Part 1. Genetic diversity
Genetic diversity usually refers to the variation, or differences between organisms at the DNA sequence level. This can be affected by natural or artificial (i.e. human) selection, mutation, recombination and other mechanisms. Genetic diversity can be considered in many different ways. Figure 1 shows the electrophoretic separation of a number of maize SSR markers all in one gel. Each lane represents a different maize line and the banding patterns identify allelic differences at each of the SSR loci.
Crop improvement is predicated on identifying new alleles and introgressing them into breeding lines, particularly in those crops that have low levels of genetic diversity. Shuffling alleles by crossing among very genetically similar lines cannot produce continued improvements over the long term.
Many disease resistance genes now in our current cultivars were introgressed from wild relatives. For example, most cultivated tomato varieties contain nematode resistance genes introgressed from wild relatives of tomato. Figure 2 shows Solanum peruvianum, a wild relative of tomato that has been used as a source of disease resistance genes. Notice the small fruit size, compared to what we are used to eating, as well as the green color. The fruit of this species is inedible but backcrossing can be used to combine traits of interest with the large, red, tasty tomatoes we like to eat.
In addition to the importance of genetic diversity to crop improvement in general, it is very important to the use of molecular markers - we need to see differences between alleles to be able to map genes or do any studies involving markers. Look back at the gel image in Figure 1. It is only because there is some difference – polymorphism – between the plant lines which makes any analysis based on markers possible (mapping, diversity studies, etc.). The differences in band mobility through the gels (distances traveled) reflects differences in the DNA sequence of the amplicons produced from the five templates.
When describing genetic diversity, the term “polymorphism” might be used. The word polymorphism literally means ‘presence of many forms’. In genetic terms, it refers to the coexistence of two or more alternative phenotypes in a population or among populations. In general, these diverse phenotypes are caused by alternative alleles of one gene/locus. An example of this would be at the molecular level, polymorphism refers to the coexistence of alternative banding patterns or DNA variants when revealed by a given detection method, such as that shown on the gel diagram in Figure 1.