The Tammar Wallaby, one species or two?

Despite being one of the most intensively studied marsupials, recent genetic studies of the Tammar Wallaby has revealed some surprises.

The first comprehensive genetic study of wild Tammar Wallabies has revealed surprisingly high levels of genetic divergence between populations in Western Australia (WA) and South Australia (SA). In addition, most sampled island populations showed low genetic diversity and strong differentiation from mainland Tammars despite their relatively recent isolation.

You may have never heard of a Tammar Wallaby (Notamacropus eugenii), but they are scientifically amongst the best known marsupials in the world. Since these diminutive (4-6 kg) kangaroos are easy to maintain and breed in captivity, they have been widely used as the marsupial biologist’s ‘lab rat’ for studies of marsupial reproduction, physiology, genetics, behaviour and biochemistry. As a consequence, the Tammar Wallaby was the obvious choice to be the first Australian marsupial species, and only the second marsupial in the world, to have its genome sequenced. So while the Tammar genome provides a huge amount of information about the genetics of one tammar, the distribution of genetic diversity within Tammar Wallabies as a species has remained largely unstudied.

At the time of European settlement, Tammar Wallabies naturally occurred across semi-arid southern Australia from southern South Australia (SA) to south-western Western Australia (WA). They were also found on 10 offshore islands, including SA’s Kangaroo Island and islands in the Recherché and Abrolhos Archipelagos, WA. Since European settlement, Tammar Wallabies have declined as a result of land clearing and introduction of foxes and cats. Tammar populations on the SA mainland and four islands are now extinct, and although they also declined significantly in south-west WA, Tammars have persisted on the WA mainland and on five islands.

During this study we sampled all surviving Tammar Wallaby populations including those on Kangaroo Island, SA and Garden and the Wallabi Islands in WA. Our genetic analysis revealed that SA and WA Tammar populations were genetically highly distinct. These populations, currently separated by the arid Nullabor Plain, appear to have not exchanged genes for almost 1 million years. In fact, the level of genetic divergence between SA and WA Tammars is similar to that seen between recognised different species the Eastern (Macropus giganteus) and Western Grey Kangaroos (Macropus fuliginosus). However, unlike Eastern and Western Greys, which mostly produce sterile hybrids when crossed, SA and WA Tammar Wallabies are known to be fully inter-fertile when hybridised. As a consequence, SA and WA Tammars are probably best regarded as divergent members of the same biological species, although each could be recognised as distinct subspecies: Notamacropus eugenii eugenii in SA and Notamacropus eugenii derbianus in WA.

Most studies of Tammar Wallabies to date have focused on animals derived from the Kangaroo Island, SA population. We hope that the extensive genetic diversity and inter-population differentiation identified in this study within Tammar Wallabies will stimulate similar intensive research of the diverse WA populations. Such comparative studies will further increase the Tammar Wallabies value and usefulness as a model organism and will significantly add to our understanding of macropodid and marsupial evolutionary biology.

Dr Mark Eldridge, Principal Research Scientist, AMRI

More information:

• Eldridge, M.D.B., Miller, E.J., Neaves, L.E., Zenger, K.R., and Herbert, C.A. 2017.Extensive genetic differentiation detected within a model marsupial, the tammar wallaby (Notamacropus eugenii). PLoS ONE 12(3): e0172777.