Conclusions and other considerations towards a potential model
-The deathbed behavior of spiders infected with Gibellula spp.likely represents an extended phenotype of parasite gene expression.
Parasite induced changes in this host-pathogen system fulfil all four
criteria set forth by Poulin (1995) to assess if the observed
manipulation is ‘truly’ adaptive for the parasite. However, web building
prior to death is not ubiquitous among spiders infected by Torrubielloid
anamorphs and other species in this genus exhibit similar phenotypes but
with characterizable differences in host choice and death location. For
example, another Torrubielloid anamorph (Gibellula leiopus ) from
the same area infecting the spider Trachelas tranquillus ,
invariably cause hosts to die on elevated tree branches and never on
leaves. In this species, a fungal mat appears to form the only
attachment of the host spider to the tree surface. Nonetheless, this
behavior of dying in elevated (1-3 meters) locations on the underside of
tree branches also provides fitness benefits for the parasite via
efficient dispersal of infectious propagules (Araujo and Hughes 2016).
The differences in host death locations between these two related
pathogens likely reflects the differences in the specific ecologies of
the host species, and like O. unilateralis infected ants, may be
indicative of adaptive manipulation by the parasite.
Parasitized hosts dying on the underside of leaves is a common feature
in the life cycle of many entomopathogenic fungi and represents a clear
fitness advantage for the parasite (Araújo and Hughes 2016). The
locations where infected hosts die can be phylogenetically informative
for some highly specialized species and relates directly to the specific
ecological niche exploited by the host (Evans et al. 2018, Araújo et al.
2018). The Torrubielloid anamorphs such as previously mentioned species
of Gibellula appear to follow the same trend, though no studies
have yet explored specialized fungal pathogens of spiders with similarly
comprehensive detail. Despite compelling empirical evidence, the current
literature is bereft of any experimental support for adaptive
manipulation in this particular parasite-host system. These specialized
fungal parasites of spiders may therefore provide a valuable
experimental model for further exploring the dynamics of parasitic
manipulation since rearing of large numbers of hosts in the laboratory
is fairly simple and low cost. Such models will endeavor to better
define the elusive distinction between adaptive and non-adaptive
manipulation, as it will likely provide valuable insight into the
evolution of specialized parasite-host systems.