Cytoplasmic incompatibility (CI) is a mating incompatibility that is induced by maternally inherited endosymbionts in many arthropods. These endosymbionts include, most famously, the alpha-proteobacterium Wolbachia pipientis (Yen & Barr 1971; Werren et al. 2008) but also the Bacteroidetes bacterium Cardinium hertigii (Zchori-Fein et al. 2001), a gamma-proteobacterium of the genus Rickettsiella (Rosenwald et al. 2020) and another, as yet undescribed alpha-proteobacterium (Takano et al. 2017). CI manifests as embryonic mortality in crosses between infected males and females that are uninfected or infected with a different strain, whereas embryos develop normally in all other crosses. This phenotype may enable the endosymbionts to spread rapidly within their host population. Exploiting this, CI-inducing Wolbachia are being harnessed to control insect-borne diseases (e.g., O'Neill 2018). Much progress elucidating the genetic basis and developmental mechanism of CI has been made in recent years, but many open questions remain (Shropshire et al. 2020).
Immediately following the discovery and early study of CI in mosquitoes, Laven (1959, 1967) proposed that CI could be an important driver of speciation. Indeed, bi-directional CI can strongly reduce gene flow between two populations due to the elimination of F1 embryos, so that CI can act as a trigger for genetic differentiation in the host (Telschow et al. 2002, 2005). This idea has received much attention, and a potential role for CI in incipient speciation has been demonstrated in several species (e.g., Bordenstein et al. 2001; Jaenike et al. 2006). However, we still don’t know how commonly CI actually triggers speciation, rather than being merely a minor player or secondary phenomenon. The problem is that in addition to CI, postzygotic reproductive isolation can also be caused by host-induced, nuclear incompatibilities. Determining the relative contributions of these two causes of isolation is difficult and has rarely been done.
The study by Cruz et al. (2020) addresses this problem head-on, using a study system of Tetranychus urticae spider mites. These cosmopolitan mites are infected with different strains of Wolbachia. They come in two different colour forms (red and green) that can co-occur sympatrically on the same host plant but exhibit various degrees of reproductive isolation. A complicating factor in spider mites is that they are haplodiploid: unfertilised eggs develop into haploid males and are therefore not affected by any postzygotic incompatibilities, whereas fertilised eggs normally develop into diploid females. In haplodiploids, Wolbachia-induced CI can either kill diploid embryos (as in diplodiploid species), or turn them into haploid males. In their study, Cruz et al. used three different populations (one of the green and two of the red form) and employed a full factorial experiment involving all possible combinations of crosses of Wolbachia infected or uninfected males and females. For each cross, they measured F1 embryonic and juvenile mortality as well as sex ratio, and they also measured F1 fertility and F2 viability. Their results showed that there is strong reduction in hybrid female production caused by Wolbachia-induced CI. However, independent of this and through a different mechanism, there is an even stronger reduction in hybrid production caused by host-associated incompatibilities. In combination with the also observed near-complete sterility of F1 hybrid females and full F2 hybrid breakdown (neither of which is caused by Wolbachia), the results indicate essentially complete reproductive isolation between the green and red forms of T. urticae.
Overall, this is an elegant study with an admirably clean and comprehensive experimental design. It demonstrates that Wolbachia can contribute to reproductive isolation between populations, but that host-induced mechanisms of reproductive isolation predominate in these spider mite populations. Further studies in this exiting system would be useful that also investigate the contribution of pre-zygotic isolation mechanisms such as assortative mating, ascertain whether the results can be generalised to other populations, and – most challengingly – establish the order in which the different mechanisms of reproductive isolation evolved.
Bordenstein, S. R., O'Hara, F. P., and Werren, J. H. (2001). Wolbachia-induced incompatibility precedes other hybrid incompatibilities in Nasonia. Nature, 409(6821), 707-710. doi: https://doi.org/10.1038/35055543
Cruz, M. A., Magalhães, S., Sucena, É., and Zélé, F. (2020) Wolbachia and host intrinsic reproductive barriers contribute additively to post-mating isolation in spider mites. bioRxiv, 2020.06.29.178699, ver. 4 peer-reviewed and recommended by PCI Evolutionary Biology. doi: https://doi.org/10.1101/2020.06.29.178699
Jaenike, J., Dyer, K. A., Cornish, C., and Minhas, M. S. (2006). Asymmetrical reinforcement and Wolbachia infection in Drosophila. PLoS Biol, 4(10), e325. doi: https://doi.org/10.1371/journal.pbio.0040325
Laven, H. (1959). SPECIATION IN MOSQUITOES Speciation by Cytoplasmic Isolation in the Culex Pipiens-Complex. In Cold Spring Harbor Symposia on Quantitative Biology (Vol. 24, pp. 166-173). Cold Spring Harbor Laboratory Press.
Laven, H. (1967). A possible model for speciation by cytoplasmic isolation in the Culex pipiens complex. Bulletin of the World Health Organization, 37(2), 263-266.
O’Neill S.L. (2018) The Use of Wolbachia by the World Mosquito Program to Interrupt Transmission of Aedes aegypti Transmitted Viruses. In: Hilgenfeld R., Vasudevan S. (eds) Dengue and Zika: Control and Antiviral Treatment Strategies. Advances in Experimental Medicine and Biology, vol 1062. Springer, Singapore. doi: https://doi.org/10.1007/978-981-10-8727-1_24
Rosenwald, L.C., Sitvarin, M.I. and White, J.A. (2020). Endosymbiotic Rickettsiella causes cytoplasmic incompatibility in a spider host. doi: https://doi.org/10.1098/rspb.2020.1107
Shropshire, J. D., Leigh, B., and Bordenstein, S. R. (2020). Symbiont-mediated cytoplasmic incompatibility: what have we learned in 50 years?. Elife, 9, e61989. doi: https://doi.org/10.7554/eLife.61989
Takano et al. (2017). Unique clade of alphaproteobacterial endosymbionts induces complete cytoplasmic incompatibility in the coconut beetle. Proceedings of the National Academy of Sciences, 114(23), 6110-6115. doi: https://doi.org/10.1073/pnas.1618094114
Telschow, A., Hammerstein, P., and Werren, J. H. (2002). The effect of Wolbachia on genetic divergence between populations: models with two-way migration. the american naturalist, 160(S4), S54-S66. doi: https://doi.org/10.1086/342153
Telschow, A., Hammerstein, P., and Werren, J. H. (2005). The effect of Wolbachia versus genetic incompatibilities on reinforcement and speciation. Evolution, 59(8), 1607-1619. doi: https://doi.org/10.1111/j.0014-3820.2005.tb01812.x
Werren, J. H., Baldo, L., and Clark, M. E. (2008). Wolbachia: master manipulators of invertebrate biology. Nature Reviews Microbiology, 6(10), 741-751. doi: https://doi.org/10.1038/nrmicro1969
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The authors have done an excellent job addressing the reviewers' and my own comments, and I find the new version to be much improved. In particular, I'm happy with the extended introduction providing background on the spider mite system and the improved discussion of pre-mating isolation. The summary sentences at the end of the Results subsections are also very helpful, and I really like the new figures with their consistent colour scheme. I only have a few more very minor comments:
The first sentence in the Abstract seems to be its own paragraph, please merge the entire abstract into a single paragraph.
It seems that some of the packages loaded and presumably used in the R scripts (e.g. car and lme4) have not been cited. Please make sure that all packages are mentioned in the methods with citations.
-l.721: Perhaps "narrows" --> "limits", or somehow rephrase this sentence? It's a bit hard to understand at first.
Additional comment of the managing board
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Wolbachia is a widespread maternally inherited endosymbiont of arthropods. These bacteria often induce mating incompatibilities between males and females with different infection status, which suggests that they may play a role in host speciation. In this article, Cruz et al. investigate the contribution of Wolbachia to reproductive isolation between different population of the spider mite Tetranychus urticae, relative to host-related incompatibilities.
Both reviewers commented positively on the experimental design of this study, and I agree that the experiments, based on a full factorial design testing all possible types of matings, are impressive in their scope and well suited to answering the questions posed. However, the reviewers also raise a number of issues that should be addressed in a revised manuscript. Most importantly, both reviewers suggest a thorough discussion of the role of pre-mating isolation in this system.
In addition, and related to this, I would also like see more context provided on the system in the introduction. The MD and FM types of CI are well explained but very little background information is given on the "biogeography" of these mites and their symbionts. E.g., do the green and red types co-occur symmetrically or only in allo- or parapatry? Is Wolbachia fixed in these populations or is there generally a polymorphism between infected and uninfected individuals? Are populations from the red and green type that are compatible infected with the same strain of Wolbachia, or are both uninfected? Given all that is known about this system, are the three populations chosen representative of the species (complex) as a whole?
Some other comments:
Abstract: "However, most studies focus on closely-related populations of single species" - this sounds as if the authors are setting up the stage for a follow-up sentence like "Here, we instead do ...", but the next sentence doesn't quite live up to that expectation (partly because the study system is also a single species).
Abstract: "these two sources": not quite clear which two sources are referred to because there's a sentence in-between, perhaps be more explicit here.
Fig.1 & 2: I was wondering why some of the crosses aren't shown here. Are those the ones that aren't as interesting for the questions asked, or is there another reason? Might be good to either add them or to mention why not in the figure legend.
Fig.1: I think the colour scheme used in Fig.2 is quite nice, perhaps the same scheme could be used for Fig. 1? (E.g., coloured circles above the plots instead of the numbers?)
Box S1 heading: I think it should be "relating to" instead of "relative to"
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