The current lack of a functional GSS has led to the mosquito population suppression projects to use alternative ways in the sex-sorting process. In addition, several transgenic genetic sexing strains developed for different mosquito vector species are also of limited applied potential due to either lack of stability, low male performance or subject to extensive regulation. A genetic sexing strain based on the tolerance to dieldrin has been developed for Anopheles arabiensis however, this strain presents several problems and has limited potential for SIT applications. GSS strains that can be sex-sorted at early developmental stages (eggs or L1) are generally accepted to be the optimal solution for mass-scale SIT and related techniques. The successful use of genetic sexing strains (GSS) for the sex sorting of Ceratitis capitata and other fruit fly species has encouraged researchers to develop similar GSS strains for mosquitoes. Given that only the female mosquitoes bite and transmit the human pathogens, those methods must be capable of ensuring a predefined acceptable level of female contamination while maximizing the male pupae recovery. The scaling-up of these projects from pilot to operational has been hampered by several problems, the most significant one being the lack of an efficient sex-sorting method. Several small-scale projects have demonstrated the high potential of these strategies to suppress mosquito populations.
These techniques are usually referred to as genetic control methods and include, among others, the sterile insect technique (SIT), the incompatible insect technique (IIT) and the release of insects carrying a dominant lethal gene (RIDL). There is a global renewed interest in area-wide integrated mosquito management strategies based on the mass production and release of sterile males to suppress target populations. We also recommend a strategy whereby an acceptable level of purity is pre-selected and remains constant across the different batches of pupae while the performance varies from batch to batch to fit with the desired purity. Adaptive and precise thresholds will allow the sex-sorting of mixed batches in operational conditions maintaining the target purity at the cost of a reduction in performance. For new developments, we recommend using adaptive and precise threshold selection methods applied individually to each batch or to a mix of batches. The heterogeneity between batches does not strongly affect the quality of the sex sorting, as long as a specific separation threshold is not pre-set before the sorting process. The efficiency of the SSD-sorting methods can be improved by reducing the heterogeneity of pupae size within rearing containers. The conclusions of this analysis are applicable to all the existing SSD-sorting methods. The purity and performance of a sex sorting operation in the tested Aedes species are linked parameters whose relation can be modeled. Sex sorting of pupae based on size dimorphism can be achieved with a high performance (% males recovery) and a reasonably high purity (% males on the sorted sample) for the different Aedes species and strains. Rearing conditions have a strong influence in the performance of the SSD-sorting methods and non-standard rearing can lead to increase pupae size heterogeneity. The optimal sorting results are predicted for the highest values of SSD and lowest values of intra-batch variance. Two dimensionless parameters that measure the suitability for SSD-based sorting of a specific batch of pupae are provided. For a given population, each size threshold is linked to a specific outcome of male recovery. Resultsįor the three Aedes species, the distribution of the pupae size can be modeled by a mixture of two Gaussian distribution functions and the proposed model fitted the experimental data. We propose a general model for the analysis of the frequency distribution of mosquito pupae in the context of SSD-sorting methods, which is based on a Gaussian mixture distribution functions, thus making possible the analysis of performance (% males recovery) and purity (% males on the sorted sample). The frequency distribution of the pupal size was analyzed.
We applied an automated pupal size estimator developed by Grupo Tragsa with laboratory samples of Anopheles arabiensis, Aedes albopictus, Ae. The currently available sorting methods have not been developed based on biometric analysis, and there is therefore potential for improvement.
Currently, most of these programmes targeting Aedes mosquitoes rely on sorting methods based on the sexual size dimorphism (SSD) at the pupal stage. However, the lack of an efficient male selection method has hampered the expansion of these approaches into large-scale operational programmes. Several mosquito population suppression strategies based on the rearing and release of sterile males have provided promising results.