Wallon Center for Industrial Biology
Serratia symbiotica from Aphis fabae: a step forward in understanding the genomic evolution of a lifestyle transition from free-living to aphid-dependent
Author(s): Grigorescu, AS, Foray, V, Sabri, A, Haubruge, E, Lognay, G, Francis, F, Wathelet, J, Hance, T, Thonart, P
Bacterial intracellular symbiosis is widespread in insects and exhibits a large variety of phenotypes, ranging from parasitism to mutualism. Aphids developed mutualistic relationships with different levels of dependency, from obligate to facultative endosymbiosis, and with various effects (e.g., metabolic complementation, increased resistance to parasites and thermal stress). Serratia symbiotica was described as a secondary endosymbiont found in many aphid families and playing a role in heat tolerance and protection against parasitoids. We argue that this bacterium represents a good model for studying the evolution of the aphid-endosymbiont relationship parallel with a lifestyle transition of this microorganism, from free-living to aphid-dependent. S. symbiotica can be a primary-like endosymbiont in the aphid Cinara cedri and a facultative endosymbiont in Acyrtosiphon pisum and Aphis fabae. The S. symbiotica strain found in A. fabae can also exist in free-living style, cultivated on artificial rich medium. In the present study the genome of the previously isolated strain of S. symbiotica CWBI 2.3 was sequenced and compared to the genome sequences of the uncultivable strains from A. pisum (strain Tucson) and C. cedri. The genome of S. symbiotica CWBI 2.3 was sequenced using Illumina and PacBio sequencing, resulting in a genome size of ~3.5 Mb, which is larger than both previously sequenced genomes of S. symbiotica but smaller than the genome sizes of free living Serratia species. The genome of S. symbiotica CWBI 2.3 is phylogenetically close to that of the strain Tucson, but it also presents some features that are more related to free-living bacteria, such as the capacity of synthesizing most of the amino acids. The functional and synteny analyses performed so far suggest that the genome of S. symbiotica CWBI 2.3 represents a relevant step in understanding the evolution of a lifestyle transition towards dependency on another organism.
Northern Research & Development
A cryptic co-evolution between an obligatory pathogen and its vector when each has a different distinct host plant
Author(s): Sharon, R, Naor, V, Raz, R, Harari, AR, Dafni-Yelin, M, Tomer, M, Sofer-Arad, C, Weintraub, P, Zahavi, T
Vector-Pathogen-Plant dynamics are complex due to both direct and indirect interactions between the organisms involved. Key factors for the pathogen dissemination are the survival of its vector and its ability to transmit the pathogen into the plant.
A possible conflict may arise between the pathogen and the vector if the pathogen host preference does not coincide with that of its vector. This is the case of the obligatory pathogen- Stolbur type phytoplasma (SP) and its vector- the planthopper Hyalsthes obsoletus; In Europe both share the same preferred host plant whereas in Israel the preferred host of one reduces life history fitness of the other.
This conflicting situation calls for a local ecological solution. We found that (1) in Israel SP is not present in the vector's preferred host- Vitex agnus-castus, although the vector feeding on this plant harbors SP. (2) Vines are the preferred and only host of SP in the area. (3) The vector can acquire SP from vines. (4) The vector cannot complete its life cycle on vines. (5) There is no vertical transmission of SP from the vector mother to her offspring. In Israel, unlike in Europe, the vector host plant is not commonly neighboring vineyards. Thus, reaching vines involves a cost to the vector of distance flying and leaving a suitable host for non-suitable plant. Apparently, the pathogen must apply a different strategy in order to disseminate between vines. Previous studies have shown that vector-borne pathogens can alter traits of their host plants and vectors in ways that influence their physiology and behavior. We suggest that the vector is "seduced" by the pathogen to change its behavior by real or assumed benefits. These benefits may be directly induced by the pathogen in the vector or indirect through a change caused by the pathogen to its host plant to attract the vector.
Ecologie & Biologie des Interactions, UMR CNRS 7267
Bacterial communities influenced by Wolbachia? Bacterial community structure and major players in the terrestrial isopod microbiome
Author(s): Dittmer, J, Lesobre, J, Johnson, M, Bouchon, D
Endosymbiotic communities, interactions between community members and their synergistic effects on host evolution have become important aspects in symbiosis research. Terrestrial isopods represent an excellent model system for multipartite symbioses due to their well-characterised association with Wolbachia. These endosymbiotic bacteria act as reproductive parasites in terrestrial isopods, inducing the feminisation of genetic males in Armadillidium vulgare. To date, three different feminising Wolbachia strains have been identified in this host and these strains vary in their distribution across host tissues. However, the microbiome of terrestrial isopods has never been analysed on a large scale and the role of Wolbachia within the bacterial community remains unknown. Here, we characterized the tissue-specific bacterial communities in A. vulgare, combining qPCR and 16S rRNA gene amplicon sequencing. To gain insight into intra-community dynamics, we analysed bacterial communities present in hosts from both laboratory and field populations, depending on (i) presence or absence of Wolbachia and (ii) infection with different Wolbachia strains. Wolbachia infection was an important factor influencing bacterial community structure. Furthermore, Wolbachia represented the predominant member of the bacterial community in infected individuals. These findings indicate that Wolbachia plays an important role within the terrestrial isopod microbiome. Apart from Wolbachia, we detected a second highly abundant bacterium: Candidatus Hepatoplasma crinochetorum, a facultative symbiont previously reported from the midgut caeca, was for the first time observed in all tested host tissues. Moreover, Ca. H. crinochetorum co-occurred with Wolbachia, although it was not observed in co-existence with all three Wolbachia strains. A better knowledge of the potential interactions of Wolbachia and Ca. H. crinochetorum will contribute to a better understanding of multipartite symbiotic interactions.
Bargaining power and a biological market analysis of the plant-mycorrhizal symbiosis
Author(s): Wyatt, GAK, Kiers, ET, Gardner, A, West, SA
Mutualistic cooperation can easily be stabilized when one partner is in relative control, interacts with a number of potential partners from the other species, and has some mechanism to enforce cooperation. In contrast, in the mycorrhizal mutualism, there is no apparent partner ‘in control’. Here, both partners may interact with multiple symbionts simultaneously. This complex series of interactions means that neither partner determines the terms of the interaction. Biological market theory models offer an approach for modeling and conceptualising such coevolutionary interactions between multiple partners. However, previous biological market models have made two assumptions that limit the extent to which they can be applied to mutualisms such as that between plants and fungi. First, they assumed that markets consist of an infinite number of partners. This prevents any partner having the bargaining power that comes from being one of a limited number of partners. Second, they assumed that cooperation is stable, rather than asking when it would be stable. Consequently, we have developed and analysed a market model without these assumptions and thus more readily applicable to the mycorrhizal mutualism. First, we found that whether or not individuals engage in mutualistic trade depends both qualitatively and quantitatively upon the number of partners of each species, as well as their degree of specialisation. Second, we found that, in a market model, mutualistic trade implies that at least one species relies completely on the other for at least one resource. Finally, we found that incorporating bargaining power may mean that many of the potential gains from trade are not exploited.
Chrysomelina leaf beetles profit from widely exploiting their host plants: ABC transporters are involved in the sequestration of poisonous phytochemicals
Author(s): Burse, A, Strauss, A, Peters, S, Boland, W
The ubiquitous consumption of plants by insect herbivores requires in many cases detoxification of noxious phytochemicals. One of the most ingenious detoxification strategies widespread in insects is sequestration which involves uptake, transfer and concentration into specialized tissues. Frequently, sequestered compounds are not only stored securely by the insects, but further; insects evolved the ability to use an overwhelming diversity of exogenous compounds for their own benefit within the competitive interactions in natural ecosystems, like for repelling insects’ enemies. Here, we report the first example of a transport protein crucial for the translocation of phytochemicals in insects. The transporter, referred to here as CpMRP, belongs to the ATP binding cassette (ABC) transporter family and functions in the defensive exocrine glands of juvenile poplar leaf beetle, Chrysomela populi, for shuttling plant derived metabolites into deterrent secretions. Silencing of CpMRP in vivo by using RNAi creates a defenseless phenotype which indicates its key function also for the secretion process. Our analyses of the transport activity suggest that involving a broad-spectrum carrier in the sequestration of phytochemicals may be advantageous for insects to affiliate alternative host plants. Indeed, we identified transporter sequences highly similar to CpMRP in the larval glands of a related Chrysomelina leaf beetle species which forms allopatric populations and shifts hosts between chemically different plant families. This demonstrates the importance of the substrate flexibility provided by ABC transporters to avoid feeding specialization and host-derived chemical defense leading to an evolutionary dead end. Moreover, ancestral species of C. populi synthesizing defensive compounds endogenously already possess in their glands CpMRP-similar proteins which shed new light on the evolution of sequestration in the taxon Chrysomelina.
Faculty of Science
Comparative analysis of symbiotic communities in Hippoboscidae
Author(s): Novakova, E, Husnik, F, Hypša, V
Symbioses between bacteria and insects can range from simple bilateral relationships, with a single obligate mutualist, to multilateral models with complex bacterial communities. In hematophagous hosts, typical examples of such associations are provided by the symbiosis of human body louse Pediculus humanus with its primary symbiont Rieisa pediculicola, and tsetse fly Glossina morsitans with bacterial community containing a primary symbiont Wigglesworthia glossinidia, a facultative associate Sodalis glossinidius and Wolbachia. To allow for broader comparison of the symbiotic communities and their effect on the biology of different hosts, we analyzed symbiotic bacteria in hosts closely related to Glossinidae, the louse flies of the family Hippoboscidae. These organisms share many unique biological characteristics with Glossinidae, e.g. strict hematophagy, adenotrophic vivipary and nourishment through milk glands, and a specialized midgut section (bacteriome). Using Illumina sequencing, we characterized genomes of the symbionts associated with two biologically different species, namely Melophagus ovinus and Lipoptena fortisetosa. We demonstrated that the complexity of the associations differs among these species. In analogy to tsetse symbiosis, Melophagus ovinus carries complex symbiotic community involving obligate mutualist from the genus Arsenophonus (Gammaproteobacteria), a facultative symbiotic bacterium originated within Sodalis lineage (Gammaproteobacteria), a widespread bacterial associate of the genus Wolbachia, and two additional microorganisms Bartonella melophagi (Alphaproteobacteria) and Trypanosoma melophagium (Excavata). In contrary, Lipoptena represents a model of bilateral symbiosis harboring Arsenophonus bacteria as a sole symbiont. We discuss possible consequences of the symbiotic community structure on the metabolic functions within the host.
Department of Cell and Molecular Biology
Comparative genomics of Wolbachia and the bacterial species concept
Author(s): Ellegaard, KM, Klasson, L, Näslund, K, Bourtzis, K, Andersson, SG
Bacteria display a wide diversity of specialized interactions with insects. In the intracellular niche, bacteria evolve under selection for host-interaction processes, at the same time as being members of what may be a complex community of other endosymbionts.
Among the obligate endosymbionts, Wolbachia have an unparalleled host range infecting at least 20% of all insect species as well as other invertebrates. The Wolbachia are divided into supergroups, where supergroup A and B strains are the most common in insects. Interestingly, multiple infections with strains of both supergroups are common, but how they interact with each other and the host is largely unknown.
Evolutionary studies of endosymbiont communities are challenging, due to the fastidious nature of these bacteria. We have developed a novel method, based on multiple-displacement amplification (MDA), which has allowed us to sequence and assemble two new Wolbachia strains co-infecting the fruit fly Drosophila simulans, and representing supergroup A and B respectively.
A comparative genomics study including additional strains revealed 24 and 33 supergroup-specific genes, putatively involved in host-adaptation processes. We are now investigating the genome expression of Wolbachia in D.simulans at the proteome level, in order to evaluate the role of the supergroup-specific genes.
Recombination frequencies were high for strains of the same supergroup, while the inferred recombination fragments for strains of different supergroups were of short sizes. Surprisingly, the genomes of the co-infecting Wolbachia strains were not more similar to each other and did not share more genes than other A- and B-group strains that infect different hosts. This suggests that the supergroups are irreversibly separated and that barriers other than host-specialization can maintain distinct clades in recombining endosymbiont populations.
Department of Cell and Molecular Biology
Comparative genomics of insect endosymbionts extracted from single hosts
Author(s): Tamarit, D, Klasson, L, Andersson, SGE
Insect endosymbionts represent the paradigm of bacterial adaptation to a host-dependent life and their study contributes to our understanding on microbial ecology and genome evolution. Their informative nature is nonetheless counterpoised to their inaccessibility. Here we demonstrate that single-host endosymbiotic communities can be studied from next-generation sequencing data after purification and whole-genome amplification of endosymbionts. We focus on the comparative genomic study of two samples belonging to pure Hamiltonella defensa sequences extracted from two individual aphids from the tribe Macrosiphini. The assembly of their genomes show that they are 1.8 and 2.0 Mb-long, respectively, and their gene content is shown to be similar to that of other H. defensa genomes. We used this new data to fully resolve the phylogenetic relationships between the known H. defensa and related enterobacteria, and to reconstruct the history of gene gains and losses in their evolutionary lineage. Finally, we performed an in-depth analysis of the genome content and evolution of the APSE bacteriophage, inserted in the H. defensa genomes, which has a remarkable ecological importance in protecting the aphid host against parasitoid wasp infections.
Biology, Ecology and Evolution
Culture-independent characterisation of the core gut microbiome of fungus-growing termites
Author(s): Otani, S, Mikaelyan, A, Nobre, T, Hansen, LH, Sørensen, SJ, Aanen, DK, Boomsma, JJ, Brune, A, Poulsen, M
Fungus-growing termites (subfamily Macrotermitinae, family Termitidae) live in an obligate mutualistic symbiosis with the fungus Termitomyces. All other termites rely on gut microbes for the breakdown of plant material and other forage, and it has been generally assumed that the association with Termitomyces has reduced the need for fermentative gut microbes after the Macrotermitinae became fungus-farmers. Only few studies have explored this in any detail and the identities, levels of interaction-specificity with the termite host, and consistency in bacterial communities between host species have remained largely unknown. Here, we employ bacterial 16S rRNA 454 high-throughput pyrosequencing to identify a potential core microbiome in the fungus-growing termites - i.e., a distinct set of bacteria present across lineages in the termite phylogeny. Comparative analysis of 9 fungus-growing termite species from 5 genera suggests that a core gut microbiome indeed exists, as all bacterial taxa of high abundance were present in all termite species examined. However, quantitative differences in microbiome composition between termite species and genera were also noticed, possibly associated with differences in substrate use and Termitomyces lineage reared. Our results are consistent with major changes in gut microbiomes having occurred when fungus farming evolved 30 MYA, followed by relatively modest elaborations in response to ecological conditions. This might help explain why neither the termites nor Termitomyces ever abandoned the symbiosis or teamed-up with another termite of fungal partner lineage.
Dynamics of protective symbiosis
Author(s): Polin, S, Leclair, M, Simon, J, Outreman, Y
Symbiosis, in which different species engage in prolonged and intimate associations, is gaining recognition as a ubiquitous feature of animal life. In many species, associations with symbiotic microorganisms are pervasive. These microbial associates are often heritable, transmitted with high fidelity from parent to offspring. Because host species and their symbionts share fates, inherited symbionts may exert beneficial effects on the hosts like conferring protection against adverse conditions. Among insects, aphids represent the best-studied case of protective symbioses. These sap-feeding insects may harbour one or several heritable bacterial symbionts, some of them providing protection against various natural enemies. To understand the dynamics of these protective symbioses in host populations, two barriers have to be investigated: the ability of microbial symbionts to infect a new host individual and the maintenance of symbionts infection over host generations. Aphids' biotic environment includes both the plants they feed on and the natural enemies they encounter (predators and parasitoids). Aphid individuals harbouring or not protective symbionts may co-occur on common plants, and horizontal transfers (transmission between host individuals) could potentially occur through direct contacts between aphids and/or the shared plant. Once present in a host population, the maintenance of protective symbionts depends on costs and benefits associated with such a symbiosis. In nature, selection pressures exerted by natural enemies may strongly fluctuate. By affecting host’s ecology through protective phenotype, the evolutionary relationship between a host and its protective symbionts would therefore be temporally and spatially dynamic. Horizontal transmission of protective symbionts and their maintenance in host populations, studied by various empirical approaches, will be presented to contribute to our understanding of symbionts’ dynamics in natural host populations.