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Alina Grigorescu
University of Liege
Wallon Center for Industrial Biology
Belgium

Serratia symbiotica from Aphis fabae: a step forward in understanding the genomic evolution of a lifestyle transition from free-living to aphid-dependent

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Author(s): Grigorescu, AS, Foray, V, Sabri, A, Haubruge, E, Lognay, G, Francis, F, Wathelet, J, Hance, T, Thonart, P

Summary:

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.

Kirsten Ellegaard
Faculty of Biology of Uppsala University
Department of Cell and Molecular Biology
Sweden

Comparative genomics of Wolbachia and the bacterial species concept

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Author(s): Ellegaard, KM, Klasson, L, Näslund, K, Bourtzis, K, Andersson, SG

Summary:

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.

Sämi Schär
University of Copenhagen, Faculty of Science
Department of Biology
Denmark

Ecological consequences of different kinds and degrees of ant-butterfly symbioses

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Author(s): Schär, S, Pierce, NE, Vila, R, Nash, DR

Summary:

Lycaenid butterflies show a wide continuum of symbiotic associations with ants, ranging from no to obligate interactions. Mutualistic caterpillars generally feed their ant symbionts with honeydew from their dorsal nectary organ, and in return receive protection from predators and parasitoids. At the other end of the virulence spectrum are species that exploit ants as inquiline social parasites or brood predators, often drastically reducing the fitness of the host ant colony. Here I will report on a project exploring the population level genetic consequences of this variety of lifestyles among lycaenids, using worldwide sample collections and a range of species. The focus is on how kind and degree of interactions with ants affect dispersal (transmission) and population size of the butterflies. Evolutionary theory suggests that virulent parasitic organisms have higher rates of transmission than mutualistic ones (with negative virulence). At the same time, virulence of the butterflies is expected to be negatively correlated with population size, allowing sustainable recovery of the host ant populations. Finally, the challenge of expanding classical population genetic studies beyond the level of congeneric populations will be addressed.

Lee Henry
University of Oxford
Zoology Department
United Kingdom

Facultative symbionts as a eukaryotic horizontal gene pool

talk 

Author(s): Henry, LM

Summary:

Facultative or ‘secondary’ bacterial symbionts are very common in eukaryotes, particularly insects. While not essential, they often provide beneficial functions that can profoundly affect host biology. It has been hypothesised that secondary symbionts may form a “horizontal gene pool” shuttling adaptive genes amongst lineages in an analogous manner to plasmids and other mobile genetic elements in bacteria. However, the viability of this hypothesis rests upon a key unknown. We do not know if the distributions of symbionts across host populations reflect random acquisitions followed by maternal inheritance, or if the associations have occurred repeatedly in a manner that is consistent with a dynamic horizontal gene pool from which adaptations can be readily gained and lost in response to environmental changes. Here we test the importance of horizontal transfer using the phylogenetic and ecological distributions of secondary symbionts carried by 1104 pea aphids collected from 14 countries and 11 plant-associated populations. This reveals that not only is horizontal transfer common, it is associated with aphid lineages colonizing new ecological niches, such as switching to novel host plants or moving into new climatic regions. Moreover, aphids that share the same ecologies in different regions worldwide have independently acquired related symbiont genotypes, suggesting a central role of the symbionts in their host’s niches. In sum, our data reveals symbiont populations are dynamic with their distributions shaped by horizontal transmission and differential retention amongst aphid lineages, both of which are strongly influenced by ecological factors. We conclude that the secondary symbiont community forms a horizontal gene pool that is central to the adaptation and distribution of their insect hosts

Ana Duarte
University of Cambridge
Department of Zoology
United Kingdom

Interactions between species shape parental investment in the burying beetle

talk 

Author(s): Duarte, ALF, Welch, M, Kilner, RM

Summary:

Parental care studies typically focus on intraspecific interactions occurring during reproductive events, such as between mates or between parents and offspring. Interactions between species are often neglected, yet feedbacks between species may shape parental investment. This is particularly important when parents breed on temporary resources for which there is high competition. The burying beetle Nicrophorus vespilloides breeds on small vertebrate carcasses which they bury and cover in antimicrobial exudates. Investment in antimicrobial exudates increases in response to carcass microbial load. Females investing more in antimicrobial exudates seem to protect their current brood at the cost of future reproduction. Beetles also carry phoretic mites that breed alongside them on a carcass. We investigate interactions between beetles, mites and microbes, and how these affect reproductive strategies in the burying beetle. Using breeding experiments we test whether mites have beneficial or detrimental effects on beetles breeding in carcasses presenting different levels of microbial challenge. To gauge the microbial challenges faced by wild beetles, we use metagenomics to assess the microbiome of prepared and unprepared carcasses. We found that females breeding with mites have lower levels of antimicrobial activity, potentially safeguarding future reproduction. However, male lifespan is reduced when breeding with mites, suggesting that interacting with mites is costly for males. Beetles radically change the microbiome of carcasses: prepared carcasses, relative to unprepared, show a shift towards overrepresentation of Gram negative bacteria. In conclusion, characterizing relationships between species in this system unravels ecological pressures shaping parental investment and trade-offs between future and current reproduction. Using a metagenomics approach we are able to identify key players in the bacterial communities which challenge carrion beetles.

Ellie Harrison
University of York
Department of Biology
United Kingdom

No host is an island: the impact of multiple symbionts on pairwise interactions

talk 

Author(s): Harrison, E, Truman, J

Summary:

Host – symbiont interactions, though often studied as a pairwise process, rarely occur in isolation. In most cases individual host populations often interact, and potentially coevolve, with multiple symbiont populations concurrently. These pairwise coevolutionary interactions will be affected by the biotic environment in which they take place and vice versa, however the complexity of natural populations make these effects hard to unravel in the wild. Experimental evolution provides an excellent framework in which to examine the impact of these rival symbiotic interactions, as (co)evolutionary dynamics can be observed through time in communities of known biotic structure. Using this approach we examine the conflict between two otherwise independent symbionts of the same host; a parasitic lytic phage and a facultatively mutualistic plasmid, both native to the bacteria Pseudomonas fluorescens. We find that the presence of a second symbiont has major effects on the ecology and evolution of these interactions in both directions. Firstly we show that the presence of coevolving phages accelerates the loss of plasmids under conditions in which they are parasitic, but not mutualistic. Secondly we demonstrate that plasmid carriage alters the trajectory of bacteria-phage coevolution, favouring generalist phage-resistance strategies among hosts also carrying the plasmid. This study therefore shows the impact community complexity has on pairwise host – symbiont interactions, and in turn how these interactions can shape the community in which they are embedded.

Celine Leroy
IRD-UMR AMAP
UMR AMAP
France

Plant performance disparities in tripartite mutualistic associations

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Author(s): Leroy, C, Lauth, J, Malé, PG, Ruiz-González, MX, Séjalon-Delmas, N, Dejean, A, Orivel, J

Summary:

Ant-plants are interesting model systems of protective mutualisms in which the associated ants are assimilated as agents of constitutive indirect defense. The plants provide ants with nesting space and food in exchange for indirect benefits such as protection. The outcomes of the relationship might, however, vary according to the behavior of the interacting ant partners, thus differently influencing plant fitness. Here, we explore this framework by studying a two-to-one ant-plant mutualism between two closely-related ant species, Allomerus decemarticulatus and A. octoarticulatus, competing for the same host-plant species, Hirtella physophora. In these systems, both ant species specifically cultivate a third partner, an Ascomycota fungus. The fungus plays a structural role that the ants use it in constructing gallery-shaped traps to ambush prey and by penetrating into the plant tissue, the hyphae also enhance plant nutrition in both systems. We also examine how the behavioral ecology of these two ant species affects host-plant fitness. First, we show that differences in the efficacy of the predation strategy (ambushing and patrolling leaves) induce a variation in the intensity of the biotic defense, affecting plant growth and fruit production. Moreover, these two ant species impose reproductive costs on their host-plant by partially destroying floral buds. This castration behavior favors the production of more leaves for the host-plant (nesting space for the ants) thanks to the reallocation of plant energy from reproduction to growth. However, plants totally castrated experimentally produced notably smaller domatia and extrafloral nectaries. Such a decrease in the investment in myrmecophytic traits demonstrates the existence of inducible sanctions against too virulent castrating ants. Altogether, these results show how multiple, co-occurring symbiont species (and their traits) influence host-plant performance by modulating their context-dependent outcomes.

Sylvain Charlat
CNRS - University of Lyon 1
Biometry and Evolutionary Biology Lab
France

Quantifying the Wolbachia turnover

talk 

Author(s): Charlat, S, Simoes, P, Cariou, M, Bailly-Bechet, M, Mialdea, G, Sagot, M

Summary:

Wolbachia bacteria infect millions of arthropod species, with diverse and extreme consequences, ranging from sex-ratio distortion, mating incompatibilities, to protection against viruses, including vector-born human pathogens. In contrast to other maternally inherited intracellular bacteria such as Buchnera in Aphids, Wolbachia is not engaged in long-term, evolutionary stable associations with its arthropod hosts, as evidenced by the variability of Wolbachia infections between closely related host lineages. Its current distribution is thus best seen as resulting from a dynamic interplay between infection acquisition, through horizontal transfer, and infection loss, possibly fuelled by Wolbachia-host evolutionary conflicts. Here we globally quantify this process by tracking recent acquisition / loss events among 3,600 arthropod lineages spanning 1,100 species from Tahiti and surrounding islands, that is, an estimated 70% of the local biodiversity. Reduced gene flow among islands deepens coalescence time, thus providing a mean to reveal recent changes in Wolbachia infection status. Using DNA barcoding approaches to characterise the host cytoplasmic lineages and their Wolbachia infections, we show that infected specimens within species (that is, differing by no more than 3% substitutions per site at the mitochondrial locus CO1) have an average 88% chance to share the same infection by descent. This probability sharply drops and reaches 22% when host mtDNA divergence ranges from 3 to 10%, that is, after approximately 3 to 10 million years of divergence. Fitting an epidemiological model to this data, we estimate that uninfected lineages acquire Wolbachia every 2 to 7 million years in average, while infections are lost every 1 to 3 million years.

Sandra Andersen
University of Oxford
Zoology
United Kingdom

Specificity and stability of the Acromyrmex-Pseudonocardia symbiosis in changing environments

talk 

Author(s): Andersen, SB, Hansen, LH, Sapountzis, P, Sørensen, SJ, Boomsma, JJ

Summary:

Fungus-growing ants live in a complex symbiosis involving both fungal and bacterial partners. Among these are Actinobacteria of the genus Pseudonocardia that are maintained on the ant cuticle to produce antibiotics, primarily against a parasitic fungus of the garden symbiont. The symbiosis has been assumed to be a hallmark of evolutionary stability, but this notion has been challenged by culturing and sequencing data. We used 454 pyrosequencing of 16S rRNA to estimate the diversity of the cuticular bacterial community of the leaf-cutting ant Acromyrmex echinatior from Panama. We used field and lab samples of the same colonies, the latter after colonies had been kept under laboratory conditions for up to 10 years. We show that the bacterial communities are highly colony-specific and stable over time. The majority of colonies (25/26) had a single dominant Pseudonocardia strain and only two strains were found in the Gamboa population across 17 years, confirming an earlier study. The microbial community on newly hatched ants consisted almost exclusively of Pseudonocardia while other Actinobacteria were identified in lower abundances on older ants. These findings are consistent with recent theory predicting that mixtures of antibiotic-producing bacteria can remain mutualistic when dominated by a single vertically transmitted strain.

Steven Parratt
University of Liverpool
Institute of Integrative Biology
United Kingdom

Symbiont Mediated Extinction: examining the spread and effects of a male-killer

talk 

Author(s): Parratt, SR, King, KC, Hurst, GDD

Summary:

The ubiquity of reproductive manipulating symbionts in arthropods makes them a key component in the ecology and evolution of natural populations. Yet questions surround the dynamics of symbiont spread within and between host populations, and the extent to which their phenotype influences host populations in ecological time remains enigmatic. Ultimately these two issues are linked, since the severity of the symbiont’s effect on its host population will be determined by its prevalence, which is a product of its transmission dynamics. These organisms, which manipulate the reproductive biology of their host to aid their own transmission, have a dramatic impact on key fitness traits of their host such as sex-ratio, reproductive behaviors and fecundity. They are therefore an important selection pressure on the host population and, by terminally reducing the effective population size through a combination of these effects, they have the potential to drive their host to extinction.

Here we experimentally investigate the spread, maintenance and loss of a heritable male-killing symbiont under differing transmission regimes. We demonstrate that ecological conditions favouring the spread of the male-killing symbiont, Arsenophonus nasoniae, reduced the effective host population size through male-death, causing severe bottlenecks and eventually driving experimental wasp populations extinct. If reproductive symbionts can sweep to near fixation in local populations, such microbes can have catastrophic effects on the survival of their hosts in the field. This study highlights the potential importance of other symbionts or sex-ratio distorters in the persistence of natural populations.

Contacts

Chairman: Octávio S. Paulo
Tel: 00 351 217500614 direct
Tel: 00 351 217500000 ext22359
Fax: 00 351 217500028
email: mail@eseb2013.com

Address

XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon
Portugal

Website

Computational Biology & Population Genomics Group 
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