Driving sex conversion for the genetic control of agricultural pests

Many insect species feed on and damage crop plants. Some flies for example lay their eggs in ripening fruit which are then decomposed by feeding larvae. This reduces agricultural production and can cause both empty stomachs and economic losses for farmers. The world's climate is changing allowing insects to invade new regions and reproduce faster. A growing human population puts a rising strain on the worlds agricultural systems while at the same time insects are becoming increasingly resistant to pesticide chemicals. These challenges are forcing us to envision novel ways in which we can sustainably control insects.

Genetic control of insects is a species-specific form of biological control where the release of modified insects reduces or eliminates the potential of wild insect populations to do harm. For example, the mass-release of sterilized males has been used to reduce the population size of many insect pests for many decades.
One genetic control strategy is the use of genetic elements called gene drives. These elements, introduced by a few modified insects into a population, can propagate and affect the genetic makeup of the entire pest population, for example reducing the fertility of females. Another approach is affecting the population's sex ratio and shifting it towards males. Since the number of females is important for the size of an insect pest population and since crop damage occurs solely through the activities of females, such approaches that interfere with female fertility and female development are very promising.

In this research project we are developing a novel control strategy that seeks to combine these two approaches for an even more powerful intervention. We will pioneer this technology in a well studied insect pest species Ceratitis capitata, the Mediterranean fruit fly (medfly) capable of damaging many fruit crops and a model insect for a whole range of related pest species. In the medfly true-sex conversion i.e. the transformation of females into fully fertile males is possible and, when effected by a gene drive that also reduces female fertility, is predicted to lead to the rapid elimination of pest populations. We will design and test the molecular building blocks for both gene drive and sex conversion in the medfly and combine them to demonstrate that caged populations of this pest species can be eliminated in the lab.

We intend to demonstrate the feasibility of a novel gene drive approach for the genetic control of Tephritid agricultural pests, using the medfly Ceratitis capitata, a major crop and fruit pest of significant economic importance. Sex-determination in Tephritids shows an unusual level of malleability and in the medfly the generation of XX fertile males and XY fertile females has been demonstrated. This suggests genetic control by way of true genetic sex conversion is possible in this pest species.
Modelling suggests that a CRISPR/Cas9 gene drive targeting female fertility, while simultaneously triggering sex-conversion by cleaving the transformer gene is a highly effective control strategy. This 'driving sex-conversion' (DSC) strategy triggers rapid population suppression compared to a range of related gene drive approaches, a highly attractive feature for genetic control in agriculture. Inducing female sterility and shifting the sex ratio towards males are two independent yet mutually enhancing approaches for population suppression and this strategy is also robust with regards to target site resistance. We will first complete the CRISPR/Cas9 toolset and establish the parameters for enabling efficient gene drive in the medfly. We will characterize female fertility targets for gene drive and test the proposed sex-conversion mechanism. Finally, we will combine the insights gained into a best-shot construct to demonstrate the power of this approach in eliminating caged medfly populations.