The first genetically modified (GM) insects are due for release in the United States this summer. Prof. Joe Cummins takes us through the basic genetics and the hazards involved.
The first GM insects to be released are modified with a marker to allow them to be easily traced. Later, female killing traits will be introduced in GM insects in the hopes that insect pest can be eradicated by mating with GM insects.
The GM insects to be released are modified using mobile genetic elements called transposons. Such mobile genetic elements are being promoted as a means of controlling weeds as well as insect pests. However, the dangers of using mobile genetic elements to eradicate pests should be recognized and discussed, and not left to the fatuous embellishments of those profiting from the technology.
The piggyBac transposon is used in genetic engineering of pest insects because it can be transferred to a number of insect pests from moths and flies to mosquitoes. Gene transfer in insects began in the early 1980s with the P transposon of the fruit fly. That transposon was first recognized as the cause of a phenomenon called hybrid dysgenesis. In crosses between species of the fruit fly certain crosses led to severe genetic damage in off springs, there were high levels of gene mutations and chromosome rearrangement leading to weak and infertile individuals.
The cause of hybrid dysgenesis was traced to the release and spread of the P transposon among the chromosomes of the affected individual. The P transposon proved valuable in gene transfer among Drosophila species but was not active in other insects.
The piggyBac transposon was discovered in cell cultures of the moth Trichopulsia, also known as the cabbage looper. The looper cells caused extensive mutation in an insect Baculovirus (Fraser et al 1985). The transposon is being used extensively as it acts as a gene transfer vector for a number of unrelated insects. Transposons are related to viruses but lack the ability to be packaged in a virion (virus capsule).
The piggyBac transposon is flanked by short inverted repeats that initiate non-replicative insertion into the insect chromosome, mediated by an enzyme called transposase that targets the sequence TTAA on the chromosome. When the transposon inserts into a gene sequence, it may cause a mutation, or the transposon may act as a recombination site leading to duplication and deletion of sequences in the chromosomes.
The transposon is usually multiplied by being inserted in a bacterial plasmid. The transposon bearing plasmids are injected into early insect embryos, where they may insert into the genome of germ cells and become maintained in a lineage arising from the germ cells. The plasmids injected into an embryo include the transposon carrying the gene(s) to be transferred and lacking a transposase, to ‘disable’ the transposon and prevent it from moving by itself. The transposase function is provided by a helper plasmid that has a transposon with the transposase gene, but is otherwise also disabled because it has one terminal short repeat missing, so it cannot insert into the chromosome.
The piggyBac transposon has been used, for example. to transform fruit fly, yellow fever mosquito and cabbage looper (Lobo et al 1999); the silkworm has been transformed using piggyBac (Toshiki et al 2000) as has pink bollworm, a cotton pest (Peloquin et al 1999).
The United States Department of Agriculture (USDA) APHIS approved field release of a genetically modified (GM) insect pest, the pink bollworm. Some 2,350 adult insects will be released over the year beginning July 15 2001 and ending July 14 2002 in a small cotton field (3 acres) located .5 miles from the Plant Protection Center Rearing facility, Phoenix, AZ, USA. The field test application is available on line: http://www.aphis.usda.gov/biotech/arthropod/permits/0102901r/0102901r.html
The application was submitted 17 January 2001. The GM insects have been modified with a gene for the green fluorescent protein (GFP) from a jellyfish. The GFP is regulated from the heat shock promoter (hsp promoter) of the fruit fly. The bollworm was modified using the piggyBac transposon that inserts at the sequence, TTAA in the chromosome ( see above). The genetic modification includes the transposon short terminal repeats but lacks the trasposase gene needed to mobilize the chromosomal insert. It is presumed that the pink bollworm lacks the competent transposon and will not encounter viruses harboring the piggyBac transposon or any other transposon that can mobilize the insert in the host chromosome. The release experiment is believed to be a precursor to using the GFP marker in evaluating the use of irradiated males to control the bollworm pest and to lead to development of female killing genetic systems capable of eradicating the bollworm pest.
The proposal for the field test is based on the belief of the researchers that the piggyBac gene inserts would be stable. However, no direct evidence of stability under laboratory conditions was provided. Furthermore, piggyBac may be carried by insect baculovirus and such virus certainly produce transposase that can mobilize the gene-bearing transposon. Virus can act as a vector for rapid spread of the modified transposon to a variety of insects.
The proposal argue that no human health concerns are involved in the field trail. It argues "Lepidoptera, in general, do not pose a threat to human health and welfare and should remain a guiding principle in deciding on human risks related to their genetic manipulation." Even though the principle may have some merit it is clear that the piggyBac transposon was also used for gene transfer in the human pathogen related yellow fever mosquito.
Even more significantly, the insect Baculovirus is known to carry piggyBac transposon and that insect virus is used for human gene therapy. The spread of piggyBac to produce green neighbors that fluoresce may be greeted with hilarity by USDA. However, the inserted transposon efficiently creates mutations in genes such as recessive oncogenes associated with cancer .
The proposed field trials should be suspended until adequate laboratory experiments are conducted on transposon gene stability and on both viral and proviral helper functions that can mobilize disabled transposons carrying transgenes.
In a following essay I will discuss the construction and the use of female killing genes in GM insects to eradicate insect pests.
Article first published 16/03/01
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