Introduction
In nature, all plants and animals are colonised by microbes (Barriere,
2006; Ley et al., 2006; Vántus et al., 2014). The composition of these
microbial communities is highly diverse and includes harmful, neutral,
and beneficial microbial species (Ley et al., 2006), including those
that can be important players in host defence against parasites, a
phenomenon referred to as ‘defensive mutualism’ (King, 2019; May and
Nelson, 2014). Recognised for over a century, defensive mutualism has
been observed in plants (Mendes et al., 2011) and in a range of animals
(Dillon et al., 2000; Dong et al., 2009; Jaenike et al., 2010; Koch and
Schmid-Hempel, 2011), including humans (Kamada et al., 2013; Ley et al.,
2006; Maynard et al., 2012) wherein microbes can supplement host immune
systems (Abt and Artis, 2013; Hooper et al., 2012; McFall-Ngai et al.,
2013).
The net benefits of defensive mutualism are dependent upon the presence
of pathogens (Clay et al., 2005; King and Bonsall, 2017; Lively et al.,
2005). Whilst hosts can benefit from microbe-mediated protection,
defensive symbionts can be less beneficial to the host in the absence of
enemies, due to metabolic and physiological costs (King, 2019). For
example, in the interaction of aphids and the bacteriumHamiltonella defensa , the host tissue is harmed by defensive
toxins that protect against infection from parasitoids (Vorburger and
Gouskov, 2011). In some cases, possessing protective microbes might be
more beneficial to the host than investing in its own immune system
(Martinez et al., 2016). From the perspective of the symbiont, it is
most useful to its host under high pathogen prevalence, and thus can
persist in the host population (Palmer et al., 2008). Nevertheless, a
stable symbiotic interaction is hypothesized to be evolved and
maintained (Kwiatkowski and Vorburger, 2012) only when the host benefit
of carrying defensive symbionts outweighs any costs. The interactions of
obligate and defensive symbionts and hosts can be stable for millions of
years (Moran et al., 2005).
Not all environments are constantly pathogen rich which might shift the
balance of costs and benefits during defensive mutualisms, particularly
during coevolutionary interactions (King and Bonsall, 2017). Pathogen
prevalence can be spatially (King et al., 2009) or temporally variable,
the latter in the case of seasonal epidemics (e.g., flu peaks each
winter in the northern hemisphere (Finkelman, 2007) or rabies in North
American skunks which peaks in Autumn (Gremillion-Smith and Woolf,
1988)). Different environmental factors can influence disease
transmission such as an increase in malaria risk in warmer regions after
rainfall (Altizer et al., 2006), or an increase in contact rate and thus
higher flu infection rate during the winter months (London and Yorke,
1973). The impact of other temporally heterogeneous factors on the
strength and direction of selection on species interactions have been
explored (oxygen concentration (Dey et al., 2016), resource availability
(Friman et al., 2011; Friman and Laakso, 2011; Hiltunen et al., 2012),
environmental productivity (Harrison et al., 2013)). Whether the varied
presence of pathogens can similarly alter selection for symbiotic
interactions has been explored theoretically (Fenton et al., 2011), but
remains to be empirically tested.
Here, we examined the impact of temporal variation in pathogen infection
on the evolution of microbe-mediated protection. We usedCaenorhabditis elegans as a worm host and allowed it to be
colonised by a bacterium (Enterococcus faecalis ) that protects
against infection by Staphylococcus aureus (King et al., 2016).Enterococcus faecalis has been shown to be protective across
animal microbiomes (Kommineni et al., 2015; Martín-Vivaldi et al.,
2010). It has been previously shown that E. faecalis can evolve
to provide enhanced protection when residing inC. elegans hosts during constant pathogen infection (King et al.,
2016; Rafaluk-Mohr et al., 2018). From this, we predict that variation
in pathogen infection might limit the evolution of microbe-mediated
protection. In the present study, we experimentally co-passaged C.
elegans with protective E. faecalis and infected the host with
evolutionary static pathogenic S. aureus at different intervals
of host evolution. We also examined whether pathogen presence at the
initial formation of the coevolving interaction is crucial to the
evolution of protection. We show that enhanced microbe-mediated
protection emerged out of novel coevolutionary host-microbe interactions
and during pathogen infection, regardless of its temporal variability or
the time point of first infection. Enhanced protection was only
effective during pathogen infection. If hosts survived infection, they
could recover and had the same longevity and reproductive output across
treatments. These results thus suggest that even occasional pathogen
infection can select for defensive mutualism, revealing the potential
for this phenomenon to be widespread in nature.