Sexually transmitted mutualist nematodes promote host growth across dung beetle species

Many symbionts are sexually transmitted and impact their host’s
development, ecology, and evolution. While the significance of symbionts
that cause sexually transmitted diseases (STDs) is relatively well
understood, the prevalence and potential significance of the sexual
transmission of mutualists remain elusive. Here, we study the effects of
sexually transmitted mutualist nematodes on their dung beetle hosts.
Symbiotic Diplogastrellus monhysteroides nematodes are present on the
genitalia of male and female Onthophagus beetles and are horizontally
transmitted during mating and vertically passed on to offspring during
oviposition. A previous study indicates that the presence of nematodes
benefits larval development and life history in a single host species,
Onthophagus taurus. However, Diplogastrellus nematodes can be found in
association with a variety of beetle species. Here, we replicate these
previous experiments, assess whether the beneficial effects extend to
other host species, and test whether nematode-mediated effects differ
between male and female host beetles. Rearing three relatively distantly
related dung beetle species with and without nematodes, we find that the
presence of nematodes benefits body size, but not development time or
survival across all three species. Likewise, we found no difference in the
benefit of nematodes to male compared to female beetles. These findings
highlight the role of sexually transmitted mutualists in the evolution and
ecology of dung beetles.