Genetic control

ByDr. Girish Chandra

GENETIC CONTROL

(Dr. Girish Chandra)

 

            Genetic control involves manipulation of genetic material of a pest species so as to confer lethality on the species. It is also called autocidal control.

Inherited Sterility

 

            Inherited sterility is an approach to the genetic manipulation of a pest population in which the reared and released insects are fertile but their progenies are sterile. This effect has also been referred to as delayed sterility, F1 sterility, partial sterility, etc. This phenomenon occurs in insects such as Lepidoptera and Hemiptera that contain polycentric chromosomes. It involves transmission of aberrant chromosomes (usually by translocations) by the release of laboratory modified population to the native population.

 

            There are various genetic alterations that can introduce inherited sterility into a population, for example, cytoplasmic incompatibility, interspecific hybridization and multiple ploidy.

            Cytoplasmic incompatibility can be accomplished in two ways:

 1) By treating a laboratory-reared population of insects with a dose of radiation which is sufficient to break chromosomes but low enough not to produce sterility.

 2) By culturing stocks of the target species which contains homozygous translocations.

            When the aberrant chromosomes are passed on to the native populations, the individuals carrying the aberrations in the heterozygous state will show sterility. The amount of sterility will depend on the number and size of the translocations carried by each individual.

            Some advantages of this method include:

1) Enhanced reproductive competitiveness of the partially sterile released individuals compared to the fully sterilized individuals used in SIRM.

2) The F1 individuals are produced in the native population, thus freeing rearing facilities for the production of higher numbers of the primary release organism.

Conditional Lethal Mutations

            In this technique, strains of insects are produced by genetic manipulation so that they carry traits that are detrimental 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 necessary.

             Some other types of conditional lethal traits which could be used are:

·        Cold-sensitive lethal mutations,

·        High-temperature-sensitive lethal mutations,

·        Inability to fly,

·        Lack of sex pheromone production or lack of response to pheromone,

·        Lack of ability to develop on a particular host,

·        Change in protective coloration,

·        Any other genetic change which could affect an individual’s ability to survive or reproduce in nature.

            In the past, introduction of conditional lethal mutations was dependent upon our ability to identify and isolate induced or naturally occurring mutations in the target species. But now such mutations can be taken from genetically well-known organisms, such as Drosophila melanogaster and introduced into economically important organisms using biotechnological techniques.

            Translocations or other genetic alterations often reduce the fitness of organisms as compared to the wild type and the creation of a population of ecologically fit, yet genetically altered insects is often a difficult task. This problem could be overcome by the use of a dominant, conditional lethal trait. A good trait to look for would be temperature sensitivity, which would allow the mutant to be reared in the laboratory but which would be lethal in the natural habitat. Creating such a mutation using classical genetic techniques has proven to be exceedingly difficult, since most of the temperature-sensitive lethal mutants turn out to be recessive.

            A dominant, cold-sensitive lethal gene (Notch60g11) has been found in Drosophila melanogaster, and it has been used to drive populations of normal flies to extinction under experimental conditions. New genetic engineering techniques could be used to transform insect pest species and produce homozygous cold-sensitive strains that could be mass-reared in the laboratory and released in the field to drive the normal pest population to extinction.

Behavioral Changes

            It would be advantageous to cause genetic changes in the specific insect behaviors that would make them undesirable or self-destructive. For example, in many pest species, a change from multivoltine to univoltine development would eliminate the economic destructiveness of the pest. If we could learn enough about insect physiology to be able to identify the genetic determinants behind such behavior, then we should be able to genetically manipulate the species and release the new strain in such large numbers that the new trait would become predominant in the population.

            
Hybrid Sterility

            Sterile hybrids could be raised in the laboratory and released into native populations. This phenomenon has been clearly demonstrated in crosses between Heliothis virescens males and Heliothis subflexa females (Laster et al. 1996) but needs to be more widely investigated.

Simply Inherited Mutations

            Genetic changes in a single gene could lead to decreased fitness in the wild population if the gene could be introduced into the native population in large numbers. Some examples of such changes would be recessive lethal mutations, eye color changes to increase or decrease light sensitivity, pupal or adult body colour changes, deformities in antennae, legs or wings.

Double stranded RNA mediated inhibition

            Double stranded RNA (dsRNA) mediated inhibition of specific genes in various pests has been previously demonstrated. dsRNA mediated approaches to genetic control have been tested in the fruit fly Drosophila melanogaster (Kennerdell and Carthew, 1998; Kennerdell and Carthew, 2000), who described a method for the delivery of dsRNA involved generating transgenic insects that express double stranded RNA molecules or injecting dsRNA solutions into the insect body or within the egg sac prior to or during embryonic development.

 

               A reduction in pest infestation is obtained through suppression of gene expression. There are methods for making transgenic plants that express the double stranded RNA molecules that can be used in protecting plants from pest infestation.

  

Reciprocal translocations

 

            When a chromosome breaks, the broken piece sometimes attaches itself to another chromosome, and when two non-homologous chromosomes swap broken ends, it is called a reciprocal translocation. This can occur naturally and spontaneously, but it  can be induced by low doses of ionizing radiation. If the break occurs without damaging a critical area of the genome, the organism still has the full complement of the original genetic information, and it can appear perfectly normal. It is in the production of gametes where the genetic abnormality manifests itself during metaphase I, when due to reciprocal translocation, the homologous parts of the original chromosomes fail to line up with one another.

 

 

Genetically modified mosquitoes

 

            Setting aside concerns over the release of genetically modified mosquitoes, there is reason to be optimistic that this strategy might work. In 2002, a team led by Marcelo Jacobs-Lorena of Case Western Reserve University, modified Anopheles stephensi with genes that blocked the development of Plasmodium berghei inside mosquito body. In March 2006, a team led by Ken E. Olson of Colorado State University showed that inserting a specific transgene (a piece of foreign, engineered DNA) into the genome of Aedes aegypti could substantially deactivate the dengue virus within hours of the female’s blood meal from an infected person. Other scientists have developed genetic strategies that simply kill the mosquitoes that carry a specific engineered gene.

 

Factors for the success of release of modified mosquitoes

 

  • Males must mate with wild females at the release site because laboratory competitiveness of males does not ensure mating with wild females.
  • Sex separation strains greatly improve efficiency and effect, and are essential for mosquitoes.
  • Release methods and sites must be suitable for all weather conditions anticipated, and are established and tested before control releases begin.
  • Whereas female sterility provides an indicator of mating frequency, vector density is a more relevant indicator of release effect on disease control.
  • Full-scale production levels must be stable before releases begin.
  • Releases must be timed for maximum effect into suppressed populations.
  • Control areas with similar monitoring must be available for comparison during sterile insect technique development.
  • Dispersal and mating characteristics are paramount quality control factors to assess the fitness of release material.
  • Isolation of test areas must be accomplished and conclusively demonstrated for all weather conditions.
  • Dispersion information from the literature provides a reasonable starting point for barrier design, but independent monitoring is essential to demonstrate the effectiveness before releases commence.
  • Political stability, and healthy relationships with the public, press and political entitities are essential for success.

 


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