On a seemingly unremarkable hill in north Queensland something remarkable appears to be happening …. to wallaby’s tails.
We have recently spent over 2 weeks in far north Queensland studying an unusual population of rock-wallabies. These wallabies are rare natural hybrids that are only produced where the distributions of two species meet and individuals interbreed. By studying these hybrids we hope to learn more about how species form – a fundamental question in biology.
Northeast Queensland is remarkable for many reasons, tropical forests, coral reefs, diverse wild landscapes and its amazing biodiversity. For mammal enthusiasts northeast Queensland is also famous as the home to an incredible six of Australia’s 17 species of rock-wallabies. These small (4-6 kg) wallabies are specially adapted to living in rocky areas, such as boulder piles, outcrops, cliffs and gorges. As a result, their distributions tend to be very patchy with populations often separated by tens to hundreds of kilometres. Each of these species has small but discreet distributions and geographically they replace each other as you travel northwards from the Whitsundays to northern Cape York Peninsula.
Although populations of many of these species come within a few kilometres of each other, at only one place, a small non-descript rocky hill on southern Cape York Peninsula, have scientists documented a place where two species meet and actually interbreed, forming what we call a hybrid zone. Scientifically, this hybrid zone is a very special place and is one of only two hybrid zones known from Australian mammals. Hybrid zones are interesting, because how species behave when they meet, interact and interbreed, can tell us a lot about the process by which species form, i.e. speciation.
Scientifically, Australia’s endemic rock-wallabies are widely known as an excellent model of chromosomal speciation, with most species having unique chromosomes that differ in their shape and number from all other species. The two species that meet and hybridise northwest of Cairns are the Mareeba rock-wallaby (Petrogale mareeba) and Godman’s rock-wallaby (Petrogale godmani). These two species have different chromosome numbers (18 vs 20) and differ by at least six major changes in chromosome shape or arrangement. They also show marked differences in their DNA sequences. Although all northeast Queensland rock-wallaby species are morphologically very similar, Mareeba rock-wallabies tend to have dark tails, while Godman’s rock-wallabies tend to have pale silvery whiter tails.
With their major chromosome and other genetic differences, theory would predict that hybrids between these species should be sterile. When the hybrid zone was first discovered and sampled in the 1980s, several sterile male hybrids were found although remarkably some female hybrids appeared to be able to produce young. There was also evidence of some genes moving between the species, a process termed introgression. Clearly these wallabies had not read the text books and some gene flow was occurring between these species despite their differences.
Another puzzle is if the male hybrids are sterile, then how does the colony persists and why doesn’t it die out? And who keeps fathering the offspring that we find in the females’ pouches? Are some hybrid males fertile, or do new wallabies keep arriving on the hill from the surrounding populations 10-15 km away?
Over 30 years since these initial studies, we wanted to revisit this hybrid colony, collect a larger sample size and then apply high-powered modern genetic and genomic techniques to test new theories on the role of chromosome change in producing species.
So, escaping a cold and bleak Sydney winter we headed north to the warmth and sunshine of the dry season in the savannah woodlands northwest of Cairns. Our aim was to try and collect genetic samples from most of the rock-wallabies living on this small hill – a unique natural laboratory where the genomes of two species have come back together. With new genomic techniques this population sample would give us the best chance of unravelling the mysteries of the hybrid zone and so expanding our knowledge of fundamental biological processes.
After failing to catch any wallabies at a nearby site in August 2016, we were a little unsure of our chances of success this year. After a bumper summer wet season, the tropical savannah woodland had a dense understory of grasses, some over 1.5 m tall. Now well into the dry season the grasses were sandy brown, dead and tinder dry. On our little rocky hill, the shrubs and some of the trees had lost most of their leaves, covering the ground in a deep crunchy leaf litter. Remarkably, less than an hours’ drive away the tropical forests of the coastal ranges were cloaked by cloud and dripping wet, yet at our study site it was hot and dry and as a result the rock-wallabies were hungry. This made the lure of some fresh apple placed into our soft-sided box-traps irresistible.
Over the course of our stay we caught 14 different wallabies, ranging from a juvenile just out of his mother’s pouch to a battle-scarred old male - the ‘King of the Hill’. Each was weighed, measured and had a small biopsy sample taken for genetic studies before being released. After just over a week, we stopped catching new individuals and all of the individuals we caught in the following days were recaptures of individuals we had sampled previously. It seems like apples are just too delicious.
After our first few days trapping, we noticed a remarkable thing – every wallaby had a different coloured tail some were almost black, some were pale silver, others were reddish, some had dark tips others had light tips. Soon we could identify each animal just by looking at their tails – ‘Kinky Silver-tail’ was one of our favourites. To find such large amounts of morphological variation within a single wallaby population is highly unusual and is likely a product of the hybrid origin of this colony. While these unusually high levels of morphological variation are enough to make a geneticist’s mouth water, much more information on genes and their expression will be revealed by our planned detailed genomic analysis. But that will take many months of work so now we all just have to wait impatiently….