Knockout of Anopheles stephensi immune gene LRIM1 by CRISPR-Cas9 reveals its unexpected role in reproduction and vector competence.

PfSPZ Vaccine against malaria is composed of Plasmodium falciparum (Pf) sporozoites (SPZ) manufactured using aseptically reared Anopheles stephensi mosquitoes. Immune response genes of Anopheles mosquitoes such as Leucin-Rich protein (LRIM1), inhibit Plasmodium SPZ development (sporogony) in mosquitoes by supporting melanization and phagocytosis of ookinetes. With the aim of increasing PfSPZ infection intensities, we generated an A. stephensi LRIM1 knockout line, Δaslrim1, by embryonic genome editing using CRISPR-Cas9. Δaslrim1 mosquitoes had a significantly increased midgut bacterial load and an altered microbiome composition, including elimination of commensal acetic acid bacteria. The alterations in the microbiome caused increased mosquito mortality and unexpectedly, significantly reduced sporogony. The survival rate of Δaslrim1 mosquitoes and their ability to support PfSPZ development, were partially restored by antibiotic treatment of the mosquitoes, and fully restored to baseline when Δaslrim1 mosquitoes were produced aseptically. Deletion of LRIM1 also affected reproductive capacity: oviposition, fecundity and male fertility were significantly compromised. Attenuation in fecundity was not associated with the altered microbiome. This work demonstrates that LRIM1's regulation of the microbiome has a major impact on vector competence and longevity of A. stephensi. Additionally, LRIM1 deletion identified an unexpected role for this gene in fecundity and reduction of sperm transfer by males.

Gal4-based enhancer-trapping in the malaria mosquito Anopheles stephensi.

Transposon-based forward and reverse genetic technologies will contribute greatly to ongoing efforts to study mosquito functional genomics. A piggyBac transposon-based enhancer-trap system was developed that functions efficiently in the human malaria vector, Anopheles stephensi. The system consists of six transgenic lines of Anopheles stephensi, each with a single piggyBac-Gal4 element in a unique genomic location; six lines with a single piggyBac-UAStdTomato element; and two lines, each with a single Minos element containing the piggyBac-transposase gene under the regulatory control of the hsp70 promoter from Drosophila melanogaster. Enhancer detection depended upon the efficient remobilization of piggyBac-Gal4 transposons, which contain the yeast transcription factor gene Gal4 under the regulatory control of a basal promoter. Gal4 expression was detected through the expression of the fluorescent protein gene tdTomato under the regulatory control of a promoter with Gal4-binding UAS elements. From five genetic screens for larval- and adult-specific enhancers, 314 progeny were recovered from 24,250 total progeny (1.3%) with unique patterns of tdTomato expression arising from the influence of an enhancer. The frequency of piggyBac remobilization and enhancer detection was 2.5- to 3-fold higher in female germ lines compared with male germ lines. A small collection of enhancer-trap lines are described in which Gal4 expression occurred in adult female salivary glands, midgut, and fat body, either singly or in combination. These three tissues play critical roles during the infection of Anopheles stephensi by malaria-causing Plasmodium parasites. This system and the lines generated using it will be valuable resources to ongoing mosquito functional genomics efforts.

piggyBac transposon remobilization and enhancer detection in Anopheles mosquitoes.

Technical advances in mosquito biology are enabling the development of new approaches to vector control. Absent are powerful forward-genetics technologies, such as enhancer and gene traps, that permit determination of gene functions from the phenotypes arising from transposon insertion mutations. We show that the piggyBac transposon is highly active in the germline of the human malaria vector Anopheles stephensi. Up to 6% of the progeny from transgenic A. stephensi containing a single 6-kb piggyBac element with a marker gene expressing EGFP had the vector in new genomic locations when piggyBac transposase was provided in trans from a second integrated transgene. The active transposition of piggyBac resulted in the efficient detection of enhancers, with ~10% of the progeny with piggyBac in new locations with novel patterns of EGFP expression in third and fourth instar larvae and in adults. The availability of advanced transgenic capabilities such as efficient transposon-based forward-genetics technologies for Anopheles mosquitoes not only will accelerate our understanding of mosquito functional genomics and the development of novel vector and disease transmission control strategies, but also will enable studies by evolutionary developmental biologists, virologists, and parasitologists.