Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly to maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density can vary among host and symbiont genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied variation in symbiont density using the pea aphid (Acyrthosiphon pisum) and the bacterium Regiella insecticola, a symbiont that can protect its host against fungal pathogens. We found that relative symbiont density varies both between two Regiella phylogenetic clades and among aphid "biotypes." Higher density symbiont infections are correlated with stronger survival costs, but variation in density has little effect on the protection Regiella provides against fungi. Instead, we found that in some aphid genotypes, a dramatic decline in symbiont density precedes the loss of a symbiont infection. Together, our data suggest that the optimal density of a symbiont infection is likely different from the perspective of aphid and microbial fitness. Regiella might prevent loss by maintaining high within-host densities, but hosts do not appear to benefit from higher symbiont numbers and may be advantaged by losing costly symbionts in certain environments. The standing variation in symbiont density observed in natural populations could therefore be maintained by antagonistic coevolutionary interactions between hosts and their symbiotic microbes.

Department of Biology University of Rochester Rochester NY USA

Department of Microbiology University of Tennessee Knoxville TN USA

Department of Zoology University of Oxford Oxford UK

Institute of Entomology Biology Centre of the Czech Academy of Sciences Ceske Budejovice Czech Republic

See more in PubMed

Ballinger, M. J., & Perlman, S. J. (2017). Generality of toxins in defensive symbiosis: ribosome-inactivating proteins and defense against parasitic wasps in Drosophila. PLoS Pathogens, 13(7), e1006431. https://doi.org/10.1371/journal.ppat.1006431

Bates, D., Mächler, M., Bolker, B., & Walker, S. (2015). Fitting linear mixed-effects models Usinglme4. Journal of Statistical Software, 67(1), 1-48. https://doi.org/10.18637/jss.v067.i01

Brownlie, J. C., & Johnson, K. N. (2009). Symbiont-mediated protection in insect hosts. Trends in Microbiology, 17(8), 348-354. https://doi.org/10.1016/j.tim.2009.05.005

Chong, R. A., & Moran, N. A. (2016). Intraspecific genetic variation in hosts affects regulation of obligate heritable symbionts. Proceedings of the National Academy of Sciences of the United States of America, 113(46), 13114-13119. https://doi.org/10.1073/pnas.1610749113

Chrostek, E., Marialva, M. S., Esteves, S. S., Weinert, L. A., Martinez, J., Jiggins, F. M., & Teixeira, L. (2013). Wolbachia variants induce differential protection to viruses in Drosophila melanogaster: a phenotypic and phylogenomic analysis. PLoS Genetics, 9(12), e1003896. https://doi.org/10.1371/journal.pgen.1003896

Chrostek, E., & Teixeira, L. (2015). Mutualism breakdown by amplification of Wolbachia genes. PLoS Biology, 13(2), e1002065. https://doi.org/10.1371/journal.pbio.1002065

Douglas, A. E. (2014). The molecular basis of bacterial-insect symbiosis. Journal of Molecular Biology, 426(23), 3830-3837. https://doi.org/10.1016/j.jmb.2014.04.005

Ferrari, J., & Vavre, F. (2011). Bacterial symbionts in insects or the story of communities affecting communities. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 366(1569), 1389-1400. https://doi.org/10.1098/rstb.2010.0226

Ferrari, J., Via, S., & Godfray, H. C. (2008). Population differentiation and genetic variation in performance on eight hosts in the pea aphid complex. Evolution, 62(10), 2508-2524. https://doi.org/10.1111/j.1558-5646.2008.00468.x

Ferrari, J., West, J. A., Via, S., & Godfray, H. C. (2012). Population genetic structure and secondary symbionts in host-associated populations of the pea aphid complex. Evolution, 66(2), 375-390. https://doi.org/10.1111/j.1558-5646.2011.01436.x

Funkhouser-Jones, L. J., van Opstal, E. J., Sharma, A., & Bordenstein, S. R. (2018). The Maternal effect gene Wds controls Wolbachia titer in Nasonia. Current Biology, 28(11), 1692 e1696-1702 e1696. https://doi.org/10.1016/j.cub.2018.04.010

Gerardo, N. M., & Parker, B. J. (2014). Mechanisms of symbiont-conferred protection against natural enemies: an ecological and evolutionary framework. Current Opinion in Insect Science, 4, 8-14. https://doi.org/10.1016/j.cois.2014.08.002

Guay, J. F., Boudreault, S., Michaud, D., & Cloutier, C. (2009). Impact of environmental stress on aphid clonal resistance to parasitoids: Role of Hamiltonella defensa bacterial symbiosis in association with a new facultative symbiont of the pea aphid. Journal of Insect Physiology, 55(10), 919-926. https://doi.org/10.1016/j.jinsphys.2009.06.006

Hansen, A. K., Jeong, G., Paine, T. D., & Stouthamer, R. (2007). Frequency of secondary symbiont infection in an invasive psyllid relates to parasitism pressure on a geographic scale in California. Applied and Environment Microbiology, 73(23), 7531-7535. https://doi.org/10.1128/AEM.01672-07

Henry, L. M., Peccoud, J., Simon, J. C., Hadfield, J. D., Maiden, M. J., Ferrari, J., & Godfray, H. C. (2013). Horizontally transmitted symbionts and host colonization of ecological niches. Current Biology, 23(17), 1713-1717. https://doi.org/10.1016/j.cub.2013.07.029

Herren, J. K., & Lemaitre, B. (2011). Spiroplasma and host immunity: Activation of humoral immune responses increases endosymbiont load and susceptibility to certain Gram-negative bacterial pathogens in Drosophila melanogaster. Cellular Microbiology, 13(9), 1385-1396. https://doi.org/10.1111/j.1462-5822.2011.01627.x

Hothorn, T., Bretz, F., & Westfall, P. (2008). Simultaneous inference in general parametric models. Biometrical Journal, 50(3), 346-363. https://doi.org/10.1002/bimj.200810425

Hrcek, J., Parker, B. J., McLean, A. H. C., Simon, J. C., Mann, C. M., & Godfray, H. C. J. (2018). Hosts do not simply outsource pathogen resistance to protective symbionts. Evolution, 72(7), 1488-1499. https://doi.org/10.1111/evo.13512

Hughes, G. L., Koga, R., Xue, P., Fukatsu, T., & Rasgon, J. L. (2011). Wolbachia infections are virulent and inhibit the human malaria parasite Plasmodium falciparum in Anopheles gambiae. PLoS Pathogens, 7(5), e1002043. https://doi.org/10.1371/journal.ppat.1002043

Ives, A. R., Barton, B. T., Penczykowski, R. M., Harmon, J. P., Kim, K. L., Oliver, K., & Radeloff, V. C. (2020). Self-perpetuating ecological-evolutionary dynamics in an agricultural host-parasite system. Nature Ecology & Evolution, 4(5), 702-711. https://doi.org/10.1038/s41559-020-1155-0

Jaenike, J. (2012). Population genetics of beneficial heritable symbionts. Trends in Ecology & Evolution, 27(4), 226-232. https://doi.org/10.1016/j.tree.2011.10.005

Jaenike, J., Unckless, R., Cockburn, S. N., Boelio, L. M., & Perlman, S. J. (2010). Adaptation via symbiosis: recent spread of a Drosophila defensive symbiont. Science, 329(5988), 212-215. https://doi.org/10.1126/science.1188235

Kambris, Z., Blagborough, A. M., Pinto, S. B., Blagrove, M. S., Godfray, H. C., Sinden, R. E., & Sinkins, S. P. (2010). Wolbachia stimulates immune gene expression and inhibits plasmodium development in Anopheles gambiae. PLoS Pathogens, 6(10), e1001143. https://doi.org/10.1371/journal.ppat.1001143

Login, F. H., Balmand, S., Vallier, A., Vincent-Monegat, C., Vigneron, A., Weiss-Gayet, M., Rochat, D., & Heddi, A. (2011). Antimicrobial peptides keep insect endosymbionts under control. Science, 334(6054), 362-365. https://doi.org/10.1126/science.1209728

Lukasik, P., Guo, H., van Asch, M., Henry, L. M., Godfray, H. C., & Ferrari, J. (2015). Horizontal transfer of facultative endosymbionts is limited by host relatedness. Evolution, 69(10), 2757-2766. https://doi.org/10.1111/evo.12767

Martinez, J., Ok, S., Smith, S., Snoeck, K., Day, J. P., & Jiggins, F. M. (2015). Should symbionts be nice or selfish? antiviral effects of wolbachia are costly but reproductive parasitism is not. PLoS Pathogens, 11(7), e1005021. https://doi.org/10.1371/journal.ppat.1005021

Martinez, J., Tolosana, I., Ok, S., Smith, S., Snoeck, K., Day, J. P., & Jiggins, F. M. (2017). Symbiont strain is the main determinant of variation in Wolbachia-mediated protection against viruses across Drosophila species. Molecular Ecology, 26(15), 4072-4084. https://doi.org/10.1111/mec.14164

McLean, A. H. C., Parker, B. J., Hrcek, J., Kavanagh, J. C., Wellham, P. A. D., & Godfray, H. C. J. (2018). Consequences of symbiont co-infections for insect host phenotypes. Journal of Animal Ecology, 87(2), 478-488. https://doi.org/10.1111/1365-2656.12705

McLean, A. H., van Asch, M., Ferrari, J., & Godfray, H. C. (2011). Effects of bacterial secondary symbionts on host plant use in pea aphids. Proceedings of the Royal Society B: Biological Sciences, 278(1706), 760-766. https://doi.org/10.1098/rspb.2010.1654

Niepoth, N., Ellers, J., & Henry, L. M. (2018). Symbiont interactions with non-native hosts limit the formation of new symbioses. BMC Evolutionary Biology, 18(1), 27. https://doi.org/10.1186/s12862-018-1143-z

Oliver, K. M., Campos, J., Moran, N. A., & Hunter, M. S. (2008). Population dynamics of defensive symbionts in aphids. Proceedings of the Royal Society B: Biological Sciences, 275(1632), 293-299. https://doi.org/10.1098/rspb.2007.1192

Oliver, K. M., Degnan, P. H., Hunter, M. S., & Moran, N. A. (2009). Bacteriophages encode factors required for protection in a symbiotic mutualism. Science, 325(5943), 992-994. https://doi.org/10.1126/science.1174463

Oliver, K. M., Moran, N. A., & Hunter, M. S. (2006). Costs and benefits of a superinfection of facultative symbionts in aphids. Proceedings of the Royal Society B: Biological Sciences, 273(1591), 1273-1280. https://doi.org/10.1098/rspb.2005.3436

Oliver, K. M., Russell, J. A., Moran, N. A., & Hunter, M. S. (2003). Facultative bacterial symbionts in aphids confer resistance to parasitic wasps. Proceedings of the National Academy of Sciences of the United States of America, 100(4), 1803-1807. https://doi.org/10.1073/pnas.0335320100

Pan, X., Pike, A., Joshi, D., Bian, G., McFadden, M. J., Lu, P., Liang, X., Zhang, F., Raikhel, A. S., & Xi, Z. (2018). The bacterium Wolbachia exploits host innate immunity to establish a symbiotic relationship with the dengue vector mosquito Aedes aegypti. ISME Journal, 12(1), 277-288. https://doi.org/10.1038/ismej.2017.174

Parker, B. J., Hrcek, J., McLean, A. H. C., Brisson, J. A., & Godfray, H. C. J. (2021). Intraspecific variation in symbiont density in an insect-microbe symbiosis. Molecular Ecology. Dryad, Dataset. https://doi.org/10.5061/dryad.j3tx95xcv

Parker, B. J., Hrcek, J., McLean, A. H. C., & Godfray, H. C. J. (2017). Genotype specificity among hosts, pathogens, and beneficial microbes influences the strength of symbiont-mediated protection. Evolution, 71(5), 1222-1231. https://doi.org/10.1111/evo.13216

Parker, B. J., McLean, A. H. C., Hrcek, J., Gerardo, N. M., & Godfray, H. C. J. (2017). Establishment and maintenance of aphid endosymbionts after horizontal transfer is dependent on host genotype. Biology Letters, 13(5), 20170016. https://doi.org/10.1098/rsbl.2017.0016

Parker, B. J., Spragg, C. J., Altincicek, B., & Gerardo, N. M. (2013). Symbiont-mediated protection against fungal pathogens in pea aphids: a role for pathogen specificity? Applied and Environment Microbiology, 79(7), 2455-2458. https://doi.org/10.1128/AEM.03193-12

Peccoud, J., de la Huerta, M., Bonhomme, J., Laurence, C., Outreman, Y., Smadja, C. M., & Simon, J. C. (2014). Widespread host-dependent hybrid unfitness in the pea aphid species complex. Evolution, 68(10), 2983-2995. https://doi.org/10.1111/evo.12478

Peccoud, J., Ollivier, A., Plantegenest, M., & Simon, J. C. (2009). A continuum of genetic divergence from sympatric host races to species in the pea aphid complex. Proceedings of the National Academy of Sciences of the United States of America, 106(18), 7495-7500. https://doi.org/10.1073/pnas.0811117106

Russell, J. A., Weldon, S., Smith, A. H., Kim, K. L., Hu, Y. I., Łukasik, P., Doll, S., Anastopoulos, I., Novin, M., & Oliver, K. M. (2013). Uncovering symbiont-driven genetic diversity across North American pea aphids. Molecular Ecology, 22(7), 2045-2059. https://doi.org/10.1111/mec.12211

Sandström, J., & Pettersson, J. (1994). Amino acid composition of phloem sap and the relation to intraspecific variation in pea aphid (Acyrthosiphon pisum) performance. Journal of Insect Physiology, 40(11), 947-955. https://doi.org/10.1016/0022-1910(94)90133-3

Scarborough, C. L., Ferrari, J., & Godfray, H. C. (2005). Aphid protected from pathogen by endosymbiont. Science, 310(5755), 1781. https://doi.org/10.1126/science.1120180

Tsuchida, T., Koga, R., & Fukatsu, T. (2004). Host plant specialization governed by facultative symbiont. Science, 303(5666), 1989. https://doi.org/10.1126/science.1094611

Tsuchida, T., Koga, R., Shibao, H., Matsumoto, T., & Fukatsu, T. (2002). Diversity and geographic distribution of secondary endosymbiotic bacteria in natural populations of the pea aphid, Acyrthosiphon pisum. Molecular Ecology, 11(10), 2123-2135. https://doi.org/10.1046/j.1365-294x.2002.01606.x

Unckless, R. L., Boelio, L. M., Herren, J. K., & Jaenike, J. (2009). Wolbachia as populations within individual insects: causes and consequences of density variation in natural populations. Proceedings of the Royal Society B: Biological Sciences, 276(1668), 2805-2811. https://doi.org/10.1098/rspb.2009.0287

Via, S. (1991). The genetic structure of host plant adaptation in a spatial patchwork: Demographic variability among reciprocally transplanted pea aphid clones. Evolution, 45(4), 827-852. https://doi.org/10.1111/j.1558-5646.1991.tb04353.x

Vogel, K. J., & Moran, N. A. (2011). Effect of host genotype on symbiont titer in the aphid-buchnera symbiosis. Insects, 2(3), 423-434. https://doi.org/10.3390/insects2030423

Vorburger, C., & Gouskov, A. (2011). Only helpful when required: a longevity cost of harbouring defensive symbionts. Journal of Evolutionary Biology, 24(7), 1611-1617. https://doi.org/10.1111/j.1420-9101.2011.02292.x

Wilkinson, T. L., & Douglas, A. E. (1998). Host cell allometry and regulation of the symbiosis between pea aphids, Acyrthosiphon pisum, and bacteria, Buchnera. Journal of Insect Physiology, 44, 629-635.

Seasonal wild dance of dual endosymbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea)

See more in PubMed

Dryad
10.5061/dryad.j3tx95xcv