Engineered Underdominance Allows Efficient and Economical Introgression of Traits into Pest Populations

A novel form of underdominance is suggested as a mechanism that is able to drive desired genes into pest populations through the release of transgenic individuals over one or more generations. Such a mechanism is urgently needed by those working to reduce the impact of malaria by releasing strains of Anopheles, the vector of the disease, that are not susceptible to malaria parasites. We use simple population genetics models to quantify the benefits conferred when heterozygous genotypes, arising from matings between introduced and wild individuals, are not viable. In a randomly mating population, underdominant systems accelerate introgression of desired alleles and allow the release of individuals to be discontinued once the frequency of transgenic alleles attains a threshold. A set of two constructs, which together are selectively neutral but lethal when one is carried without the other, are found to produce dynamics that are characteristic of underdominant systems. When these constructs are carried on non-homologous chromosomes, then the ratio of released to natural born individuals need only be greater than 3:100 for introgression to occur. Furthermore, the threshold for the gene frequencies over which the introduced genes are expected to become fixed upon discontinuing the release of transgenic individuals is surprisingly low. The location of the threshold suggests that the introduced genes are expected to spread in space, at least locally. For the first time, the prospect of a practical drive mechanism for the genetic manipulation of pest populations is raised.