Population suppression with dominant female-lethal alleles is boosted by homing gene drive

Abstract

Background: Methods to suppress pest insect populations using genetic constructs and repeated releases of male homozygotes have recently been shown to be an attractive alternative to older sterile insect techniques based on radiation. Female-specific lethal alleles have substantially increased power, but still require large, sustained transgenic insect releases. Gene drive alleles bias their own inheritance to spread throughout populations, potentially allowing population suppression with a single, small-size release. However, suppression drives often suffer from efficiency issues, and the most well-studied type, homing drives, tend to spread without limit.

Results: In this study, we show that coupling female-specific lethal alleles with homing gene drive allowed substantial improvement in efficiency while still retaining the self-limiting nature (and thus confinement) of a lethal allele strategy. Using a mosquito model, we show the required release sizes for population elimination in a variety of scenarios, including different density growth curves, with comparisons to other systems. Resistance alleles reduced the power of this method, but these could be overcome by targeting an essential gene with the drive while also providing rescue. A proof-of-principle demonstration of this system in Drosophila melanogaster was effective in both biasing its inheritance and achieving high lethality among females that inherit the construct in the absence of antibiotic.

Conclusions: Overall, our study shows that substantial improvements can be achieved in female-specific lethal systems for population suppression by combining them with various types of gene drive.

Keywords: Drosophila; Gene drive; Modeling; Population suppression; RIDL.

Fig. 1

The Drive-RIDL system. Females with one or more drive alleles are nonviable unless reared with an antibiotic (pink circles). Male homozygotes are released into a population, and their daughters will be nonviable. Drive conversion can take place in male germline cells, allowing over half of the progeny to inherit the drive. A resistance allele can form as an alternative to successful drive conversion, and such alleles cannot be converted into drive alleles

Fig. 2

Effect of density dependence in the mosquito model. Males homozygous for Drive-RIDL were released into a population every week based the drop ratio, which specifies the relative number released each generation (3.17 weeks) compared to the male population at equilibrium. The density dependence of the model was varied (see Fig. S4), with a fixed low-density growth rate of 6. Gray indicates failure to eliminate the population after 100 generations (317 weeks). Each spot shows an average of 20 simulations

Fig. 3

Self-limiting nature of Drive-RIDL. Male mosquitoes homozygous for Drive-RIDL were released into a population every week between weeks 10 and 20 with a drop ratio of 3, which specifies the relative number released each generation compared to the male population at equilibrium. The low-density growth rate was 6, and the drive conversion efficiency was varied as shown. The graphs show the drive frequency among juvenile mosquitoes as well as the adult female population size. Population elimination occurred in half the simulations with 100% drive efficiency. Each line is an average of 20 simulations

Fig. 4

Effect of fitness costs in the mosquito model on different suppression systems. Males of the indicated types (Drive-RIDL males for both the complete drive and split drive had a 50% drive conversion efficiency, and Drive-RIDL TARE males had a 100% germline cut rate) were released into a population every week based the drop ratio, which specifies the relative number released each generation (3.17 weeks) compared to the male population at equilibrium. The fitness of each construct was varied (the Cas9 allele for the split drive had no fitness costs), with a fixed low-density growth rate of 6 and a linear density response curve. Grey indicates failure to eliminate the population after 100 generations (317 weeks). Each spot shows an average of 20 simulations

Fig. 5

Effect of resistance alleles in the mosquito model. Males homozygous for Drive-RIDL were released into a population every week based the drop ratio, which specifies the relative number released each generation compared to the male population at equilibrium. With a low-density growth rate of 6 and a drive conversion efficiency of 50%, the resistance allele formation rate was allowed to vary. The left panel assumes all resistance alleles are functional (note the logarithmic scale), while the center and right panel assume nonfunctional resistance alleles. Grey indicates failure to eliminate the population after 100 generations (317 weeks). Each spot shows an average of 20 simulations

Fig. 6

Schematic of Drive-RIDL and homing drive performance. A The Drive-RIDL allele is transformed into a previous split homing rescue drive, rendering the DsRed marker inactive. The Drive-RIDL allele has an EGFP gene expressed in the eyes, and a tTAV gene that can activate itself in the absence of tetracycline. The drive retains a recoded rescue copy of its haplolethal target and two gRNAs. B The drive showed high inheritance from male and female heterozygote parents (with one paternal copy of Cas9) in the presence of tetracycline. Each dot represents progeny from a single drive individual. Black bars represent the average and standard error

Fig. 7

Female lethal effect of the Drive-RIDL allele. Egg viability was recorded in vials in various crosses, all of which were with a Cas9 homozygous background. Adult progeny were also phenotyped for sex. Crosses were between drive homozygotes males and wild-type females A with and B without tetracycline, C between drive heterozygous males and wild-type type females without tetracyclines, and between drive homozygous males and females D with and E without tetracycline. Each dot represents progeny from a single drive individual. The green dot represents the mean for all individuals, and black bars represent the average and standard error