Maintenance of algal endosymbionts in P aramecium bursaria : a simple model based on population dynamics

Modeling endosymbioses: Insights and hypotheses from theoretical approaches

Endosymbiotic relationships are pervasive across diverse taxa of life, offering key avenues for eco-evolutionary dynamics. Although a variety of experimental and empirical frameworks have shed light on critical aspects of endosymbiosis, theoretical frameworks (mathematical models) are especially well-suited for certain tasks. Mathematical models can integrate multiple factors to determine the net outcome of endosymbiotic relationships, identify broad patterns that connect endosymbioses with other systems, simplify biological complexity, generate hypotheses for underlying mechanisms, evaluate different hypotheses, identify constraints that limit certain biological interactions, and open new lines of inquiry. This Essay highlights the utility of mathematical models in endosymbiosis research, particularly in generating relevant hypotheses. Despite their limitations, mathematical models can be used to address known unknowns and discover unknown unknowns.

Cellular interactions and evolutionary origins of endosymbiotic relationships with ciliates

As unicellular predators, ciliates engage in close associations with diverse microbes, laying the foundation for the establishment of endosymbiosis. Originally heterotrophic, ciliates demonstrate the ability to acquire phototrophy by phagocytizing unicellular algae or by sequestering algal plastids. This adaptation enables them to gain photosynthate and develop resistance to unfavorable environmental conditions. The integration of acquired phototrophy with intrinsic phagotrophy results in a trophic mode known as mixotrophy. Additionally, ciliates can harbor thousands of bacteria in various intracellular regions, including the cytoplasm and nucleus, exhibiting species specificity. Under prolonged and specific selective pressure within hosts, bacterial endosymbionts evolve unique lifestyles and undergo particular reductions in metabolic activities. Investigating the research advancements in various endosymbiotic cases within ciliates will contribute to elucidate patterns in cellular interaction and unravel the evolutionary origins of complex traits.

Comparative Analysis Between Paramecium Strains with Different Syngens Using the RAPD Method

Paramecium spp. are a genus of free-living protists that live mainly in freshwater environments. They are ciliates with high motility and phagocytosis and have been used to analyze cell motility and as a host model for pathogens. Besides such biological characteristics, apart from the usual morphological and genetic classification of species, the existence of taxonomies (such as syngens) and mating types related to Paramecium's unique reproduction is known. In this study, we attempted to develop a simple method to identify Paramecium strains, which are difficult to distinguish morphologically, using random amplified polymorphic DNA (RAPD) analysis. Consequently, we can observe strain-specific band patterns. We also confirm that the presence of endosymbiotic Chlorella cells affects the band pattern of P. bursaria. Furthermore, the results of the RAPD analysis using several P. caudatum strains with different syngens show that it is possible to detect a band specific to a certain syngen. By improving the reaction conditions and random primers, based on the results of this study, RAPD analysis can be applied to the identification of Paramecium strains and their syngen confirmation tests.

Francisella novicida can utilize Paramecium bursaria as its potential host

Francisella novicida is a facultative intracellular pathogen and the causative agent of tularemia. Although cases of infection caused by exposure to contaminated water have been reported, its natural host and ecology in the environment remain unclear. In this study, we investigated in vitro the possibility that Paramecium bursaria may be a useful tool as a protist host model of F. novicida. Experimental infection with F. novicida resulted in a stable intracellular relationship within P. bursaria. This symbiotic intracellular relationship was not observed in experimental infections with other Francisella species and Legionella pneumophila. We found that F. novicida showed similar behaviour to that of the eukaryotic endosymbiont of P. bursaria, the green algae Chlorella, in the internalization process. In addition, stable intracellular localization of F. novicida was possible only when Chlorella was not present. Although we investigated the type VI secretion system of F. novicida as a candidate for the bacterial factor, we found that it was not involved in the establishment of an intracellular relationship with P. bursaria. These results suggested that P. bursaria is potentially a protist host model for F. novicida and may be a useful tool for understanding the relationship between protist hosts and their symbionts.

The ciliate Paramecium bursaria allows budding of symbiotic Chlorella variabilis cells singly from the digestive vacuole membrane into the cytoplasm during algal reinfection

The ciliate Paramecium bursaria harbors several hundred symbiotic Chlorella spp. cells in the cytoplasm. Algal re-endosymbiosis can be artificially induced using alga-removed P. bursaria. During algal re-endosymbiosis, algae ingested into the host digestive vacuoles (DVs) avoid digestion by the host lysosomal enzymes and then escape into the cytoplasm by budding off of the DV membrane. The budded alga-enclosing DV membrane then differentiates into the symbiosome or perialgal vacuole (PV) membrane and is localized beneath the host cell cortex. In this study, we determined whether the PV membrane has the ability to recognize the symbiotic alga singly by eliminating other small microspheres in the same DV. To clarify the accuracy of the budding process, we mixed fluorescent-labeled microspheres of diameter 0.20 µm with isolated symbiotic algae during algal re-endosymbiosis. No fluorescence was observed from the PV membrane, as expected, and the budding DVs that enclosed both undigested and digested algae. Additionally, the algal re-endosymbiosis rate was significantly reduced in the presence of microspheres. These observations showed that the host P. bursaria allowed budding of the algae singly from the membranes of DVs without microspheres and this process required close contact between the DV membrane and the algal cell wall.

Efficient isolation and cultivation of endosymbiotic Chlorella from Paramecium bursaria on agar plates by co-culture with yeast cells

Paramecium bursaria is a ciliate that harbors Chlorella-like unicellular green algae as endosymbionts. The relationship between the host P. bursaria and the endosymbiotic Chlorella is facultative; therefore, both partners can be cultured independently and re-combined to re-establish symbiosis, making this system suitable for studying algal endosymbiosis. However, despite many previous studies, cultivation of endosymbiotic Chlorella remains difficult, particularly on agar plates. Here we describe a simple agar plate method for efficiently isolating and culturing cells of the endosymbiotic alga Chlorella variabilis from an individual P. bursaria cell, by co-culturing them with yeast Saccharomyces cerevisiae. The co-culture with the yeast significantly improved the colony-forming efficiency of the alga on agar. Growth assays suggest that the main role of the co-cultured yeast cells is not to provide nutrients for the algal cells, but to protect the algal cells from some environmental stresses on the agar surface. Using the algal cells grown on the plates and a set of specially designed primers, direct colony PCR can be performed for screening of multiple endosymbiont clones isolated from a single host ciliate. These methods may provide a useful tool for studying endosymbiotic Chlorella species within P. bursaria and various other protists.

Evolution of self-limited cell division of symbionts

In mutualism between unicellular hosts and their endosymbionts, symbiont's cell division is often synchronized with its host's, ensuring the permanent relationship between endosymbionts and their hosts. The evolution of synchronized cell division thus has been considered to be an essential step in the evolutionary transition from symbionts to organelles. However, if symbionts would accelerate their cell division without regard for the synchronization with the host, they would proliferate more efficiently. Thus, it is paradoxical that symbionts evolve to limit their own division for synchronized cell division. Here, we theoretically explore the condition for the evolution of self-limited cell division of symbionts, by assuming that symbionts control their division rate and that hosts control symbionts' death rate by intracellular digestion and nutrient supply. Our analysis shows that symbionts can evolve to limit their own cell division. Such evolution occurs if not only symbiont's but also host's benefit through symbiosis is large. Moreover, the coevolution of hosts and symbionts leads to either permanent symbiosis where symbionts proliferate to keep pace with their host, or the arms race between symbionts that behave as lytic parasites and hosts that resist them by rapid digestion.

Host control and nutrient trading in a photosynthetic symbiosis

Photosymbiosis is one of the most important evolutionary trajectories, resulting in the chloroplast and the subsequent development of all complex photosynthetic organisms. The ciliate Paramecium bursaria and the alga Chlorella have a well established and well studied light dependent endosymbiotic relationship. Despite its prominence, there remain many unanswered questions regarding the exact mechanisms of the photosymbiosis. Of particular interest is how a host maintains and manages its symbiont load in response to the allocation of nutrients between itself and its symbionts. Here we construct a detailed mathematical model, parameterised from the literature, that explicitly incorporates nutrient trading within a deterministic model of both partners. The model demonstrates how the symbiotic relationship can manifest as parasitism of the host by the symbionts, mutualism, wherein both partners benefit, or exploitation of the symbionts by the hosts. We show that the precise nature of the photosymbiosis is determined by both environmental conditions (how much light is available for photosynthesis) and the level of control a host has over its symbiont load. Our model provides a framework within which it is possible to pose detailed questions regarding the evolutionary behaviour of this important example of an established light dependent endosymbiosis; we focus on one question in particular, namely the evolution of host control, and show using an adaptive dynamics approach that a moderate level of host control may evolve provided the associated costs are not prohibitive.

Did Gause Have a Yeast Infection?

We planned to develop predator-prey models using Paramecium and yeast, but they have not been empirically examined since work by Gause in the 1930s. Therefore, we evaluated if Paramecium aurelia ingests and grows on eight yeasts. Recognising that it ingested yeasts but could not grow, we assessed if it might grow on other yeasts, by empirically parameterising a predator-prey model that relies on ingestion, not growth. Simulations were compared to P. aurelia-yeast time-series data, from Gause. We hypothesised that if the model simulated predator-prey dynamics that mimicked the original data, then possibly P. aurelia could grow on yeast; simulations did not mimic the original data. Reviewing works by Gause exposed two issues: experiments were undoubtedly contaminated with bacteria, allowing growth on bacteria, not yeast; and the population cycle data cannot be considered a self-sustaining time series, as they were manipulated by adding yeast and ciliates. We conclude that past and future work should not rely on this system, for either empirical or theoretical evaluations. Finally, although we show that P. aurelia, P. caudatum, Euplotes patella, and Blepharisma sp. cannot grow on yeast, Tetrahymena pyriformis and Colpidium striatum can; these may provide models to explore predator-prey dynamics.