- Introduction
- Non-response to fertility control
- Fertility control & reduced genetic diversity
- Conclusions
- Acknowledgments
- References
Session 2 Identification & Testing of Targets for Fertility Control
Immunologically Based Fertility Control for Possums - is Non-response an Issue?
Phil Cowan
Marsupial Cooperative Research Centre
Landcare Research, Private Bag 11052, Palmerston North, New Zealand
Introduction
New Zealand invests significant funds of about $5 million per year towards the development of methods of fertility control to help reduce the impact of possums on the country's conservation resources and agricultural production (NSSC 2000). Direct impacts of possums cost New Zealand about $80-100 million per year, and possums spreading bovine Tb to livestock threaten the export market for meat and dairy products (Montague 2000).
Immunologically based fertility control involves using a vaccine to induce the possum's immune system into making antibodies against its own reproductive system that will block reproduction (Cowan 2000). For example, immunocontraception involves possums making antibodies against proteins from their own sperm or eggs - the antibodies then block fertilisation so no or few eggs are fertilised. A number of the approaches to possum fertility control being researched in New Zealand are likely to involve the possum's immune system (Sutherland 1999).
One contentious issue surrounding this new technology is the possibility that there will be natural selection for possums that fail to mount a significant anti-fertility response to the vaccine and remain fertile (Bomford 1990; Nettles 1997; Cooper 1999). If this lack of effect on fertility is inherited by offspring, then immunologically based fertility control could become increasingly ineffective over time due to selection in favour of the animals that do not respond.
Non-response to fertility control
The Marsupial Cooperative Research Centre is developing fertility control for possums. Because of concerns about this issue, it therefore recently commissioned independent expert opinion about non-response to vaccines and fertility control to assess the significance of and likely solutions to this potential problem. Advice was sought on immunological aspects from a number of research groups, particularly the Centenary Centre of Cancer Medicine and Cell Biology, University of Sydney, and a literature review was commissioned (Rush 2001). A report on genetic aspects was also prepared by a group from the Centre for Environmental Stress and Adaptation Research (CESAR), Victoria (Magiafoglou et al. 2001)
Information was obtained relating to the variability and genetic control of immune responses; the heritability of immune responses; the nature of selection for resistance to fertility control; whether it was possible to model and predict the likely outcome of possum fertility control given the current state of knowledge; and whether there were strategies that could be adopted to minimise any problems of non-response. Both reports acknowledged a lack of information in a number of areas, and specifically about fertility control.
The conclusions were that most, if not all, immune responses vary between individuals in a species; that this variation is under genetic control, particularly (but not exclusively) by genes of the major histocompatibility complex (MHC); and that high or low immune responsiveness is partly heritable. So, in the case of possums, there are likely to be some individuals that will show reduced anti-fertility effects if treated with a fertility control vaccine, and that lower response will be able, at least in part, to be passed on to their offspring, leading to increase in the frequency of non-response in the population. The extent and time frame for such an effect are unknown and not easily modelled given the lack of relevant data.
However, the CESAR report suggested that the degree to which effects on fertility from altered antibody response levels are inherited is likely to be low, suggesting that resistance to possum fertility control will also have a low heritability.
Fertility control & reduced genetic diversity
Advice was also sought on a second suggestion - that because of selection for non-response to fertility control there could be a reduction in genetic variability, particularly in that part of the immune system that controls an animal's ability to respond to parasites and diseases. Such loss of variability might make animals more susceptible to infection. While this might not be an issue when dealing with a pest such as the possum, it could be an issue if the technology was applied to help control overabundant native species, such as koalas in Australia. The CESAR report concluded that low responder animals appeared unlikely to be more susceptible to environmental pathogens as a result of reduced variability at the MHC. This opinion was supported by observations that some wild animals, such as musk ox, have increased in numbers in recent years, despite limited MHC diversity for at least 100,000 years (Mikko & Andersson 1995). However, as pointed out by Rush (2001), the issue remains unclear, as no published evidence has specifically investigated the relationship of MHC monomorphism or polymorphism with disease resistance and susceptibility.
Conclusions
Overall, for immunologically based fertility control for possums to be effective in the long term, it was recommended that the evolution of fertility resistance will need to be investigated and managed. A number of strategies were suggested for managing the proportion of low responders, including engineering vaccines to maximise their ability to evoke a targeted immune response; varying selection pressure by, for example, using multiple fertility-inhibiting antigens or rotating the use of different antigens; and optimising the combination of conventional possum control using toxins with anti-fertility vaccines.
Together, by assessing the likely impact of potential problems and identifying solutions, the reports provide support for the continuation of the research effort into immunologically based fertility control for possums in New Zealand.
Acknowledgments
The research was supported by the Foundation for Research, Science and Technology Contract C09X0009, Landcare Research and the Marsupial CRC. J. Rush, A. Hoffmann, S. McKechnie and T. Basten provided helpful comments on the manuscript.
References
Bomford, M. 1990: Role of fertility control in wildlife management? Canberra, Australia, Bureau of Rural Resources Bulletin 7. Pp. 1-50.
Cooper, D. 1999: Evolution of resistance to biocontrol agents. In: Sutherland, G. ed. Advances in the biological control of possums. The Royal Society of New Zealand Miscellaneous Series 56. Pp. 96-99.
Cowan P. E. 2000: Biological control of possums: prospects for the future. In: Montague, T. ed. The brushtail possum: biology, impact and management of an introduced marsupial. Lincoln, New Zealand, Manaaki Whenua Press. Pp. 262-270.
Magiafoglou, A.; Schiffer, M.; Hoffmann, A. A.; McKechnie, S. W. 2001: Immunocontraception: a discussion paper on predicting the consequences of the occurrence of immunocontraceptive-resistant individuals. Report prepared by the Centre for Environmental Stress and Adaptation Research, La Trobe and Monash Universities, for the Marsupial Cooperative Research Centre, (unpublished). 37 p.
Mikko, S.; Andersson, L. 1995: Low major histocompatibility complex class II diversity in European and North American moose. Proceedings of the National Academy of Sciences, USA 92: 4259-4263.
Montague, T. L. 2000: The brushtail possum: Biology, impact and management of an introduced marsupial. Lincoln, New Zealand, Manaaki Whenua Press. 292 p.
Nettles, V. F. 1997: Potential consequences and problems with wildlife contraceptives. Reproduction, Fertility and Development 9: 137-143.
NSSC 2000: National Science Strategy Committee for the Control of Possums and Bovine Tuberculosis. Annual Report 2000. Wellington, New Zealand, Foundation for Research, Science and Technology.
Rush, J. S. 2001: Variability and genetic control of immune response: implications for management of wildlife populations by immunocontraception. Report prepared for the Marsupial CRC, (unpublished). 82 p.
Sutherland G. ed. 1999: Advances in the biological control of possums. The Royal Society of New Zealand Miscellaneous Series 56.
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