Example 2: Transgenic Fish
The modification of animals is very different from that of plants. Although we are able to
regenerate animals directly from a single cell, the bombardment of a tissue as used in transforming
plants is not available. The generation time of an animal (particularly large animals rather than
insects) is too long to allow selection techniques which allow the transformation of a very few
cells to provide a significant source of transgenic animals. For large animals the transformation
must result in a very high success rate, probably more than 50%.
Although modification of animals is very different from that of plants, the questions which have
to be considered in order for a risk assessment to be performed are very similar.
Techniques for transforming animals include microinjection, where the DNA is injected directly
into a cell or the use of retroviruses which should result in stable integration of the genes within
the genome of the transformed cells.
One group of animals which has proved controversial is fish, both because they may prove
relatively easy to modify, and because they are not easily contained. Recommendations on marine
releases made by the International Council for exploration of the Sea (ICES) are published in the
"1994 Code of Practice on the Introductions and Transfers of Marine Organisms". The discussion
which follows is largely based on a publication by the British Department of the Environment
entitled "Guidance for the experimental release of Genetically modified fish" referenced in
If fish are modified for aquaculture the likelihood of escape into natural ecosystems is thought to
be high, and the fish might well carry traits which might result in significant impacts on
ecosystems. In 1998 there had been experiments on 31 species using approximately 40 different
DNA constructs. Most of the work has been on
- growth enhancement,
- environmental tolerance, including tolerance to extremes of temperature, salinity, heavy metals
- reproduction, where inhibition of early maturation would reduce loss in aquaculture, and
- disease resistance, where there is potential for research on viral, bacterial and parasitic
A major problem when compared to plants is the generation time for species of fish used in
aquaculture -- generation times may range from less than one year for Tilapia to a few years for
"Microinjection into the fertilised egg is the most commonly used technique for introduction of a
DNA construct. Unlike mammalian eggs, it is generally not possible to visualise the pronuclei or
nucleus of fish eggs, and microinjection is generally into the egg cytoplasm before the first cell
division. This often leads to high levels of mosaicism, with integration of one or more copies of
the introduced DNA often occurring after the first cell division". Other techniques which have
been used include electroporation of eggs and sperm, or soaking of sperm in the DNA construct
prior to fertilisation. Biolistic techniques have also been attempted.
The hazards which could be identified for which likelihood would have to be considered include
knowledge of the molecular biology and physiology of fish. The basic science 'database' for fish is
limited; although the potential for modification for use in aquaculture is vast, detailed knowledge
particularly of the physiology of most fish species is limited
Infomation requirements include:
- capacity to survive, breed, establish and spread to other habitats
If the GM fish proposed for release has very specific habitat requirements the distribution of the
fish after release may be able to be predicted, and monitored. If the fish roams over a wide variety
of habitats, the capability to monitor will be greatly decreased. The impact and survival of the
modified fish depends on their ecological fitness relative to wild stocks. The performance of these
fish, however, could be very different from that observed in containment. The relative fitness
could be unchanged, increased or decreased due to the modification. If decreased, the new form
would be less likely to establish self-sustaining populations, but it could take many generations
before the genetically modified fish disappeared.
The capacity to breed will be influenced by the ability of the fish to survive in the open
environment but may also be dependent on the transgene or on the site of insertion of the gene
into the genome. If the GM fish is capable of reproduction in the open environment, it may be
able to spread to other habitats, establish viable populations, transfer genetic material and
compete with other organisms.
- Behavioural changes in the modified fish and their descendants
- Physical and physiological changes in the modified fish
The genetic modification may cause physical or physiological changes. An introduction of a
growth hormone to increase growth rate may require a greater food intake, or result in the
modified fish eating smaller relatives. This would deplete the stocks of 'wild-type', even if the
modified fish was less fit, or even sterile. A fish modified for increased tolerance to low
temperatures may have an increased capacity to spread into a wider range of habitats. Phenotypic
changes may provide the fish with a competitive advantage for food, shelter, mates and suitable
breeding sites. Damaging competition with wild stocks has to be considered as a negative
outcome of an introduction.
- Potential for and consequences of transfer of the inserted genetic material to other fish or
Successful breeding between modified fish and wild relatives could result in a change of the
genomes of the wild stocks. Problems which might arise would then depend on the inserted gene,
the frequency of transfer and the fate of the offspring. The spread of the gene from modified fish
to other species by hybridisations would be of concern if the resultant hybrid offspring carried an
- Competition with other organisms
- Phenotypic and genotypic stability
Damage to the environment may be delayed and long term as it may be caused by the descendants
of the originally released fish.
Once the hazards have been identified, the likelihood of these occurring must be estimated, and
risk management procedures must be considered. A serious problem is that physical containment
is rarely perfect -- Penman (reference ) suggests that the introduction of modified fish into
aquaculture should be considered as an intentional introduction.
It may be possible to use biological containment, by (for example) ensuring sterility.