Introduction
Amphibians have suffered immense rates of population declines and extinctions over recent decades (Houlahan et al. 2000; McCallum 2007; Grant et al. 2019) and are considered the most threatened vertebrate class on the planet (Howard & Bickford 2014). A significant contributor to that mortality has been the invasive chytrid fungus (Batrachochytrium dendrobatidis ), which attacks the host’s epidermis (Lötters et al. 2009; Fisher & Garner 2020). The catastrophic impact of chytrid has drawn attention to the vulnerability of anurans to a diverse array of pathogens, such as ranaviruses (Grayet al. 2009) and parasites (Hartson et al. 2011; Gustafsonet al. 2018). In response to challenges induced by pathogens, anurans exhibit several lines of defense including immune responses and behavioral avoidance of pathogens (Hossack et al. 2013; Koprivnikar et al. 2014; McMahon et al. 2014). Moreover, amphibian skin secretions consist of secretions produced by the amphibian itself (Clarke 1997) and skin microbiota (mostly bacteria (Federici et al. 2015)), hereafter, we refer to this combination as ‘skin secretions’. These secretions contain many antimicrobial properties (Gustavo Tempone et al. 2007; Govender et al.2012), which might help to fight off pathogens (Weitzman et al.2019; Christian et al. 2021). Simultaneously, pathogens are under selection to overcome those barriers, generating an ‘arms race’ of adaptations and counter-adaptations in the host and its adversaries (Sorci & Faivre 2008). Those host-parasite interactions may drive the rapid evolution of spatial variation in the attributes of both participants, and selection should favor hosts that can either tolerate infection (limiting the harm caused by a given parasite infection) or reduce infection probability and burden, i.e., resistance (Råberget al. 2009).
Although some studies showed that amphibian skin secretions contain properties to resist chytrid fungus (Rollins-Smith 2009; Niederleet al. 2019) and bacteria (Quintana et al. 2017), little is known about the role of secretions as parasite defense mechanisms more generally. Similarly, we know little about how parasitic nematodes find their hosts. Generally, nematodes can utilize olfaction, gustation, thermosensation, and humidity to locate hosts, which they seek via strategies ranging from ambushing to actively crawling toward host-emitted cues (Castelletto et al. 2014; Gang & Hallem 2016). If such strategies for infection avoidance (in hosts) and host detection/recognition (in parasites) exist, they are expected to vary geographically between different populations, due to co-evolved local adaptations between host and parasite which occur on small spatial scales (Schmid-Hempel 2011).
Biological invasions provide unparalleled opportunities to investigate arms races between hosts and their parasites, because they create spatial heterogeneity in transmission rates and impose novel selective forces on one or both participants. Recently, we showed that the invasion of cane toads (Rhinella marina , Fig. 1) through tropical Australia has generated substantial spatial divergence in host-parasite interactions (Kelehear et al. 2012; Brown et al. 2016; Mayer et al. 2021). The toads have carried with them a native-range lungworm (Rhabdias pseudosphaerocephala , Fig. 1) (Dubey & Shine 2008; Selechnik et al. 2017) that can reduce viability of the host (Kelehear et al. 2011; Finnerty et al. 2018). The risk of parasite infection has been modified by the invasion process, with low population densities of hosts at the invasion-front reducing opportunities for parasite transfer among toads (Phillips et al. 2010). Apparently as a result, toads at the western invasion-front have evolved a greater resistance to parasite infection (Mayer et al. 2021), and nematodes close to the invasion-front have evolved a higher infectiviy (Kelehear et al.2012), continuing the arms race. That situation provides an ideal opportunity to investigate the biological role of anuran skin secretions in host-parasite interactions.
Because both parasites and their hosts adapt to local conditions, geographic variation in the skin secretions of cane toads might have both positive and negative consequences for host fitness. Skin secretions might render it more difficult for a parasite to infect their host, if secretions inhibit the parasite’s ability to locate and/or enter the host’s body or find its way to the target organ where it can grow and mature (here termed the ‘host defense ’ hypothesis). Alternatively, the parasite might evolve to use the host’s skin secretions as a signal for host location, or as a cloak to hide from the host’s immune system during the parasite’s migration through the body of the host (here termed the ‘parasite cue ’ hypothesis). Notably, these two hypotheses are not entirely exclusive; for example, skin secretions might facilitate host location by the parasite, but still defend against penetration by the parasite. By studying a system where host-parasite interactions have diverged rapidly, we have an opportunity to detect a range of such outcomes. We conducted experiments to (1) clarify the role of toad skin secretions as a host-finding cue for lungworms, and (2) test the function of toad skin secretion as defense mechanism against parasite infection (Table 1).