Cooperative secretion of virulence factors by pathogens can lead to social conflict
when cheating mutants exploit collective secretion, but do not contribute to it. If
cheats outcompete cooperators within hosts, this can cause loss of virulence. Insect
parasitic nematodes are important biocontrol tools that secrete a range of significant
virulence factors. Critically, effective nematodes are hard to maintain without
live passage, which can lead to virulence attenuation. Using experimental evolution,
we tested whether social cheating might explain unstable virulence in the
nematode *Heterorhabditis floridensis* by manipulating relatedness via multiplicity
of infection (MOI), and the scale of competition. Passage at high MOI, which
should reduce relatedness, led to loss of fitness: virulence and reproductive rate
declined together and all eight independent lines suffered premature extinction. As
theory predicts, relatedness treatments had more impact under stronger global
competition. In contrast, low MOI passage led to more stable virulence and
increased reproduction. Moreover, low MOI lineages showed a trade-off between
virulence and reproduction, particularly for lines under stronger between-host
competition. Overall, this study indicates that evolution of virulence theory is valuable
for the culture of biocontrol agents: effective nematodes can be improved and
maintained if passage methods mitigate possible social conflicts.
biological control, cooperation, evolution of virulence, Heterorhabditis floridensis, pest management, stability
Adaptation, Behavior & Social Evolution, Evolutionary Applications, Evolutionary Dynamics, Evolutionary Ecology, Evolutionary Epidemiology, Evolutionary Theory, Experimental Evolution, Population Genetics / Genomics, Reproduction and Sex
