Discussion
Our data confirm that anuran skin secretions play important and complex roles in host-parasite interactions. In line with the host defense hypothesis, skin secretions can decrease a toad’s vulnerability to nematodes, by reducing larval longevity and by reducing the ability of larvae to penetrate to the host’s lungs. However, skin secretions also provide chemical cues that can attract infective larvae (parasite cue hypothesis) as suggested by other studies (Theodoropoulos et al. 2001). Moreover, skin secretions can enhance rather than decrease the ability of parasite larvae to penetrate the toad’s body and establish an infection in the lungs. Thus, some effects of skin secretions appear to represent adaptations of the host that reduce vulnerability to parasite attack (Tempone et al.2008; Christian et al. 2021), whereas other effects are more consistent with adaptations of parasites to exploit the traits of their host (Gang & Hallem 2016). The most striking result from our studies is the magnitude of geographic divergence in host-parasite interactions within our study system. Although cane toads have been spreading through tropical Australia for only 85 years (Urban et al. 2008), they appear to have evolved major divergences in the roles that skin secretions play in host-parasite biology.
Broadly, the skin secretions of cane toads appear to act as a defense against lungworms in invasion-front populations in Western Australia (Christian et al. 2021), but not in range-core populations in Queensland. Thus, the presence of skin secretions reduced the rate at which invasion-front L3 were able to establish infections in invasion-front hosts, whereas the reverse was true for range-core L3 attacking range-core hosts. In regard to the host, the shift was from comparatively low protection (or the absence thereof) against parasite infection by skin secretions at the range-core, to increased protection at the invasion-front. That geographic divergence fits well with the hypothesis that cane toads in the range-core have low resistance to parasite infection (Mayer et al. 2021), because parasites are ubiquitous due to high host densities, and toads thus rely on tolerating rather than resisting pathogens (Adelman & Hawley 2017). Conversely, at the invasion-front, where there is a strong evolutionary pressure for dispersal ability via both natural selection and spatial sorting (Brownet al. 2014; Phillips & Perkins 2019), increased host resistance might be favored if parasite infection reduces dispersal ability. Moreover, some sets of immune genes are upregulated at the invasion-front (Selechnik et al. 2019a), suggesting that heightened, possibly non-specific, immune responses arising from exposure to other pathogens or conditions at the invasion-front may increase resistance of toads (Brown et al. 2018; Mayer et al. 2021). Although mechanisms leading to shifts in host immune responses are unclear, our study demonstrates that changes in defense strategies against parasite infection can evolve rapidly.
In regard to the parasite, one surprising result was that in range-core toads, skin secretions enhanced rather than reduced the rate at which larvae were able to reach the lungs and establish an infection. Theodoropoulos et al. (2001) suggested that gastro-intestinal helminth parasites express mucin-like molecules to avoid detection by the host’s immune system (mucin is the main molecule of the intestinal mucus barrier (Carlisle et al. 1991; Sharpe et al. 2018)). If so, larvae entering the host’s body may benefit by cloaking themselves in host-derived skin secretions that enable them to evade detection by the host’s immune system. That tactic apparently does not work at the invasion-front, perhaps because the immune system of invasion-front toads is more active against pathogens (Brown et al. 2018) as indicated by the effects of host skin secretion on larval longevity. Additionally, the larger larvae at invasion-front sites potentially cannot be cloaked as effectively, leading to increased susceptibility to the host’s immune system. An alternative scenario for the reduced infection success of range-core L3 when skin secretions were reduced might be a reduced ability to detect the host. Range-core L3 used skin secretions as a cue and olfaction is important for host-finding in other species (Gang & Hallem 2016). However, we found no significant difference in the number of L3 entering the host as a function of the presence of skin secretions. Invasion-front L3 also used skin secretions as a cue – but only from range-core toads, not invasion-front toads. This latter finding might have been caused by the negative effect of skin secretions from invasion-front toads on longevity of L3. Thus, invasion-front L3 might still be able to use skin secretions of invasion-front toads as a cue, but avoid exposure to evade detrimental effects. In contrast, the non-coevolved range-core L3 might not detect the detrimental effects of the secretions during the short exposure time of the experiment.
Much remains to be learned about the mechanisms underlying the interactions between amphibian hosts and the organisms that infect them. For example, we do not know which components of the skin secretion function as host defense or parasite cue, respectively. Cane toads from different regions have different proportions of bacteria with antifungal properties (Weitzman et al. 2019), suggesting that microbial properties might also contribute to defense against parasites. It would be of great interest to tease apart the degree to which the functional attributes of skin secretions (e.g., in reducing or enhancing parasite success) derive from molecules produced by the amphibian versus the microbiota that live on its skin. Moreover, the impacts of skin secretions ideally need to be seen in a wider context, as part of a suite of responses that also encompass behavior, morphology and physiology (especially, immune function: Brown et al. (2018)). Thus, for example, a highly effective immune response to pathogens that enter the body might relax selection for barriers on the skin. Additionally, it is important to remember that the skin has many other functions, such as respiration, regulating water flow and as the location of toxin-excreting glands (Huang et al. 2016; Senzano & Andrade 2018; Blennerhassettet al. 2019; Kosmala et al. 2020).
The devastating impacts of diseases (especially chytrid-driven) on anuran amphibians worldwide places a high priority on the need to understand factors that render an anuran more or less vulnerable to infection (Rollins-Smith 2009). Strong phylogenetic and geographic variation in the magnitude of disease impacts on amphibians (e.g. Savage & Zamudio 2016; Fisher & Garner 2020) suggest that spatially variable outcomes of host-parasite arms races may be key to understanding – and hopefully, ameliorating – some of those impacts. Our data indicate that at least part of the diversity in host-parasite interactions can involve diversity in the role of skin secretions, and that location-specific selective forces can generate rapid changes in the ways that anuran hosts interact with their pathogens.