TECHNIQUES FOR GENETIC DIVERSITY EVALUATION


Marcelino Perez de la Vega
Dpto. de Genetica. Universidad de Leon


During this course (Conservation and Use of Genetic 
Reosurces), among other questions a question will be arise 
repeteadly: what kind and amount of genetic variability 
should be preserved?. This question is intimately related 
to two other questions: How much variability is there in a 
species or population? and, how much of this variability 
is relevant?. This presentation is mainly focussed on the 
methods to evaluate genetic variability, and in particular 
on those biochemical or molecular methods to do so. 
Likewise, some evidence of the usefulness of the 
assessment of "molecular" variability will be given.

Genetic diversity within populations and within species 
determines the rates of adaptative evolution and the 
extent of response in traditional crop improvement. 
Natural and artificial selection choose among the variants 
that occur within populations, based om their adaptation 
to the inmediate environment or their fitting to the 
breeder's interest. The goal for crop improvement is to 
agronomically fix useful genetic variants within cultivars 
by selective breeding. Therefore, breeders, 
conservationists and evolutionists are concerned with the 
extent and quality of genetic variability.

The traditional approach to characterization and 
evaluation of populations involves morphological and 
agronomic description. Considered as a whole, numerous 
morphological data are difficult to comprehend in terms of 
patterns of variation in populations. For this reason, 
numerical taxonomic techniques are needed to simplify and 
handle these complex patterns of variation. Traits of 
agronomic interest such as vigor, disease resistance and 
cold tolerance, and so on, are usually under high 
genotype-environment interactions. Morphological 
evaluation of population variability may be supplemented 
and generally surpassed by a more direct study of the 
genome by means of the analysis of biochemical markers. 
These markers, in particular isozymes, have been extremely 
useful in improving our knowledge of the genetic 
composition of populations and for determining the 
magnitude of various evolutionary forces involved in 
molding the genetic architecture of plant populations. In 
the future, DNA polymorphism studies will be a further and 
definitive step in this knowledge. Biochemical markers are 
less affected, if any, by environmental factors and 
numerous data can be handled and statistically analyzed in 
terms of patterns of genetic variation, at least those 
traits such as isozyme and DNA polymorphisms whose genetic 
control is easily understood.

Several kinds of biochemical markers have been used for 
the characterization of plant populations. These markers 
can be grouped in three classes:

1) A heterogeneous pool of biochemical compounds including 
phenolics, alkaloids, cyanogens and non-protein amino-
acids, that can be designated as low molecular weight 
markers.

2) Proteins, including both isozymes and storage proteins.

3) DNA markers, including fragments of variable length 
obtained by digestion with restriction enzymes 
(restriction fragment length polymorphism, RFLP), DNA 
polymorphism shown by PCR, and base sequences.

In this presentation the techniques and the relevance of 
the first group of markers will only be briefly commented 
on. It will be devoted to describe and compare the 
usefulness and limitations of protein/isozyme 
electrophoretic techniques, still the most widely used 
technique in the estimation of genetic variability in 
plant species and populations, and the techniques to 
analyze DNA polymorphism whose use is increasing 
exponentially. In particular DNA fingerprinting by random 
amplified polymorphism DNA (RAPDs) is being rapidly 
introduced into the method for genetic variability 
assessment.


References


Helentjaris T, Burr B (eds.). Development and application 
of Molecular Markers to Problems in Plant Genetics. Cold 
Spring Harbor Laboratory Publ. Cold Spring Harbor, New 
York. 1989.


Innis MA, Gelfand DH, Sninsky JJ, White TJ (eds.). PCR 
Protocols. A guide to methods and Applications. Academic 
Press, San Diego, California, 1990.