sample.The main method of complexity reduction used relies on a combination of restriction enzyme digestion and adapter ligation, followed by amplification.However, an infinite range of alternative methods can be used to prepare genomic representations for DArT analysis.DArT markers for a new species are discovered by screening a library of several thousand fragments from a genomic representation prepared from a pool of DNA samples that encompass the diversity of the species.The microarray platform makes the discovery process efficient because all markers on a particular DArT array are scored simultaneously.For each complexity reduction method, an independent collection of DArT markers can be assembled on a separate DArT array.The number of markers for a given species, therefore, is only dependent on: (i) the level of genetic variation within the species (or gene pool); and (ii) the number of complexity reduction methods screened.

New Generation of Sequencing Technology
The recent development "high throughput sequencing" technologies make DNA sequencing particularly important to conservation biology.These technologies have the potential to remove one of the major impediments to implementing genomic approaches in non-model organisms, including many of conservation relevance, i.e, the lack of extensive genomic sequence information.These technologies, in fact avoid the expense, complication, and biases associated with traditional clone-based sequencing by using direct amplification of DNA templates [58][59][60].The three pre-eminent technologies to be commercialized are 454 (Roche), Solexa (Illumina), and SOLiD (Applied Biosystems).The 454 sequencing is a pyrosequencing-based method that utilizes emulsion PCR to achieve high throughput, parallel sequencing [61].Solexa's sequencing-by-synthesis (SBS) approach is based on a simplified library construction method and reversible fluorescence termination chemistry in the sequencing reaction, which produces 35-bp reads [58].Supported oligonucleotide ligation and detection (SOLiD) sequencing has some features in common with the other two technologies but, unlike the other two technologies, uses ligation-based sequencing technology [62].These new approaches to DNA sequencing enable the generation of 0.1-4 gigabases of DNA sequence in one to seven days with reagent costs being between US$ 3,400 and 8,500.Due to the differences in fragment read lengths of sequencing, the target of each of these technologies is different: the shorter length and lower price per base of Solexa and SOLiD.This makes these approaches well suited to whole genome resequencing, where a novel genome sequence can be assembled and then compared to a reference sequence, that is, when the genome sequence of the species already exists.The 454 sequencing, on the other hand, with longer read lengths (soon to be upward of 400 bp per sequence) can also be used for obtaining the first glimpse of a species' genome or transcriptome.

Conclusions
Molecular markers represent a class of molecular tools that are particularly sensitive to new genome-based discoveries and technical advancements and are, therefore, subject to continuous evolution.Most molecular marker techniques are employed in the evaluatation of genetic diversity and constructruction of genetic and physical maps.Physical mapping of linked markers helps in relating genetic distances to physical distances.Correlating patterns of inheritance in a meiotic-mapping population to those of individually-mapped genetic markers has led to construction of genetic linkage maps by locating many monogenic and polygenic traits within specific regions of the plant genome.Greater and greater amounts of sequence data, genomic and cDNA libraries, and isolated chromosomes will be increasingly available with time.This information and material will be of major importance in the future due to the present rate of extinction and diversity reduction.For example, they could be used to draw genes coding for potentially useful traits.Data obtained through PCR analysis of DNA fragments from ancient DNA samples have shown evolutionary changes within the genepool over long time periods.The information available is now also key in devising suitable conservation strategies.It is highly unlikely, however, that these data and molecular sample collections can replace the germplasm conservation of whole organisms.Most agronomically important characters are coded by polygenes, and it would be virtually impossible, with our current knowledge state, to reconstitute all the implied gene blocks with their regulatory elements.
Molecular markers are used also to assess plant response to climate change, which is a major issue at a global level.Changes, such as rapid warming, have been seen to cause a decrease in the variability of those loci controlling physical responses to climate [63].Jump and Peñuelas [63] conducted a review of climatic factors correlated with microgeographical genetic differences, and the various molecular markers used for each study.They concluded that although phenotypic plasticity buffers against environmental changes over a plant's life cycle, it will weaken over time as climatic event become more extreme and over longer time spans.The assessment and maintenance of genetic diversity, through the use of molecular markers is crucial as it provides a repository of adaptability to environmental and other changes.

Table 1 .
non-PCR-based techniques or hybridization based techniques; and (2) PCR-based techniques.See Table 2 for a comparison of the most commonly used markers.Acronyms commonly used for different molecular markers.

Table 2 .
Comparison of different characteristics of most frequently used molecular markers techniques.