Differentiation of a free-living alga into forms with ecto- and endosymbiotic associations with heterotrophic organisms in a 5-year microcosm culture

Group-selection via aggregative propagule-formation enables cooperative multicellularity in an individual based, spatial model

The emergence of multicellularity is one of the major transitions in evolution that happened multiple times independently. During aggregative multicellularity, genetically potentially unrelated lineages cooperate to form transient multicellular groups. Unlike clonal multicellularity, aggregative multicellular organisms do not rely on kin selection instead other mechanisms maintain cooperation against cheater phenotypes that benefit from cooperators but do not contribute to groups. Spatiality with limited diffusion can facilitate group selection, as interactions among individuals are restricted to local neighbourhoods only. Selection for larger size (e.g. avoiding predation) may facilitate the emergence of aggregation, though it is unknown, whether and how much role such selection played during the evolution of aggregative multicellularity. We have investigated the effect of spatiality and the necessity of predation on the stability of aggregative multicellularity via individual-based modelling on the ecological timescale. We have examined whether aggregation facilitates the survival of cooperators in a temporally heterogeneous environment against cheaters, where only a subset of the population is allowed to periodically colonize a new, resource-rich habitat. Cooperators constitutively produce adhesive molecules to promote aggregation and propagule-formation while cheaters spare this expense to grow faster but cannot aggregate on their own, hence depending on cooperators for long-term survival. We have compared different population-level reproduction modes with and without individual selection (predation) to evaluate the different hypotheses. In a temporally homogeneous environment without propagule-based colonization, cheaters always win. Predation can benefit cooperators, but it is not enough to maintain the necessary cooperator amount in successive dispersals, either randomly or by fragmentation. Aggregation-based propagation however can ensure the adequate ratio of cooperators-to-cheaters in the propagule and is sufficient to do so even without predation. Spatiality combined with temporal heterogeneity helps cooperators via group selection, thus facilitating aggregative multicellularity. External stress selecting for larger size (e.g. predation) may facilitate aggregation, however, according to our results, it is neither necessary nor sufficient for aggregative multicellularity to be maintained when there is effective group-selection.

Biodiversity and Constrained Information Dynamics in Ecosystems: A Framework for Living Systems

The increase in ecosystem biodiversity can be perceived as one of the universal processes converting energy into information across a wide range of living systems. This study delves into the dynamics of living systems, highlighting the distinction between ex post adaptation, typically associated with natural selection, and its proactive counterpart, ex ante adaptability. Through coalescence experiments using synthetic ecosystems, we (i) quantified ecosystem stability, (ii) identified correlations between some biodiversity indexes and the stability, (iii) proposed a mechanism for increasing biodiversity through moderate inter-ecosystem interactions, and (iv) inferred that the information carrier of ecosystems is species composition, or merged genomic information. Additionally, it was suggested that (v) changes in ecosystems are constrained to a low-dimensional state space, with three distinct alteration trajectories-fluctuations, rapid environmental responses, and long-term changes-converging into this state space in common. These findings suggest that daily fluctuations may predict broader ecosystem changes. Our experimental insights, coupled with an exploration of living systems' information dynamics from an ecosystem perspective, enhance our predictive capabilities for natural ecosystem behavior, providing a universal framework for understanding a broad spectrum of living systems.

Synthetic symbiosis between a cyanobacterium and a ciliate toward novel chloroplast-like endosymbiosis

Chloroplasts are thought to have co-evolved through endosymbiosis, after a cyanobacterial-like prokaryote was engulfed by a eukaryotic cell; however, it is impossible to observe the process toward chloroplasts. In this study, we constructed an experimental symbiosis model to observe the initial stage in the process from independent organisms to a chloroplast-like organelle. Our system of synthetic symbiosis is capable of long-term coculture of two model organisms: a cyanobacterium (Synechocystis sp. PCC6803) as a symbiont and a ciliate (Tetrahymena thermophila) as a host with endocytic ability. The experimental system was clearly defined, because we used a synthetic medium and the cultures were shaken to avoid spatial complexity. We determined the experimental conditions for sustainable coculture, by analyzing population dynamics using a mathematical model. We experimentally demonstrated that the coculture was sustainable for at least 100 generations, through serial transfers. Moreover, we found that cells isolated after the serial transfer improved the probability of coexistence of both species without extinction in re-coculture. The constructed system will be useful for understanding the initial stage of primary endosymbiosis from cyanobacteria to chloroplasts, i.e., the origin of algae and plants.

Symbiogenesis is driven through hierarchical reorganization of an ecosystem under closed or semi-closed conditions

Ecosystems generate selective environments and function as sources of various metabolic systems for symbiogenesis. In this study, we have explored how symbiogenesis occurs in the living world, from a holistic perspective, by observing a long-term experimental culture of an ecosystem model (CET microcosm) and using related findings in laboratory and field studies of endosymbiosis between auto- (photo-) and heterotrophic organisms. The results obtained suggest that symbiogenesis can occur in the mature stages of semi-closed ecosystems and lead to a new ecosystem-oriented perspective of symbiogenesis. Symbiogenesis is an aspect of ecosystem evolution in which whole ecosystem dynamics generate selective conditions operating on the component species, favoring symbiotic associations among some of them. The development of symbiotic associations then modifies the organization and material/energy flow structure of the ecosystem, which, in turn, modifies their selective environments.

Endosymbiosis before eukaryotes: mitochondrial establishment in protoeukaryotes

Endosymbiosis and organellogenesis are virtually unknown among prokaryotes. The single presumed example is the endosymbiogenetic origin of mitochondria, which is hidden behind the event horizon of the last eukaryotic common ancestor. While eukaryotes are monophyletic, it is unlikely that during billions of years, there were no other prokaryote-prokaryote endosymbioses as symbiosis is extremely common among prokaryotes, e.g., in biofilms. Therefore, it is even more precarious to draw conclusions about potentially existing (or once existing) prokaryotic endosymbioses based on a single example. It is yet unknown if the bacterial endosymbiont was captured by a prokaryote or by a (proto-)eukaryote, and if the process of internalization was parasitic infection, slow engulfment, or phagocytosis. In this review, we accordingly explore multiple mechanisms and processes that could drive the evolution of unicellular microbial symbioses with a special attention to prokaryote-prokaryote interactions and to the mitochondrion, possibly the single prokaryotic endosymbiosis that turned out to be a major evolutionary transition. We investigate the ecology and evolutionary stability of inter-species microbial interactions based on dependence, physical proximity, cost-benefit budget, and the types of benefits, investments, and controls. We identify challenges that had to be conquered for the mitochondrial host to establish a stable eukaryotic lineage. Any assumption about the initial interaction of the mitochondrial ancestor and its contemporary host based solely on their modern relationship is rather perilous. As a result, we warn against assuming an initial mutually beneficial interaction based on modern mitochondria-host cooperation. This assumption is twice fallacious: (i) endosymbioses are known to evolve from exploitative interactions and (ii) cooperativity does not necessarily lead to stable mutualism. We point out that the lack of evidence so far on the evolution of endosymbiosis from mutual syntrophy supports the idea that mitochondria emerged from an exploitative (parasitic or phagotrophic) interaction rather than from syntrophy.

The role of exploitation in the establishment of mutualistic microbial symbioses

Evolutionary theory suggests that the conditions required for the establishment of mutualistic symbioses through mutualism alone are highly restrictive, often requiring the evolution of complex stabilising mechanisms. Exploitation, whereby initially the host benefits at the expense of its symbiotic partner and mutual benefits evolve subsequently through trade-offs, offers an arguably simpler route to the establishment of mutualistic symbiosis. In this review, we discuss the theoretical and experimental evidence supporting a role for host exploitation in the establishment and evolution of mutualistic microbial symbioses, including data from both extant and experimentally evolved symbioses. We conclude that exploitation rather than mutualism may often explain the origin of mutualistic microbial symbioses.

How did cyanobacteria first embark on the path to becoming plastids?: lessons from protist symbioses

Symbioses between phototrophs and heterotrophs (a.k.a 'photosymbioses') are extremely common, and range from loose and temporary associations to obligate and highly specialized forms. In the history of life, the most transformative was the 'primary endosymbiosis,' wherein a cyanobacterium was engulfed by a eukaryote and became genetically integrated as a heritable photosynthetic organelle, or plastid. By allowing the rise of algae and plants, this event dramatically altered the biosphere, but its remote origin over one billion years ago has obscured the sequence of events leading to its establishment. Here, we review the genetic, physiological and developmental hurdles involved in early primary endosymbiosis. Since we cannot travel back in time to witness these evolutionary junctures, we will draw on examples of unicellular eukaryotes (protists) spanning diverse modes of photosymbiosis. We also review experimental approaches that could be used to recreate aspects of early primary endosymbiosis on a human timescale.

Experimental evolution in photoautotrophic microorganisms as a means of enhancing chloroplast functions

The term 'experimental evolution' refers to short-term evolutionary experiments with microorganisms under controlled conditions in which selection is expected to occur. In combination with whole-genome sequencing and genetic engineering, the method has become a powerful tool to study evolutionary mechanisms and engineer new microbial variants. It has been most extensively used in the model species Escherichia coli and Saccharomyces cerevisiae, but more recently photosynthetic microorganisms have been subjected to experimental evolution. In such assays, strains were generated that had become more tolerant to certain abiotic environmental factors or evolved new traits during co-propagation with other organisms. These strains were viable under conditions that were lethal to the non-adapted progenitor and in a few cases, the causative mutations were identified. Because cyanobacteria like Synechocystis or green algae like Chlamydomonas reinhardtii share many features with crop plants - which are not amenable to such experiments - experimental evolution with photosynthetic microorganisms has the potential to identify novel targets for improving the capacity of plants to acclimate to environmental change. Here, I provide a survey of the experiments performed so far in cyanobacteria and green algae, focusing on Synechocystis and C. reinhardtii, and discuss the promise and the challenges of such approaches.

Evolution on the bright side of life: microorganisms and the evolution of mutualism

Mutualistic interactions, where two interacting species have a net beneficial effect on each other's fitness, play a crucial role in the survival and evolution of many species. Despite substantial empirical and theoretical work in past decades, the impact of these interactions on natural selection is not fully understood. In addition, mutualisms between microorganisms have been largely ignored, even though they are ecologically important and can be used as tools to bridge the gap between theory and empirical work. Here, I describe two problems with our current understanding of natural selection in mutualism and highlight the properties of microbial mutualisms that could help solve them. One problem is that bias and methodological problems have limited our understanding of the variety of mechanisms by which species may adapt to mutualism. Another problem is that it is rare for experiments testing coevolution in mutualism to address whether each species has adapted to evolutionary changes in its partner. These problems can be addressed with genome resequencing and time-shift experiments, techniques that are easier to perform in microorganisms. In addition, microbial mutualisms may inspire novel insights and hypotheses about natural selection in mutualism.