Abstract
Cultures of the stored product mite Tyrophagus putrescentiae are often single-infected by one of intracellular bacteria Cardinium or Wolbachia. No naturally occurring multi-infected Cardinium and Wolbachia a T. putrescentiae mites are known. Under laboratory conditions, we mixed two single-infected mite cultures of each endosymbiont to obtain four multi-infected cultures and to test the intraspecific competition among individuals under the influence of Cardinium and Wolbachia. The mite population growth, as a fitness indicator, and endosymbiont prevalence were estimated repeatedly during five months, repeated in three separate experiments. The fitness of four single-infected populations varied significantly. The proportion of infected individuals was higher in the two Cardinium infected populations than in the two Wolbachia infected populations. The fitness of multi-infected populations fluctuated over time in all three experiments. After six months of experiments, Cardinium infected individuals prevailed in 50% of the populations, the Wolbachia infected individuals prevailed in 41.7%, and asymbiotic individuals in 8.3% of populations (N = 12). The fitness of two multi-infected populations was like the fitness of single-infected populations. In one of them, the proportion of Cardinium or Wolbachia infected individuals was, on average, similar during the experiment. At the same time, symbionts disappeared, resulting in the majority of asymbiotic individuals in the second population. No correlation between symbionts proportion and mite fitness was found. The results indicated that the outcome of the battle between Cardinium and Wolbachia is strongly stochastic at the population level.