More examples of breakdown the 1:1 partner specificity between figs and fig wasps

Background The obligate mutualism between fig trees (Ficus, Moraceae) and pollinating fig wasps (Agaonidae) is a model system for studying co-evolution due to its perceived extreme specificity, but recent studies have reported a number of examples of trees pollinated by more than one fig wasp or sharing pollinators with other trees. This will make the potential of pollen flow between species and hybridization more likely though only few fig hybrids in nature have been found. We reared pollinator fig wasps from figs of 13 Chinese fig tree species and established their identity using genetic methods in order to investigate the extent to which they were supporting more than one species of pollinator (co-pollinator). Results Our results showed (1) pollinator sharing was frequent among closely-related dioecious species (where pollinator offspring and seeds develop on different trees); (2) that where two pollinator species were developing in figs of one host species there was usually one fig wasp with prominent rate than the other. An exception was F. triloba, where its two pollinators were equally abundant; (3) the extent of co-pollinator within one fig species is related to the dispersal ability of them which is stronger in dioecious figs, especially in small species. Conclusions Our results gave more examples to the breakdown of extreme specificity, which suggest that host expansion events where pollinators reproduce in figs other than those of their usual hosts are not uncommon among fig wasps associated with dioecious hosts. Because closely related trees typically have closely related pollinators that have a very similar appearance, the extent of pollinator-sharing has probably been underestimated. Any pollinators that enter female figs carrying heterospecific pollen could potentially generate hybrid seed, and the extent of hybridization and its significance may also have been underestimated.

Transitions in symbiosis: evidence for environmental acquisition and social transmission within a clade of heritable symbionts

A dynamic continuum exists from free-living environmental microbes to strict host-associated symbionts that are vertically inherited. However, knowledge of the forces that drive transitions in symbiotic lifestyle and transmission mode is lacking. Arsenophonus is a diverse clade of bacterial symbionts, comprising reproductive parasites to coevolving obligate mutualists, in which the predominant mode of transmission is vertical. We describe a symbiosis between a member of the genus Arsenophonus and the Western honey bee. The symbiont shares common genomic and predicted metabolic properties with the male-killing symbiont Arsenophonus nasoniae, however we present multiple lines of evidence that the bee Arsenophonus deviates from a heritable model of transmission. Field sampling uncovered spatial and seasonal dynamics in symbiont prevalence, and rapid infection loss events were observed in field colonies and laboratory individuals. Fluorescent in situ hybridisation showed Arsenophonus localised in the gut, and detection was rare in screens of early honey bee life stages. We directly show horizontal transmission of Arsenophonus between bees under varying social conditions. We conclude that honey bees acquire Arsenophonus through a combination of environmental exposure and social contacts. These findings uncover a key link in the Arsenophonus clades trajectory from free-living ancestral life to obligate mutualism, and provide a foundation for studying transitions in symbiotic lifestyle.

Cycad-Weevil Pollination Symbiosis Is Characterized by Rapidly Evolving and Highly Specific Plant-Insect Chemical Communication

Coevolution between plants and insects is thought to be responsible for generating biodiversity. Extensive research has focused largely on antagonistic herbivorous relationships, but mutualistic pollination systems also likely contribute to diversification. Here we describe an example of chemically-mediated mutualistic species interactions affecting trait evolution and lineage diversification. We show that volatile compounds produced by closely related species of Zamia cycads are more strikingly different from each other than are other phenotypic characters, and that two distantly related pollinating weevil species have specialized responses only to volatiles from their specific host Zamia species. Plant transcriptomes show that approximately a fifth of genes related to volatile production are evolving under positive selection, but we find no differences in the relative proportion of genes under positive selection in different categories. The importance of phenotypic divergence coupled with chemical communication for the maintenance of this obligate mutualism highlights chemical signaling as a key mechanism of coevolution between cycads and their weevil pollinators.

The Extent of Pollinator Sharing Among Fig Trees in Southern China

Background: The obligate mutualism between fig trees (Ficus, Moraceae) and pollinating fig wasps (Agaonidae) is a model system for studying co-evolution due to its perceived extreme specificity, but recent studies have reported a number of examples of trees pollinated by more than one fig wasp or sharing pollinators with other trees. This makes pollen flow between species and hybridization more likely. We reared pollinator fig wasps from figs of 13 Chinese fig tree species trees and established their identity using genetic methods in order to investigate the extent to which are they were supporting more than one species of pollinator.Results: Our results showed 1) pollinator sharing was frequent among closely-related dioecious species (where pollinator offspring and seeds develop on different trees), but not monoecious species and 2) that where two pollinator species were developing in figs of one host species there was usually one fig wasp that was far rarer than the other. An exception was F. triloba, where its two pollinators were equally abundant. Conclusions: Our results suggest that host expansion events where pollinators reproduce in figs other than those of their usual hosts are not uncommon among fig wasps associated with dioecious hosts. Because closely related trees typically have closely related pollinators that have a very similar appearance, the extent of pollinator-sharing has probably been underestimated. Any pollinators that enter female figs carrying heterospecific pollen could potentially generate hybrid seed, and the extent of hybridization and its significance may also have been underestimated.

Engineering cooperation in an anaerobic co-culture

Over the past century, microbiologists have studied organisms in pure culture, yet it is becoming increasingly apparent that the majority of biological processes rely on multispecies cooperation and interaction. While little is known about how such interactions permit cooperation, even less is known about how cooperation arises. To study the emergence of cooperation in the laboratory, we constructed both a commensal community and an obligate mutualism using the previously non-interacting bacteria Shewanella oneidensis and Geobacter sulfurreducens. Incorporation of a glycerol utilization plasmid (pGUT2) enabled S. oneidensis to metabolize glycerol and produce acetate as a carbon source for G. sulfurreducens establishing a cross-feeding, commensal co-culture. In the commensal co-culture, both species coupled oxidative metabolism to the respiration of fumarate as the terminal electron acceptor. Deletion of the gene encoding fumarate reductase in the S. oneidensis pGUT2 strain shifted the co-culture with G. sulfurreducens to an obligate mutualism where neither species could grow in absence of the other. A shift in metabolic strategy from glycerol catabolism to malate metabolism was associated with obligate co-culture growth. Further targeted deletions in malate uptake and acetate generation pathways in S. oneidensis significantly inhibited co-culture growth with G. sulfurreducens. The engineered co-culture between S. oneidensis and G. sulfurreducens provides a model laboratory system to study the emergence of cooperation in bacterial communities, and the shift in metabolic strategy observed in the obligate co-culture highlights the importance of genetic change in shaping microbial interactions in the environment. Importance Microbes seldom live alone in the environment, yet this scenario is approximated in the vast majority of pure-culture laboratory experiments. Here we develop an anaerobic co-culture system to begin understanding microbial physiology in a more complex setting, but also to determine how anaerobic microbial communities can form. Using synthetic biology, we generated a co-culture system where the facultative anaerobe Shewanella oneidensis consumes glycerol and provides acetate to the strict anaerobe Geobacter sulfurreducens. In the commensal system, growth of G. sulfurreducens is dependent on the presence of S. oneidensis. To generate an obligate co-culture, where each organism requires the other, we eliminated the ability of S. oneidensis to respire fumarate. An unexpected shift in metabolic strategy from glycerol catabolism to malate metabolism was observed in the obligate co-culture. Our work highlights how metabolic landscapes can be expanded in multi-species communities and provides a system to evaluate the evolution of cooperation under anaerobic conditions.

Transitions in symbiosis: evidence for environmental acquisition & social transmission within a clade of heritable symbionts

A dynamic continuum exists from free-living environmental microbes to strict host associated symbionts that are vertically inherited. However, knowledge of the forces that drive transitions in the modes by which symbioses form is lacking. Arsenophonus is a diverse clade of bacterial symbionts, comprising reproductive parasites to coevolving obligate mutualists, in which the predominant mode of transmission is vertical. We describe a symbiosis between a member of the genus Arsenophonus and the Western honey bee. We then present multiple lines of evidence that this symbiont deviates from a heritable model of transmission. Field sampling uncovered marked spatial and seasonal dynamics in symbiont prevalence, and rapid infection loss events were observed in field colonies and individuals in the laboratory. Fluorescent in-situ hybridization showed Arsenophonus localised in the gut, and detection of the bacterium was rare in screens of early honey bee life stages. We directly show horizontal transmission of Arsenophonus between bees under varying social conditions. We conclude that honey bees acquire Arsenophonus through a combination of environmental exposure and social contacts. Together these findings uncover a key link in the Arsenophonus clades trajectory from free-living ancestral life to obligate mutualism, and provide a foundation for studying transitions in symbiotic lifestyle.

Evolution of division of labour in mutualistic symbiosis

Mutualistic symbiosis can be regarded as interspecific division of labour, which can improve the productivity of metabolites and services but deteriorate the ability to live without partners. Interestingly, even in environmentally acquired symbiosis, involved species often rely exclusively on the partners despite the lethal risk of missing partners. To examine this paradoxical evolution, we explored the coevolutionary dynamics in symbiotic species for the amount of investment in producing their essential metabolites, which symbiotic species can share. Our study has shown that, even if obtaining partners is difficult, ‘perfect division of labour’ (PDL) can be maintained evolutionarily, where each species perfectly specializes in producing one of the essential metabolites so that every member entirely depends on the others for survival, i.e. in exchange for losing the ability of living alone. Moreover, the coevolutionary dynamics shows multistability with other states including a state without any specialization. It can cause evolutionary hysteresis: once PDL has been achieved evolutionarily when obtaining partners was relatively easy, it is not reverted even if obtaining partners becomes difficult later. Our study suggests that obligate mutualism with a high degree of mutual specialization can evolve and be maintained easier than previously thought.

Symbiotic interactions between ciliates (Ciliophora) and diatoms (Bacillariophyceae)

Introduction: Diatoms and ciliates are important components of the marine plankton community, and some species are able to develop symbiotic associations in the tropical seas. Objective: To describe the nature of the symbioses, the morphological adaptations of the members, and the possible ecological advantages of the symbiotic life versus the free-living forms. Methods: Plankton samples were collected from Mediterranean and Caribbean Seas, and the South Atlantic Ocean. Consortia were examined during laboratory incubations, including studies of the motility and the feeding currents by high-speed video recordings, and culture tests of the species as symbiotic or free-living forms. Results: The consortia of the diatoms Chaetoceros dadayi and C. tetrastichon with the tintinnid Eutintinnus spp., and C. coarctatus with the peritrich ciliate Vorticella oceanica are examples of an obligate mutualism. The cultures of the host diatoms as free-living organism were unsuccessful. The consortia between Eutintinnus lususundae and the diatoms Chaetoceros peruvianus, Hemiaulus hauckii, H. membranaceus, and Thalassionema sp. are facultative symbioses. These are examples of three or four partner consortia because Hemiaulus spp. is the host of the diazotrophic cyanobacteria Richelia intracellularis. Other example of facultative three partner consortium is the peritrich ciliate Zoothamnium pelagicum with an ectobiont bacteria, and the diatom Licmophora sp. The barrel-shaped chains of the diatom Fragilariopsis dolious encircled the lorica of Salpingella spp., while these chains were almost flat in the free-living stage. The peritrich ciliate Pseudovorticella coscinodisci lives on large pelagic diatoms such as Coscinodiscus and Palmerina. These symbioses are facultative for the diatoms, but they extended their survival under unfavorable conditions. High-speed video recordings of the consortium of Vorticella oceanica and Chaetoceros coarctatus revealed that during the stalk contraction the zooid of reached 5 400 body length s-1, being the fastest organism with respect to its size. The consortia of Chaetoceros densus and an undescribed species of Vorticella is re-discovered. Conclusions: These symbioses have allowed that the sessile peritric ciliates colonize the pelagic environment and the proliferation of diatoms with a polar origin in the tropical sea.

Loss of top-down biotic interactions changes the relative benefits for obligate mutualists

The collapse of mutualisms owing to anthropogenic changes is contributing to losses of biodiversity. Top predators can regulate biotic interactions between species at lower trophic levels and may contribute to the stability of such mutualisms, but they are particularly likely to be lost after disturbance of communities. We focused on the mutualism between the fig tree Ficus microcarpa and its host-specific pollinator fig wasp and compared the benefits accrued by the mutualists in natural and translocated areas of distribution. Parasitoids of the pollinator were rare or absent outside the natural range of the mutualists, where the relative benefits the mutualists gained from their interaction were changed significantly away from the plant's natural range owing to reduced seed production rather than increased numbers of pollinator offspring. Furthermore, in the absence of the negative effects of its parasitoids, we detected an oviposition range expansion by the pollinator, with the use of a wider range of ovules that could otherwise have generated seeds. Loss of top-down control has therefore resulted in a change in the balance of reciprocal benefits that underpins this obligate mutualism, emphasizing the value of maintaining food web complexity in the Anthropocene.