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.