The Genetics of Vector-Host Interactions:
Alternative Strategies for Genetic Engineering
for Malaria Control
Willem Takken* and Carlo Costantini
As long as behavioural genetics remains a scientific backwater, much of the genome sequence willlook like uninterruptible gibberish.
—Of Flies and Men
, by Dean H. Hamer, Scientific American
, June 1999
Malaria transmission is accomplished byry the innate behavioural trait of mosquitoes to
ingest vertebrate blood required for egg production. As human malaria parasites are,by definition, cir
e factors that affect mosquito-host interactions to assess ho ute
culating between humans and certain anopheline species, dis-
ruption of mosquito-human contact will effectively inhibit transmission of the malaria parasite.
Here we explor
exploited to reduce malaria transmission. Host preference in mosquitoes is geneticallycontrolled, and it is argued that a change in host preference could result in less human bitingand in reduced parasite transmission. The effect of this is being demonstrated using the vecto-rial capacity equation, in which the human biting index and mosquito survival are represented.
It is argued that effective malaria control strategies should be based on a reduction of humanbiting preference coupled with reduced survival. Strategic interventions based on behaviouralmanipulation and ecological change may affect the biting fraction of the vector population tosuch an extent that the vectorial capacity is significantly affected. In some cases this mayrequire genetic modification of organisms (GMO) technology, but mechanical or physicaltechniques should also be considered.
, the causative agents of one of the deadliest diseases on Earth: malaria,
are unquestionably among the most successful of the vector-borne parasites, overcoming thenatural r
esistance mechanisms of their vertebrate and arthropod hosts, and showing strong
resilience against conventional methods of disease control. This results in more than one milliondeaths every year due to this disease. Genetic variability of the Plasmodia
and their associationwith a relatively small group of mosquitoes provide the key to interpret this success. As sexualrecombination in the Plasmodia
, and hence the mechanism insuring a higher degree of geneticvariability, is accomplished in the mosquito midgut, mosquito fitness and behaviour are ofcritical impor Eurekah / Landes Bioscience
tance for the parasite. The concept of genetic engineering technologies as an
alternative method for malaria control is dominated by the notion of manipulation of vectorcompetence through modification of the mosquito natural immunity against the parasite.1,2
*Corresponding Author: Willem Takken—Laboratory of Entomology, Wageningen University,
P.O Box 8031, 6700 EH Wageningen, The Netherlands. Email: [email protected]
Genetically Modified Mosquitoes for Malaria Control
, edited by Christophe Boëte.
Genetically Modified Mosquitoes for Malaria Control
Here we argue that other traits of anopheline mosquitoes could represent adequate targets
for intervention by genetic manipulation, and might result as effective means for the interruptionof malaria parasite transmission. This argument is encouraged by the recent publication of themalaria mosquito complete genome sequence and the continuing development ofhigh-throughput genomic technologies, which everyone hopes will provide in the future thetechnological basis to investigate and identify novel targets for intervention.3 However, as ajudicious reminder evoked in the citation opening this chapter, we need to take into accountand relate such technological advances with the grassroots biology of the vectors, hence to adeeper understanding of their field ecology and behaviour, or our efforts are bound to fail.4 Asmany studies on insect transgenesis originate in the laboratory, it is obvious that the transfer ofthis technology from the bench to the field requires specific attention lest one ends up with amosquito that has lost several of its natural traits.5 It is didactic and perhaps farsighted thatsuch a reminder comes from the community of scholars studying the fruit fly Drosophilamelanogaster
as a model organism (whose genome sequence was completed well before that ofthe malaria mosquito), as their intimate knowledge of the biology and genetics of this species isarguably unparalleled in the animal kingdom.6,7 Intrinsic and extrinsic factors determinemosquito fitness and vector competence.8 B
ehavioural traits like host preference, diurnal rhythms
and locomotion affect the uptake and spread of the parasite. The implications of these aspectsof vector biology with respect to malaria transmission and control are now discussed in thelight of the pr
oposed GMO technology for malaria control. Different aspects of mosquito
y a discussion about whether genetic modification of
behavioural traits might be considered as a potential strategy for disease control radicallydifferent from the strategies based on vector competence.
The success of a malaria parasite can be measured by the rate of its spread through a human
community, expressed as Basic Reproductive Rate.9 In practical terms, we measure this throughthe vectorial capacity,10 a derivative of the basic reproductive rate (see Box 1). Apart from ademographic factor expressing the mean longevity of the vector population (p), this equationalso contains a behavioural component (a) which is the frequency of mosquito bites onhumans, which in turn depends on the proportion of the vector population selecting humansas a blood host. This factor is squared, because the mosquito needs to bite two subsequenttimes to transmit the parasite, first to become infected, and second to pass the parasite on to
another human host, after having allowed for the completion of the parasite sporogonic cyclein the mosquito. This extrinsic incubation time (n) is also dependent on the behaviour of themosquito: should she choose to spend a lot of time in environments having favourablemicro-climatic conditions (constant and relatively high temperatures), the development of theparasite from ookinete to sporozoites will occur faster compared to siblings remaining at lowerambient temperatures.
ed is the natural susceptibility of the mosquito for parasite
development, expressed as vector competence. This is determined by the genetic make up ofboth the vector and the parasite, and possibly explains why only some 60 anopheline speciesare suitable vectors for human Plasmodia
.11,12 A “good” malaria vector is therefore characterized byhigh longevity
, a high degree of anthropophily, and a tendency to seek shelter in an environ-
ment with relatively high ambient temperatures while digesting the blood meal, as well as ahigh susceptibility for parasite development. Less successful vector species fail to have some ofthese characters. Yet, some anopheline vectors expressing a favourable combination of theseparameters, such as species of the An. dirus
complex in Southeast Asia, are not among the worldchampions of malaria transmission, if judged by the incidence of infections they cause, becausethey live in forested areas at the margins of the human environment, hence are not favourablyimpacted by human modifications of the natural habitat.13 The degree of sinanthropomorphism,or anthropophily in its loosest meaning, is therefore another important biological trait insuringthe success of an anopheline species as a malaria vector. This is not accounted for in the
The Genetics of Vector-Host Interactions
Box 1. The vectorial capacity equation
Vector capacity is the daily rate at which new human infections arise due to the introduction in amalaria-free area of a single gametocyte carrier, i.e., the malaria multiplying potential in thehuman population due to the vector.
m - total number of Anopheles per persona - frequency of bites on humans per vector per dayp - vector mean daily survival raten - Plasmodium extrinsic incubation duration (in days)
formulation of vectorial capacity, but it is arguably one of the reasons why the vectors in
sub-saharan Africa champion malaria transmission and have the regretful repute of accountingfor 90% of the world malaria burden.
It follows that strategies for malaria contr
ol should be directed to impact some or all of these
factors. Usually this is accomplished by spraying of insecticides that cause reduced longevity orby the use of insecticide-impregnated bed nets that r
educe indoor biting and resting behaviour.
These interventions can cause a reduction in parasite transmission, but have not been shown toaffect the genetically-determined traits of anthropophily and v
All anautogenous mosquitoes require vertebrate blood for egg production. Some species are
opportunistic in this behaviour, and feed on any type of blood host, provided sufficient quantitiesof blood can be ingested to permit egg development.14 Others, by contrast, have evolvedoligotrophic habits and feed on a limited number of hosts or even on a single host species suchas Deinocerites dyari
.15 Most malaria vectors belong to the first category, but several importantvectors feed preferentially on humans. To this group of anopheline mosquitoes belong Anophelesgambiae sensu stricto
, An. funestus
and An. nili
in Africa and An. fluviatilis
species S in Asia.16-20These species have evolved a strong association with humans by adapting to human habitationas feeding and r
esting ground, finding shelter inside people’s dwellings and biting preferentially
at times when the host is asleep.21,22 The acquired endophilic and endophagic feeding behavioursaccidentally enhance the mosquito’s survival because the human home offers a relatively stableenvironment with protection from predators and extreme meteorological events. Furthermore,for endophilic and anthr
opophilic mosquito species such as An. gambiae s.s
. the human host is
always close by, unlike outdoors, where host availability can be haphazard causing the insect toloose precious energy during host searching, thereby augmenting the general fitness of suchspecies.22,23 Nevertheless, in specific circumstances, normally-endophagic mosquitoes can biteexcessively outdoors, presumably in response to ambient conditions.24
The degree of anthropophily, i.e., the intrinsic or endogenous preference for feeding on
human hosts, is an important character in the equation of malaria transmission. This characterhas a genetic basis, as demonstrated by experiments selecting for higher or lower degrees ofanthropophily than baseline strains in species of the An. gambiae
complex (H.V Jamet (Pates),PhD dissertation, London 2002). The evolution of anthropophily might have followed differentpaths in separate species, and at least three processes can be suggested: (i) shift from primitivesimian host preferences, under the assumption that the host profile of monkeys or apes is themost similar to that of Homo; (ii) preliminary adaptation to the domestic environment; (iii)exploitation of anthropogenic features of the environment as ecological markers of the mostsuitable habitat.25 Mosquitoes exhibit a wide range of host preference, varying from reptiles to
Genetically Modified Mosquitoes for Malaria Control
birds to mammals, and sometimes leading to specialized behaviour such as the anthropophilicspecies. From the malariological aspect, the variation in host preference can be complicatedbecause within anopheline species complexes the host preference can be highly divergent. Forexample, the Anopheles gambiae
complex consists of seven species,26 of which only Anophelesgambiae sensu stricto
is highly anthropophilic. An. arabiensis
can at times feed preferentially onhumans, but is behaviourally distinctly different from An. gambiae s.s
. with a greater tendencyto feed on other mammals as well.17,27 This difference has been shown to be mediated byolfactory behaviour, An. arabiensis
responding more strongly to carbon dioxide and less tohuman-specific emanations.19 A similar phenomenon is present in the An. fluviatilis
species complexes, where only one species each has a very strong degree ofanthropophily.18,28,29 Thus, closely related sibling species sharing the same ecological niche canexhibit widely different host preferences. As a consequence, their role as malaria vectors is alsolikely to be different. More specifically, in the proposed strategy of release of transgenicmosquitoes for malaria control30 it is possible that the target species may be replaced by anincompetent mosquito, but ignorance of the other sympatric sibling species and their potentialrole as malaria vectors may result in a continuation of malaria transmission, albeit with reducedintensity, just as in the case of an incomplete intr
oduced refractoriness.31 Alternatively, the
vector competence of the less suitable vector species may be enhanced by parasite-inducedbehavioural changes, for example by enhanced attractiveness of Plasmodium
carriers32 or byrepetitive biting of mosquitoes carr
ying infectious sporozoites.33 For this reason, the bionomics
and behaviours of all potential malaria vectors in the target ar
planning a GMO approach for malaria control.
Host Abundance and Vector Behaviour
e developed a strong association with their human
hosts. In uninhabited regions and nature reserves that are situated in habitats suitable for thesevectors, these species are absent. For instance, the Kruger National Park in South Africa isdevoid of An. gambiae s.s
., while the sibling species An. arabiensis
and An. quadriannulatus
arewidely present, feeding on the abundant wildlife.34,35 In lowland rainforests An. gambiae s.s
. ismostly found near human settlements, being absent in remote forests presumably due to lackof suitable hosts (M. Coluzzi, personal communication). Because humans provide the principlefood source for the anthropophilic anopheline species, the transmission of human malariaparasites between humans is reinforced by the specialised feeding habits of the vectors. The
density of the human population is rarely considered a factor that inhibits malaria transmission. Itis not known how many mosquitoes can feed on one human host, but there is no evidence ofdensity dependence in the population regulation of the African malaria vectors.36 Estimates ofanopheline numbers in an African village suggest that it was not the number of humans thatdetermined the mosquito abundance in the village.37,38 It has been suggested that zooprophylaxismight be a means for diver
ting mosquitoes to alternative hosts and thus reducing the human
biting rate. Although this idea has been shown to work in Asia,39,40 the African vectors cannotbe sufficiently diverted to serve as effective tool for malaria control.41
For the vectorial capacity, however, human density is an important parameter because the
human biting rate (ma) is determined by both the mosquito density and the human popula-tion density.
Thus a high mosquito density with low human abundance may result in higher
vectorial capacity than in a situation with median or high human abundance.10 At present,only in urban settings with a high human density per km,2 can the figure of human densitycause for sufficient dilution to affect the vectorial capacity negatively.42
Synchronization with the Host Habits
The synanthropomorphic anopheline mosquitoes have not only adjusted to the human
environment, having developed endophagic and endophilic traits, but they have also adopted afeeding habit convergent with times when the host exhibits the least defensive responses. These
The Genetics of Vector-Host Interactions
anophelines blood feed between midnight and sunrise, a time when the host is usually asleep.21,43This enables the mosquitoes to complete their blood meals undisturbed, as during sleep thehost defensive responses are likely to be small and ineffective. It is perhaps for these reasons thatthe use of insecticide-impregnated bed nets has been highly successful in Africa, at least inthose areas where such nets have been introduced (many areas have not yet been given access tosuch nets), because it prevents the mosquitoes from biting when the hosts are not available,being protected by a physical barrier.44
Other Factors Affecting Vector-Host Contact
The development of the malaria parasite in the mosquito vector requires 10-14 days under
tropical conditions. During this time, the insect will pass several gonotrophic cycles. Each cycleis initiated by a blood meal, after which the insect enters a resting stage in which its behaviouris significantly modified, with no response to host odours.45 The suppression of host-responsivebehaviour during this time serves to enhance the completion of egg maturation at a time whenthe insect should be left undisturbed. The traditional African mud house offers an idealenvironment for this purpose, providing a dark and relatively moist environment. Malariavectors with an opportunistic feeding prefer
ence tend to spend less time indoors, and complete
the gonotrophic cycle elsewhere, where they are more exposed to environmental extremes.
Implications of V
ector Behaviour for Malaria Transmission
ve all contribute to enhance the transmission of malaria parasites,
and it has been shown that those mosquito species with strong anthropophilic habits are highlyeffective malaria vectors. When considering effective inter
malaria transmission using GMO techniques, several behavioural aspects can be considered.
About ten years ago, Curtis46 proposed that malaria vectors could be rendered zoophilic
through manipulation of their genome by introgressing genes for zoophily between closelyrelated species like the sibling members An. quadriannulatus
and An. gambiae s.s.
of the gambiae
complex. The host preference is a genetic trait that may be modified, depending on the intensity ofmalaria transmission. In India, much of the malaria transmission is caused by An. culicifacies
, acomplex of sibling mosquitoes with mostly zoophilic species. Many of these mosquitoes biteoutdoors and rest in cattle sheds. In spite of this behaviour, malaria is widespread in India, andonly indoor spraying or the use of insecticide-impregnated bed nets have shown to reducetransmission effectively.47 Anopheles darlingi
is an important vector in South America. This
species, too, is zoophilic, but can at times become associated with human settlements where itcan efficiently transmit due to its high biting densities.48,49 However, the force of malariatransmission in regions where the main vectors are mainly zoophilic is generally much lowerthan where vectors are highly anthropophilic, and reduced entomological inoculation ratesincrease the likelihood of good impact on epidemiological parameters such as malaria morbid-ity and mortality by traditional v
ector control methods. By contrast, the two most important
malaria vectors in Africa, An. gambiae s.s
. and An. funestus
, are highly anthropophilic, endophagicand endophilic. Current control methods based on insecticide-impregnated materials wherethese anthropophilic vectors are present have usually had a significant impact on malariamortality, but generally much less spectacular results on malaria morbidity.44 In the African
continent, the force of transmission is too high to achieve its interruption, or for endemicity tobe destabilized.50 It can be inferred, therefore, that for a genetic strategy based on manipulationof anthropophily to be successful, the level of penetration of the induced zoophilic trait mustbe complete, otherwise transmission will not be interrupted solely by partial zoophily.
It is worth distinguishing between obligate and facultative zoophily (S. Torr, C. Costantini
and G. Gibson, unpublished data). Among the constraints posed by the maintenance orresidual anthropophily in a facultative zoophilic vector, is the general trend for urban malariato become the predominant epidemiological facies of the disease in Africa during the nextcentury.51 In the urban environment, the lack of nonhuman hosts favours human-vector
Genetically Modified Mosquitoes for Malaria Control
contact by disallowing the normal expression of the zoophilic tendencies of the vector. Casesare known of malaria resurgence following the disappearance of the main nonhuman hosts ofzoophilic vectors. In the Guyana, An. aquasalis
, a mostly zoophilic species, shifted to bitinghumans and caused a malaria epidemic in Georgetown after the replacement of its main host,buffaloes used in the culture of rice, with mechanical equipment.52 Nevertheless, integratedvector control management with existing technologies can greatly benefit from a population ofvectors whose degree of anthropophily is less. Examples of successful vector control withzoophilic vectors are described in.39,53,54
Behavioural Genetics of Vectors
The biological basis of animal behaviour is well established: behaviour is often species-specific,
it can be reproduced or altered in successive generations, it can be changed in response toalterations in biological structures or processes, and it has an evolutionary history that canpotentially be traced in the genome of related organisms. The debate of the relative importanceof nature vs. nurture in the ontogeny of behavioural repertoires has animated the early days ofethology when this science was still a novel scientific discipline. Nowadays, the genetic bases of
behaviour cannot be denied, and the challenge for scientists in the post-genomic era is to findand disentangle the complex interaction between genes and environment at several levels oforganismal organization, i.e., from the molecular interaction between stimuli and their receptorsto the integration of an individual’
s behaviour in populations and ecosystems.
Evidences for a genetic basis of host preference are provided by thr
selection experiments association between chromosomal polymorphism and feeding behaviour55and indirect evidence from behavioural bioassays in standardized envir
(Pates), W. Takken and C.F. Curtis, unpublished data). Host preferences in malaria vectorshave been shown to be already expressed early on in the behavioural sequence leading ahost-seeking mosquito to its preferred host, when olfactory responses to host volatiles play akey role in the behavioural repertoire of the foraging mosquito.57 The suitability of a host istherefore ‘judged’ by the profile of odorants emitted by the host. Alteration of the perceivedhost profile can result in the nonacceptance of the host by the questing mosquito. By manipu-lating the perception abilities of mosquitoes for key host volatiles, it might be possible to altertheir expression of host preference. Thus, genes coding for key receptor molecules (e.g., odorantbinding proteins), or promoters of receptor sensitivity are candidate targets for geneticmanipulation of host preference.58
anipulation for Behavioural Change?
Strongly anthropophilic mosquitoes are considered good disease vectors because of the close
association with the human host (see above). For this reason, classical methods of vectorcontrol have been directed to either vector killing, for example with residual insecticides onresting sites, or prev
ention of mosquito bites by placing the human host under a bed net. If the
bed nets are impregnated with insecticides, such nets may also result in killing mosquitoes thatland on the net, although this method does not affect the entire mosquito population.59
Mosquito species with a more opportunistic taste for blood will be present in higher densities
compared to anthropophilic species in order to cause a similar degree of transmission intensity
as their anthropophilic cousins. Manipulation of the host-preference trait in malaria mosquitoescould render them less anthropophilic or even completely zoophilic, as many of the non-malaria vectors are. For instance, in tropical Africa An. coustani
and An. ziemanni
are both verycommon animal biters, occurring in high densities. Yet, these species have never been considereda vector because of their zoophilic nature. The publication of the genome of An. gambiae s.s
and the recent discovery of An. gambiae
specific olfactory receptor genes58,60 suggest that itmight be possible to manipulate the odour recognition of this mosquito so that the anthropo-philic trait is modified or even made extinct. It is not to be expected that mosquitoes that havethus been manipulated, will revert to anthropophilic behaviour because there are usually more
The Genetics of Vector-Host Interactions
animal feeds available than those on human. It is also likely that changes in the local ecosystemwill render the survival chance of anopheline mosquitoes less favourable, leading to enhancedmortality or reduced adult population density. Small changes in human biting habits (parameter a,Box 1) and mosquito survival (parameter p; Box 1) can have a large impact on the vectorialcapacity, thus effectively contributing to malaria reduction. These proposed changes will be lessdependent on the use of genetically modified mosquitoes and therefore may be moreacceptable for environmental and sociological reasons.4 It is even conceivable that behaviouralmodifications can be achieved by classical selection and hybridization.46 The factors that drivethe ecology of vector behaviour and population dynamics are still poorly understood, andshould be more fully explored to exploit these characters for malaria control.
Although the genome of An. gambiae
has been identified, most of the genes that control the
insect’s behaviour and physiology need to be discovered. Until such information becomesavailable, the potential use of GMO technology other than that based on modification ofvector competence,61 remains speculative. Even then, the evolutionary forces that have resultedin the current genetic traits of the mosquito are likely to kick into higher gear to counteract theintrusion of new genetic material. For this reason we argue that ecological studies on thisimportant gr
oup of insects, in their native habitat, should be increased to better understand the
often unpredictable behaviours of entire vector populations in response to their environment.
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LETTERS TO THE EDITOR low dopamine neurotransmission. This keeps prolactin levelsHowever, as I argued in detail in my recent book (2), family,normal, spares cognition, and obviates extrapyramidal signs. twin, and adoption studies do indeed suffer from “strong andThis letter provides data on the off-rates of additionalconsistent methodologic biases operating across study de-newer atypica
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