A 鈥淛UMPING GENE鈥� being used to genetically engineer organisms has crossed the
species barrier at least seven times in evolutionary history, in one instance
between flies and humans, according to a study commissioned by the British
government. If organisms modified using this mobile element are released, there
will be a risk of genes spreading to other species, the report says.
The so-called mariner element can move around in the genome of individual
species thanks to the transposase enzyme it encodes, which 鈥渃uts and pastes鈥� it
from one place in a cell鈥檚 DNA to another. Such jumping genes litter most
creatures鈥� genomes. Their ability to insert themselves into chromosomes makes
them attractive to genetic engineers as a means of moving genes from one species
into another.
In a project for the Ministry of Agriculture, Fisheries and Food, Andy Brass
and two colleagues at the University of Manchester compared the DNA of 80 000
different organisms, using over five million sequences. They found seven pairs
of similar mariner sequences. For instance, 83 per cent of the sequence in the
tsetse fly Glossina palpalis, a blood-sucker that spreads human
sleeping sickness, was the same as a sequence in humans.
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Such a close match is 鈥渟trong suggestive evidence鈥� that mariner has moved
between tsetse flies and humans, Brass says. The transfer occurred recently in
evolutionary terms, he thinks, although it is not clear whether mariner jumped
from the fly to humans or vice versa.
The researchers also found close matches between the mariner sequences in a
tsetse fly and a mosquito; a bee and a blister beetle; and a cat flea and a
rusty grain beetle. This shows that, while it鈥檚 very rare, mariner can jump
species. So if it is used to insert genes into animals, these genes might spread
into other species. 鈥淚t might not be the smartest thing to use,鈥� warns
Brass.
Dan Hartl of Harvard University, who discovered mariner in the 1980s, agrees
that caution is necessary, given that we know so little about how mariner moves
around. He is worried that if, for example, it were used to insert an
insecticide into a food crop, it could spread the toxin to wild flowers and
poison beneficial insects such as bees. 鈥淵ou have to tread carefully,鈥� he says.
鈥淵ou can鈥檛 use it to move genes by just making assumptions beforehand.鈥�
While no modified organisms created using mariner have reached the market
yet, several groups around the world are working with it. Genetic engineers in
the US have demonstrated that mariner can transfer an eye colour gene from a fly
into a mosquito, while the Roslin Institute near Edinburgh has inserted mariner
into chickens, with the aim of getting them to produce pharmaceuticals in their
eggs.
David Finnegan at the University of Edinburgh, who works with the Roslin
team, accepts that mariner might enable inserted genes to jump species, but
argues that it could be designed so that its transposase only functioned for a
short while. 鈥淲e can engineer things so that the risk is acceptably small,鈥� he
says.