While many technologies for halting the spread of deadly mosquito-borne pathogens exist, none are without significant drawbacks such that additional measures are needed. By disrupting essential genes throughout development, we demonstrate efficient production of short-lived, flightless pgSIT♀’s and fit sterile pgSIT♂’s. Importantly, when repeatedly released into caged populations, the pgSIT♂’s competed with WT ♂’s thereby suppressing, and even eliminating, populations using release ratios that are achievable in the field (4–6). Mathematical models suggest that population elimination could be accomplished in the field through sustained releases of ~ 100–200 or more pgSIT eggs per wild Ae. aegypti adult, even if fitness costs significantly exceed those measured in contained laboratory experiments.
For pgSIT to be realized in the wild, the two strains will first need to be separately and continuously mass-reared in a facility, without contamination, and crossed to produce sterile ♂’s. While this can be viewed as rate-limiting (21), it offers stability, as the binary CRISPR system will remain inactive until crossed—thereby reducing the evolution of suppressors or mutations that could disrupt the system. Additionally, each sorted ♀ can produce up to 450 eggs in her lifetime (22), which improves scalability. Moreover, once crossed, the resulting progeny are essentially dead-ends (i.e. sterile ♂’s /flightless ♀’s), hatched among high numbers of sterile pgSIT♂’s, and should not contribute to the gene pool (23). We demonstrate here that the technology is fully penetrant by screening > 100K individuals.
pgSIT offers an alternative approach to scalability that should help decrease costs and increase efficiency. For instance, the required genetic cross at scale can be initiated using existing robotic sex sorting devices (www.senecio-robotics.com) or (5). Upon sex sorting and crossing, the resulting pgSIT progeny can be distributed and released at any life stage, mitigating requirements for sex separation at field sites. This strategy will be especially effective for mosquitoes that diapause during the egg stage (e.g. Aedes species) because it will enable long-term egg accumulation. Eggs could be distributed to logistically spaced remote field sites where they can hatch, develop, and compete with wild mosquitoes (Fig. S11). This attractive feature should reduce the costs of developing multiple production facilities requiring on-site sex separation for manual release of fragile adults.
It should be noted that the releases of adult pgSIT♂’s unexpectedly resulted in faster population suppression as compared to egg releases in multigenerational population cage experiments. We believe this to result from the slightly reduced egg hatching rates of pgSIT♂’s and their delayed larva-pupa development time, which likely enabled the co-released WT ♂’s first access to WT ♀’s. While this could impact the discrete generation population cage experiments conducted here, it should not be problematic for suppressing continuous populations in the wild.
Finally, notwithstanding its inherently safe nature, pgSIT requires genetic modification, and regulatory use authorizations will need to be granted prior to implementation. While this could be viewed as a limitation (21), we don’t expect obtaining such authorizations to be insurmountable. In fact, we envision pgSIT to be regulated in a similar manner to Oxitec’s RIDL technology, which has been successfully deployed in many locations and recently received experimental use authorizations in the USA.
Overall, the inherent self-limiting nature of pgSIT, offers a controllable safe alternative to technologies that can persist and spread in the environment, such as gene drives (8). Going forward, pgSIT may provide an efficient, safe, scalable, and environmentally friendly alternative next-generation technology for wild population control of mosquitoes resulting in wide-scale prevention of human disease transmission.
Mosquito rearing and maintenance
Ae. aegypti mosquitoes were derived from the Liverpool strain (wildtype [WT]) previously used to generate the reference genome (24). Mosquitoes were raised in incubators at 27.0°C with 20–40% humidity and a 12-hour light/dark cycle in cages (Bugdorm, 24.5 cm 𝗑 24.5 cm 𝗑 24.5 cm). Adults were provided 0.3 M aqueous sucrose ad libitum, and ♀’s were blood fed on anesthetized mice for two consecutive days for ~ 15 minutes at a time. Oviposition substrates were provided ~ 3 days following the second blood meal. Eggs were collected and aged for ~ 4 days to allow for embryonic development, then were hatched in deionized H2O in a vacuum chamber. Roughly ~ 400 larvae were reared in plastic containers (Sterilite, 34.6 cm 𝗑 21 cm 𝗑 12.4 cm, USA) with ~ 3 liters of deionized H2O, and fed fish food (TetraMin Tropical Flakes, Tetra Werke, Melle, Germany). For genetic crosses, to ensure ♀ virginity, pupae were separated and sexed under the microscope by sex-specific morphological differences in the genital lobe shape (at the end of the pupal abdominal segments just below the paddles) before being released to eclose in cages. These general rearing procedures were followed unless otherwise noted. Mosquitoes were examined, scored, and imaged using the Leica M165FC fluorescent stereo microscope equipped with the Leica DMC2900 camera. For higher resolution images, we used a Leica DM4B upright microscope equipped with a VIEW4K camera enabling time lapse videos. Time lapse videos of caged adult mosquitoes were taken with a mounted Canon EOS 5D Mark IV using a 24–105mm image stabilizer ultrasonic lens.
Guide RNA design and testing
Two target genes were selected for gRNA design: β-Tubulin 85D (βTub, AAEL019894) and myosin heavy chain (myo-fem, AAEL005656). For each target gene, DNA sequences were first identified using reference genome assembly (24), and genomic target sites were validated using PCR amplification and Sanger sequencing (Table S18 for primer sequences). Gene structures, transcripts, and exon-intron junction boundaries were carefully evaluated using comprehensive developmental transcriptome data (11, 24) loaded into an internal genome browser. Target gRNA sequences were selected to be 20 bp (N20) in length, excluding the PAM (NGG) (25). For in silico gRNA selection, we used either CHOPCHOP V3.0.0 (https://chopchop.cbu.uib.no) or CRISPOR (http://crispor.tefor.net) to minimize potential genomic off-target cleavage events. In total, we designed four gRNAs targeting βTub and four gRNAs targeting myo-fem (Table S18). To confirm gRNA activity in vivo, each gRNA was in vitro synthesized prior to construct design (Synthego, CA, USA). Then 100 ng/ul of gRNA was individually injected into fifty preblastoderm stage embryos (0.5–1 hr old) derived from Exu-Cas9 maternally depositing mothers, per previous embryo-injection protocols (16, 17). The surviving G0 progeny were pooled (2–5 individuals per pool), and genomic DNA was extracted using the DNeasy blood and tissue kit (Qiagen, Cat No./ID: 69506) following the manufacturer's protocols. To molecularly characterize the induced mutations, target loci were PCR amplified from extracted genomic DNA, and the PCR products were gel purified (Zymo Research, Zymoclean Gel DNA Recovery Kit, Cat No./ID: D4007). The purified products were either sent directly for sequencing or subcloned (Invitrogen, TOPO-TA, Cat No./ID: LS450641), wherein single colonies were selected and cultured in Laurel Broth (LB) with ampicillin before plasmid extraction (Zymo Research, Zyppy plasmid miniprep kit, Cat No./ID: D4036) and Sanger sequencing. Mutated alleles were identified in silico by alignment with WT target sequences. All primers used for PCR and sequencing, including gRNA target sequences, are listed in Table S18.
Construct molecular design and assembly
The Gibson enzymatic assembly method was used to engineer all constructs in this study (26). To generate the Nup50-Cas9 construct marked with CFP, OA-874PA (Addgene #164846), we used our previous plasmid for Cas9 expression (Addgene plasmid #100608) as the backbone (17). The fragments of T2A-eGFP-P10-3’UTR and OpIE2-dsRed-SV40 were removed by cutting with restriction enzyme FseI. Then, the P10-3’UTR fragment was amplified from Addgene plasmid #100608 with primers 874-P10 and 777B. Another fragment, OpIE2-CFP-SV40, was synthesized using gBlocks® Gene Fragment service (Integrated DNA Technologies, Coralville, Iowa). Both fragments were provided for the Gibson assembly into the cut backbone. We designed two constructs, OA-1067A1 (Addgene #164847) and OA-1067K (Addgene #164848), each carrying four different gRNAs targeting either β-Tubulin 85D (βTub, AAEL019894) or myosin heavy chain (myo-fem, AAEL005656) genes.
To engineer these plasmids, four intermediate plasmids, OA-1055A (gRNAβTub1&2), OA-1055B (gRNAβTub3&4), OA-1055W (gRNAmyo−fem1&2), and OA-1055X (gRNAmyo−fem3&4), each harboring two gRNAs, were generated by cutting a backbone plasmid OA-984 (Addgene plasmid #120363), which contains piggyBac elements and the 3xP3-tdTomato transformation marker, with the restriction enzymes AvrII and AscI. Two gBlocks® Gene Fragments were then cloned in, each containing two gRNAs: one driven by U6b (AAEL017774) and one by U6c (AAEL017763) promoters (17). To assemble the final plasmid OA-1067A1, an intermediate plasmid OA-1067A was generated by linearizing the plasmid OA-1055B with the restriction enzyme BglII and inserting in the fragment of U6b-gRNAβTub1-U6c-gRNAβTub2 amplified with primers 1167.C1 and 1067.C2 from plasmid OA-1055A. Then, the fragment of 3xP3-tdTomato was removed from plasmid OA-1067A using the restriction enzymes AscI and NotI and replaced with the 3xP3-eGFP transformation marker amplified with primers 1067A1.C1 and 1067A1.C2 from the plasmid OA-961B (Addgene plasmid #104967). To assemble the final plasmid OA-1067K, OA-1055W was linearized with the restriction enzyme FseI, and the insertion of U6b-gRNAmyo−fem3-U6c-gRNAmyo−fem4 was amplified with primers 1167.C5 and 1067.C6 from the plasmid OA-1055X. During each cloning step, single colonies were selected and cultured in LB medium with ampicillin, and then the plasmids were extracted (Zymo Research, Zyppy plasmid miniprep kit, Cat No./ID: D4036) and Sanger sequenced. Final plasmids were maxi-prepped using (Zymo Research, ZymoPURE II Plasmid Maxiprep kit, Cat No./ID: D4202) and Sanger sequenced. All primers are listed in Table S18. Complete plasmid sequences and plasmid DNA are available at www.addgene.com.
Generation of transgenic lines
Transgenic lines were generated by microinjecting preblastoderm stage embryos (0.5–1 hr old) with a mixture of the piggybac plasmid (200 ng/ul) and a transposase helper plasmid (phsp-Pbac, (200 ng/ul). Embryonic collection and microinjections were performed following previously established procedures (17). After 4 days of development post-microinjection, G0 embryos were hatched in deionized H2O in a vacuum chamber. Surviving G0 pupae were separated and sexed and divided into separate ♀ or ♂ cages (~ 20 cages total). The pupae eclosed inside these cages along with added WT ♂ pupae (added into the ♀ cages) or WT ♀ pupae (added into the ♂ cages) at 5:1 ratios (WT:G0). Several days post-eclosion (~ 4–7), enabling sufficient time for development and mating, a blood meal was provided, and eggs were collected, aged, then hatched. The hatched larvae with positive fluorescent markers were individually isolated using a fluorescent stereo microscope (Leica M165FC). To isolate separate insertion events, selected transformants were individually crossed to WT (5:1 ratios of WT:G1), and separate lines were established (Table S2). These were subjected to many generations of backcrosses to WT to isolate single insertion events. Each of these individual gRNA lines (OA-1067A1: gRNAβTub and OA-1067K: gRNAmyo−fem) were maintained as mixtures of homozygotes and heterozygotes with periodic selective elimination of WTs. The Cas9 line (OA-874PA: Nup50-Cas9) was homozygosed by ~ 10 generations of single-pair sibling matings selecting individuals with the brightest expressing transformation markers. Homozygosity was confirmed genetically by repeated test crosses to WT.
Genetic testing of established lines
To assess the activity of the transgenic lines generated, we performed a series of genetic crosses by releasing sexed pupae into cages. We first crossed gRNA lines (gRNA ♂ 𝗑 WT ♀) to generate heterozygotes. We next reciprocally crossed heterozygous gRNAβTub/+ (lines #1–10) and the heterozygous gRNAmyo−fem/+ (lines #1–5), with homozygous Cas9 (1 ♂ 𝗑 10 ♀). To measure the fecundity, the resulting transheterozygous F1 progeny (gRNAβTub/+; Cas9/+), or (gRNAmyo−fem/+; Cas9/+), were reciprocally crossed to WT’s (50 ♂ 𝗑 50 ♀), keeping track of the grandparents genotypes (Fig. S2, S3, Table S3). Control crosses of: WT ♂ 𝗑 WT ♀; WT ♂ 𝗑 Cas9 ♀; Cas9 ♂ 𝗑 WT ♀; gRNA/+ ♂ 𝗑 Cas9 ♀; gRNA/+ ♀ 𝗑 Cas9 ♂; gRNA/+ ♀ 𝗑 WT ♂; and gRNA/+ ♂ 𝗑 WT ♀ were also set up for comparisons (50 ♂ 𝗑 50 ♀). Adults were allowed to mate in the cage for 4–5 days, then blood meals were provided, and eggs were collected and hatched. The percentage of egg hatching (i.e. fertility) was estimated by dividing the total number of eggs laid by the total number of hatched eggs. Larvae-to-adult survival rates were calculated by dividing the total number of adults that emerged by the total number of larvae. Pupae-adult survival rates were calculated by dividing the number of dead pupae by the total number of pupae. Flight capacity for each sex was calculated by dividing the total number that were flightless (observed by eye) by the total of number of adult mosquitoes of that sex. Blood acquisition rates were calculated by dividing the number of blood-fed ♀’s by the total number of ♀’s. To investigate ♂ internal anatomical features, testes and ♂ accessory glands (n = 20) were dissected in 1% PBS buffer for imaging.
Generation and characterization of gRNAβTub+myo−fem
To generate gRNAβTub+myo−fem, we genetically crossed gRNAβTub#7 (marked with 3xp3-GFP) with gRNAmyo−fem#1 (marked with 3xp3-tdTomato). Resulting F1 transheterozygotes gRNAβTub#7 /+ ; gRNAmyo−fem#1 /+ were subjected to multiple generations of single-pair sibling matings, carefully selecting individuals with the brightest expressing transformation markers, to generate a transhomozygous stock (termed: gRNAβTub+myo−fem). Zygosity was confirmed genetically by repeated test crosses to WT. To measure efficacy, we bidirectionally crossed gRNAβTub+myo−fem with Cas9 (50 ♂ 𝗑 50 ♀), generating F1 transheterozygotes gRNAβTub+myo−fem/+ ; Cas9/+. Control crosses were also setup for comparisons: gRNAβTub+myo−fem ♂ 𝗑 gRNAβTub+myo−fem ♀; gRNAβTub+myo−fem ♂ 𝗑 WT ♀; gRNAβTub+myo−fem ♀ 𝗑 WT ♂; Cas9 ♂ 𝗑 Cas9 ♀; Cas9 ♂ 𝗑 WT ♀; and Cas9 ♀ 𝗑 WT ♂; (50 ♂ 𝗑 50 ♀). To determine the fecundity and fertility, resulting transheterozygous F1’s (~ 3 days old) were bidirectionally crossed to WT’s (50 ♂ 𝗑 50 ♀; 10 replicates each). These were allowed to mate for ~ 2 days and then blood fed. Afterwards, eggs were collected for up to five consecutive gonotrophic cycles and hatched.
Determination of transgene integration sites and copy number
To determine the transgene insertion site(s) and copy number(s), we performed Oxford Nanopore DNA sequencing. We extracted genomic DNA using the Blood & Cell Culture DNA Midi Kit (Qiagen, Cat# 13343) from twenty adult transheterozygous pgSIT♂’s (3 days old) harboring all three transgenes (Cas9/+ ; gRNAβTub#7 /+ ; gRNAmyo−fem#1 /+), following the manufacturer’s protocol. The sequencing library was prepared using the Oxford Nanopore SQK-LSK109 genomic library kit and sequenced on a single MinION flowcell (R9.4.1) for 72 hrs to generate an N50 read length for the set of 4088 bp. Basecalling was performed using ONT Guppy basecalling software version 4.4.1, generating 2.94 million reads above quality threshold Q ≧ 7, which corresponds to 8.68 Gb of sequence data. To determine transgene copy number(s), reads were mapped to the AaegL5.0 reference genome (24) supplemented with transgene sequences (OA-1067A1: gRNAβTub; OA-1067K: gRNAmyo−fem; and OA-874PA: Nup50-Cas9) using minimap2 (27). In total, 2,862,171 out of 2,936,275 reads (97.48%) were successfully mapped with a global genome-wide depth of coverage of 5.495. We calculated the mean coverage depth for all contigs in the genome (2310) and the three plasmids (OA-1067A1: gRNAβTub; OA-1067K: gRNAmyo−fem; and OA-874PA: Nup50-Cas9) as well as normalized coverage (Table S9-S10). Transgene coverage ranged from 5.1 to 7.6, and normalized coverage ranged from 0.93 to 1.38. As compared to the three chromosomes, the coverages are consistent with the transgenes present at a single copy (Fig. S7).
To identify transgene insertion sites, we inspected reads that aligned to the transgenes in the Interactive Genomics Viewer (IGV) browser. The reads extending beyond the boundaries of the transgenes were then analyzed to determine mapping sites within the genome. For OA-874PA, one read spanned the whole transgene (~ 11.5 kb) and extended 4 and 3.5 kb on both sides. The extending portions mapped to both sides of the position on NC_035109.1:33,210,105 (chromosome 3), with the nearest gene being AAEL023567, which is ~ 5 kb away. For OA-1067K, one read covered ~ 7 kb of the transgene extending ~ 10 kb off the 3' end, 9 kb of which map to the NC_035108.1:287,686 − 296,810 region (chromosome 2). A few other shorter reads map to the same location. The site is located in the intron of AAEL005206, which is a capon-like protein, and based on the RNA-seq data, it's expression does not appear to be affected in pgSIT animals. For OA-1067A1, the nanopore sequencing was unable to resolve the insertion site, presumably due to its insertion in one of the remaining gaps in the genome. Finally, using nanopore data, we confirmed genomic deletions in both pgSIT target genes - see AEL019894 and AAEL005656 as expected (Fig. S8-S9). The nanopore sequencing data has been deposited to the NCBI sequence read archive (SRA) under BioProject ID is PRJNA699282 with accession number SRR13622000.
Transcriptional profiling and expression analysis
To quantify target gene reduction and expression from transgenes as well as to assess global expression patterns, we performed Illumina RNA sequencing. We extracted total RNA using miRNeasy Mini Kit (Qiagen, Cat# 217004) from ten sexed pupae: WT ♀, WT♂, transheterozygous pgSIT♂’s, and pgSIT♀ harboring all three transgenes Cas9/+; gRNAβTub#7 /+; gRNAmyo−fem#1 /+ with each genotype in biological triplicate (12 samples total), following the manufacturer’s protocol. DNase treatment was conducted using DNase I, RNase-free (ThermoFisher Scientific, Cat# EN0521), following total RNA extraction. RNA integrity was assessed using the RNA 6000 Pico Kit for Bioanalyzer (Agilent Technologies #5067 − 1513), and mRNA was isolated from ~ 1 µg of total RNA using NEBNext Poly(A) mRNA Magnetic Isolation Module (NEB #E7490). RNA-seq libraries were constructed using the NEBNext Ultra II RNA Library Prep Kit for Illumina (NEB #E7770) following the manufacturer’s protocols. Briefly, mRNA was fragmented to an average size of 200 nt by incubating at 94°C for 15 min in the first strand buffer. cDNA was then synthesized using random primers and ProtoScript II Reverse Transcriptase followed by second strand synthesis using NEB Second Strand Synthesis Enzyme Mix. Resulting DNA fragments were end-repaired, dA tailed, and ligated to NEBNext hairpin adaptors (NEB #E7335). Following ligation, adaptors were converted to the “Y” shape by treating with USER enzyme, and DNA fragments were size selected using Agencourt AMPure XP beads (Beckman Coulter #A63880) to generate fragment sizes between 250 and 350 bp. Adaptor-ligated DNA was PCR amplified followed by AMPure XP bead clean up. Libraries were quantified using a Qubit dsDNA HS Kit (ThermoFisher Scientific #Q32854), and the size distribution was confirmed using a High Sensitivity DNA Kit for Bioanalyzer (Agilent Technologies #5067 − 4626). Libraries were sequenced on an Illumina HiSeq2500 in single read mode with the read length of 50 nt and sequencing depth of 20 million reads per library. Base calls were performed with RTA 1.18.64 followed by conversion to FASTQ with bcl2fastq 1.8.4. The reads were mapped to the AaegL5.0 (GCF_002204515.2) genome supplemented with OA-874PA, OA-1067A1, and OA-1067K sequences using STAR. On average, ~ 97.5% of the reads were mapped (Table S11). Gene expression was then quantified using featureCounts against the annotation release 101 GTF downloaded from NCBI (GCF_002204515.2_AaegL5.0_genomic.gtf). TPM values were calculated from counts produced by featureCounts and combined (Table S12).
PCA and hierarchical clustering of the data show that the samples generally behaved as expected in clustering by sex and genotype (Fig. S10). DESeq2 was then used to perform differential expression analyses between pgSIT vs WT samples within each sex (Fig. S10, Table S13, S14), and a two-factor design consistently showed what changed in response to the genotype in both sexes (Table S15). In a comparison between pgSIT♀ and WT ♀, 660 genes were upregulated in pgSIT♀ and 392 were downregulated at an adjusted p-value < 0.05. The target gene, AAEL005656, was significantly downregulated in pgSIT♀ (Fig. S10C). In a comparison between pgSIT♂’s andWT♂ (Table S13), 2067 genes were upregulated in pgSIT♂’s and 2722 were downregulated at an adjusted p-value < 0.05. The target gene, AEL019894, was strongly downregulated in pgSIT♂ (Fig. S10D). It’s important to note here that the CRISPR/Cas9 pgSIT system disrupts the DNA (not the RNA) so transcription is expected to occur; however, the transcripts produced will encode mutations and should be degraded by nonsense -mediated mRNA decay (NMD) mechanisms. Indeed, these mutant RNA’s can be observed in the IGV (Fig. S8, S9). In the two-factor comparison, 1447 genes were upregulated in pgSIT and 2563 were downregulated at an adjusted p-value < 0.05 (Fig. S10E). For each DESeq2 comparison, gene ontology enrichments were performed on significantly differentially expressed genes, and these are provided as tabs in the corresponding tables (Table S13-S15). All Illumina RNA sequencing data has been deposited to the NCBI sequence read archive (SRA) under BioProject ID is PRJNA699282 with accession numbers SRR13620773-SRR13620784.
Amplicon Sequencing of Target Loci
To sample a variety of molecular changes at the gRNA target sites (myo-fem and βTub), we used the Amplicon-EZ service by Genewiz® and followed the Genewiz® guidelines for sample preparation. Genomic DNA from 50 WT and 50 pgSIT sexed pupae (25♀ + 25♂) were extracted separately using DNeasy Blood and Tissue Kit (Qiagen, Cat No./ID: 69506) following the manufacturer's protocols. Primers with Illumina adapters (Table S18) were used to PCR amplify the genomic DNA. PCR products were purified using the Zymoclean Gel DNA Recovery Kit (Zymo Research, Cat No./ID: D4007). Roughly 50,000 one-directional reads were generated by Genewiz® and uploaded to Galaxy.org for analysis. Quality control for the reads was performed using FASTQC. Sequence data were then paired and aligned against the myo-fem or βTub sequence using Map with BWA-MEM under “Simple Illumina mode”. Sequence variants were detected using FreeBayes, with parameter selection level set to “simple diploid calling.” The amplicon sequencing data has been provided as File S1.
Prior mating with pgSIT♂’s suppress ♀ fertility
To determine whether prior matings with pgSIT♂ could reduce ♀ fertility, we initiated 15 cages each consisting of 250 mature (4–5 days old) pgSIT♂ combined with 50 mature (4–5 days old) WT virgin ♀. We allowed the pgSIT♂’s to mate with these ♀’s for a limited period of time (including: 2, 6, 12, 24, and 48 hrs; 3 replicate cages each). Cages were shaken every 3 minutes for the first half hour to increase mating opportunities. Following these time periods, all ♀’s were removed and transferred to new cages along with 250 WT mature ♂’s, cages were again shaken every 3 minutes for the first half hour to increase mating opportunities and left to mate for an additional 2 days. The ♀’s were then blood fed, and each blood fed ♀ was individually transferred to a single narrow Polystyrene vial (Genesee Scientific Cat# 32–116), and eggs were collected and hatched for fertility determination. Following this, non-fertile ♀’s were then placed back into cages along with the original WT ♂’s, plus an additional 50 mature WT ♂’s, for another chance to produce progeny. This was repeated for up to five gonotrophic cycles. As controls, cages with 250 WT ♂’s and 50 WT ♀’s, or 50 unmated blood fed WT ♀’s with no ♂’s added, or 50 unmated blood fed WT ♀’s with 250 WT ♂ adults were also set up (Table S8).
Life table parameters
Life table parameters were assessed by comparing WT, homozygous gRNAβTub+myo−fem, homozygous Cas9, and transheterozygous pgSIT (gRNAβTub+myo−fem/+; Cas9/+) generated with Cas9 inherited from either the mother (maternal Cas9) or father (paternal Cas9). Larva/pupae development times were recorded as the number of days from hatched larvae to pupae and then to adults. One hundred larvae from each line were placed in separate larval rearing containers (Sterilite, 34.6 cm 𝗑 21 cm 𝗑 12.4 cm, USA), each with 3 liters of deionized water, and fed once a day. Larvae were counted twice daily until pupation, and then the date of pupation and emergence were recorded. Larval to pupae development time was calculated for each sex. Pupae were transferred to plastic cups (Karat, C-KC9) with 100 ml of water, and survivors were recorded until adulthood. ANOVA and Tukey post-hoc tests were performed to compare differences in larval and pupal development among all groups.
For measuring ♂ /♀ longevity, we tested the variation in ♂ and ♀ longevity among different lines using two methods: (i) released along with WT of the opposite sex or (ii) without WT of the opposite sex. (i) One hundred WT, homozygous gRNAβTub+myo−fem, homozygous Cas9 newly eclosed adult mosquitoes (fifty ♂’s and fifty ♀’s) were maintained in a cage; fifty newly-eclosed pgSIT ♂’s (maternal cas9) and fifty newly-eclosed pgSIT ♂’s (paternal cas9) were caged with fifty newly-eclosed WT ♀’s; and finally, fifty newly-eclosed pgSIT ♀’s (maternal cas9) and fifty newly-eclosed pgSIT ♀’s (paternal cas9) were caged with fifty newly-eclosed WT ♂’s. (ii) Fifty ♂’s or ♀’s from each line were released into a cage separately without the opposite sex. Adults were provided with 10% sucrose and monitored daily for survival until all mosquitoes had died (3 replicates).
For measuring ♀ fecundity and fertility, ♀’s (n = 50) and ♂’s (n = 50) three days post-emergence raised under the same standardized larval conditions were placed into a cage and allowed to mate for 2 days. ♀ mosquitoes were blood fed until fully engorged and were individually transferred into plastic vials with oviposition substrate. Eggs were stored in the insectary for 4 days to allow full embryonic development and then were hatched in a vacuum chamber. Fecundity was calculated as the number of eggs laid per ♀, and fertility was calculated as the percentage of eggs hatched per ♀. An analysis of variance (ANOVA) and a Tukey post-hoc test were performed to compare differences in fecundity and fertility among all groups.
♂ mating capacity (how many ♀’s can be mated by one mature ♂ ) was measured as follows. Fifteen mature WT ♀’s were caged with 1 mature ♂ of each genotype for 24 hours (1♂:15♀ ratio). After 24 hours, the single ♂ was removed from all cages. Two days after the single ♂ was removed, 75 WT ♂’s were added to each cage that previously had a pgSIT ♂ (5♂:1♀ ratio). Blood meals were provided, and each blood fed ♀ was individually transferred to a single vial for egg collection. The fecundity and fertility of each ♀ was determined. The mating capacity was calculated as the total number of ♀’s - total number of fertile ♀’s. The mating capacity of WT, homozygous gRNAβTub+myo−fem, and homozygous Cas9 ♂ was equal to the number of fertile ♀’s. All statistical analyses were performed using GraphPad Prism software (GraphPad Software, La Jolla, California, USA). P values > 0.05 were considered not significant.
Flight activity quantification
Mosquitoes were reared at 28°C, 80% relative humidity under a 12:12 hr light:dark regime, and measurements of flight activity were performed using a Drosophila Activity Monitoring (DAM) System (TriKinetics, LAM25) using large tubes designed for mosquitoes (TriKinetics, PGT 25 x 125 mm Pyrex Glass). Individual 4–7 day-old, non-blood fed virgin ♀ and non-mated ♂ mosquitoes were introduced into the monitoring tubes, which contained 10% sucrose (Sigma, Cat. S0389) at both ends of the tube as the food source. The DAM System was positioned vertically during the assays. Flight activity was measured over a period of 24 hrs by automatically calculating the number of times that mosquitoes passed through the infrared beam in the center of the tubes. The walls of the monitoring tubes were coated with Sigmacote (Sigma, Cat. SL2) to inhibit mosquitoes from walking upward. For preparing the wingless mosquitoes, the animals were anesthetized on ice, and the wings were removed using Vannas Scissor (World Precision Instruments, Cat. 14003). The wingless mosquitoes were allowed to recover for 12 hrs before recording. Mosquitoes were manually checked after flight activity recording to ensure survival. Data acquisition was performed using the DAMSystem (TriKinetics) (Fig. 2D, Video S5, Table S6).
Sound attraction assay
The sound attraction assay was performed in a chamber with a temperature of 28°C and humidity of 80%. Seven-day old ♂’s were sex separated after the pupae stage. The day before testing, 30–40 ♂’s were transferred by mouth aspiration to a 15-cm3 mesh cage with a 10% sucrose bottle. ♂ mosquitoes were allowed to recover in the cage under a 12 hr:12 hr light:dark regime for 24 hrs. For each trial, a 10-second 600 Hz sine tone was applied on one side of the cage as a mating behavior lure, mimicking ♀ flight tones. The number of mosquitoes landing on the mesh area around the speaker box(10 cm2)was quantified at 5-second intervals throughout the stimulus. The average percent of mosquitoes landing around the speaker area out of the total cage post-sound presentation was calculated (Fig. 2E, Video S6, Table S7). Heatmaps were generated using Noldus Ethovision XT.
Multigenerational population cage trials
To perform multigenerational population cage trials, two strategies were employed: (i) release of eggs; (ii) release mature adults (Fig. 3, Table S16). Cage trials were carried out using discrete non-overlapping generations. For the first release of eggs strategy (i), WT eggs and pgSIT eggs were hatched together using the following ratios of 1:1 (100:100), 1:5 (100:500), 1:10 (100:1000), 1:20 (100:2000), and 1:40 (100:4000), and three biological replicates for each ratio (15 cages total). All eggs were hatched simultaneously, then separated into multiple plastic containers (Sterilite, 34.6 cm 𝗑 21 cm 𝗑 12.4 cm, USA). Roughly 400 larvae were reared in each container using standard conditions with 3 liters of deionized water and were allowed to develop into pupae. Pupae were placed in plastic cups (Karat, C-KC9) with ~ 100 ml of water (~ 150 pupae per cup) and transferred to large cages (BugDorm, 60cm 𝗑 60cm 𝗑 60cm) to eclose. All adults were allowed to mate for ~ 5–7 days. ♀’s were blood fed, and the eggs were collected. Eggs were counted and stored for ~ 4 days to allow full embryonic development, then 100 eggs were selected randomly and mixed with pgSIT eggs with ratios of 1:1 (100:100), 1:5 (100:500), 1:10 (100:1000), 1:20 (100:2000), and 1:40 (100:4000) to seed for the following generation, and this procedure continued for all subsequent generations. The remaining eggs were hatched to measure hatching rates and to screen for the possible presence of transformation markers. The hatching rate was estimated by dividing the number of hatched eggs by the total number of eggs.
For the release of mature adults strategy (ii), 3–4-days-old mature WT adult ♂’s were released along with mature (3–4 days old) pgSIT adult ♂’s at release ratios: 1:1 (50:50), 1:5 (50:250), 1:10 (50:500), 1:20 (50:1000), and 1:40 (50:2000), with three biological replicates for each release ratio (15 cages total). One hour later, 50 mature (3–4 days old) WT adult ♀’s were released into each cage. All adults were allowed to mate for 2 days. ♀’s were then blood fed and eggs were collected. Eggs were counted and stored for four days to allow full embryonic development. Then, 100 eggs were randomly selected, hatched, and reared to the pupal stage, and the pupae were separated into ♂ and ♀ groups and transferred to separate cages. Three days post eclosion, 50 (1:1), 250 (1:5), 500 (1:10), 1000 (1:20), and 2000 (1:40) age-matched pgSIT mature ♂ adults were caged with these mature ♂’s from 100 selected eggs. One hour later, mature ♀’s from 100 selected eggs were transferred into each cage. All adults were allowed to mate for 2 days. ♀’s were blood fed, and eggs were collected. Eggs were counted and stored for 4 days to allow full embryonic development. The remaining eggs were hatched to measure hatching rates and to screen for the possible presence of transformation markers. The hatching rate was estimated by dividing the number of hatched eggs by the total number of eggs. This procedure continued for all subsequent generations.
To model the expected performance of pgSIT at suppressing and eliminating local Ae. aegypti populations, we used the MGDrivE simulation framework (19). This framework models the egg, larval, pupal, and adult mosquito life stages with overlapping generations, larval mortality increasing with larval density, and a mating structure in which females retain the genetic material of the adult ♂ with whom they mate for the duration of their adult lifespan. The inheritance pattern of the pgSIT system was modeled within the inheritance module of MGDrivE, along with impacts on adult lifespan, male mating competitiveness, and pupatory success. We distributed Ae. aegypti populations according to human structures sourced from OpenStreetMap on the basis that Ae. aegypti is anthropophilic. Each human structure was assumed to have an equilibrium population of 16 adult Ae. aegypti, producing an equilibrium island population of 992. We implemented the stochastic version of the MGDrivE framework to capture random effects at low population sizes and the potential for population elimination. Weekly releases of up to 400 pgSIT eggs were simulated in all human structures of Onetahi over a period of 10–24 weeks. 200 repetitions were carried out for each parameter set, and mosquito genotype trajectories, along with the proportion of simulations that led to local population elimination, were recorded. Complete model and intervention parameters are listed in Table S17.