While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites (Macrocheles subbadius) prefer to infect Spiroplasma poulsonii MSRO (Melanogaster sex ratio organism)-infected flies and if MSRO infection impairs fly resistance against secondary mite attack. First, we tested if mites prefer MSRO+ or MSRO- flies using pairwise choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO2 production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-day-old MSRO+ flies relative to MSRO- flies (31% infection) but preferred MSRO+ flies (64% infection) among 26-day-old flies. Using flowthrough respirometry, we found 14-day-old MSRO+ flies had higher CO2 emissions than MSRO- flies (32% greater), whereas at 26 days old the CO2 production among MSRO+ flies was 20% lower than that of MSRO- flies. Thus, mite preferences for high-metabolic-rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. IMPORTANCE Male-killing endosymbionts are transmitted from mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here, we test if (i) mites prefer to infect flies that harbor Spiroplasma poulisoni MSRO and (ii) flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance, but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.
Keywords: ectoparasites; host behavior; host physiology; host resistance; melanogaster sex ratio organism; mites; parasite preference; secondary infection.