Trends in Parasitology
Volume 32, Issue 3, March 2016, Pages 219-229
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Review
Special Issue: Vectors
Control of Mosquito-Borne Infectious Diseases: Sex and Gene Drive

https://doi.org/10.1016/j.pt.2015.12.003Get rights and content

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Predominantly, male progeny may be obtained when mosquito sex determination is manipulated, either by knocking down the female-specific transcript of dsx in Aedes aegypti or by expressing an endonuclease that specifically destroys the X chromosome during spermatogenesis in Anopheles gambiae.

The first insect M factor, Nix, has been discovered in Ae. aegypti. Ectopic expression of Nix is sufficient to initiate male development in genetic females. Two candidate M factors have been discovered in Anopheles mosquitoes. M factors and other intermediary genes in the sex determination pathway provide new ways to promote ‘maleness,’ the ultimate disease-refractory trait in mosquitoes.

The CRISPR/Cas9-based gene drive system has been successfully demonstrated in Drosophila melanogaster, which could be developed in mosquitoes to self-propagate dominant-acting maleness genes.

Sterile male releases have successfully reduced local populations of the dengue vector, Aedes aegypti, but challenges remain in scale and in separating sexes before release. The recent discovery of the first mosquito male determining factor (M factor) will facilitate our understanding of the genetic programs that initiate sexual development in mosquitoes. Manipulation of the M factor and possible intermediary factors may result in female-to-male conversion or female killing, enabling efficient sex separation and effective reduction of target mosquito populations. Given recent breakthroughs in the development of CRISPR-Cas9 reagents as a source of gene drive, more advanced technologies at driving maleness, the ultimate disease refractory phenotype, become possible and may represent efficient and self-limiting methods to control mosquito populations.

Section snippets

Why Sex Matters

Anopheles mosquitoes are the primary vectors of malaria, one of the most deadly and costly diseases in human history. Aedes aegypti is a major vector for dengue, yellow fever, and chikungunya viruses. Over the past 40 years, the increase in the burden of dengue disease has been driven by many factors, primarily urbanization, globalization, and an inability to effectively control Ae. aegypti breeding [1]. More recently, similar factors have led to the large-scale emergence of viruses such as

Sex in Insects: Diversity and Model Species

Sex is critical to the survival and evolution of sexually reproducing organisms. Among different insect species, diverse chromosome systems are employed to determine the sex of an individual. Such diversity can manifest within the same order, family, or even more closely related taxonomic groups. In the order Diptera, for example, sex-determining chromosome systems encompass a wide spectrum including XX/XY in the Anopheles mosquitoes and in Drosophila fruit flies, ZW/ZZ and XX/XO in some

Sex as a Target for Vector Control

The sex determination pathway can provide novel targets for generating genetic sexing strains [35] for sex separation and for interference of the sex ratio to reduce mosquito populations. For example, knocking down the female isoform of dsx by RNAi in the larval stage resulted in female lethality [36]; alternative RNAi attempts resulted in reduced female fecundity [37]. If full sex conversion is to be achieved, it will at a minimum require manipulating both dsx and fru. Approaches that

Gene Drive: Is CRISPR/Cas9 Different?

The concept of driving genes into wild populations to control vector-borne diseases is not new [6], and has been reviewed extensively over the past few decades 7, 8, 9, 10. More recently, such strategies have caught the attention of other disciplines 39, 40, 41. However, engineering or harnessing chromosomal translocations [6], meiotic drive systems [42], transposable elements (TEs) 43, 44, maternal effect dominant embryonic arrest 45, 46, engineered underdominance 47, 48, and homing

Gene Editing and Drive with CRISPR/Cas9 in Mosquitoes

The rapid development of CRISPR-based editing reagents 72, 73, 74 introduced a new programmable nuclease that does not suffer from the problems of HEGs (difficult to engineer) or TALENs (poor repair substrates). Gantz et al. [52] recently described the successful CRISPR/Cas9-based drive of a 17-kbp synthetic construct in the Asian malaria vector An. stephensi. For several generations, males containing the gene drive construct produced almost all transgenic offspring, defying Mendelian

Engineering Requirements for Safe Drive: The Advantage of Targeting Maleness

Ten years ago, James [83] laid out a set of engineering challenges that would have to be overcome to realize the potential for using a gene drive approach to achieve fixation of a synthetic allele in a wild target population. These included challenges of genetic engineering (drive mechanism must be sufficient to overcome fitness costs, must work rapidly, be adaptable to many species, be reusable, and retain linkage to any cargo), epidemiology (gene drive system must not change the behavior of

Concluding Remarks

Mosquitoes are highly sexually dimorphic. For the major vectors of malaria and dengue there is also a clear separation between harmless, nectar-feeding males and deadly blood-feeding females. The identification of molecular switches and genetic programs that control the decision made in the early developing mosquito embryo to proceed as male or female may be used to improve existing sterile insect strategies for controlling mosquito-borne disease agents. Combined with the sudden prospects for

Disclaimer Statement

This work and its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIAID.

Questions related to the sex determination pathway in Aedes aegypti

Is NIX a splicing factor? What are the RNA targets of NIX? Does NIX directly regulate the splicing of the dsx/fru pre-mRNA, or does NIX function through other intermediates? Is there a tra-like gene in Ae. aegypti and if so is its processing regulated by NIX?

Questions related to the sex

Acknowledgments

This work was supported by the National Institute of Allergy and Infectious Diseases (NIAID) (AI113643, AI105575, and AI077680). We thank Brantley Hall for drafting Figure 1.

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