POISONOUS TOADS WERE SPREADING ALONG THE SILK ROAD. COMENIUS UNIVERSITY ZOOLOGIST CONTRIBUTES TO SURPRISING EVOLUTIONARY FINDINGS
Bratislava, 9 January 2025: One of the most invasive poisonous frogs in the world, the Asian black-spined toad, whose genetic diversity in the Indian Ocean region bears traces of human activity, actually consists of two species. This discovery was published in Nature Communications by a team of scientists, one of whom was Daniel Jablonski from the Faculty of Natural Sciences of Comenius University. The discovery changes our understanding of the evolution of these amphibians in Asia, but also highlights the long-term impact of human globalisation on biodiversity.
The toad that travels the world
The Asian black-spined toad is commonly found near people and their settlements from Pakistan to eastern Indonesia and often becomes a stowaway on ships or planes. This species has previously been transported to Madagascar and the Wallacea region, an area in Indonesia that is among the world's key centres of biodiversity. In Australia, the species is blacklisted due to a potential ecological threat, similar to that caused by the cane toad (Rhinella marina) brought from South America. Cane toad was introduced to Australia in 1935 to control insect pests on sugarcane plantations. Instead, it has become an invasive species affecting local ecosystems in a negative way. Its poison poses a threat to native predators that are not adapted to its toxins, and its rapid spread and high reproductive capacity have led to the displacement of Australian species. It is assumed that this was also the case of the Asian black-spined toad.
However, using DNA analyses of populations across Asia, scientists have now shown that the Asian black-spined toad is not a single species. "Our research has shown that what we thought to be a single species is actually at least a pair of evolutionarily distinct populations that diverged about 7 million years ago," explains Dr. Jablonski, one of the study's lead authors. The first species, currently inhabiting the Indian subcontinent and Indonesia, found a new home in the Wallacea region, while the second, originally from Southeast Asia, reached as far as Madagascar.
Traces of human migration in toad DNA
Perhaps the most surprising finding is that toads from Indonesia are genetically identical to populations from southern India. However, these countries are separated by thousands of kilometres of land and sea. This suggests that they did not arrive in Indonesia naturally. "They were probably brought there by humans, several centuries or even millennia ago. This must have happened a long time ago, given that they spread throughout the entire Indonesian archipelago and zoologists were already aware of their presence in the early 20th century. What they did not know was that these frogs weren't native there," says Jablonski. Scientists link this colonization to the Maritime Silk Road, which connected India and Indonesia. The ancient economic ties along this route likely contributed both to the shared history and cultural similarities between Indians and Indonesians, and helped the frogs to spread.
Toads may have been transported intentionally because they represent an important element in Asian culture through their religious or pharmaceutical significance; or unintentionally as stowaways. "It's a fascinating example of how trade, culture and migration in ancient times shaped today's biodiversity,” adds Jablonski.
Threat to ecosystems and new challenges for nature conservation
Not only are Asian black-spined toads invasive, but their high toxicity poses a serious threat to many predators, including the largest lizard in the world, the iconic Komodo dragon. The study also points out that Wallacea and Madagascar face different ecological challenges, because each region hosts a different species of the toad. "We should analyse the toxicity of these populations to better predict the impacts of invasions and design effective conservation measures," Jablonski states.
Why does it matter?
The discovery that the Asian black-spined toad consists of two species represents a ground-breaking insight into the evolution of these amphibians in Asia and the dynamics of their invasions. The study also highlights the danger that can arise from the spread of species due to human activity. "To this day we observe the consequences of what our ancestors were doing hundreds or thousands of years ago, and there even weren't 8 billion of them in the world.” concludes Jablonski. The study has the potential to improve our understanding of invasions and to help prevent further biodiversity loss – an issue that is becoming increasingly urgent today.
Dufresnes C, Jablonski D, Ambu J, Prasad VK, Bal Gautam K, Kamei RG, Mahony S, Hofmann S, Masroor R, Alard B, Crottini A, Edmonds D, Ohler A, Jiang J, Khatiwada JR, Gupta SD, Borzée A, Borkin LJ, Skorinov DV, Melnikov DA, Milto KD, Konstantinov EL, Künzel S, Suchan T, Arkhipov DV, Trofimets AV, Nguyen TV, Suwannapoom C, Litvinchuk SN, Poyarkov NA. 2025. Speciation and historical invasions of the Asian black-spined toad (Duttaphrynus melanostictus). Nature Communications. 16: 298. https://doi.org/10.1038/s41467-024-54933-4
Daniel Jablonski is a recipient of a NextGenerationEU scholarship through the Slovak Recovery and Resilience Facility under project no. 09I03-03-V04-00306.