Arthropods have a bunch of really cool symbiotic bacteria that are vertically transmitted from parent to offspring. Some of these symbionts reduce their hosts’ susceptibility to infection by parasites and parasitoids. Because host susceptibility is very important to parasite transmission, symbiotic bacteria that reduce host susceptibility can influence epidemics of parasites and parasitoids in arthropod populations. However, arthropods have other symbiotic bacteria that don’t have any measureable effect on arthropod susceptibility to parasites and pathogens. Can those other symbiotic bacteria still influence interactions between arthropods and their natural enemies?
One of the cool things about vertically transmitted symbionts is that they are often male-killers (that’s cool for me, but maybe not for the male arthropods). Male arthropod embryos infected with the bacteria die, and this produces female biased sex ratios. This is just what happens when lady beetles are infected with Spiroplasma bacteria, where lady beetle populations infected by Spiroplasma are 74% female (Ryder et al. 2014). So…what does this have to do with parasite epidemics? Well, lady beetles are also infected with sexually transmitted mites, and the dynamics of epidemics of sexually transmitted parasites might be very sensitive to sex ratios.
To explore this possibility, Ryder et al. (2014) made a really pretty model to explore how female biased sex ratios might influence parasite epidemics. They found that female biased sex ratios result in “male first” epidemics, where mite epidemics happened faster in the male population than the female population. Then Ryder et al. (2014) observed a bunch of mite epidemics in areas with and without male-killing bacterial symbionts. And guess what: in the areas with male-killing symbionts, there were male-first mite epidemics, while in areas without male-killing symbionts, male and female lady beetles had similar epidemic dynamics. Beautiful!
There is a lot more cool stuff in the paper, so go check it out! You should be at least a little bit jealous of how much data they had from experiments and field surveys. I know I am.
[My nerdiest cartoon yet? Perhaps.]
Ryder, J.J., M-J. Hoare, D. Pastok, M. Bottery, M. Boots, A. Fenton, D. Atkinson, R.J. Knell, and G.D.D. Hurst. 2014. Disease epidemiology in arthropods is altered by the presence of nonprotective symbionts. The American Naturalist 183(3): E89-E104.