The evolution of host-parasite interactions during a biological invasion
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Hosts and parasites are in a perpetual co-evolutionary “arms race” that might be affected during a biological invasion, where both host and parasite are exposed to powerful new selective forces. The cane toad (Rhinella marina) invasion in Australia provides a unique opportunity to study the initial stages by which hosts and parasites adapt to each other in novel environments. To explore this issue, we raised the offspring of cane toads from three regions across its invasion range under standard conditions, and used reciprocal cross-infection experiments with lungworm nematodes (Rhabdias pseudosphaerocephala) to clarify geographical coevolution between toad fitness measures and parasite infectivity. Our results show rapid shifts in hostparasite interactions during a biological invasion. Whereas parasites evolved higher infectivity close to the invasion front, hosts seem to develop rapid specific resistance to co-evolved parasites after separation by a few generations. Indeed, our results suggest a parasite local maladaptation (i.e., local adaptation of hosts to parasites). Importantly, we found that the main mechanism of toad resistance to infection is prevention of the worms reaching and establishing in the lungsrather than immune reaction once worms have reached the lungs. No major impacts on cane toad fitness measures were found during the lungworms migrations through the metamorphs bodies, though negative impacts could appear once they are established in the lungs.