Molecular markers to assess genetic diversity

Molecular markers play an essential role today in all aspects of plant breeding, ranging from the identification of genes responsible for desired traits to the management of backcrossing programs. The emergence of marker systems has, for the last 30 years, closely tracked developments in biochemistry and molecular biology. Following the demonstration that retrotransposons are ubiquitous, active, and abundant in plant genomes, markers were developed that are based on the insertional polymorphism they create upon replication. They virtually all exploit the joint that is formed, during retrotransposon integration, between genomic DNA and the long terminal repeats at either end of retrotransposon. The various retrotransposon marker systems differ in the nature of the second primer used in the amplification reactions. All except one of these marker methods is dominant (RBIP is co-dominant). Hence, the availability of effective means to generate doubled haploid populations through gametophytic embryogenesis is valuable for the efficient use of these markers. Over the last eight years, retrotransposon-based markers have been developed for crop species and trees across the plant kingdom, as well as for fungi and insects. Many retrotransposons features make them appealing as the basis of molecular marker systems. They are usually dispersed throughout the genome and produce large genetic changes at the point of insertion that can be detected with family-specific amplification primers. The ancestral state of a retrotransposon insertion is known, and subsequent changes at the locus are not subject to homoplasy. Retrotransposon markers are useful in accelerating backcrossing programs, tagging genes of interest, tracking germplasm, verifying and producing pedigrees, and examining crop evolution.