Download Genetic control of arthropods

Medical Importance of Arthropods
A. Arthropods as direct agents of disease or discomfort
-Entomophobia
-Annoyance (head and pubic lice) and blood loss.
-Envenomization (black widow and brown recluse spiders).
-Dermatosis (scabies mite).
-Myiasis and related infestations.
-Allergy.
B. Arthropods as vectors of developmental hosts
-Mechanical carriers.
-Obligatory vectors (involves some degree of development
within the arthropod).
Examples of diseases transmitted by arthropods :
- Tick borne: as Lyme disease or Lyme arthritis, Babesiosis
and Rocky Mountain spotted fever.
- Mosquito borne: as Malaria, Dengue fever, Yellow fever,
Eastern equine encephalitis, Elephantiasis.
- Flea borne : as Bubonic plague and Sylvatic plague
- Black flies : as Onchocerciasis or River Blindness
- Hemipteran borne: as Chagas disease
-Tsetse borne: as
African sleeping sickness
Some Important Approaches
of Control
A- Chemical Control
1- No. 2 diesel oil may be applied to the surface of water
which leads to deprivation of larvae from air besides its direct
toxic effect.
2- Repellants as Deet.
3- Laboratory bioassay of the essential oil extracted from a
plant, commonly known as 'Railway creeper', was carried out
against the larvae of mosquitoes and it was found to have a
mosquito larvicidal effect.
4- Insecticides are placed in the water to eliminate mosquitolarval habitats or sprayed on adult as DDT, Organochlorine,
Organophosphorous compounds ect..
5-Insect growth regulators, which disrupt insect growth and
development (Juvenile hormone analogue, Ecdysone
inhibitors, Chitin synthesis inhibitors).
There are three types of insect growth regulators:
Hormonal, enzymatic, and chitin synthesis inhibitors.
Juvenile hormone analogs and ecdysone inhibitors both
disrupt the ratio of hormones with the young insect. For
an insect to molt to the next stage, the correct ratio of
juvenile hormone and ecdysone must be present.
Juvenile hormone analogs
Interferes with molting process which causes premature
molting. Also causes deformations of wings and
reproductive parts. Ovaries produce infertile eggs.
Ecdysone is a primary molting hormone that is
necessary for insects to go from the larval to pupal
stage. If we change the ratio of one to the other, the
insect will not become an adult---thus reducing
reproduction and population increase.
Ecdysone inhibitors Blocks ecdysone which signals
the insect to molt. This causes pupae to die.
The chitin synthesis inhibitor
Since insects molt many times, the ability to create
chitin is vital to form the insect’s exoskeleton. Without
the proper cuticle (exoskeleton), the insect will die.
B- Physical Control
1- Filling or drainage swampy areas.
2- Removal of aquatic plants (deweeding).
3- Using irrigation water properly.
4- Screening of houses and use of mosquito netting at night
5- Using of barrier animals (zooprophylaxis).
6-Raising houses reduce mosquito bites.
7- Traps and Sound generators with certain frequencies
drive away mosquitoes.
8- The use of expanded polystyrene (EPS) beads
The use of expanded polystyrene beads (as a blanket over
the water surface) is a potential alternative safe applicable
method. EPS beads not only prevent oviposition but also
kill the immature by forming a thick blanket on the water
surface..
The EPS beads are
the expanded form of polystyrene granules which are
(petrochemical) hard translucent glass like beads with
diameter ranging from 0.6 to 2.5 mm. The unexpanded
beads contain an expanding agent (pentan). When
exposed to super heated steam, they expand about 35
to 40 times of their original volume.
The EPS beads are light in weight, inert, nontoxic, and
resistant (do not interact with) to water content.
Applied at the rate of 500g to 1 kg/m2 in different
habitats, float on the water surface in several layers,
physical barrier formed by the floating blanket of EPS
beads prevent egg laying. Furthermore immature
stages of mosquito trapped under the layer of the
beads die of suffocation.
C- Biological Control
Agents of biological control (natural enemies) of insects
include predators, parasitic insects, and insect
pathogens.
-Predators may be insects or other insectivorous
animals, that consumes many insect preys during its
lifetime. such as Gambusia fish, Toxorhnchites mosquito
larvae.
-Parasites (also called parasitoids) of insects are other
insects which lay their eggs in or on the host insect.
When they hatch, the young parasite larvae feed on the
host (the pest) and kills it. As Trichogramma.
- Insect pathogens include viruses, bacteria, fungi,
nematodes, and other microorganisms that cause insect
diseases.
- Bacterial insecticides such as Bacillus thuringiensis
israelensis and B. sphaericus,
-Mermithid nematodes such as Romanomermis
culicivorax.
-Microsporidia such as Nosema algerae,
Entomopathogenic fungi such as Lagenidium giganteum.
Success in infecting Glossina.
D- Genetic Control:
The principles of genetic control are based of sterility
or other desirable genetic factors in successive
generations. The general goal is birth control through
reduction or replacement of a population following
release of a desirable genotype.
Basis of genetic control:
1- population suppression
2- population replacement
1-Population suppression
The most common strategy, It include the following
following-A-The sterile insect technique (SIT)
B-Release of insects with a dominant lethal gene (RIDL).
C-Inherited or delayed sterility.
D- Autocidal control methods:
a-conditional lethal mutations
b- simple inherited mutations.
E-Cytoplasmic incompitability.
A- Sterile insect technique (SIT)
Or sterile insect release method (SIRM)
This technique relies on the mass rearing of the target
insect in large biobio-factories, their sterilization by
ionizing radiation or chemical sterilants and then
aerially releasing them in the field in sufficient
numbers. The wild female will have no offsprings
following mating with the released sterile male, leading
to reduction in the natural pest population.
The major technical aspects of SIT are:
i-Automated mass rearing (Breeding biofactory)
ii--Genetic sexing (only male)
ii
iii--Sterilization (irradiation or chemically)
iii
iv--Release (in the air as pupae)
iv
Colonization of target insect
Mass rearing in
Insect Factory----Factory-----Breeding
Breeding population
Genetic sexing (Males reared)
Sterilization
Release
Mate with wild insects
Reduced reproductive potential
Population Suppression/ Eradication
Flowchart of SIT
Applications of the SIT:
1-Successful areaarea-wide SIT program has been
conducted against the Screwworm fly
Cochliomyia hominivorax in Libya (1988
(1988--1991
1991).
).
Adult Cochliomyia hominivorax
Larvae of Cochliomyia hominivorax in
wound
Larvae of Cochliomyia hominivorax in
wound
3-The SIT has been conducted against the
Mediterranean fruit fly (Medfly) Ceratitis
capitata,, the Stable fly Stomoxys calcitrans
capitata
and the pink Bollworm, Pectinophora
gossypiella.
4-Application of SIT against mosquitoes.
The production of rearing mosquitoes does
not exceed 7 million per week. This compares
with 500 million per week for the screwworm
fly, and natural mosquito densities are usually
much higher than those of screwworm flies.
Major improvements will need to be made in
larval rearing and pupal collection. Also other
techniques involving the release of fragile
sterile pupae need to be considered.
2-The SIT was tried in the control of Glossina
transmitting animal trypanosomes in South
Africa, Zinzibar and now tried in Ethiopia.
5-Application of SIT against Glossina
transmitting sleeping sickness:
The use of SIT is unfortunately impractical with
the present technique. Release of Tsetse flies
(males or females) close to human habitations
leads to an unacceptable increase in biting.
B- Release of insects with a dominant lethal
gene (RIDL)
The transgenic female insects will be reared on a
medium supplied with special additive to develop and
express certain reporter gene.
then when they are released to the natural environment
free from the additive used to suppress the system in
the rearing facility will result in females death.
The mating of released RIDL males with wild females will
result in viable male offspring, but no viable daughters
as the dominant lethality would be expressed in the
natural environment. Moreover, any males produced by
RIDL males would contribute to the elimination of
females in later generations.
generations.
The RIDL system has been exemplified in
Drosophila and is under development for
Aedes aegypti,
aegypti, the vector of dengue fever and
yellow fever.
C- Inherited or (Delayed) Sterility
"Inherited sterility" is an alternative genetic strategy
that generally requires fewer insects to be reared and
released. It also requires lower doses of irradiation
than those necessary for sterilization.
It involves the transmission of aberrant chromosomes
(usually in the form of translocations) from the
released population to the native population. When the
aberrant chromosomes are passed on to the native
populations, the individuals carrying the aberrations in
the heterozygous state will show sterility.
sterility.
D- Autocidal control methods :
1-Conditional lethal mutations
In this technique, strains of insect are produced (by genetic
manipulation) carry traits that are detrimental (lethal) to the
species in the native environment, but which are not
detrimental under laboratory conditions.
For example, the inability to diapause would be a conditional
lethal in an insect that had to go into diapause to survive a
host--free period, but in the laboratory no diapause would be
host
necessary..
necessary
2- Simply inherited mutations
Genetic changes in a single gene could lead to decreased
fitness in the native if the gene could be forced into the native
population in large numbers. Some examples of such
changes would be recessive lethal mutations, eye colour
changes (to increase or decrease light sensitivity), pupal or
adult body colour changes, and leg, antenna or leg
deformities...
deformities.
Such technique requires significant research data on insect
genetics, physiology, behaviour and reproduction .
E- Cytoplasmic incompatibility
sterility between populations of the same
species can be made by microbial infection of
the genus Wolbachia
Wolbachia.. The bacterium is
transmitted through the egg cytoplasm. Sperm,
thus can’t fertilize any ova except the one
carrying the same kind of Wolbachia
In its simplest form cytoplasmic incompatibility
results when a Wolbachia
Wolbachia--negative female
mates with a Wolbachia
Wolbachia--positive male,
male, no
offsprings are produced from such an
incompatible cross. In using cytoplasmic
incompatibility as a substitute for SIT, the idea
would be to release males of a strain that is
uniformly incompatible with the local wild
population. Such males are considered as
functionally sterile without the expense of
artificial sterilization.
♀× ♂
↓
C
♀×♂
↓
C
♀ ×♂
↓
C
♀× ♂
↓
I
II Population replacement
A-Use of transmission blocking vaccine
B- Blocking malaria parasite invasion of
mosquito salivary glands
C-Transgenesis
D-Paratransgenesis
A-Use of transmission blocking vaccine
Transmission blocking vaccines consist of
antibodies that are ingested by the mosquito
with blood meal and interfere with parasite
development. They block ooknite invasion of
the midgut epithelium.
B-Blocking malaria parasite invasion of
mosquito salivary glands
-In general, the three lobes of male salivary glands
appear similar to each other and likely all have the
same secretory capabilities.
Female glands are differentiated into two lateral and
one medial lobe. The proximal regions of the lateral
lobes are involved in sugar feeding.
In contrast, the medial lobe and distal-lateral lobes
express genes whose products such as enzymes,
anticoagulants and vasodilatory agents are involved in
hematophagy.
a number of studies have been interpreted to
indicate that sporozoites preferentially invade
the distal-lateral and medial lobes of the female
glands. A certain peptide was found to bind to
the distal-lateral and medial lobes of female
glands of An. gambiae and An. stephensi, and
blocks slightly more than 90% of P. berghei
sporozoite invasion in the latter species. The
authors concluded that the peptide competes
with the sporozoites for a salivary ligand.
C-Transgenesis
It is chromosomal insertion and expression
of a gene derived from one organism in a
second heterologous organism. Major
components of the transformation system
include:
1-Potential DNA vectors (transposable
elements or viralviral-transducting agent).
2-A specific refractory gene that express the
desired phenotype (i.e a factor that would
inhibit transmission of the pathogen).
Examples:
1-Transgenic Anopheles stephansi were
produced by expressing either of two genes, a
tetramer of SMI peptide or the phospholipase
A2 gene (PL A2
A2) from honey bee venom.
Mosquitoes carrying either of these two
transgenes were impaired to Plasmodium
berghei transmission.
2-There is recorded genetic variability for
mosquito refractoriness against malaria
parasites based on Melanotic Encapsulation of
its early developmental stages. In
encapsulation an electronelectron-dense melanin like
substance begins to coalesce around the
parasite in the mosquito midgut. Identification
of the genes responsible for encapsulation and
melanization and the possible mechanisms of
introducing these genes into the mosquito
genome could be one strategy of genetic
control.
D-Paratransgenesis
It is the extrachromosomal expression of
exogenous genes in arthropods by genetically
modifying the symbiotic bacteria.
1-Wolbachia pipientis are intracellular bacteria
found in many species of arthropods and are
maternally transmitted from parents to
offsprings. Since Wolbachia are frequently
observed in the reproductive tissues of
arthropods, one potential approach that they
might be used to disrupt transmission of a
pathogen.
Genetically modified Wolbachia can be used to
block transovarian transmission of arboviruses
as Rift valley fever virus which is dependant on
vertical transmission from adult mosquito to
its progeny for maintenance of the virus in
nature. The disruption of transovarian
transmission is an application that could
potentially be adopted for use in a number of
species of mosquitoes
mosquitoes,, ticks and mites in
which transovarian transmission of pathogen
is important.
2- In the midgut of triatomine bugs there is a
symbiotic bacterium Rhodococcus rhodnii
(vital for sexual maturity of bugs).
It produce antimicrobial peptide cecropin A
renders a bugs refractory to infection by
Trypanosoma cruzi.
cruzi.
This peptide has a strong lytic activity against
T. cruzi but no effect on bacteria or gut tissues
of bugs (which is vector of Chagas’ disease).
Genetically altered symbiotic bacteria can be
introduced into aposymbitic first instar
nymphs of R. prolixus where they allow normal
growth and reproduction while expressing
specific gene products of interest.