Continuously high Wolbachia incidence in flea populations may result from dual-strain infections with divergent effects.

The continuously high incidence of some endosymbionts in arthropods despite potential conflicts with their hosts is often explained by obligatory relationships, in which the host is fully dependent on its endosymbiont, fitness advantages conferred on hosts by facultative endosymbionts, or reproductive manipulation of hosts by endosymbionts (typically facultative). Yet continuously endosymbiont high incidence is sometimes observed without clear evidence supporting any of these mechanisms. This situation could potentially be explained by the presence of several coinfecting strains of the same endosymbiont species, each affecting the host differently such that their effects counteract one another when studied collectively. Here, we investigated Wolbachia endosymbionts of fleas, which stably persist in high loads in all females, with no indication that any of the above mechanisms explain their continuously high incidence. We sequenced fleas and identified two Wolbachia strains, designated as wSc1 and wSc2. We then correlated the strain composition in fleas with measures of their reproductive success. We found that fleas with high wSc1 and low wSc2 loads had a higher reproductive success than fleas that had high loads of both strains, low loads of both strains, or no Wolbachia, suggesting that wSc1 may provide a direct fitness advantage to their hosts. Conversely, the number of males and total offspring was negatively correlated with wSc2 levels, supporting male killing. Our research demonstrates that endosymbionts' continuously high incidence may persist through intricate relationships in nature.