We predicted that butterflies belonging to the same genus would share similar strains of Wolbachia due to recent common ancestry, and the possibility of HT by the means of hybridization events, and/or shared resources. We did not expect the same to be true between genera as any hybridization is impossible, and the butterflies of the two genera considered in this study only share similar macro-habitats (i.e. forest and open savannah), but not micro-habitats (i.e. larval host-plant). Our data did not support co-cladogenesis of Wolbachia in the African butterflies, but still partially supported the first prediction. Within each genus, many species carry similar strains to the one found in congeneric species, but not always. However, the same was also true between genera, which contrasts with our second prediction. The occurrence of similar Wolbachia strains in both of the two Lepidoptera families (Pieridae and Nymphalidae) is unlikely to occur through shared ancestry, nor through horizontal transfer via the larval host-plants. This is in a clear contrast with the insect communities associated with fleshy mushrooms [14], and pumpkin plants [15]. These results suggest that factors other than the larval host-plants must support the transfer of Wolbachia between host species. The study of the horizontal transfer of Wolbachia between host species might however be currently skewed by (1) our restricted knowledge of the ecology of each species within insect communities, (2) the strong biases associated with the available Wolbachia strain diversity dataset, and (3) the way we characterize the different strains of the bacterium.
As it is the case for many species, especially in the Afrotropics, many aspects of the ecology of the Mylothris and Bicyclus butterflies remain unfortunately poorly studied. To date, almost all ecological studies of the Mylothris butterflies focus on their association at the larval stage to mistletoe plants (e.g. Santalaceae family) [28-30, 51] in their native Afrotropical range [23-25], neglecting other aspects of their life history. There is currently no available comprehensive record, or formal study looking at the community of parasitoid wasps or mite communities associated with any Mylothris or Bicyclus butterflies. To our knowledge, Gupta et al. [52] provided the only description of Cotesia pistrinariae as a parasitoid wasp of M. chloris; but it remains unknown whether C. pistrinariae could also parasitize any Bicyclus species, or vector Wolbachia between insect hosts. Our phylogenetic tree suggests several examples of parasitoid wasps sharing similar infection to Bicyclus or Mylothris butterflies, however in each case the direct contact between the Hymenoptera and the Lepidoptera species are unlikely [9], due to geographical or ecological reasons, or both. For example, despite sharing similar Wolbachia strains, the braconid parasitoid wasp Apanteles chilonis, an endoparasitoid of the rice stem borer Chilo suppressalis [53] in the Palearctic, is unlikely to parasitize B. vulgaris or B. auricruda in the Afrotropics. Similarly, Evania appendigaster, a parasitoid of cockroaches [54], is also unlikely to predate on B. ignobilis or B. xeneas. Only systematic surveys of the Wolbachia strains from species communities, rather than individual species or clades, could potentially offer the material currently lacking for testing how a single strain of Wolbachia may occur in highly different hosts and environments. Investigating the Wolbachia infection status of the community of endo- and ectoparasites associated with the Bicyclus and Mylothris butterflies, should thus inform whether these parasites can act as vectors of Wolbachia among the two genera of butterflies, as it was previously suggested in other insects, including flies, mosquitoes and ants [7, 19, 55].
Wolbachia is known to survive in an extracellular phase in the laboratory for up to a week [56]. Although Mylothris and Bicyclus larvae use very different host-plants and adult food resources [57, 58], the adult butterflies of both genera have occasionally been observed sucking from the same mud-pools or animal feces. By potential being the only nutrient resources shared by the two genera (Tropek, pers. comm.), mud-pools and feces could thus represent suitable short-term environments supporting the survival of Wolbachia until its successful horizontal transfer to a new host niche. This, however, remains to be tested.
Although the origin of Wolbachia supergroups A and B is estimated to be 200 My ago [based on whole genome data, 59], the divergence of the strains within each supergroup is most likely much younger (e.g. estimated around 28 My ago by [9] based on the MLST markers only), and does not match the divergence between Pieridae and Nymphalidae butterflies [97 My ago, 20]. This further support our claim that co-cladogenesis is improbable, and strains have not been passed down from their common ancestor or transferred via hybridization events between the butterfly species. Additionally, the ecological links described so far as potential routes for the recent transfer of Wolbachia between species can only explain local HTs of the bacterium. Nonetheless, Ahmed et al. [9] found that strain type ST-41, a strain type commonly characterized in butterflies [9, 40], was found in species from Africa (i.e. Azanus mirza; Lycaenidae), Japan (i.e. Eurema hecabe; Pieridae), Borneo (i.e. Nacaduba angusta; Lycaenidae) and North America (i.e. Celastrina argiolus; Lycaenidae). Following these results, we show that Mylothris and Bicyclus butterflies in Africa share similar Wolbachia strains to, for example, Lycaenidae from South Africa (with ST-19) or Malaysia (with ST-40) [9], or moths from the Pacific islands [60], and potentially to many other species in between these two geographical regions. None of the geographically distant host species described in these two studies are likely to share the same host-plants, parasitoids nor mite parasites. Despite the lack of a clear understanding of ‘how’, the research community however agrees that the ability of Wolbachia to transfer horizontally has without a doubt contributed to the global pandemic of the bacterium [61].
A recent study by Detcharoen et al [62] estimated that, to date, more than 99% of all existing Wolbachia strains have yet to be characterized; worse: that strong biases occur in the database. The PubMLST-Wolbachia database [18] currently includes over 2000 strains. Out of those, 370 are from Lepidoptera species (18.3%), which is more than for the Coleoptera (92; 4.6%), the Hemiptera (297; 14.7%), and the Hymenoptera (359; 17.8%), but less than the Diptera (473; 23.4%). Thus, strains from particular insect orders, but also host families are more represented. Furthermore, in Lepidoptera for example, most of the Wolbachia strains were characterized from species inhabiting the Palearctic ecoregion (N=107; 29%), while very few are from the Afrotropics (N=18; 5%). And this pattern at the ecoregion level is similarly found in the other insect orders, representing another important bias in the PubMLST-Wolbachia database. Although the present study brings new data for the Afrotropic region, showing for example that the ST-41 commonly found in Lepidoptera [9], is not found in the Mylothris and Bicyclus, many biases still remain, and they will continue to impede the comprehensive study of the diversity and geographical distribution of Wolbachia strains, as well as our understanding of the mechanisms behind their pandemic.
The commonly applied method to characterized Wolbachia strains is based on the sequences of six markers for a maximum length of about 3,000bp [18]. This molecular technique has recently been highly criticized [63]. New studies are pushing towards the use of whole genome data, which seems to more accurately infer Wolbachia supergroup phylogeny and origin [59, 64]. Although still rather expensive, whole genome sequencing can not only provide the material to improve our understanding of Wolbachia strain diversity, its diversification rates, and its HT, but can also support the investigation of the ecology and evolution of the bacterium, including for example its ability to modify its host phenotype [65], and maybe, one day, its ability to establish in a wide range of host species.
The horizontal transfer of Wolbachia between insect hosts was already suggested in the early 90’s [66, 67]. Our study contributes to the growing literature showing that ecological links between species can act as platforms to the between species transfer of the symbiont, however no common understanding of this process and the relative importance of each transfer route has yet been proposed. Furthermore, our study also re-enforces the idea that biases in the dataset, and restrictions in the methodological approaches associated with such study, will, until solved, continue to impede our comprehensive analyses and understanding of the global Wolbachia pandemic.