Gene Duplication in Insecticide Resistance

Gene duplication is a widely observed phenomenon in all three kingdoms of life and is considered to be a major driving force in the evolution of genomes and organisms. Gene duplication refers to any duplication event of a region of DNA that contains genes, eventually giving rise to gene families. In a classical sense, gene duplication is considered to predate functional diversification. When a duplicated copy is generated, the surplus copy is released from the selective pressure that is posed by random mutations, which allows the rapid accumulation of mutations without deleterious consequences to the organism (Zhang, 2003). The accumulated mutations can increase the fitness of the organism or create a novel function, thereby playing a major role in evolution through functional divergence (Ohno, 1970; Taylor and Raes, 2004). Paralogous gene family members that share a common ancestor gene are generated from a duplication event, which is distinguished from the orthologous genes in different genomes that share a common ancestor as a result of a speciation event (Hurles, 2004). In another theory, instead one copy retains the original function after gene duplication, both of the two copies become to undergo complementary functional diversification, allowing the evolution of an organism over generations (Force et al., 1999). Whole genome duplication events are also common particularly in plant species having polyploidy genomes. Whole genome duplication has influenced the evolutionary path in many species. One example of extensive gene duplication is the gene amplification. Contrary to gene duplication, which is a doubling mechanism of one gene, gene amplification refers to the process by which the copy number of a particular gene is specifically increased to a greater extent compared to those of other genes, resulting in a dramatic increase in gene dosage. Gene amplification generally results from the repeated replication of a stretch of DNA in a specific region of a genome. Because gene amplification increases the copy number of a gene relatively quickly, it is commonly involved in gene expression control during the development of an organism. Increased copy numbers of a particular gene enables rapid production of a large amount of protein within a short period. The most common mechanism of gene duplication is homologous recombination by unequal crossing-over between short repeated sequences on homologous segments of chromosomes during meiosis. The replication slippage is also responsible for the duplication of small contiguous repeats of DNA. The possibility and frequency of gene duplication depend on the degree of repetitive sequence distribution between two homologous chromosomes. Detailed information on gene duplication mechanisms can be found elsewhere in this book.