Making a name for itself: the laboratory opossum
Mice, rats, macaques, ferrets … there are quite a few well-known laboratory mammals. But opossums? What can these solitary marsupials offer science? Katherine Nightingale went to the MRC National Institute for Medical Research (MRC NIMR) to find out.
It’s Thursday morning at the MRC NIMR and the institute’s 100 opossums are going about their usual business. Being nocturnal, that means not a lot. In their room, most of the female opossums are burrowed into their nests, big eyes peering out from under shredded paper. Next door, the males are settling into their new double-height cages, tails dangling from one end of their cardboard tubes and long noses protruding from the other.
Native to South America, the grey short-tailed opossum (Monodelphis domestica), also known as the laboratory opossum, is a fairly unusual research animal. The NIMR is the only place in the UK to keep a colony, with other larger colonies at the University of Trieste in Italy, and the Texas Biomedical Research Institute in the United States.
Yet in recent years, scientists have begun to recognise just how useful the opossum can be to medical research. Being mammals, they share many genes and biological processes with humans, and so can be used as model animals in much the same way as mice or rats.
But opossums have other tricks up their sleeves. They are marsupials, meaning their branch of the mammal family tree split from ours around 148 million years ago. This means they differ from us in crucial ways, and by comparing these differences, researchers can learn more about the biology of people.
The NIMR’s James Turner has used opossums in his research to look into a process called X-inactivation. This is the way that one of the X chromosomes in female cells is silenced, and can result in diseases such as cancer when it goes wrong. “Opossums are great models for comparative genetics because they throw up surprises,” he says.
There are other unique features of opossums that make them useful in research. They differ from ‘true’ marsupials, such as kangaroos and possums, by not having a pouch. Pups are born after 13.5 days gestation at half the size of a little fingernail and with no back legs. They crawl to the teat and latch on, continuing their development there in the same way that a kangaroo joey develops in the pouch.
Newborn opossums are at a similar stage of development to a six-week old human fetus, so their body systems are immature and ideal for studying the development of organs such as the brain. And because they are already outside the mother’s body, there is no need to cull or operate on her to retrieve the pups.
Opossums are also the only animals other than humans which develop skin cancer caused solely by UV light so they are good for studying the development, prevention and treatment of melanoma. Newborn opossums can also regenerate their spinal cords completely, with no effect on spinal function, up to 9-12 days after birth. If researchers can find out which genes are active during the healing process, they might be able to find the equivalent genes in humans and use this knowledge to develop treatments for damage to spinal cords in people.
Keeping the radio on
These unique features mean that looking after the opossums in a research environment is also different to that for mice and rats. Each female opossum can have up to 13 pups, one for each of their 13 teats, though a typical litter size is more like seven or eight. In the wild, opossums are solitary and hostile to each other, interacting — violently — only when breeding.
Their solitary nature means they can easily be kept in their own cages, but also that breeding is a taxing nine-day affair. The keepers sensitise the opossums to each other by placing their cages side by side, before swapping the male and female into each other’s cages and finally putting them into a large single cage. Because they can act so aggressively, the NIMR technicians keep a radio on in each room so that the opossums aren’t too agitated when someone walks in.
Still relatively rare in science, specialist equipment for opossums is lacking, so each is housed in the clear yellow plastic cages designed for rats, and much of the knowledge about keeping them comes from pet-owners rather than researchers. As well as standard food pellets, their diet is enriched with live mealworms and fruit.
The males are currently testing out double-decker cages that are intended to satisfy their desire to climb, and the females will receive the same if all goes well. When the animals become old and are no longer productive breeders, they are culled humanely before their health begins to deteriorate. Their tissues are then stored at the NIMR or sent to other researchers in the UK to maximise the research opportunities from each animal.
While interest is growing, opossums are far from the research stars that mice are, says James. “Their genome has been sequenced, but it’s not possible to genetically engineer them to make opossums without particular genes, for example, to explore their function. And there are no opossum stem cells. But these things will come.”
This article is one of two about research using opossums at the MRC NIMR. You can read about the work of researcher James Turner in his post Why I use marsupials in research.