A recent study published in Proceedings of the Royal Society B reveals that the lungworm parasites found in cane toads in Australia are evolving to be better at infecting their hosts. The study focused on the lungworms that were brought to Queensland along with the cane toads in 1935. Researchers discovered that the lungworms taken from the invasion front in Western Australia were more effective at infecting toads compared to their Queensland counterparts.
Nematode lungworms are small threadlike creatures that reside in a toad’s lungs, feed on its blood, and release eggs through the host’s digestive system. The larvae that hatch from these eggs wait for a new host to pass by, then enter through its skin and migrate through the toad’s body to reach the lungs, where they establish themselves and start the cycle again.
Parasites and hosts engage in an ongoing evolutionary arms race. Any characteristic that allows a parasite to better find a new host, establish an infection, and evade the host’s defenses will be favored by natural selection. Similarly, hosts evolve mechanisms to detect, avoid, or repel parasites. However, parasites often have the advantage due to their high reproductive rates and short generation times, allowing them to evolve new strategies faster than hosts can adapt.
The co-evolution between hosts and parasites is most pronounced when they are in close proximity and regularly encounter each other. However, when hosts invade new territories, it disrupts the evolutionary balance between local hosts and parasites. Since their introduction in 1935, cane toads have spread over 2,500 kilometers across Australia, reaching Western Australia. Along the way, the toads have changed significantly.
Queensland toads tend to stay in a small area and reuse the same shelter repeatedly, leading to high population densities. This makes it easier for lungworm larvae to find new hosts. However, at the invasion front in Western Australia, toads are highly mobile and rarely stay in one place for more than a night. As a result, lungworm larvae at the invasion front have fewer opportunities to encounter and infect new hosts.
The study conducted experiments using toads and lungworms from different locations in Australia to understand how co-evolution is occurring between the two. The researchers collected toads from various areas, bred them in captivity, and exposed them to lungworm larvae from different regions. They found that lungworms from the invasion front were the most effective at infecting toads, even those from different locations. In contrast, hosts from intermediate and old populations were better able to resist infection from their local parasites compared to those from distant populations.
While the study observed significant differences in infection outcomes, the specific biochemical mechanisms behind these differences and how changes in genetic variation of host and parasite populations contribute to them are yet to be determined.
