Studies about genetic
diversity have been of great importance for the purposes of genetic improvement
and to evaluate the impact of human activity on biodiversity. They are equally
important in the understanding of the microevolutionary and macroevolutionary
mechanisms that act in the diversification of the species, involving population
studies, as well as in the optimization of the conservation of genetic
diversity. They are also fundamental in understanding how natural populations
are structured in time and space and the effects of anthropogenic activities on
this structure and, consequently, on their chances of survival and/or
extinction. This information provides an aid in finding the genetic losses
generated by the isolation of the populations and of the individuals, which
will be reflected in future generations, allowing for the establishment of
better strategies to increase and preserve species diversity and diversity within
the species.
Genetic resources are established by accesses that represent the genetic variability organized in a set of different materials called germplasm. They comprise the diversity of genetic material contained in old, obsolete, traditional, and modern varieties, wild relatives of the target species, wild species and primitive lines, which can be used in the present or in the future, for food, agriculture, and other purposes.
The most important causes of
loss of biodiversity and of genetic resources are: the destruction of habitats
and natural communities; genetic vulnerability; genetic erosion; and genetic
drift. The genetic diversity of the species is an important way to maintain the
natural capacity to respond to climatic changes and all types of biotic and
abiotic stress. In actuality, there exists great concern in evaluating
biodiversity, because of the marked loss of genetic diversity, mostly due to
the actions of man, replacing local varieties with modern varieties, hybrids,
and, most recently, clones, so that large expanses of area are occupied by one
or a few varieties or narrow genetic base.
The loss of this diversity
will probably decrease the organisms’ capacity to respond to environmental
changes and will eliminate potentially useful biological information, as the
genetic diversity of cultivated species and valuable biochemical compounds is
still unknown.
In this sense, an important
evaluation parameter is the fraction of intrapopulational and interpopulational
components of the total genetic variability of a given species. In species with
low genetic variability, the component interpopu lational variation can be
large, due to local adaptation or simply to divergence as a result of low
genetic flux (Frankham et al., 2003). Erosion constitutes the reduction of
genetic diversity, with loss of individual genes and of particular combinations
of genes (gene pools), like those manifested in locally adapted breeds. It has
been reported that only 15 to 30 superior plant species have been responsible
for 90% of food grown for human consumption. This situation reflects
considerable loss of genetic diversity in plants, which represents genetic
erosion over a long period of time. The principal cause of this genetic erosion
of the crops is also the replacement of local varieties with exotic species and
improved varieties. Thus, breeding programs – responsible for the development
of superior varieties based on modern agriculture – are considered the major
cause of genetic erosion. Genetic drift is one of the factors that alter gene
frequency in the form of dispersion, that is, it has a quantifiable magnitude,
but the direction of its action is unpredictable and can either increase or
decrease the frequency of a particular allele in the population. This is a stochastic
process, acting on the populations, modifying the frequency of those alleles,
and, consequently, contributing to the predominance of certain genotypic
combinations in the populations. Its effects are manifested more frequently in
populations with reduced
effective sizes.
The evaluation of genetic
diversity was originally carried out starting with phenotypic information
related to morphologic characteristics or agronomic performance. However, the
recent advances in molecular biology have opened new perspectives for research
in species conservation and for the study of populational biology. With the use
of molecular markers, it is possible to detect the existent variability
directly in the DNA. Thus, studies about diversity are also distinguished by
their goals, which may be targeted for genetic improvement, for evolutionary
associations and for the conservation and management of genetic material.
Currently, the analysis of
polymorphisms of DNA fragments has been one of the main tools in the study of
biodiversity, allowing, as inferred from special distribution patterns of
genetic diversity, to test even explicit hypotheses of historic biogeography
and define priority areas for conservation. The evaluation of molecular
polymorphisms in noncoding regions has provided important information about the
various levels of genetic diversity, intra × interpopulational and intra ×
interspecific. Accordingly, the microsatellites (SSRs), repetitions in tandem
of one to six nucleotides, are ideal tools, which are randomly distributed in
the genome of most eukaryotic organisms, because they present codominance, a
high degree of polymorphism, and are easily detected by PCR (Glowatzki-Mullis
et al., 1995).
In a review paper Silva and
Russo (2000) grouped the molecular techniques, applied to populational biology,
into four main categories:
1. The
first comprises problems related to the analysis of genetic variation within
individuals, covering areas such as heteroplasmy (variation evalu ated in the
mitochondrial DNA), evolution of multigene families, as well as problems
related to forensic medicine.
2. Included
in the second cluster were problems that involve the variation within
populations, such as the effect of endogamy and genetic bottlenecking in
hereditary variation, consanguinity, nepotism, social structure, and reproductive
success.
3. Clustered
into the third category were studies directly related to genetic variation
between populations, involving problems like bioinvasion and gene flow,
structuring of natural stock in populations of extractive exploitation, as well
as problems related to the taxonomic status of morphotypes or ecotypes.
4. Finally,
grouped into the fourth category were the problems that involve the genetic
variation above the species level, including the effects of the different life
cycles on the process of differentiation and isolation of the species, in the
hybridization, and in the establishment of hybrid zones, as well as the phylogenetic
reconstruction of the taxa.
It is worth mentioning that
the genetic material used in the works with molecular markers is, in the last
analysis, a sample of the original population. Consequently, the variability
contained within the sample will depend on the polymorphism and the genetic
structure existent in the population and on the way in which the sampling was
performed (Robinson, 1998). The polymorphism accessed, on the other hand,
depends in large part upon the molecular technique adopted. In whatever
situation, however, the basic presupposition is that the molecular markers
analyzed are inheritable, reproducible, and independent (Silva and Russo,
2000).
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