How many species are infected with Wolbachia?--A statistical analysis of current data

Abstract

Wolbachia are intracellular bacteria found in many species of arthropods and nematodes. They manipulate the reproduction of their arthropod hosts in various ways, may play a role in host speciation and have potential applications in biological pest control. Estimates suggest that at least 20% of all insect species are infected with Wolbachia. These estimates result from several Wolbachia screenings in which numerous species were tested for infection; however, tests were mostly performed on only one to two individuals per species. The actual percent of species infected will depend on the distribution of infection frequencies among species. We present a meta-analysis that estimates percentage of infected species based on data on the distribution of infection levels among species. We used a beta-binomial model that describes the distribution of infection frequencies of Wolbachia, shedding light on the overall infection rate as well as on the infection frequency within species. Our main findings are that (1) the proportion of Wolbachia-infected species is estimated to be 66%, and that (2) within species the infection frequency follows a 'most-or-few' infection pattern in a sense that the Wolbachia infection frequency within one species is typically either very high (>90%) or very low (<10%).

Fig. 1

Estimated distribution B_(iii) of the frequency of Wolbachia within species. The underlying data set includes only the samples in which fewer than 100 individuals were tested.

Fig. 2

Numbers of species with infection densities in the particular intervals. Gray bars describe the observations made in samples with sample size n_j≥22. The black bars indicate the number of species expected based on B_(iii). The value of the χ²- statistic is 8.4 (<14, error probability 5%), thus we can accept this distribution as an underlying density function. Here, also B_(i) could be accepted, whereas B_(ii) had to be rejected.