Auswahl der wissenschaftlichen Literatur zum Thema „Host-Symbiont specificity“

Host specificity is a key element to our understanding of symbiont diversification and is driven by multiple macro- and microevolutionary processes. Broad scale (e.g., species-level) studies can uncover relevant processes such as cospeciation and host-switching that shape host-symbiont evolutionary histories.

Host/symbiont specificity has been investigated in non-symbiotic and aposymbiotic brown and green hydra infected with various free-living and symbiotic species and strains of Chlorella and Chlorococcum. Morphology and ultrastructure of the symbioses obtained have been compared. Aposymbiotic Swiss Hydra viridis and Japanese H. magnipapillata served as controls. In two strains of H. attenuata stable hereditary symbioses were obtained with Chlorococcum isolated from H. magnipapillata. In one strain of H. vulgaris, in H. oligactis and in aposymbiotic H. viridis chlorococci persisted for more than a week. Eight species of free-living Chlorococcum, 10 symbiotic and 10 free-living strains of Chlorella disappeared from the brown hydra within 1–2 days. In H. magnipapillata there was a graded distribution of chlorococci along the polyps. In hypostomal cells there were greater than 30 algae/cell while in endodermal cells of the mid-section or peduncle less than 10 algae/cell were found. In H. attenuata the algal distribution was irregular, there were up to five chlorocci/cell, and up to 20 cells/hydra hosted algae. In the dark most cells of Chlorococcum disappeared from H. magnipapillata and aposymbiotic hydra were obtained. Chlorococcum is thus an obligate phototroph, and host-dependent heterotrophy is not required for the preservation of a symbiosis. The few chlorococci that survived in the dark seem to belong to a less-demanding physiological strain. In variance with known Chlorella/H. viridis endosymbioses the chlorococci in H. magnipapillata and H. attenuata were tightly enveloped in the vacuolar membrane of the hosting cells with no visible perialgal space. Chlorococcum reproduced in these vacuoles and up to eight daughter cells were found within the same vacuole. We suggest that the graded or scant distribution of chlorococci in the various brown hydra, their inability to live in H. viridis and the inability of the various chlorellae to live in brown hydra are the result of differences in nutrients available to the algae in the respective hosting cells. We conclude that host/symbiont specificity and the various forms of interrelations we show in green and brown hydra with chlorococci and chlorellae are based on nutritional-ecological factors. These interrelations demonstrate successive stages in the evolution of stable obligatoric symbioses from chance encounters of preadapted potential cosymbionts.

Despite the omnipresence of specific host–symbiont associations with acquisition of the microbial symbiont from the environment, little is known about how the specificity of the interaction evolved and is maintained. The bean bug Riptortus pedestris acquires a specific bacterial symbiont of the genus Burkholderia from environmental soil and harbors it in midgut crypts. The genus Burkholderia consists of over 100 species, showing ecologically diverse lifestyles, and including serious human pathogens, plant pathogens, and nodule-forming plant mutualists, as well as insect mutualists. Through infection tests of 34 Burkholderia species and 18 taxonomically diverse bacterial species, we demonstrate here that nonsymbiotic Burkholderia and even its outgroup Pandoraea could stably colonize the gut symbiotic organ and provide beneficial effects to the bean bug when inoculated on aposymbiotic hosts. However, coinoculation revealed that the native symbiont always outcompeted the nonnative bacteria inside the gut symbiotic organ, explaining the predominance of the native Burkholderia symbiont in natural bean bug populations. Hence, the abilities for colonization and cooperation, usually thought of as specific traits of mutualists, are not unique to the native Burkholderia symbiont but, to the contrary, competitiveness inside the gut is a derived trait of the native symbiont lineage only and was thus critical in the evolution of the insect gut symbiont.

In the ocean, most hosts acquire their symbionts from the environment. Due to the immense spatial scales involved, our understanding of the biogeography of hosts and symbionts in marine systems is patchy, although this knowledge is essential for understanding fundamental aspects of symbiosis such as host–symbiont specificity and evolution. Lucinidae is the most species-rich and widely distributed family of marine bivalves hosting autotrophic bacterial endosymbionts. Previous molecular surveys identified location-specific symbiont types that “promiscuously” form associations with multiple divergent cooccurring host species. This flexibility of host–microbe pairings is thought to underpin their global success, as it allows hosts to form associations with locally adapted symbionts. We used metagenomics to investigate the biodiversity, functional variability, and genetic exchange among the endosymbionts of 12 lucinid host species from across the globe. We report a cosmopolitan symbiont species, Candidatus Thiodiazotropha taylori, associated with multiple lucinid host species. Ca. T. taylori has achieved more success at dispersal and establishing symbioses with lucinids than any other symbiont described thus far. This discovery challenges our understanding of symbiont dispersal and location-specific colonization and suggests both symbiont and host flexibility underpin the ecological and evolutionary success of the lucinid symbiosis.

The specificity of the relationship between cnidarian hosts and symbiotic dinoflagellates (zooxanthellae) differs among host species. Some cnidarian hosts can establish symbiotic relationship with various types of zooxanthellae, while others exhibit high fidelity to specific symbiont type. It is not known how compatibility or specificity of the relationship is determined. We hypothesized that some cnidarian hosts select symbiont type that leads to highest fitness when the host is flexible with symbiont type and more than one types of symbionts are available. As a first step to study this possibility, compatibility of clonal polyps of Cassiopea sp. with six strains of cultured zooxanthellae and the fitness of the host associated with different types of symbionts were studied. Polyp diameter was measured and the number of asexual buds were calculated as a measure of host fitness. The number of zooxanthellae in host and in asexual buds was also measured as a measure of symbiont fitness. Three strains KB8 (clade A), Y106 (clade A), and K100 (clade B) were compatible with the Cassiopea polyps, while other three strains, Y103 (clade C), K111 (clade D), and K102 (clade F) were incompatible. No clear difference in the fitness was found among the polyps inoculated with compatible and incompatible symbiont strains. In one experiment, a compatible strain Y106 seemed to decrease host fitness, but this should be checked by further studies. This study suggests that feeding regimes and long observation period might be important when fitness of hosts associated with different types of symbionts is investigated.

Symbioses between eukaryotes and sulfur-oxidizing (thiotrophic) bacteria have convergently evolved multiple times. Although well described in at least eight classes of metazoan animals, almost nothing is known about the evolution of thiotrophic symbioses in microbial eukaryotes (protists). In this study, we characterized the symbioses between mouthless marine ciliates of the genus Kentrophoros , and their thiotrophic bacteria, using comparative sequence analysis and fluorescence in situ hybridization. Ciliate small-subunit rRNA sequences were obtained from 17 morphospecies collected in the Mediterranean and Caribbean, and symbiont sequences from 13 of these morphospecies. We discovered a new Kentrophoros morphotype where the symbiont-bearing surface is folded into pouch-like compartments, illustrating the variability of the basic body plan. Phylogenetic analyses revealed that all investigated Kentrophoros belonged to a single clade, despite the remarkable morphological diversity of these hosts. The symbionts were also monophyletic and belonged to a new clade within the Gammaproteobacteria, with no known cultured representatives. Each host morphospecies had a distinct symbiont phylotype, and statistical analyses revealed significant support for host–symbiont codiversification. Given that these symbioses were collected from two widely separated oceans, our results indicate that symbiotic associations in unicellular hosts can be highly specific and stable over long periods of evolutionary time.

ABSTRACT The phylogenetic relationships of bacterial symbionts from three gall-bearing species in the marine red algal genusPrionitis (Rhodophyta) were inferred from 16S rDNA sequence analysis and compared to host phylogeny also inferred from sequence comparisons (nuclear ribosomal internal-transcribed-spacer region). Gall formation has been described previously on two species ofPrionitis, P. lanceolata (from central California) and P. decipiens (from Peru). This investigation reports gall formation on a third related host,Prionitis filiformis. Phylogenetic analyses based on sequence comparisons place the bacteria as a single lineage within theRoseobacter grouping of the α subclass of the divisionProteobacteria (99.4 to 98.25% sequence identity among phylotypes). Comparison of symbiont and host molecular phylogenies confirms the presence of three gall-bearing algal lineages and is consistent with the hypothesis that these red seaweeds and their bacterial symbionts are coevolving. The species specificity of these associations was investigated in nature by whole-cell hybridization of gall bacteria and in the laboratory by using cross-inoculation trials. Whole-cell in situ hybridization confirmed that a single bacterial symbiont phylotype is present in galls on each host. In laboratory trials, bacterial symbionts were incapable of inducing galls on alternate hosts (including two non-gall-bearing species). Symbiont-host specificity in Prionitis gall formation indicates an effective ecological separation between these closely related symbiont phylotypes and provides an example of a biological context in which to consider the organismic significance of 16S rDNA sequence variation.

La stabilité évolutive des relations hôte-microbe est cruciale pour la symbiose. La transmission verticale des symbiotes microbiens des parents à la progéniture est bien établie, mais l'acquisition environnementale par transmission horizontale de symbiotes nécessite des adaptations spécifiques. Les insectes de l'infra-ordre Pentatomomorpha disposent d'un mécanisme efficace pour l'acquisition de leur symbiote à partir du sol. Ces insectes possèdent une architecture intestinale distinctive contenant une région postérieure, appelée M4, composée de centaines de cryptes, constituant une niche spécifique pour abriter des symbiotes intestinaux bénéfiques. Les Coreoidea sélectionnent spécifiquement des bactéries Caballeronia. Ma thèse explore la spécificité de cette association et les mécanismes bactériens sous-jacents. Trois espèces de Coreoidea (Riptortus pedestris, Leptoglossus occidentalis, Coreus marginatus) montrent une préférence pour des sous-clades spécifiques de Caballeronia, influencée par la compétition interspécifique. La région M4 est dominée par une seule espèce bactérienne, suggérant une forte pression de sélection. La spécificité de la souche est alignée avec un avantage en termes de fitness reproductif. Des criblages génétiques ont révélé des fonctions cruciales pour la colonisation des cryptes, notamment le chimiotactisme, la résistance aux peptides antimicrobiens et de la capacité à utiliser des sources de carbone néoglucogéniques, la taurine et l'inositol, suggérant que l'hôte fournit ce type de métabolites comme nutriments aux symbiotes. Ces découvertes démontrent que malgré une grande diversité microbienne environnementale, les insectes sélectionnent des symbiotes spécifiques grâce à des mécanismes multifactoriels
The evolutionary stability of host-microbe relationships is crucial for symbiosis. Vertical transmission of microbial symbionts from parents to offspring is well established, but environmental acquisition through horizontal transmission of symbionts requires specific adaptations. Insects of the infraorder Pentatomomorpha have an effective mechanism for acquiring their symbionts from the soil. These insects possess a distinctive intestinal architecture with a posterior region called M4, composed of hundreds of crypts that provide a specific niche for harboring beneficial gut symbionts. Coreoidea specifically select Caballeronia bacteria. My thesis explores the specificity of this association and the underlying bacterial mechanisms. Three species of Coreoidea (Riptortus pedestris, Leptoglossus occidentalis, Coreus marginatus) show a preference for specific subclades of Caballeronia, influenced by interspecific competition. The M4 region is dominated by a single bacterial species, suggesting strong selective pressure. Strain specificity is aligned with a reproductive fitness advantage. Genetic screenings revealed crucial functions for crypt colonization, including chemotaxis, resistance to antimicrobial peptides, and the ability to utilize neoglucogenic carbon sources such as taurine and inositol, suggesting that the host provides these metabolites as nutrients to the symbionts. These findings demonstrate that despite high environmental microbial diversity, insects select specific symbionts through multifactorial mechanisms

Abstract Research on the formation of symbioses includes four topics. 1. Identification of the source of the partner. A host may acquire its symbionts either from the environment or directly from another host. In many associations, the symbionts are transferred directly from a hostparent to its offspring, and this process is known as vertical transmission. 2. Establishment of the symbiosis. It is usual to consider the development of an association as a series of stages, each stage dependent on the successful completion of the previous stage. As a hypothetical example, an association between an animal host and microbial symbiont may include contact, internalization of the microorganism, and initiation of nutrient transfer. 3. Specificity of the association. This refers to the taxonomic range of partners with which an organism can form a symbiosis. Specificity is a consequence of both the degree of specialization of an organism for its partner, and its capacity to select and discriminate between alternative potential partners.