Does a biological invasion modify host immune responses to parasite infection?

Biological invasions disrupt the spatial structuring of antagonistic co-evolution between host and parasites. At the same time, the shifting demographic and selective landscapes during invasion can result in rapid evolution of traits in both host and parasite. Hosts at the invasion front may reduce investment into costly immune defences and redistribute those resources to other fitness-enhancing traits. Parasites at the invasion front may have reduced pathogenicity because traits that negatively impact host dispersal are left behind in the expanding range. The host’s immune system is its primary arsenal in the coevolutionary ‘arms race’ with parasites. To assess the effects of invasion history on immune responses to parasite infection we conducted a cross-infection experiment which paired common-garden reared cane toads and lungworm parasites originating from various sites in their invaded range across northern Australia. Infected toads had larger spleens and higher concentrations of eosinophils than did uninfected toads. Infected toads also exhibited lower bacteria-killing ability, perhaps reflecting a trade-off of resources towards defences that are more specifically anthelminthic. The impact of infection intensity on multiple immune measures differed among toads and parasites from different parts of the invasion trajectory, supporting the hypothesis that invasion has disrupted patterns of local adaptation.

Methods
Data are immunological measures taken from common-garden reared cane toads at the end of an experimental crossinfection study. Toads whose parents orignated from different parts of toads' invaded range in Australia were exposed to infective larvae of a lungworm parasite (brought with the toads when they were introduced from South America) from various parts of their range in Australia. A sample of the toads were Conmtrols who were never exposed to infective lungworm larvae.

3-5 months after experimental infections,ads were euthanised and we recorded data on their white blood cell populations, the ability of their plasma to kill bacteria, and the phagocytic ability of whole blood.