The genes of an invading toad

Why do some animals steam-roll across the landscape, commanding new territory in the manner of Genghis Khan, while others skulk around a tiny patch of marsh?

The question can also be applied to smaller groups of animals, like the toads. At one extreme, the spray toad lives only in the mist of one waterfall and has gone extinct in its native Tanzania. Meanwhile, the cane toad, a monster native of South America that was deliberately distributed to control insects on farms, has colonized Australia and Caribbean and Pacific islands, where it is busily crowding out native animals.

Photo: New South Wales Government

The cane toad is an ecological pest in Australia, but it carries a full set of genes that enable it to occupy new habitat.

Both species are among the 500-plus members of the family bufonidae, called the “true toads.” But what distinguishes toads that can dominate new landscapes from those that must struggle to survive, and what can that tell us about how species form from their ancestors?

In a study published this week, Ines Van Bocxlaer, of the biology department at Vrije University in Brussels, identified “range expansion” traits that would, logically, make for successful invaders:

Image courtesy Franky Bossuyt

The Common Indian Toad (Duttaphrynus melanostictus), an optimal range-expansion phenotype that originated from tropically-adapted ancestors endemic to the Western Ghats mountain range of the Indian subcontinent.

The making of a new species

“We chose characteristics that might be related to being able to disperse,” says Franky Bossuyt of Vrije University, the study’s corresponding author. “If they don’t need too much water, or are poisonous, that’s an advantage.”

Van Bocxlaer and colleagues correlated the range expansion traits with the size of habitats occupied by particular species, and concluded that all seven traits were “highly correlated” with the area occupied by the toads in their new homes, Bossuyt said.

The researchers produced a branched genetic tree that traces back to the ancestral toads in South America, which suggested that the range expansion traits were present when the toads began dispersing to new locations between about 37 million and 24 million years ago. Afterwards, these traveling toads branched into many of the species that survive today.

The species that now live in South America, however, have fewer of the range expansion traits. “We think it is the first time this was studied in this way, correlating the range-expansion characteristics and mapping them on a phylogenetic tree,” says Bossuyt.

Photo: Simeon

A sugar cane farm in Northern New South Wales, Australia. Cane toads were brought here to control insect pests, and then became pests themselves.

Traits tell tales

These results shine a beacon on a venerable evolutionary question: do new species arise before or after they begin occupying new ground? Which is more important for promoting the development of new species: new habitat, or the traits needed to occupy it?

“Links between geographic expansions and speciation have rarely been demonstrated,” says Carol Lee, an associate professor of zoology at the University of Wisconsin-Madison. “The authors first found a correlation between life history traits that might promote range expansion and current distribution, suggesting that such traits are plausible candidates for promoting range expansions.” Then they constructed a genetic tree of the apparent range-expansion traits, and found that the traits correlated with the transcontinental movements, adds Lee, who focuses on the genetics of invasive organisms.

Finally, the researchers calculated that species were forming extra-fast during the global colonization, Lee notes. “The authors argue that this coincidence suggests that range expansion itself might have been an important driver of diversification and speciation. Thus, they argue that this might be a case where dispersal ability might drive range expansions, followed by speciation.”

“Range expansion itself,” the researchers concluded, “was an important driver of diversification in bufonids.” In other words, animals that can adapt are more likely to invade and conquer new habitats.

Photo: Image courtesy Bert Willaert

The common European toad has all the talents needed to colonize new habitat, and is widespread in Europe.

Which came first?

The differentiation of newcomers into many different species is the standard explanation for the many unique species found in islands like Hawaii. Although this “adaptive radiation” is often thought to result from the availability of new ecological niches rather than the genetic talents of the arriving organism, the new study suggests that these genetic talents may need more focus.

Because the study looks at only one example of global expansion, “the co-occurrence of acceleration of speciation with global colonization could be purely coincidental,” says Lee. “Nevertheless, this study is an elegant attempt to test the links between range expansions and speciation.”

Although Bossuyt concedes that the study may not aid the battle against the pestiferous cane toad, “if people wanted to try again to introduce some other species, they could use this method to predict if it was good disperser or not, and thus whether it might be dangerous.”

David J. Tenenbaum