Positive fitness effects help explain the broad range ofWolbachiaprevalences in natural populations

The bacterial endosymbiontWolbachiais best known for its ability to modify its host’s reproduction by inducing cytoplasmic incompatibility (CI) to facilitate its own spread. Classical models predict either near-fixation of costlyWolbachiaonce the symbiont has overcome a threshold frequency (invasion barrier), orWolbachiaextinction if the barrier is not overcome. However, natural populations do not all follow this pattern:Wolbachiacan also be found at low frequencies (below one half) that appear stable over time.Wolbachiais known to have pleiotropic fitness effects (beyond CI) on its hosts. Existing models typically focus on the possibility that these are negative. Here we consider the possibility that the symbiont provides direct benefits to infected females (e.g. resistance to pathogens) in addition to CI. We discuss an underappreciated feature ofWolbachiadynamics: that CI with additional fitness benefits can produce low-frequency (<1/2) stable equilibria. Additionally, without a direct positive fitness effect, any stable equilibrium close to one half will be sensitive to perturbations, which make such equilibria unlikely to sustain in nature. The results hold for both diplodiploid and different haplodiploid versions of CI. We suggest that insect populations showing low-frequencyWolbachiainfection might host CI-inducing symbiotic strains providing additional (hidden or known) benefits to their hosts, especially when classical explanations (ongoing invasion, source-sink dynamics) have been ruled out.