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.
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.