**9.1 Working model of gene drive**

When a gene drive is inserted into the genome of an animal, the progeny inherits the drive on one chromosome and a normal gene from the other parent. During early development, the CRISPR component of the drive shears the other copy. The cut is subsequently repaired using the drive as a template, resulting in two copies of the change being passed down to the progeny [183]. By suppressing the fertility gene termed doublesex on the usage of drive and thereby crashing a population of caged *Anopheles gambiae* mosquitos, Crisanti and his colleagues were able to prevent female mosquitos from biting or laying eggs while the drive was in place [184].

In *C. capitata* [185] and *Aedes aegypti*, a bisex RIDL system containing a tetracycline-repressible positive feedback transactivator (tTA) was successfully constructed, which does not require a specific promoter derived from the target species, but rather a minimal promoter used in conjunction with oligomerised tetO, the binding sequence. Under tight conditions, particularly in the absence of tetracycline, tTA (transactivator and lethal effector) accumulates to deadly levels in both sexes of the

#### *Genetically Modified Crops and Their Impact on New Era of Agriculture DOI: http://dx.doi.org/10.5772/intechopen.105937*

transgenic insect. Females must be removed from a 'male-only' release programme using an independent approach based on an underlying molecular mechanism that is female-specific. Functional Tra protein is only produced by females as it is encoded by a splice variant exclusively produced in females leading to functional tTA protein only produced in females rendering the system female-specific. Fu et al. [185] integrated the first intron of the sex determining gene Cctra into the DNA sequence coding for tTA to provide a sex selective component to the positive feedback system. If this Cctra fragment is spliced in the same as it is in its native context, tTA production is only allowed when the intron is spliced in the female-specific form, as the continuous coding frame of tTA is only restored in this variant. As this intron's full splicing in its native gene is exclusive to females, tTA expression was only expected in transgenic females. The analysis of tTA transcription in transgenic *C. capitata* revealed a sex-specific pattern similar to the natural Cctra gene. The inserted intron spliced to produce three distinct tTA transcripts: one female-specific (F1) and two nonsexspecific (M1 and M2). The female-specific transcript was the only one that encoded a complete tTA. As a result, when grown under harsh conditions, all of the transgenic female progeny died as larvae or pupae [186]. Thus, the utilisation of gene drivers in agriculture may be primarily harnessed by implementing sterile insect techniques for successful insect pest management (**Table 8**).
