Why do we use opossums in research?

The Crick is home to one of northern Europe’s very few colonies of opossums used in scientific research. Much smaller than their possum relative, grey short tailed opossums have unique features that make them useful for research. But what makes them so special, and how do we work with them at the Crick?

The Crick’s opossum colony lives in the Biological Research Facility (BRF). These rat-sized, nocturnal creatures originated in South America, and have become well-known in scientific research as the ‘model organism’ to represent all marsupials.

The colony at the Crick was originally kept at the University of Glasgow, but was rescued by Crick group leader James Turner in 2009 when there were plans to close it down. More than 100 opossums made the move from Glasgow to the MRC National Institute for Medical Research in north London, and then into the Crick in 2017.

The team in the BRF are writing the book on how to care for opossums based on their longstanding experience working with and caring for this species. They strive for continuing improvement of living conditions.

As opossums come from a tropical climate, we need to mimic this as much as possible in the laboratory setting. The holding room temperature is set between 24-28 degrees Celsius, and the relative humidity range is 55-75%. They are set on a light cycle of 14 hours light to 10 hours dark. The temperature and humidity ranges can cause some challenges with keeping such a high humidity, and with our technicians when working in the room for long periods of time. Technicians are encouraged to take regular breaks to rehydrate and cool down. As the humidity is also high, we need to be careful and monitor the cages closely as it can cause the food to deteriorate and spoil quicker. 

Opossums are solitary animals in the wild and only come together for mating. That is why we house them individually, and males and females are kept in separate rooms. This avoids them being put under any undue stress, due to pheromones in the rooms. We keep them in double story cages that allow them to climb. Our opossums are provided with aspen wood chip bedding material and crinkled paper as nesting material. In the cage, we put a card board tunnel, a crawl ball and a house as enrichment items. 

Our opossums are provided with insectivore zoo diet. They are also provided with a range of food enrichment to supplement the pelleted diet. They are given live meal worms daily which are sprinkled into the cage. The mealworms will try to burrow down into the woodchip, this noise alerts the opossum, as they have very sensitive hearing. The opossum will then dig down for them, which encourages their natural foraging behaviour, as well as providing an additional protein source. We also give them some fruit every week. The opossums have continuous access to water in drinking bottles with spouts.

Each opossum is weighed on a weekly basis apart from if the female has recently littered. This allows us to track any weight gains or losses easily and is a permanent record. Weighing the opossums gives the technician a good indication of their overall health and wellbeing, as if an opossum is sick or stressed, they can lose weight rapidly.

One of their most important features is that young opossums, or joeys, are born relatively under-developed compared to other mammals. They latch on to their mothers’ tits where they stay for the next eight weeks, meaning that scientists can keep an eye on their development without having to perform invasive procedures or use complex imaging techniques

James Turner was keen to provide a home for the opossums as they are well-suited for research, contributing to a number of different areas of science, including neuroscience and tumour biology. But James is interested in a feature which originated millions of years ago. Marsupials’ branch of the genetic family tree split off from our own 160 million years ago, and it includes animals like kangaroos and koalas. Because marsupials have evolved alongside our own branch of mammals, comparing their genome to ours lets us see how evolution has affected our genetic makeup. 

There are three main things that we can look for when comparing the opossum genome to our own. Firstly, there are the similarities: genes that have stayed constant and made it through millions of years of evolution. Then there are differences: genes that are found in both mice and humans, but not in opossums, might not be quite as important as they appear.

But the most interesting things are ones that have evolved separately in both branches of the family tree. The evolution of echolocation in both bats and dolphins is the most famous example of this. This convergent evolution points researchers towards important features that are probably the most effective way to do something, rather than just an evolutionary fluke. 

James’s group at the Crick studies sex chromosomes, including X-chromosome inactivation, the automatic ‘turning off’ of one of the X chromosomes in every cell in female mammals.

The process evolved separately in both opossums and other mammals, but not in our shared ancestor, so it’s an example of convergent evolution and a successful strategy. And we know that problems in X-chromosome inactivation can cause developmental disorders and cancer. By examining the genetic makeup of opossums and the differences and similarities in our chromosomes, the team are answering important questions about fertility and development in mammals.