Humans have moved plants and animals well beyond their native ranges, across barriers that normally prevent dispersal. As a result, people have increased the rates of hybridisation between populations that were once isolated for thousands, or even millions, of years.
Authors
- Lachie Scarsbrook
Postdoctoral Research Fellow, Genetics, School of Archaeology, University of Oxford
- Greger Larson
Professor of Palaeogenomics, University of Oxford
- Laurent Frantz
Professor of Palaeogenomics, Ludwig Maximilian University of Munich
Animal hybrids are a controversial issue among scientists, as they often suffer from health issues.
But our new study of Australian dingoes , published in the journal PNAS, found that hybridisation with introduced European dogs might have had evolutionary benefits.
New species can evolve when a subset of the population becomes separated, often by physical barriers like mountains or oceans. Over time, these isolated populations accumulate unique genetic mutations, some of which become fixed. If these populations spend long enough apart, they become so different they can no longer interbreed.
Although they were once domestic, dingoes became isolated from other dogs around 3,500 years ago and evolved into free-living apex predators. Some scientists argue that the dingoes' distinct appearance and behaviour warrant their recognition as a new species. Others claim that hybridisation with domestic dogs, which were brought to the continent by Europeans from the late-18th century onwards, has blurred this boundary.
Humans have been moving animals around for millennia. When farmers spread from the Near East into Europe around 8,500 years ago, for example, the domestic pigs that accompanied them came into contact and mated with European wild boar . In some cases where there were no closely related native populations, however, such as the import of exotic animals during the Roman period , escapees formed feral populations. Dingoes fit into this second category.
Species translocations and hybridisation accelerated during the colonial period, which reshaped local ecosystems. Hybrid offspring can lose the unique traits that allowed their parent populations to thrive in their specific habitats. Other effects are invisible, and can only be teased out of genetic studies.
For instance, across Asia, diversity in wild red jungle fowl populations is being lost through interbreeding with domestic chickens. In the Americas, almost all traces of Indigenous dog diversity was wiped out through hybridisation with introduced European dogs.
Hybridisation can also be beneficial. The acquisition of alleles (a different version of a gene) from another population may improve an animal's survival in new environments, or make them resistant to new diseases.
The ancestors of modern human populations on the Tibetan Plateau, for example, inherited an allele of the EPAS1 gene from Denisovans (a closely related human species) that improved their ability to live at high altitudes.
The dingo debate
Since dingoes were only isolated from other dog populations for a few thousand years, it is not a surprise that they can readily interbreed. The "purity" of dingoes is therefore a great source of conflict between conservationists, farmers and policy makers, and is used by both sides to justify policies to either protect or persecute dingoes .
Some genetic studies have suggested that dingo-dog hybridisation has not taken place , while others indicate most dingo populations have some level of European dog ancestry. A fundamental issue with these studies is that they require comparison against a "pure" reference population. Given centuries of overlap between dingoes and dogs, it is almost impossible to be sure that modern populations do not have mixed ancestry.
To circumvent this issue, our study sequenced genomes from ancient dingo bones recovered from caves on the Nullarbor Plain in southern Australia. Crucially, this included dingoes that lived and died prior to the arrival of the First Fleet in 1788. Establishing a precolonial baseline of ancestry for dingoes allowed us to to pinpoint the degree of European dog ancestry in dingo populations across Australia today.
Our genetic analysis showed that most dingoes living in the northwest of Australia did not have any detectable European dog ancestry. The opposite was true in the southeast, where almost a quarter of the genome of some dingoes came from European dogs.
Further investigation found that the European ancestry was in fact broken up into small chunks throughout the genome of dingoes, indicating that interbreeding took place at least ten generations (or 30 years) ago.
In fact, most of the hybrid mating coincided with the outset of aerial baiting programs in the mid-20th century , when poisoned meat was dropped from helicopters to kill dingoes en masse. This reinforces similar findings in Scottish wildcats , which shows local populations were resistant to interbreeding with invasive (domestic cat) populations until their own numbers declined to the point where finding a suitable mate (another wildcat) became too difficult.
Diversity is the key to success
Superficially, gene flow between dingoes and European dogs sounds like a negative outcome. Our research, however, suggests that dingoes have actually benefited. Hybridisation has led to an increase in genetic diversity in dingoes across southeast Australia, potentially offsetting the negative effects of inbreeding.
We also found evidence that a few alleles, which were transmitted from dogs to dingoes via interbreeding, may provide better protection against infectious diseases brought to the continent by European dogs.
Despite being an introduced species, dingoes are now adapted to Australia's varied ecosystems. Based on our results, we suggest that instead of prioritising "purity", future conservation efforts should focus on maintaining large enough populations for natural selection to operate effectively, so that dingoes can maintain their position as Australia's apex predator.
Hybrids are becoming increasingly common as humans and their domesticates continue to encroach into wild habitats, from Scottish salmon to Andean alpacas . In order to understand the impacts, both positive and negative, of this hybridisation, our results suggest we must first look to the past.
Laurent Frantz receives funding from the European Research Council, and the Deutsche Forschungsgemeinschaft
Greger Larson and Lachie Scarsbrook do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
