# The Perpetual Vector Mosquito Threat and Its Eco-Friendly Nemeses
## 4 Effective \"Green\" Technologies
### Release of Insect Carrying a Dominant Lethal Gene (RIDL)  
RIDL utilizes engineering techniques for the production and release of genetically modified mosquitoes that harbor genes that are lethal to their young offspring. The lethal gene is regulated by a molecular switch that is turned off during the mass production of the RIDL insects in insectaries. The switch utilizes a tetracycline-repressible expression system in which the tetracycline-repressible transcriptional activator protein (tTA) is placed under the regulation of a selected promoter that governs essential developmental specificities of the insect. The development-specific expression of tTA results in the tTA activator binding to a specific sequence, _tetO_, driving the expression of a lethal effector protein gene from a minimal promoter, and subsequently leading to the death of the host in which it is expressed. The system is easily manipulated when insects are reared in the presence of low levels of tetracycline, which disrupts the binding of tTA to _tetO_. Without tetracycline, RIDL adult males develop normally, and females die in the preimaginal stage of development. The selected lethal gene is turned on in the absence of tetracycline when the engineered males are released [175; 176; 177; 178; 179]. The RIDL adults then mate with their wild counterparts, and the resulting female offspring die in the larval and pupal stages when the lethal gene is expressed. Repeated mating in the field leads to a decline in females and, subsequently, a potential population crash. RIDL has been used successfully to control lepidopteran pests of agriculture and vector mosquitoes in cage studies and in the field [122; 178; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 190; 191]. RIDL OX513A--Success and Concerns  
As alluded to above, the use of \"self-limiting genetic technology\" to control vector mosquitoes, where used, has already proven to be successful. This is more specifically exemplified by the results of sustained releases of RIDL OX513A (Oxitech Ltd., Abingdon, UK). OX513A is a robust commercial fluorescent-tagged transgenic _Aedes aegypti_ that harbors a conditional lethal gene engineered to deliver a dominant non-sex-specific deadly effect on targeted natural populations of the species [186]. The release of the OX513A strain in 2010 resulted in an 80% suppression of native _Aedes aegypti_ in the Cayman Islands [187; 188]. Later, in 2012, the release of the strain in a suburb of Juazeiro, Bahia, Brazil, led to an 81% to 95% suppression of local _Aedes aegypti_ within the span of a year [189]. Despite these observations, there are reasonable concerns that lethality may not be complete, and genes can indeed be transferred from engineered strains to native populations that can then spread and be regionally established in successive generations. A study showing this to be true was conducted in Jacobina, Bahia, Brazil [190]. Approximately 450,000 OX513A males were released each week over a 27-month period. Samples were collected 6, 12, and 27-30 months after releases began, and the genotypes of 57 fluorescent larvae collected six months after the initial release that represented hybrid F1 were determined. As the OX513A and native Jacobina population were genotyped for >21,000 single nucleotide polymorphisms (SNPs), accurate genotype SNP assessments convincingly showed that viable hybrids are capable of reproducing in nature, at least within regional confines. Interestingly, OX513A was developed using a strain that originated in Cuba and outcrossed to a Mexican population. Therefore, extant _Aedes aegypti_ in Jacobina represents a mix of the three populations. It must be noted that transgene from OX513A was not found in the hybrids, and there is no evidence to suggest that the hybrids were more robust [192; 142]. Exactly how this affects the population ecology and acquisition and dissemination of vectored pathogens in the region remains to be determined.