Abstract
The diversity and infection dynamics of the endosymbiont Wolbachia can be influenced by many factors, such as transmission rate, cytoplasmic incompatibility, environment, selection and genetic drift. The interplay of these factors in natural populations can result in heterogeneous infection patterns with substantial differences between populations and strains. The causes of these heterogeneities are not yet understood, partly due to the complexity of natural environments. We present experimental evolution as a new approach to study Wolbachia infection dynamics in replicate populations exposed to a controlled environment. A natural Drosophila melanogaster population infected with strains of Wolbachia belonging to different clades evolved in two laboratory environments (hot and cold) for 1.5 years. In both treatments, the rate of Wolbachia infection increased until fixation. In the hot environment, the relative frequency of different Wolbachia clades remained stable over 37 generations. In the cold environment, however, we observed marked changes in the composition of the Wolbachia population: within 15 generations, one Wolbachia clade increased more than 50% in frequency, whereas the other two clades decreased in frequency, resulting in the loss of one clade. The frequency change was highly reproducible not only among replicates, but also when flies that evolved for 42 generations in the hot environment were transferred to the cold environment. These results document how environmental factors can affect the composition of Wolbachia in D. melanogaster. The high reproducibility of the pattern suggests that experimental evolution studies can efficiently determine the functional basis of habitat-specific fitness among Wolbachia strains.
Highlights
Wolbachia are intracellular a-proteobacteria that infect many arthropod and nematode species (Werren et al 2008)
One additional layer of complexity of Wolbachia dynamics comes from the diversity of strains in natural populations (e.g. Osborne et al 2009 for D. simulans), which were recently grouped into five distinct clades (Richardson et al 2012)
Rate of Wolbachia infection We determined the Wolbachia infection rate with an individual-based PCR assay in the base population (BP) and in the evolved populations at different time points
Summary
Wolbachia are intracellular a-proteobacteria that infect many arthropod and nematode species (Werren et al 2008). Hoffmann et al 1986; Clancy & Hoffmann 1998; Reynolds & Hoffmann 2002; Jia et al 2009), bacterial density (Mouton et al 2007), Wolbachia strain and host genotype (Olsen et al 2001; Fry et al 2004), virus protection properties (Hedges et al 2008; Teixeira et al 2008), transmission rate (Turelli & Hoffmann 1995), cytoplasmic incompatibility (Caspari & Watson 1959), Wolbachia-mediated fecundity differences Experimental evolution in controlled environments can be used to overcome these difficulties and to better understand the role of specific environmental factors in the maintenance or spread of Wolbachia infection This approach can clarify the habitat-dependent differences of specific strains and clades. We discuss to what extent our results could be extrapolated to processes in natural populations
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