The system of figs and fig wasps is considered to be a classic example of coevolved mutualism. It is well known that in general each fig species has a unique pollinator, which is called the "one-to-one" rule. However, more and more examples of co-pollinators (two or more pollinating wasp species on a fig) have broken the "one-to-one" rule [1–10]. However, we do not yet know how they evolve. They might be distantly related species, suggesting host shifts, or sister species, suggesting speciation on the current host [11, 12]. Host shifts might be more likely when a fig colonises a new habitat, or is near the edge of its geographic range, because the normal pollinator is rare or absent. In this scenario, the co-pollinators are usually not closely related species [2, 5, 8]. Alternatively, sister co-pollinators may evolve from a recent speciation event in the pollinator that is not accompanied by fig radiation [3, 11, 12]. However, the exact mode of speciation for co-pollinators has not been well understood yet.
Wolbachia bacteria are the most common intracellular bacteria in arthropods and nematodes, and can manipulate host reproduction in many ways . Cytoplasmic incompatibility (CI), the most common effect on host reproduction, usually occurs between infected males and uninfected females (or females infected by a different incompatible Wolbachia strain), inducing progeny sterility or mortality . This post-zygotic reproductive isolation can potentially cause or facilitate host speciation [14–18]. Fig wasp species have a very high incidence of Wolbachia infection [19–21], and previous work suggests that Wolbachia might have an influence on fig wasp speciation, because cryptic pollinator species have different Wolbachia infections . So, Wolbachia may play a potential important role in co-pollinator speciation.
In addition, Wolbachia can influence host mitochondrial DNA evolution. Because Wolbachia and mitochondria are co-transmitted maternally, the spread of Wolbachia can result in the hitchhiking of mitochondrial haplotypes. One particular mitochondrial haplotype can sweep through a population, via hitchhiking, associated with the sweep of Wolbachia [22, 23]. This affects mtDNA evolution by decreasing mitochondrial haplotype diversity and also sometimes the mtDNA divergence between species [15, 22–27], and thus can confound phylogenies and barcodes based on host mtDNA [28, 29]. Accordingly, we look forward to understanding the effect of Wolbachia on mtDNA evolution of fig wasps.
Ficus microcarpa Linn. is a functionally monoecious fig species. It is pollinated solely by Eupristina verticillata Waterston . Here, we investigate the genetic variance and Wolbachia infection status of Eupristina verticillata in order to explore whether co-pollinators exist, the association between Wolbachia infection and co-pollinators speciation, and the effect of Wolbachia on host mtDNA evolution.