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The discovery of CRISPR-based gene editing and its application to homing-based gene drive systems has been greeted with excitement, for its potential to control mosquito-borne diseases on a wide scale, and concern, for the invasiveness and potential irreversibility of a release. Gene drive systems that display threshold-dependent behavior could potentially be used during the trial phase of this technology, or when localized control is otherwise desired, as simple models predict them to spread into partially isolated populations in a confineable manner, and to be reversible through releases of wild-type organisms. Here, we model hypothetical releases of two recently engineered threshold-dependent gene drive systems-reciprocal chromosomal translocations and a form of toxin-antidote-based underdominance known as UD MEL-to explore their ability to be confined and remediated. We simulate releases of Aedes aegypti, the mosquito vector of dengue, Zika, and other arboviruses, in Yorkeys Knob, a suburb of Cairns, Australia, where previous biological control interventions have been undertaken on this species. We monitor spread to the neighboring suburb of Trinity Park to assess confinement. Results suggest that translocations could be introduced on a suburban scale, and remediated through releases of non-disease-transmitting male mosquitoes with release sizes on the scale of what has been previously implemented. UD MEL requires fewer releases to introduce, but more releases to remediate, including of females capable of disease transmission. Both systems are expected to be confineable to the release site; however, spillover of translocations into neighboring populations is less likely. Our analysis supports the use of translocations as a threshold-dependent drive system capable of spreading disease-refractory genes into Ae. aegypti populations in a confineable and reversible manner. It also highlights increased release requirements when incorporating life history and population structure into models. As the technology nears implementation, further ecological work will be essential to enhance model predictions in preparation for field trials.
nheritance and landscape features of the modeling framework. a Reciprocal translocations (T1 and T2) result from the mutual exchange between terminal segments of two non-homologous chromosomes (N1 and N2). The cross here depicts possible parental gametes, with respect to the translocation, and offspring that result from matings between them. Matings between wild-type organisms or translocation homozygotes result in viable offspring, but translocation heterozygotes produce unbalanced gametes, and many of the resulting offspring are unviable (shaded). This results in a heterozygote disadvantage and threshold-dependent population dynamics. b UDMEL is composed of two unlinked constructs (here referred to as A and B), each consisting of a maternally expressed toxin and a zygotically expressed antidote for the toxin on the opposite construct. The cross here represents matings between two of the nine possible parental genotypes (“+” represents the wild-type allele, and “A” and “B” represent alleles corresponding to the two UDMEL constructs). The complete inheritance pattern is depicted in Additional file 2: Fig. S1. Offspring lacking the antidotes to the maternal toxins produced by their mother are unviable (shaded). At high population frequencies, the selective advantage on the constructs, by virtue of the antidotes, outweighs the fitness load due to the toxins and hence results in frequency-dependent spread. c Distribution of households in Yorkeys Knob (blue) and Trinity Park (red) in Cairns, Queensland, Australia. Households serve as individual Aedes aegypti populations in our metapopulation framework, with movement of adult Ae. aegypti between them. Yorkeys Knob serves as a simulated release site, and Trinity Park as a simulated control site.
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Replacement, remediation, and confinement outcomes for translocations and UDMEL. Outcomes are depicted for the proportion of 50 stochastic simulations of population replacement, remediation, and confinement of translocations and UDMEL that result in fixation of each system. a–d For replacement and remediation, each cell corresponds to a given number of releases (horizontal axis) and coverage level (vertical axis), given 20 adult Ae. aegypti per household per release. For replacement, releases are of males homozygous for the system into a wild-type population. For remediation of translocations, releases are of wild-type males into a population homozygous for the translocation, and for mixed remediation of UDMEL, releases are of wild-type females and males into a population homozygous for UDMEL. Blue cells represent cases where all simulations result in fixation of the system, and white cells represent cases where the wild-type is fixed in all simulations. e, f For confinement, each cell corresponds to a daily number of batch migration events (horizontal axis) of a given size (vertical axis) from Yorkeys Knob, where the system is fixed, to Trinity Park, where the system is initially absent. Dark pink cells represent cases where all simulations result in fixation of the system in Trinity Park, and white cells represent cases where the wild-type is fixed in all simulations. These results are encouraging for translocations as systems for introducing transgenes in a local and reversible way as (i) they can be remediated through an achievable number of male-only releases and (ii) they require more batch migration events to spread to neighboring communities
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R E S E A R C H A R T I C L E Open Access
Modeling confinement and reversibility of
threshold-dependent gene drive systems in
spatially-explicit Aedes aegypti populations
Héctor M. Sánchez C.
1
, Jared B. Bennett
2
, Sean L. Wu
1
, Gordana Rašić
3
, Omar S. Akbari
4
and John M. Marshall
1,5*
Abstract
Background: The discovery of CRISPR-based gene editing and its application to homing-based gene drive systems
has been greeted with excitement, for its potential to control mosquito-borne diseases on a wide scale, and
concern, for the invasiveness and potential irreversibility of a release. Gene drive systems that display threshold-
dependent behavior could potentially be used during the trial phase of this technology, or when localized control
is otherwise desired, as simple models predict them to spread into partially isolated populations in a confineable
manner, and to be reversible through releases of wild-type organisms. Here, we model hypothetical releases of two
recently engineered threshold-dependent gene drive systemsreciprocal chromosomal translocations and a form
of toxin-antidote-based underdominance known as UD
MEL
to explore their ability to be confined and remediated.
Results: We simulate releases of Aedes aegypti, the mosquito vector of dengue, Zika, and other arboviruses, in
Yorkeys Knob, a suburb of Cairns, Australia, where previous biological control interventions have been undertaken
on this species. We monitor spread to the neighboring suburb of Trinity Park to assess confinement. Results
suggest that translocations could be introduced on a suburban scale, and remediated through releases of non-
disease-transmitting male mosquitoes with release sizes on the scale of what has been previously implemented.
UD
MEL
requires fewer releases to introduce, but more releases to remediate, including of females capable of disease
transmission. Both systems are expected to be confineable to the release site; however, spillover of translocations
into neighboring populations is less likely.
Conclusions: Our analysis supports the use of translocations as a threshold-dependent drive system capable of
spreading disease-refractory genes into Ae. aegypti populations in a confineable and reversible manner. It also
highlights increased release requirements when incorporating life history and population structure into models. As
the technology nears implementation, further ecological work will be essential to enhance model predictions in
preparation for field trials.
Keywords: Chromosomal translocations, Underdominance, Metapopulation, Population dynamics, Population
replacement, Biosafety, Field trials
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data made available in this article, unless otherwise stated in a credit line to the data.
* Correspondence: john.marshall@berkeley.edu
1
Division of Epidemiology and Biostatistics, School of Public Health,
University of California, Berkeley, CA 94720, USA
5
Innovative Genomics Institute, Berkeley, CA 94720, USA
Full list of author information is available at the end of the article
Sánchez C. et al. BMC Biology (2020) 18:50
https://doi.org/10.1186/s12915-020-0759-9
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Background
The discovery of CRISPR and its application as a gene
editing tool has enabled gene drive systems to be engi-
neered with much greater ease [1,2]. Recent attention
has focused on homing-based drive systems and their
potential to control mosquito-borne diseases on a wide
scale, either by spreading disease-refractory genes [3]or
by spreading genes that confer a fitness load or sex bias
and thereby suppress mosquito populations [4,5]. The
increased ease of gene editing has also advanced the en-
tire field of gene drive, including systems appropriate
during the trial phase of the technology [6]. Such sys-
tems would ideally be capable of enacting local popula-
tion control by (a) effectively spreading into populations
to achieve the desired epidemiological effect, and (b) be-
ing recallable from the environment in the event of un-
wanted consequences, public disfavor, or the end of a
trial period.
As gene drive technology has progressed, a number of
systems have been proposed with the potential to enact
localized population control without spreading on a wide
scale [6,7]. Sterile male releases provide one option [8],
a recent version of which is based on the same molecu-
lar components as CRISPR gene drive systems [8,9]. At
the interface between homing-based and localized
suppression systems, an autosomal X chromosome-
shredding system has been proposed that induces a tran-
sient male sex bias and hence population suppression
before being selected out of the population [10]. Popula-
tion modification drive systems that display transient
drive activity before being eliminated by virtue of a fit-
ness cost could also spread disease-refractory genes into
populations in a localized way. Examples of this variety
of drive system include split-gene drive [11], daisy drive
[12], and killer-rescue systems [13]. Each system has its
own strengths and weaknesses and could be suited to a
different situation. In this paper, we theoretically explore
the potential for two recently engineered threshold-
dependent gene drive systems to achieve localized and
reversible population modification in structured popula-
tionsreciprocal chromosomal translocations [14] and a
toxin-antidote-based system known as UD
MEL
[15].
Threshold-dependent gene drive systems must exceed a
critical threshold frequency in a population in order to
spread. Based on this dynamic, simple population models,
in which two randomly mating populations exchange
small numbers of migrants with each other, predict that
these systems can be released at high frequencies in one
population and spread to near-fixation there, but never
take off in the neighboring population because they do
not exceed the required threshold there [16,17]. These
systems can also be eliminated through dilution to sub-
threshold levels with wild-type organisms at the release
site, making them excellent candidates for the trial phase
of a population modification gene drive strategy, or when
localized population modification is desired [16]. Elimin-
ation of a non-driving transgene can in fact be more diffi-
cult, as the dynamics of threshold-dependent systems
actively drive them out of populations at sub-threshold
levels. However, whether these dynamics hold in real eco-
systems depends crucially on the dispersal patterns and
population structure of the species being considered. First
steps towards modeling the spatial dynamics of these sys-
tems have been taken by Champer et al. [18], who model
spatially structured releases of various threshold-
dependent systems without life history, and Huang et al.
[19], who model engineered underdominance [20]ona
grid-based landscape incorporating life history for Aedes
aegypti, the mosquito vector of dengue, Zika, and other
arboviruses.
Here, we present a detailed ecological analysis of the
expected population dynamics of two recently engi-
neered threshold-dependent drive systems, transloca-
tions and UD
MEL
,inAe. aegypt in a well-characterized
landscapeYorkeys Knob, a suburb ~ 17 km northwest
of Cairns, Australia (Fig. 1c)suitable for confineable
and reversible releases. Yorkeys Knob and the nearby
town of Gordonvale were field sites for releases of Wol-
bachia-infected mosquitoes in 2011 [21], and the preva-
lence of Wolbachia infection over time provided
information on the number of adult Ae. aegypti mosqui-
toes per household and other mosquito demographic pa-
rameters for that location [22], as well as an opportunity
to validate our modeling framework. Wolbachia is an
intracellular bacterium and biocontrol agent that biases
inheritance in its favor if infected females are present,
and blocks transmission of dengue and other arboviruses
[23]. Yorkeys Knob is a partially isolated suburb, sepa-
rated by a 12-km-wide uninhabited, vegetated area
from the nearest suburb, Trinity Park, a control site for
the Wolbachia trial. This allowed us to simulate trials of
transgenic mosquitoes in a well-characterized popula-
tion, while also theoretically exploring their potential of
spread to a neighboring community.
A number of other ecological details are relevant to
the spread of threshold-dependent gene drive systems
that have not been considered in previous modeling
studies. Perhaps of greatest importance, the frequency of
the introduced transgene in the mating pool is markedly
different from the frequency of introduced adults. It is
typical to release only male mosquitoes as part of an
intervention, as only females are involved in human dis-
ease transmission. Life cycle and mating structure there-
fore become relevant, as immature life stages are not
available for mating, and female adults are thought to
mostly mate only once soon after emergence [24]. This
means that many of the released adult males will not
find a mating partner, and hence, larger releases will be
Sánchez C. et al. BMC Biology (2020) 18:50 Page 2 of 14
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Fig. 1 Inheritance and landscape features of the modeling framework. aReciprocal translocations (T1 and T2) result from the mutual exchange
between terminal segments of two non-homologous chromosomes (N1 and N2). The cross here depicts possible parental gametes, with respect
to the translocation, and offspring that result from matings between them. Matings between wild-type organisms or translocation homozygotes
result in viable offspring, but translocation heterozygotes produce unbalanced gametes, and many of the resulting offspring are unviable
(shaded). This results in a heterozygote disadvantage and threshold-dependent population dynamics. bUD
MEL
is composed of two unlinked
constructs (here referred to as A and B), each consisting of a maternally expressed toxin and a zygotically expressed antidote for the toxin on the
opposite construct. The cross here represents matings between two of the nine possible parental genotypes (+represents the wild-type allele,
and Aand Brepresent alleles corresponding to the two UD
MEL
constructs). The complete inheritance pattern is depicted in Additional file 2:
Fig. S1. Offspring lacking the antidotes to the maternal toxins produced by their mother are unviable (shaded). At high population frequencies,
the selective advantage on the constructs, by virtue of the antidotes, outweighs the fitness load due to the toxins and hence results in
frequency-dependent spread. cDistribution of households in Yorkeys Knob (blue) and Trinity Park (red) in Cairns, Queensland, Australia.
Households serve as individual Aedes aegypti populations in our metapopulation framework, with movement of adult Ae. aegypti between them.
Yorkeys Knob serves as a simulated release site, and Trinity Park as a simulated control site
Sánchez C. et al. BMC Biology (2020) 18:50 Page 3 of 14
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required to exceed threshold frequencies than predicted
in simple population frequency models.
The nature of mosquito dispersal behavior is also rele-
vant to the spatial dispersal of transgenes. Our species of
interest, Ae. aegypti, is understood to display leptokurtic
dispersal behavior in a suburban setting, in which mosqui-
toes tend to remain in the same household for the major-
ity of their lifespan, while a few mosquitoes disperse over
larger distances [25]. With these landscape, dispersal, and
life cycle considerations accounted for, we theoretically
explore the ability to drive two threshold-dependent sys-
tems, translocations and UD
MEL
, into populations of Ae.
aegypti in one community, Yorkeys Knob, without them
spreading in significant numbers to a neighboring com-
munity, Trinity Park, and to be remediated from Yorkeys
Knob at the end of the simulated trial period.
Results
Model framework
We use the Mosquito Gene Drive Explorer (MGDrivE)
modeling framework [26] to model the spread of trans-
locations and UD
MEL
through spatially structured mos-
quito populations (Fig. 1). This is a genetic and spatial
extension of the lumped age-class model of mosquito
ecology [27] modified and applied by Deredec et al. [28]
and Marshall et al. [29] to the spread of homing gene
drive systems. The framework incorporates the egg, lar-
val, pupal, and adult life stages, with egg genotypes being
determined by maternal and paternal genotypes and the
allelic inheritance pattern of the gene drive system.
Spatial dynamics are accommodated through a metapop-
ulation structure in which lumped age-class models run
in parallel and migrants are exchanged between popula-
tions according to a zero-inflated exponential dispersal
kernel with parameters defined in Additional file 1:
Table S1 [22,3038]. Further details of the framework
are described in the Methodssection.
Applying the MGDrivE modeling framework to our re-
search questions, we incorporate the inheritance pat-
terns of reciprocal chromosomal translocations and
UD
MEL
into the inheritance module of the model (Fig. 1a,
b, Additional file 2: Fig. S1), the life cycle parameters of
Aedes aegypti (Additional file 1: Table S1) into the life
history module, and the distribution of households in
Yorkeys Knob (923 households) and Trinity Park (1301
households) along with their expected mosquito popula-
tion sizes and movement rates between them into the
landscape module (Fig. 1c). The suburb of Trinity Park
served as a control site for field releases of Wolbachia-
infected mosquitoes, to quantify the extent to which the
Wolbachia infection could spread from one community
to another, and plays a similar role for our simulated re-
leases of threshold-dependent gene drive systems.
The inheritance patterns that result from chromosomal
translocations are depicted in Fig. 1a. Chromosomal trans-
locations result from the mutual exchange between ter-
minal segments of two non-homologous chromosomes.
When translocation heterozygotes mate, several crosses re-
sult in unbalanced genotypes and hence unviable offspring,
resulting in a heterozygote reproductive disadvantage. This
results in bistable, threshold-dependent population dy-
namics, confirmed in laboratory drive experiments [14].
The inheritance patterns produced by the UD
MEL
system
are depicted in Fig. 1b. This system consists of two un-
linked constructs, each possessing a maternally expressed
toxin active during oogenesis and a zygotically active anti-
dote expressed by the opposite construct. Offspring are
more likely to have the antidote(s) to the maternal toxin(s)
when transgenes are present at moderate-to-high popula-
tion frequency. This produces threshold-dependent dy-
namics since, above a critical frequency, the selective
advantage of the antidotes outweighs the selective disad-
vantage of the toxins, and below the critical frequency, the
selective disadvantage of the toxins dominates. Mathemat-
ical models predict a threshold frequency of ~ 24% in the
absence of an additional fitness costa result consistent
with laboratory drive experiments for a construct having a
modest additional fitness cost [15].
Model validation
Using data from field trials of Wolbachia-infected Ae.
aegypti mosquitoes in Yorkeys Knob and Gordonvale,
Australia [21], we validated our modeling framework
prior to application to other threshold-dependent drive
systems. Wolbachia biases the offspring ratio in favor of
those carrying Wolbachia through a mechanism known
as cytoplasmic incompatibility, in which offspring of
matings between infected males and uninfected females
result in the death of some or all progeny, while matings
involving infected females tend to produce infected off-
spring [39]. For the wMel strain of Wolbachia that was
used in the Australian field trials, incompatible crosses
produce no viable offspring, and Wolbachia is inherited
by all offspring of infected females. There is also a fitness
cost associated with Wolbachia infection, the value of
which has been estimated between 0 and 20% [21,40,
41].
We used Wolbachia surveillance data from Fig. 1 of
Hoffmann et al. [21] to validate our model framework
and parameter values. Based on the description of the
field trials in Hoffmann et al. [21], we simulated weekly
releases of 20 Wolbachia-infected mosquitoes (10 female
and 10 male) per household at a coverage of 30% over
10 weeks in the spatially explicit landscapes of Yorkeys
Knob and Gordonvale with the exception that in Gor-
donvale, the fifth release was postponed by a week due
to a tropical cyclone. Model predictions were calculated
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for a variety of literature-based values of adult mortality
rate and Wolbachia-associated fitness cost [21,33,40
42] and were compared to observed Wolbachia preva-
lence over time. We found that model predictions most
closely matched field data for a baseline adult mortality
rate of 0.090 per day [33] and that predictions matched
field data quite well for both 5% and 10% fitness costs,
with a 10% fitness cost being closer to that estimated
elsewhere [21,40,41] (Additional file 3: Fig. S2). Model
predictions in Fig. 2use these parameter values, along
with others listed in Additional file 1: Table S1, and their
agreement with the observed field data provides good
validation of our modeling framework.
Population replacement and remediation for
translocations
The use of translocations for transforming pest popula-
tions was initially suggested by Serebrovskii [43] and
later Curtis [44] for the introduction of disease-
refractory genes into mosquito populations. A number
of models have been proposed to describe their spread
through randomly mating populations [14,16,45,46];
however, with one recent exception addressing spatial
structure [18], these have largely ignored insect life his-
tory and mating structure. Such models suggest that the
translocation need only exceed a population frequency
of 50%, in the absence of a fitness cost associated with
the translocation, to spread to fixation in a population,
which could conceivably be achieved through a single
seeding release round. Here, we find that incorporating
life history and population structure into mosquito
population dynamic models significantly increases re-
lease requirements.
In Figs. 3and 4, based on the precedent set by the
2011 Wolbcahia field trial [21], we consider weekly re-
leases of 20 adult Ae. aegypti males homozygous for the
translocation per household for given durations and
coverage levels, where coverage level is the proportion of
households that receive the releases. Releases are simu-
lated in the community of Yorkeys Knob, in which prior
releases of Wolbachia-infected mosquitoes suggested a
local population of ~ 15 adult Ae. aegypti per household
[22], and for mosquito movement rates inferred from
previous studies [25,37,38] (Additional file 1: Table S1).
For a coverage level of 100%, and in the absence of a fit-
ness cost, four weekly releases of 20 Ae. aegypti males
(~ 3:1 released to local males) are required for the trans-
location to spread to fixation in the community (Fig. 3),
as opposed to the single release expected when ignoring
life history and population structure [45]. As coverage is
reduced to 50%, the required number of releases in-
creases to 7, and for a coverage level of 25%, as seen for
the World Mosquito Program in Yorkeys Knob, the re-
quired number of releases increases to 16 (Figs. 3and 4).
Although large, these releases are achievable, considering
the much larger releases conducted for sterile insect pro-
grams [47].
To simulate remediation of a translocation, we con-
sider weekly releases of 20 adult Ae. aegypti wild-type
males in the community of Yorkeys Knob, whereby the
translocation has already reached fixation in that com-
munity. In the absence of a fitness cost associated with
the translocation, translocations are symmetrical in their
threshold dynamics, and so, for a coverage level of 100%,
four weekly releases are required for the translocation to
be completely remediated from the community, and for
Fig. 2 Comparison of model predictions to Wolbachia field trial data from Yorkeys Knob and Gordonvale, Australia. Field observations of
Wolbachia population frequency are depicted in light blue, with 95% binomial confidence intervals based on the frequency and sample size
reported for the 2011 field trials in Yorkeys Knob and Gordonvale [21]. Model predictions are depicted for an analogous release scheme
consisting of 20 Wolbachia-infected mosquitoes (10 female and 10 male) per household at a coverage of 30% over 10 weeks with the exception
that in Gordonvale, the fifth release was postponed by a week due to a tropical cyclone. Parameter values listed in Additional file 1: Table S1.
Wolbachia infection is associated with a 10% fitness cost. Agreement between observations and predictions is strong, providing good validation
of the modeling framework
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a coverage of 25%, 16 weekly releases are required for
the translocation to be completely remediated (Figs. 3
and 4). Encouraging features of these results are that (i)
remediation can be achieved through releases of non-
biting, non-disease-transmitting males; (ii) release sizes
are achievable; and (iii) despite the spatial household
structure, both replacement and remediation are
complete within the community. The time to
replacement is highly dependent on the coverage level
and number of releases, but is reasonably quick given
sufficient releases. At a coverage of 50%, 20 weekly re-
leases led to the translocation spreading to a frequency >
99% within half a year of the final release (or within
300 days of the first release). For equivalent wild-type
releases, this is the same as the time to > 99%
elimination.
Fig. 3 Replacement and remediation results for translocations and UD
MEL
. Time-series results are shown for a given number of weekly releases of
20 adult Ae. aegypti per household with the intent of population replacement or remediation in the community of Yorkeys Knob (Fig. 1c), and at
given coverage levels, where coverage is the proportion of households that receive the releases. For population replacement, releases are of
males homozygous for the translocation or UD
MEL
into a wild-type population. For remediation of translocations, releases are of wild-type males
into a population homozygous for the translocation. For UD
MEL
, remediation is not possible through releases of males only, and so mixed
remediationis considered, in which releases consist of 10 females and 10 males. (Top) Replacement and remediation are symmetric for
translocations. At a coverage of 50%, seven or more releases result in the translocation being driven to fixation or remediated from the
population. (Bottom) Release requirements for UD
MEL
are smaller for population replacement, but larger for mixed remediation. At a coverage of
50%, a single release results in UD
MEL
being driven to fixation
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Population replacement and remediation for UD
MEL
UD
MEL
was the first synthetic gene drive system to be
engineered that displays threshold-dependent dynamics
[15]. The system consists of two unlinked constructs,
each possessing a maternally expressed toxin active dur-
ing oogenesis and a zygotically active antidote expressed
by the opposite construct (Fig. 1b). At low population
frequencies, the maternal toxin confers a significant se-
lective disadvantage, leading to elimination, while at high
population frequencies, the zygotic antidote confers a se-
lective benefit in the context of a prevalent toxin, leading
to fixation. The dynamics of this system in randomly
mating populations have been characterized by Akbari
et al. [15], suggesting that the system need only exceed a
population frequency of ~ 24%, in the absence of a fit-
ness cost, to spread to fixation, while the wild-type must
exceed a population frequency of ~ 76% to eliminate the
construct. Both replacement and remediation should
therefore be achievable with 12 releases of transgenic
and wild-type organisms, respectively; however, as for
translocations, we find that incorporating life history
and population structure into our models increases re-
lease requirements in both cases.
In Figs. 3and 4,weconsiderweeklyreleasesof20adult
Ae. aegypti males homozygous at both loci for the UD
MEL
system in the community of Yorkeys Knob. The lower
threshold for UD
MEL
as compared to translocations means
that replacement is much easier to achieve for UD
MEL
.
For a coverage level of 50% or higher, and in the absence
of a fitness cost, a single release of 20 Ae. aegypti males
leads to the UD
MEL
system spreading to fixation through-
out the community (Fig. 3). As coverage is reduced to
25%, the required number of releases to achieve fixation
increases to two (Figs. 3and 4). As for translocations, the
time to replacement is highly dependent on the coverage
level and number of releases. From Fig. 3,itisapparent
Fig. 4 Replacement, remediation, and confinement outcomes for translocations and UD
MEL
. Outcomes are depicted for the proportion of 50
stochastic simulations of population replacement, remediation, and confinement of translocations and UD
MEL
that result in fixation of each
system. adFor replacement and remediation, each cell corresponds to a given number of releases (horizontal axis) and coverage level (vertical
axis), given 20 adult Ae. aegypti per household per release. For replacement, releases are of males homozygous for the system into a wild-type
population. For remediation of translocations, releases are of wild-type males into a population homozygous for the translocation, and for mixed
remediation of UD
MEL
, releases are of wild-type females and males into a population homozygous for UD
MEL
. Blue cells represent cases where all
simulations result in fixation of the system, and white cells represent cases where the wild-type is fixed in all simulations. e,fFor confinement,
each cell corresponds to a daily number of batch migration events (horizontal axis) of a given size (vertical axis) from Yorkeys Knob, where the
system is fixed, to Trinity Park, where the system is initially absent. Dark pink cells represent cases where all simulations result in fixation of the
system in Trinity Park, and white cells represent cases where the wild-type is fixed in all simulations. These results are encouraging for
translocations as systems for introducing transgenes in a local and reversible way as (i) they can be remediated through an achievable number of
male-only releases and (ii) they require more batch migration events to spread to neighboring communities
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that UD
MEL
reaches total allele fixation slowly, although
the number of individuals having at least one copy of the
transgene increases quickly. At a coverage of 50%, 10
weekly releases lead to wild-type individuals falling to a
frequency < 2% within 2.6 years of the final release (or
within 3 years of the first release).
Remediation, however, is more difficult to achieve for
UD
MEL
compared to translocations due to the higher
threshold that wild-type organisms must exceed to elimin-
ate UD
MEL
. Additionally, wild-type females must be in-
cluded in the releases to propagate the wild-type allele
because, assuming continued functioning of UD
MEL
com-
ponents, the maternal toxins of females having UD
MEL
at
both loci kill all offspring that do not inherit UD
MEL
at
both loci. To simulate remediation, we first consider
weekly releases of 10 adult Ae. aegypti wild-type females
and 10 adult males in the community of Yorkeys Knob. In
the absence of a fitness cost associated with the UD
MEL
construct, and for a coverage level of 75%, nine weekly re-
leases are required for a reduction in UD
MEL
allele fre-
quency over the first year (Fig. 3); however, a closer
inspection of the simulation results reveals that complete
remediation of UD
MEL
from the community is not pos-
sible even with 20 releases, as the UD
MEL
allele frequency
bounces back. Comparison of these results to those for a
panmictic population with a population size equal to that
of Yorkeys Knob reveals that complete remediation of
UD
MEL
can occur at coverage levels as low as 25% (for 16
or more weekly releases) (Additional file 4:Fig.S3).In-
spection of the spatially explicit simulation results sug-
gests that the rebound in UD
MEL
allele frequency in the
structured population is due to UD
MEL
remaining at
super-threshold levels after the wild-type releases in a
small number of households, and slowly recolonizing the
landscape following that. Complete remediation of UD
MEL
is possible, however, for 15 or more releases at a coverage
level of 100% (Fig. 4). These results make a strong case for
translocations as preferred systems to introduce trans-
genes in a local and reversible way as (i) remediation of
UD
MEL
requires releases of biting, vector-competent fe-
males and (ii) release requirements for these biting,
vector-competent females are burdensomely high due to
the high threshold that must be surpassed consistently
throughout a spatially structured population.
Confinement of translocations and UD
MEL
to release site
Confinement of translocations and UD
MEL
to partially
isolated populations has previously been modeled by
Marshall and Hay [16] and Akbari et al. [15]. In both
cases, two randomly mating populations were modeled
that exchange migrants at given rates. Population struc-
ture was otherwise ignored, as was mosquito life history.
Results from these analyses suggest that translocations
would spread and remain confined to populations for
which the migration rate is less than ~ 5.8% per mos-
quito per generation [16], and that UD
MEL
would remain
confined to populations for which the migration rate is
less than ~ 1.6% per mosquito per generation [15]. These
migration rates are relatively low; however, this may be
beneficial for the types of landscapes we are considering
here, whereby the system may spread between neighbor-
ing households, but not from one suburb to another. Re-
cently, Champer et al. [18] showed that translocations
would remain confined to and persist in a population
connected to another by a migration corridorunder a
range of parameter values.
For our landscape of interestthe suburbs of Yorkeys
Knob and Trinity Parkit is very unlikely that Ae.
aegypti mosquitoes will travel from one suburb to an-
other by their own flight. Extrapolating the exponential
dispersal kernel used in our simulations, fitted to data
from mark-release-recapture experiments collated by
Guerra et al., [38] suggests these events to be negligible,
before accounting for the fact that the intervening vege-
tated area may serve as a barrier to Ae. aegypti flight
[36]. Furthermore, rare migrant mosquitoes are unlikely
to cause the threshold frequency for either drive system
to be exceeded, thus making spatial spread due to such
movements unlikely. In considering confineability to the
release suburb, we therefore model batch migration,in
which several mosquitoes are carried, perhaps by a ve-
hicle, from one community to another at once. Batch
migration events could be thought of as several adult mos-
quitoes being carried at once, or perhaps more likely, as a
larval breeding site, such as a tyre, being carried from one
household to another, with several adults emerging from
the tyre following transport. We model batch migration
events as occurring between randomly chosen households,
and vary the number of daily migration events and the ef-
fective number of adults carried per event. For computa-
tional simplicity, we focus on migration events from
Yorkeys Knob, in which either system has already reached
fixation, to households in Trinity Park, which is initially
fixed for wild-type mosquitoes.
In Fig. 4e, f, we see that both the number and size of
daily batch migration events affect the chance of either
system establishing itself in the neighboring suburb,
Trinity Park. For translocations, ~ 16 daily migration
events of batches of 5 adults are required for spread in
Trinity Park. For batches of 10 adults, ~ 9 daily migra-
tion events are required, and for batches of 20 adults, ~
5 daily migration events are required. For UD
MEL
,~3
daily migration events of batches of 5 adults are required
for spread in Trinity Park, and for batches of 10 adults,
~ 2 daily migration events are required.
These results continue to make a strong case for trans-
locations as preferred systems to introduce transgenes in
a local and reversible way as (i) many more batch
Sánchez C. et al. BMC Biology (2020) 18:50 Page 8 of 14
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
migration events are required to lead to spread for trans-
locations as opposed to UD
MEL
and (ii) the rate of mi-
gration events required for translocations to spread is
higher than what would be expected between these com-
munities. Specifically, Wolbachia releases in Yorkeys
Knob in 2011 provide evidence for occasional batch mi-
grations to the nearby suburb of Holloways Beach; how-
ever, the spatio-temporal pattern of Wolbachia spread,
as inferred from monitored trap data, suggests only ~ 1
2 batch migration events over the course of a month,
consisting of less than 5 adult females per event [21].
Sensitivity analysis
A theoretical study by Khamis et al. [48] on toxin-
antidote-based underdominance gene drive systems,
similar to UD
MEL
but for which the toxins are zygotic
rather than maternal [20], found that the gene drive
threshold frequency is highly sensitive to (i) the increase
in adult mortality rate in organisms having the trans-
gene, (ii) the duration of the larval life stage, and (iii) the
parameters determining the character or strength of lar-
val density dependence. In Fig. 5and Additional file 5:
Fig. S4, we explore the sensitivity of our model outcomes
of replacement, remediation, and confinement for trans-
locations and UD
MEL
as we vary (i) the duration of the
larval life stage, (ii) the baseline adult mortality rate, (iii)
the fitness cost associated with the gene drive system,
and (iv) the mean adult dispersal distance. For transloca-
tions, we model a 10% fitness cost as a 10% reduction in
mean adult lifespan for organisms homozygous for the
translocation and a 5% reduction for organisms hetero-
zygous for the translocation. For UD
MEL
, since its inher-
itance bias is induced through the action of maternal
toxins, we model a 10% fitness cost as a 10% reduction
in female fecundity for organisms homozygous for
UD
MEL
at both loci, with 2.5% additive fitness costs con-
tributed by each transgenic allele.
For translocations, the associated fitness cost had the
greatest impact on the release scheme required for the
system to be fixed or remediated from the population,
given the life parameters considered (Fig. 5). A 10% fitness
cost led to ~ 10 weekly releases at a coverage of 50% being
required for the translocation to reach fixation (an in-
crease of 3 releases), while a 20% fitness cost led to ~ 13
weekly releases being required (an increase of 6 releases).
Small changes in the duration of the larval life stage had
minor impacts on the release requirements, with an in-
crease in larval lifespan of 2 days leading to one more
weekly release being required for the translocation to
reach fixation, and vice versa. A 2% change in the baseline
adult mortality rate and 50% change in the mean migra-
tion distance had negligible impact on release require-
ments. Remediation, on the other hand, requires fewer
wild-type releases when there is a fitness cost associated
with the translocation. A 10% fitness cost led to 5 weekly
releases at a coverage of 50% being sufficient to eliminate
the translocation (a decrease of 2 releases), and a 20% fit-
ness cost led to 4 weekly releases at a coverage of 50% be-
ing sufficient for elimination (a decrease of 3 releases).
Small changes in the duration of the larval life stage had
minor impacts on the wild-type release requirements for
elimination, with an increase in larval lifespan of 2 days or
a 2% decrease in the adult mortality rate leading to one
fewer release being required.
The sensitivity of our predictions regarding confine-
ment to the release site is of particular interest, as inva-
sion of a neighboring community may be more likely
under some parameter values than others. Fortunately, a
fitness cost associated with the translocation leads to a
higher threshold and hence more batch migration events
required for invasion of a neighboring community. A
10% fitness cost led to ~ 23 additional daily migration
events of 10 adults required for spread to Trinity Park,
and a 20% fitness cost led to ~ 67 additional daily mi-
gration events required (Fig. 5). Also noteworthy, a 2%
increase in the adult mortality rate led to ~ 2 fewer daily
migration events required for spread to Trinity Park
i.e., ~ 7 migration events for batches of 10 adults and ~
14 migration events for batches of 5 adults. While still
above inferred batch migration rates, this highlights that
there could exist parameter sets beyond those explored
for which invasion is feasible.
UD
MEL
displays similar parameter sensitivities regard-
ing fixation and batch migration outcomes as for trans-
locations, with the exception that these outcomes are
less sensitive to fitness costs (Additional file 5: Fig. S4),
likely due to the fact that fitness is accommodated
through a reduction in female fecundity rather than an
increase in adult mortality. A 20% fitness cost led to ~ 1
additional weekly release being required for the system
to spread to fixation, whether at a coverage of 25% or
50%. Similarly, for an invasion of Trinity Park, a 10% fit-
ness cost required ~ 1 additional daily migration event
of 5 adults, and a 20% fitness cost required ~ 2 add-
itional daily migration events. Of note, a 2% increase in
the adult mortality rate or a 2-day increase in the dur-
ation of the larval stage led to ~ 1 fewer daily migration
event required for spread to Trinity Park, making this
now very achievablei.e., ~ 23 migration events for
batches of 5 adults and ~ 12 migration events for
batches of 10 adults.
Finally, we conducted an analysis of the sensitivity of
our results to population structure, exploring the impact
of (i) removing all population structure by treating Yor-
keys Knob and Trinity Park as randomly mixing popula-
tions and (ii) incorporating heterogeneity in mosquito
household population size. Results of the comparison to
panmictic populations are depicted in Additional file 6:
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Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Fig. S5. Previously, we had seen that introducing popula-
tion structure greatly increases the release requirements
to eliminate UD
MEL
from a community (Additional file 4:
Fig. S3). The trend of higher release requirements in
structured populations is also seen for translocations, al-
though to a lesser extent, with one additional release re-
quired for either replacement or remediation at a
coverage of 75%, two additional releases required at a
coverage of 50%, and 78 additional releases required at
a coverage of 25%. Invasion of a neighboring population,
on the other hand, requires ~ 12 fewer daily migration
events in structured populations for batches of 10 adults
having either the translocation or UD
MEL
.
Incorporating heterogeneity in household mosquito
population size, we retain a mean of 15 adults per
household, as inferred from Wolbachia field trial data in
Yorkeys Knob [22], and distribute population sizes
across households according to a zero-inflated, truncated
exponential distribution with 55% of households having
no mosquitoes and none having more than 45 adults.
This distributional form, including zero inflation, is
based on results of a large field survey conducted across
a set of households in Kamphaeng Phet province,
Thailand [49]. Including this source of heterogeneity
substantially increases release requirements for replace-
ment and remediation with translocations, with 34
additional releases required at a coverage of 75%, 56
additional releases required at a coverage of 50%, and 20
releases being insufficient at a coverage of 25% (Add-
itional file 7: Fig. S6). Release requirements are margin-
ally increased for UD
MEL
, with ~ 12 additional releases
required at coverages of 25100%. These results are
likely due to the threshold frequency being more diffi-
cult to exceed in households with large numbers of mos-
quitoes, and this being less of an issue for UD
MEL
due to
its lower threshold frequency. Fortunately, population
size heterogeneity makes confinement more promising
for both systems, increasing the required number of
daily migration events for batches of 10 adults by 34
for translocations and by 12 for UD
MEL
.
Fig. 5 Sensitivity of model outcomes for replacement, remediation, and confinement of translocations. Changes are depicted in the proportion of
50 stochastic simulations that result in fixation for replacement, remediation, and confinement of translocations. Proportions are compared to
those in the first row of Fig. 4as we vary (i) the mean dispersal distance of adult mosquitoes (± 50%), (ii) the duration of the larval life stage (± 2
days), (iii) the baseline adult mortality rate (± 2%), and (iv) the fitness cost associated with being homozygous for the translocation (+ 10% or +
20%). Fitness costs have the greatest impact on the release scheme required for the system to be fixed or remediated from the population, given
the life parameters considered. Fitness costs also lead to more batch migration events being required for invasion of Trinity Park. A small increase
in the baseline adult mortality rate leads to slightly fewer batch migration events being required for invasion of Trinity Park; however, comparison
to migration rates inferred from field data suggests that confinement is still expected
Sánchez C. et al. BMC Biology (2020) 18:50 Page 10 of 14
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Discussion
The idea of using threshold-dependent gene drive sys-
tems to replace local populations of disease vectors with
varieties that are unable to transmit diseases has been
discussed for over half a century now, since Curtis [44]
famously proposed the use of translocations to control
diseases transmitted by mosquitoes. While Curtis had
been primarily concerned with introducing and spreading
genes into a population, as the technology nears imple-
mentation, confining them is also becoming a significant
concern. As CRISPR-based homing gene drive technology
edges closer to field application, concerns are increasingly
being raised regarding the invasiveness of these systems
[50,51], and systems such as split drive [11], daisy drive
[12], and threshold-dependent underdominant systems [7]
are gaining interest, at least during the trial phase of popu-
lation replacement technology [6]. In this paper, we model
the introduction of two drive systems, chromosomal
translocations and UD
MEL
, that have been engineered in
the laboratory and shown to display threshold-dependent
spread [14,15]. While previous papers have described the
population dynamics of these two systems in randomly
mating populations ignoring life history [1416,44], with
one recent paper including spatial structure [18], we
present the first analysis of these systems in a spatially
structured population including mosquito life history and
reflecting a well-characterized landscape where field trials
could conceivably be conducted [21].
Our results provide strong support for the use of
translocations to implement confineable and reversible
population replacement in structured Ae. aegypti popu-
lations. Regarding reversibility, translocations are prefer-
able to UD
MEL
as (i) they can be remediated through
releases of non-disease-transmitting male Ae. aegypti
and (ii) required releases sizes are achievable (~ 10
weekly releases at a coverage level of 50%). UD
MEL
re-
quires less effort to introduce into a population, but is
much more difficult to remove once it has been intro-
duced, requiring a very large number of both males and
disease-transmitting females to be released. This high-
lights the benefit of a ~ 50% threshold for reversible
population replacement: the symmetry allows both re-
placement and remediation to be achieved with similar
effort. Extreme underdominance is another example of
system with a 50% threshold [20,52]. Regarding confine-
ability, translocations again outperform UD
MEL
as ~ 16
daily migration events of batches of 5 Ae. aegypti adults
are required for translocations to spread to the neigh-
boring suburb of Trinity Park, while UD
MEL
can spread
to Trinity Park given only ~ 3 daily migration events (or
23 daily migration events for alternative model parame-
terizations). The true batch migration rate between sub-
urbs is expected to be smaller than either of these [21];
however, the rate required for translocations to spread is
highly unlikely to be reached, while the rate for UD
MEL
is conceivable.
As with any modeling study, there are limitations in-
herent in our analysis. Several of the parameters we as-
sumed to be constant here would indeed be dynamic in
a real intervention scenario. At the genetic level, lab ex-
periments suggest non-outbred individuals homozygous
for the translocation had a fitness cost that largely disap-
peared once offspring were produced that were the
product of at least one wild-type individual [14]. Models
fitted to data from UD
MEL
drive experiments also sug-
gested dynamic fitness costs that depended on the fre-
quency of transgenic organisms in the population [15].
Another recent modeling study highlights the possibility
of toxin and antidote mutational breakdown for under-
dominance constructs; however, this is expected to be
impactful over a larger timescale than considered here
(hundreds of generations) [53]. At the ecological level,
our model of Ae. aegypti life history [26], based on the
lumped age-class model of Hancock and Godfray [27],
assumes the existence of a constant equilibrium popula-
tion size and other constant ecological parameters, such
as adult death rate and larval development times. These
parameters have indeed been shown to vary in space and
time, and in response to local mosquito density [54,55],
which our sensitivity analyses suggest could have signifi-
cant impacts on release thresholds and gene drive out-
comes [48] (Figs. 5&S4). At the landscape level, we
have assumed a relatively homogenous distribution of
mosquitoes per household, and movement rates between
households that depend only on distance and household
distribution. Extensive landscape heterogeneities have
been shown to slow and alter the spread of Wolbachia
[41,56] and would likely impact the spread of transloca-
tions and UD
MEL
as well. Future work that helps to
characterize the environmental drivers of mosquito
population dynamics will inform iterative model devel-
opment to address this.
Conclusions
In conclusion, our analysis supports the use of transloca-
tions as a threshold-dependent drive system capable of
spreading disease-refractory genes into structured Ae.
aegypti populations in a confineable and reversible man-
ner. If such a system were engineered in Ae. aegypti,it
would be an excellent candidate for the introduction of
disease-refractory genes during the trial phase of popula-
tion replacement technology, or whenever a localized re-
lease was otherwise desired. As the technology nears
implementation, further ecological work characterizing
the density dependencies, seasonality, and spatial hetero-
geneities of Ae. aegypti populations will be essential to
enhance model predictions in preparation for field trials.
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Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Methods
We used the MGDrivE framework [26](https://marshalllab.
github.io/MGDrivE/) to simulate releases of adult Ae.
aegypti males homozygous for one of two threshold-
dependent gene drive systemsreciprocal chromosomal
translocations or UD
MEL
in the community of Yorkeys
Knob in Queensland, Australia. To simulate remediation,
we modeled releases of wild-type adult Ae. aegypti into
populations in Yorkeys Knob already fixed for the gene
drive system. Houses receiving releases were randomly
chosen from a uniform distribution. These were conserved
within simulation runs, but varied between runs. To deter-
mine confineability, we simulated batch migration events
from Yorkeys Knob (fixed for the gene drive system) to
the neighboring community of Trinity Park (initially wild-
type). The MGDrivE framework models the egg, larval,
pupal, and adult (male and female) mosquito life stages
implementing a daily time step, overlapping generations
and a mating structure in which adult males mate
throughout their lifetime, while adult females mate
once upon emergence, retaining the genetic material
of the adult male with whom they mate for the dur-
ation of their adult lifespan. Density-independent
mortality rates for the juvenile life stages are assumed
to be identical and are chosen for consistency with
the population growth rate in the absence of density-
dependent mortality. Additional density-dependent
mortality occurs at the larval stage, the form of which
is taken from Deredec et al. [28]. Full details of the
modeling framework are available in the S1 Text of
Sánchez et al. [26]andinthesoftwaredocumentation
available at https://marshalllab.github.io/MGDrivE/
docs/reference/. Parameters describing Ae. aegypti life
history and the gene drive systems and landscape of
interest are listed in Additional file 1:TableS1.
The inheritance patterns for reciprocal chromosomal
translocations (depicted in Fig. 1a) and UD
MEL
(depicted
in Additional file 2: Fig. S1) are modeled within the in-
heritance module of the MGDrivE framework [26], and
their impacts on female fecundity and adult lifespan are
implemented in the life history module. The distribution
of households in Yorkeys Knob, Trinity Park, and Gor-
donvale was taken from OpenStreetMap (https://www.
openstreetmap.org/) (Fig. 1c). We implement the sto-
chastic version of the MGDrivE framework to capture
the randomness associated with events that occur in
small populations, such as households, which serve as
nodes in the landscape modeled here. In the stochastic
implementation of the model, the number of eggs pro-
duced per day by females follows a Poisson distribution,
the number of eggs having each genotype follows a
multinomial distribution, all survival/death events follow
a Bernoulli distribution, and female mate choice follows
a multinomial distribution with probabilities given by
the relative frequency of each adult male genotype in the
population.
Supplementary information
Supplementary information accompanies this paper at https://doi.org/10.
1186/s12915-020-0759-9.
Additional file 1: Table S1. Life history, population size and movement
parameters for Aedes aegypti in Cairns, Australia.
Additional file 2: Figure S1. Complete inheritance pattern of UD
MEL
.
UD
MEL
is composed of two unlinked constructs (here referred to as A and
B), each consisting of a maternally-expressed toxin and a zygotically-
expressed antidote for the toxin on the opposite construct (see Fig. 1b).
The cross here represents matings between all nine possible parental ge-
notypes (+represents the wild-type allele, and Aand Brepresent al-
leles corresponding to the two UD
MEL
constructs). Offspring lacking the
antidotes to the maternal toxins produced by their mother are unviable
(shaded). At high population frequencies, the selective advantage on the
constructs, by virtue of the antidotes, outweighs the fitness load due to
the toxins, and hence results in frequency-dependent spread.
Additional file 3: Figure S2. Comparison of model predictions to
Wolbachia field trial data from Yorkeys Knob and Gordonvale, Australia
for a variety of parameterizations. Field observations of Wolbachia
population frequency are depicted in light blue, with 95% binomial
confidence intervals based on the frequency and sample size reported
for the 2011 field trials in Yorkeys Knob and Gordonvale. Model
predictions are depicted for an analogous release scheme consisting of
20 Wolbachia-infected mosquitoes (10 female and 10 male) per
household at a coverage of 30% over 10 weeks with the exception that,
in Gordonvale, the fifth release was postponed by a week due to a
tropical cyclone. Parameter values are listed in Additional file 1: Table S1.
Model predictions more closely match observed field data for a baseline
adult mortality rate of 0.090 per day (Top), as compared to one of 0.050
per day (Bottom). Observations match predictions well for both 5% and
10% fitness costs associated with Wolbachia. Agreement between
observations and predictions is strong, providing good validation for the
modeling framework.
Additional file 4: Figure S3. Remediation results for UD
MEL
in spatially-
explicit and panmictic populations. Time-series results are shown for a
given number of weekly releases of 20 adult wild-type Ae. aegypti per
household (10 female and 10 male) with the intent of remediation in the
community of Yorkeys Knob (Fig. 1c), and at given coverage levels, where
coverage is the proportion of households that receive the releases. (Top)
Remediation in the spatially-explicit population is extremely difficult. At a
coverage level of 75%, nine weekly releases are required for a reduction
in UD
MEL
allele frequency over the first year; however, complete remedi-
ation is not possible even with 20 releases. Complete remediation is pos-
sible at a coverage level of 100% for 15 or more weekly releases. (Bottom)
Remediation in the panmictic population is much less demanding. At a
coverage level of 25%, it can be achieved with 16 or more releases, and
at a coverage level of 50%, it can be achieved with nine or more
releases.
Additional file 5: Figure S4. Sensitivity of model outcomes for
replacement and confinement of UD
MEL
. Changes are depicted in the
proportion of 50 stochastic simulations that result in fixation for
replacement and confinement of UD
MEL
. Proportions are compared to
those in the second row of Fig. 4as we vary: i) the mean dispersal
distance of adult mosquitoes (+/- 50%), ii) the duration of the larval life
stage (+/- 2 days), iii) the baseline adult mortality rate (+/- 2%), and iv)
the fitness cost associated with being homozygous for the translocation
(+10% or +20%). UD
MEL
displays similar parameter sensitivities regarding
fixation and batch migration outcomes as for translocations (Fig. 5), with
the exception that these outcomes are less sensitive to fitness costs,
likely due to the fact that fitness is accommodated through a reduction
in female fecundity rather than an increase in adult mortality.
Additional file 6: Figure S5. Sensitivity of model outcomes for
translocations and UD
MEL
comparing spatially-explicit and panmictic
Sánchez C. et al. BMC Biology (2020) 18:50 Page 12 of 14
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
populations. Changes are depicted in the proportion of 50 stochastic sim-
ulations that result in fixation for replacement, remediation and confine-
ment of translocations and UD
MEL
. Proportions are compared to those in
Fig. 4as we simulate a model where Yorkeys Knob and Trinity Park are
panmictic populations of equivalent size to their spatially-explicit versions.
Introducing population structure greatly increases the release require-
ments to remediate UD
MEL
(as seen in Additional file 4: Fig. S3), and sub-
stantially increases the release requirements for replacement or
remediation of translocations. Invasion of a neighboring population, on
the other hand, requires moderately fewer daily migration events in
structured populations for both translocation and UD
MEL
.
Additional file 7: Figure S6. Sensitivity of model outcomes for
translocations and UD
MEL
comparing spatially-explicit populations with
and without heterogeneity in household mosquito population size.
Changes are depicted in the proportion of 50 stochastic simulations that
result in fixation for replacement, remediation and confinement of trans-
locations and UD
MEL
. Proportions are compared to those in Fig. 4as we
simulate a model where household mosquito population size is distrib-
uted according to a zero-inflated, truncated exponential distribution with
a mean of 15 adults, 55% of households having no mosquitoes, and
none having more than 45 adults. Introducing household population size
heterogeneity substantially increases release requirements for replace-
ment and remediation with translocations, and marginally increases re-
lease requirements for replacement with UD
MEL
. Fortunately, population
size heterogeneity makes confinement moderately more promising for
both systems.
Acknowledgements
The authors would like to thank Dr. Gregory Lanzaro, Dr. Yoosook Lee, Dr.
Tomás León, and Ms. Partow Imani for discussions on Aedes aegypti life
history and dispersal behavior.
Authorscontributions
JMM and HMSC conceptualized the study. HMSC, JBB, and SLW performed
the experiments and visualized the results. GR and OSA contributed to the
interpretation of the results. JMM wrote the first draft of the manuscript. All
authors contributed to the writing of and approved the final manuscript.
Funding
This work was supported by a DARPA Safe Genes Program Grant (HR0011-
17-2-0047), awarded to OSA and JMM, and funds from the Innovative
Genomics Institute, awarded to JMM.
Availability of data and materials
The distribution of households in Yorkeys Knob, Trinity Park, and Gordonvale
was obtained from OpenStreetMap (https://www.openstreetmap.org/). The
software package, MGDrivE, is available on GitHub (https://marshalllab.github.
io/MGDrivE/)[26]. All information required to reproduce the simulations is
available in the GitHub deposition, the manuscript, and its additional files.
Ethics approval and consent to participate
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Division of Epidemiology and Biostatistics, School of Public Health,
University of California, Berkeley, CA 94720, USA.
2
Biophysics Graduate Group,
University of California, Berkeley, CA 94720, USA.
3
Mosquito Control
Laboratory, QIMR Berghofer Medical Research Institute, Brisbane, Australia.
4
Cell and Developmental Biology Section, Division of Biological Sciences,
University of California, San Diego, CA 92093, USA.
5
Innovative Genomics
Institute, Berkeley, CA 94720, USA.
Received: 18 June 2019 Accepted: 26 February 2020
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... Several synthetic RNAi-based TA gene drives have been developed theoretically or tested on mosquitos and flies to control diseases spread by those vectors. Some of these developed drives require a significant introduction threshold to ensure their spread, which makes them suitable to small secluded area populations with reduced gene flow [42][43][44][45][46][47]. The main RNAi-based TA drive systems developed to date are summarized in Table 1. ...
... Findings from this study revealed high threshold population replacement in laboratory populations, while it remains to be explored in wild populations. Likewise, a recent modeling study supported the use of translocations for spreading disease-resistant genes into Aedes aegypti populations in a confined and reversible way [47]. ...
... Population replacement [42] UD MEL Theoretical design on Aedes aegypti UD MEL system comprises two unlinked constructs. The maternally expressed toxin is designed on the first construct, whereas its zygotically expressed antidote is present on a separate construct Population replacement [47] Combined MEDEAunderdominance system ...
Article
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Ongoing pest and disease outbreaks pose a serious threat to human, crop, and animal lives, emphasizing the need for constant genetic discoveries that could serve as mitigation strategies. Gene drives are genetic engineering approaches discovered decades ago that may allow quick, super-Mendelian dissemination of genetic modifications in wild populations, offering hopes for medicine, agriculture, and ecology in combating diseases. Following its first discovery, several naturally occurring selfish genetic elements were identified and several gene drive mechanisms that could attain relatively high threshold population replacement have been proposed. This review provides a comprehensive overview of the recent advances in gene drive research with a particular emphasis on CRISPR-Cas gene drives, the technology that has revolutionized the process of genome engineering. Herein, we discuss the benefits and caveats of this technology and place it within the context of natural gene drives discovered to date and various synthetic drives engineered. Later, we elaborate on the strategies for designing synthetic drive systems to address resistance issues and prevent them from altering the entire wild populations. Lastly, we highlight the major applications of synthetic CRISPR-based gene drives in different living organisms, including plants, animals, and microorganisms.
... limited in their spread and persistence) and reversible (i.e. recallable from the environment in the event of unwanted consequences) [24,[41][42][43][44]. Several approaches have been proposed to restrict the spread of engineered GDs within a specified target population or geographic region, or to reduce their persistence in target populations over the course of several generations [44][45][46]. ...
... While current research is investigating the development of engineered GDs in insect populations and deploying them, it will take many years before they can be applied to practical disease vector/pest management. At present, some GDMIs are either in development or have been tested experimentally in the laboratory, often with multigenerational data and model simulations [17,19,20,27,28,33,35,39,43,45,46,[53][54][55][56][57][58][59][60][61][62][63][64][65][66][67]. However, no "contemporary" GDMIs have been assessed in small-scale physically and/or ecologically confined field trials, or open release trials [5,8,10,15,68]. ...
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Potential future application of engineered gene drives (GDs), which bias their own inheritance and can spread genetic modifications in wild target populations, has sparked both enthusiasm and concern. Engineered GDs in insects could potentially be used to address long-standing challenges in control of disease vectors, agricultural pests and invasive species, or help to rescue endangered species, and thus provide important public benefits. However, there are concerns that the deliberate environmental release of GD modified insects may pose different or new harms to animal and human health and the wider environment, and raise novel challenges for risk assessment. Risk assessors, risk managers, developers, potential applicants and other stakeholders at many levels are currently discussing whether there is a need to develop new or additional risk assessment guidance for the environmental release of GD modified organisms, including insects. Developing new or additional guidance that is useful and practical is a challenge, especially at an international level, as risk assessors, risk managers and many other stakeholders have different, often contrasting, opinions and perspectives toward the environmental release of GD modified organisms, and on the adequacy of current risk assessment frameworks for such organisms. Here, we offer recommendations to overcome some of the challenges associated with the potential future development of new or additional risk assessment guidance for GD modified insects and provide considerations on areas where further risk assessment guidance may be required.
... aegypti simulation was based on studies that 648 suggest this to be a reasonable estimate for the number of Ae. aegypti adults within a 649 characteristic dispersal radius in a variety of settings [19][20][21]; however, Ae. aegypti 650 adults tend to be relatively sessile, often remaining within the same household unit for 651 the duration of their lifetime [11]. With this in mind, a more accurate model might be 652 Ae. aegypti populations distributed across households with migration between them [35]. 653 Areas of future research would be to test the robustness of single-population CKMR 654 methods to data from spatially-structured simulations [36], and to incorporate spatial 655 structure into the CKMR analyses themselves, opening the potential to estimate 656 dispersal parameters using these methods. ...
Preprint
Full-text available
Close-kin mark-recapture (CKMR) methods have recently been used to infer demographic parameters such as census population size and survival for fish of interest to fisheries and conservation. These methods have advantages over traditional mark-recapture methods as the mark is genetic, removing the need for physical marking and recapturing that may interfere with parameter estimation. For mosquitoes, the spatial distribution of close-kin pairs has been used to estimate mean dispersal distance, of relevance to vector-borne disease transmission and novel biocontrol strategies. Here, we extend CKMR methods to the life history of mosquitoes and comparable insects. We derive kinship probabilities for mother-offspring, father-offspring, full-sibling and half-sibling pairs, where an individual in each pair may be a larva, pupa or adult. A pseudo-likelihood approach is used to combine the marginal probabilities of all kinship pairs. To test the effectiveness of this approach at estimating mosquito demographic parameters, we develop an individual-based model of mosquito life history incorporating egg, larva, pupa and adult life stages. The simulation labels each individual with a unique identification number, enabling close-kin relationships to be inferred for sampled individuals. Using the dengue vector Aedes aegypti as a case study, we find the CKMR approach provides unbiased estimates of adult census population size, adult and larval mortality rates, and larval life stage duration for logistically feasible sampling schemes. Considering a simulated population of 3,000 adult mosquitoes, estimation of adult parameters is accurate when a total of 1,000 adult females are sampled biweekly-to-fortnightly over a three month period. Estimation of larval parameters is accurate when adult sampling is supplemented with a total of 4,000 larvae sampled biweekly over the same period. As the cost of genome sequencing declines, these methods hold great promise for characterizing the demography of mosquitoes and comparable insects of epidemiological and agricultural significance. Author summary Close-kin mark-recapture (CKMR) methods are a genetic analogue of traditional mark-recapture methods in which the frequency of marked individuals in a sample is used to infer demographic parameters such as census population size and mean dispersal distance. In CKMR, the mark is a close-kin relationship between individuals (parents and offspring, siblings, etc.). While CKMR methods have mostly been applied to aquatic species to date, opportunities exist to apply them to insects and other terrestrial species. Here, we explore the application of CKMR to mosquitoes, with Aedes aegypti , a primary vector of dengue, chikungunya and yellow fever, as a case study. By analyzing simulated Ae. aegypti populations, we find the CKMR approach provides unbiased estimates of adult census population size, adult and larval mortality rates, and larval life stage duration. Optimal sampling schemes are consistent with Ae. aegypti ecology and field studies, requiring only minor adjustments to current mosquito surveillance programs. This study represents the first theoretical exploration of the application of CKMR to an insect species, and demonstrates its potential for characterizing the demography of insects of epidemiological and agricultural importance.
... aegypti populations, we used the MGDrivE simulation framework 23 . While MGDrivE was designed for gene drives, thanks to the modular design, it is quite flexible at handling any inheritance pattern describable using Punnett squares (see ref. 39 ). 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 ♀'s retain the genetic material of the adult ♂ with whom they mate for the duration of their adult lifespan. ...
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The mosquito Aedes aegypti is the principal vector for arboviruses including dengue/yellow fever, chikungunya, and Zika virus, infecting hundreds of millions of people annually. Unfortunately, traditional control methodologies are insufficient, so innovative control methods are needed. To complement existing measures, here we develop a molecular genetic control system termed precision-guided sterile insect technique (pgSIT) in Aedes aegypti . PgSIT uses a simple CRISPR-based approach to generate flightless females and sterile males that are deployable at any life stage. Supported by mathematical models, we empirically demonstrate that released pgSIT males can compete, suppress, and even eliminate mosquito populations. This platform technology could be used in the field, and adapted to many vectors, for controlling wild populations to curtail disease in a safe, confinable, and reversible manner.
... recallable from the environment) (e.g. Backus and Delborne, 2019;Li et al., 2020;Maselko et al., 2020;Sánchez et al., 2020b;Webster et al., 2020;Buchman et al., 2021;Hay et al., 2021;Kandul et al., 2021;Oberhofer et al., 2021;Terradas et al., 2021;Willis and Burt, 2021). Several theoretical approachessome of which have already been tested experimentally under laboratory settingshave been proposed to restrict spread of engineered gene drives within a specified target population or geographic region, or their persistence (Raban et al., 2020). ...
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Full-text available
The ability to engineer gene drives (genetic elements that bias their own inheritance) has sparked enthusiasm and concerns. Engineered gene drives could potentially be used to address long-standing challenges in the control of insect disease vectors, agricultural pests and invasive species, or help to rescue endangered species. However, risk concerns and uncertainty associated with potential environmental release of gene drive modified insects (GDMIs) have led some stakeholders to call for a global moratorium on such releases or the application of other strict precautionary measures to mitigate perceived risk assessment and risk management challenges. Instead, we provide recommendations that may help to improve the relevance of risk assessment and risk management frameworks for environmental releases of GDMIs. These recommendations include: (1) developing additional and more practical risk assessment guidance to ensure appropriate levels of safety; (2) making policy goals and regulatory decision-making criteria operational for use in risk assessment so that what constitutes harm is clearly defined; (3) ensuring a more dynamic interplay between risk assessment and risk management to manage uncertainty through closely interlinked pre-release modelling and post-release monitoring; (4) considering potential risks against potential benefits, and comparing them with those of alternative actions to account for a wider (management) context; and (5) implementing a modular, phased approach to authorisations for incremental acceptance and management of risks and uncertainty. Along with providing stakeholder engagement opportunities in the risk analysis process, the recommendations proposed may enable risk managers to make choices that are more proportionate and adaptive to potential risks, uncertainty and benefits of GDMI applications, and socially robust.
... High temperatures may reduce CI and transmissibility of some Wolbachia strains though, which may impact their establishment and persistence in some field locations 70 . Theoretical modeling suggests that some HGD systems can be established in a population within a year of the initial release 25,71 . There is no current field data on HGDs, however, to validate these models. ...
Article
Full-text available
Mosquito-borne diseases, such as dengue and malaria, pose significant global health burdens. Unfortunately, current control methods based on insecticides and environmental maintenance have fallen short of eliminating the disease burden. Scalable, deployable, genetic-based solutions are sought to reduce the transmission risk of these diseases. Pathogen-blocking Wolbachia bacteria, or genome engineering-based mosquito control strategies including gene drives have been developed to address these problems, both requiring the release of modified mosquitoes into the environment. Here, we review the latest developments, notable similarities, and critical distinctions between these promising technologies and discuss their future applications for mosquito-borne disease control. Mosquito-borne diseases pose significant global health burdens. In this review, the authors explore Wolbachia and genome engineering approaches to mosquito-borne disease population control.
... Environmental Vector Control Methods: Before the introduction of chemical insecticides, such as DDT (dichlorodiphenyl-trichloroethane) in 1940, vector control was mainly limited to environmental management, which focused on disrupting local breeding sites and manipulating vector behavior and ecology [14]. Types of environmental vector control include house screens, aquatic habitat drainage, vegetation clearance, water container coverage, hygienic measures, waste management, protective clothing, and various other agricultural and housing improvements [15]. Looking back at the history of vector control practices, a form of environmental management was always implemented since past generations successfully connected fevers to the proximity of surface waters, like swamps and marshes [16]. ...
Chapter
Despite its significance in public health, the population genomics of Aedes aegypti is in its infancy. We suspect this dichotomy is largely driven by its relatively large genome size affecting cost of sequencing. Efforts to capture the subset of genomic markers have been conducted using exome capture and microarray technologies. The focus of the population genomic studies has been largely on identifying population structure and investigating insecticide resistance. Recent advances in library preparation technology and availability of a chromosome-level reference assembly allowed whole genome sequence-based study. These developments open new questions that can be addressed with further population genomic data. As with Anopheles gambiae, interests in advancing gene drive-based mosquito control methods are propelling additional collections of Ae. aegypti datasets aimed at characterizing dispersal and population size. With increasing interest in novel genetic control and their application in the field, we expect to accelerate research in characterizing field populations using genomics approaches in the near future.
Chapter
Full-text available
The discovery of CRISPR-based gene editing and its application to homing-based gene drive has been greeted with excitement, for its potential to control mosquito-borne diseases on a wide scale, and concern, for the invasiveness and potential irreversibility of a release. At the same time, CRISPR-based gene editing has enabled a range of self-limiting gene drive systems to be engineered with much greater ease, including (1) threshold-dependent systems, which tend to spread only when introduced above a certain threshold population frequency, and (2) temporally self-limiting systems, which display transient drive activity before being eliminated by virtue of a fitness cost. As these CRISPR-based gene drive systems are yet to be field-tested, plenty of open questions remain to be addressed, and insights can be gained from precedents set by field trials of other novel genetics-based and biological control systems, such as trials of Wolbachia-transfected mosquitoes, intended for either population replacement or suppression, and trials of genetically sterile male mosquitoes, either using the RIDL system (release of insects carrying a dominant lethal gene) or irradiation. We discuss lessons learned from these field trials and implications for a phased exploration of gene drive technology, including homing-based gene drive, chromosomal translocations, and split gene drive as a system potentially suitable for an intermediate release.
Article
Full-text available
1.Malaria, dengue, Zika, and other mosquito‐borne diseases continue to pose a major global health burden through much of the world, despite the widespread distribution of insecticide‐based tools and antimalarial drugs. The advent of CRISPR/Cas9‐based gene editing and its demonstrated ability to streamline the development of gene drive systems has reignited interest in the application of this technology to the control of mosquitoes and the diseases they transmit. The versatility of this technology has enabled a wide range of gene drive architectures to be realized, creating a need for their population‐level and spatial dynamics to be explored. 2.We present MGDrivE (Mosquito Gene Drive Explorer): a simulation framework designed to investigate the population dynamics of a variety of gene drive architectures and their spread through spatially‐explicit mosquito populations. A key strength of the MGDrivE framework is its modularity: a) a genetic inheritance module accommodates the dynamics of gene drive systems displaying userdefined inheritance patterns, b) a population dynamic module accommodates the life history of a variety of mosquito disease vectors and insect agricultural pests, and c) a landscape module generates the metapopulation model by which insect populations are connected via migration over space. 3.Example MGDrivE simulations are presented to demonstrate the application of the framework to CRISPR/Cas9‐based homing gene drive for: a) driving a disease‐refractory gene into a population (i.e. population replacement), and b) disrupting a gene required for female fertility (i.e. population suppression), incorporating homing‐resistant alleles in both cases. Further documentation and use examples are provided at the project's Github repository. 4.MGDrivE is an open‐source R package freely available on CRAN. We intend the package to provide a flexible tool capable of modeling novel inheritance‐modifying constructs as they are proposed and become available. The field of gene drive is moving very quickly, and we welcome suggestions for future development.
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Underdominance gene drives are frequency-dependent drives that aim to spread a desired homozygote genotype within a population. When the desired homozygote is released above a threshold frequency, heterozygote fitness disadvantage acts to drive the desired trait to fixation. Underdominance drives have been proposed as a way to control vector-borne disease through population suppression and replacement in a spatially contained and reversible way-benefits that directly address potential safety concerns with gene drives. Here, ecological and epidemiological dynamics are coupled to a model of mosquito genetics to investigate theoretically the impact of different types of underdominance gene drive on disease prevalence. We model systems with two engineered alleles carried either on the same pair of chromosomes at the same locus or homozygously on different pairs at different loci, genetic lethality that affects both sexes or only females, and bi-sex or male-only releases. Further, the different genetic and ecological fitness costs that can arise from genetic modification and artificial rearing are investigated through their effect on the population threshold frequency that is required to trigger the drive mechanism. We show that male-only releases must be significantly larger than bi-sex releases to trigger the underdominance drive. In addition, we find that female-specific lethality averts a higher percentage of disease cases over a control period than does bi-sex lethality. Decreases in the genetic fitness of the engineered homozygotes can increase the underdominance threshold substantially, but we find that the mating success of transgenic mosquitoes with wild-type females (influenced by a lack of competitiveness or the evolution of behavioural resistance in the form of active female mate preference) and the longevity of artificially-reared mosquitoes are vitally important to the success chances of underdominance based gene drive control efforts.