Intercellular structure in a many-celled magnetotactic prokaryote

A many-called magnetotactic prokaryote obtained from brackish water was observed to possess intercellular connections at points of contact between the outer membranes of constituent cells. Each aggregate organism consisted of 10 to 30 individual Gram-negative cells containing material with the appearance of poly--hydroxybutyrate and magnetosomes of unusual arrangement, structure and composition. The aggregate, which possessed prokaryotic-type flagella arranged at the outwards surfaces of each cell, showed motility indicative of co-ordination between individual component cells. These results suggest that this organism could be a multicellular prokaryote.     

The evolutionary path to terminal differentiation and division of labor in cyanobacteria

A common trait often associated with multicellularity is cellular differentiation, which is a spatial separation of tasks through the division of labor. In principle, the division of labor does not necessarily have to be constrained to a multicellular setting. In this study, we focus on the possible evolutionary paths leading to terminal differentiation in cyanobacteria. We develop mathematical models for two developmental strategies. First, of populations of terminally differentiated single cells surviving by the exchange of common goods. Second, of populations exhibiting terminal differentiation in a multicellular setting. After testing the two strategies against the effect of disruptive mutations (i.e. “cheater” mutants), we assess the effects of selection on the optimization of the ratio of vegetative (carbon fixing) to heterocystous (nitrogen fixing) cells, which in turn leads to the maximization of the carrying capacity for the population density. In addition, we compare the performance of differentiated populations to undifferentiated ones that temporally separate tasks in accordance to a day/night cycle. We then compare some predictions of our model with phylogenetic relationships derived from analyzing 16S rRNA sequences of different cyanobacterial strains. In line with studies indicating that group or spatial structure are ways to evolve cooperation and protect against the spread of cheaters, our work shows that compartmentalization afforded by multicellularity is required to maintain the vegetative/heterocyst division in cyanobacteria. We find that multicellularity allows for selection to optimize the carrying capacity. These results and the phylogenetic analysis indicates that terminally differentiated cyanobacteria evolved after undifferentiated species. In addition, we show that, in regimes of short daylight periods, terminally differentiated species perform worse than undifferentiated species that follow the day/night cycle; indicating that undifferentiated species have an evolutionary advantage in regimes of short daylight periods.     

The evolution of multicellularity in animals as a shift in biological autonomy

The hypothesis is advanced that major evolutionary innovations are characterized by an increase of organismal autonomy in the sense of an emancipation from the environment. After a brief overview of the literature on this concept, increasing autonomy is defined as the evolutionary shift in the individual system-environment relationship, so that the direct influences of the environment are gradually reduced and a stabilization of self-referential, intrinsic functions within the system is generated. This is described as relative autonomy because numerous interconnections with the environment and dependencies upon it are retained. Elements of an increasing autonomy are spatial separations, an increase in homeostatic functions, internalizations and an increase in physiological and behavioral flexibility. These elements are described by taking the transition from single cells to metazoans as a case study. The principle of increasing autonomy is of central relevance for understanding this transition. The hypothesis does not contradict the principle of adaptation, but rather contributes to a further understanding of its elements as it supplies aspects for a reconsideration of the relationship between the outside and the inside, between organism and environment.     

Multiscale heterogeneity in filamentous microbes

Microbial cells within clonal populations can display different morphologies or carry out different tasks. This heterogeneity is beneficial at the population level and allows microbes to spread risk or separate incompatible activities. Heterogeneity is also evident in filamentous bacteria and fungi, which form mycelial networks consisting of interconnected hyphae. Here, heterogeneity is observed between clonal mycelial particles, between different zones of colonies, between adjacent hyphae and even between adjacent compartments of individual hyphae. In this review, we compare this multiscale heterogeneity in filamentous bacteria and fungi and discuss the underlying mechanisms. These mechanisms might provide targets to improve the exploitability of these organisms as cell factories in the biotech sector.     

Fungi took a unique evolutionary route to multicellularity: Seven key challenges for fungal multicellular life

The evolution of multicellularity has been one of the major transitions in the history of life. In contrast to animals and plants, how multicellularity evolved in fungi and how it compares to the general principles distilled from the study of more widely studied model systems, has received little attention. This review broadly discusses multicellular functioning and evolution in fungi. We focus on how fungi solved some of the common challenges associated with the evolution of multi-celled organisms and what unique challenges follow from the peculiar, filamentous growth form of fungi. We identify and discuss seven key challenges for fungal multicellular growth: apical growth, compartmentalization, long-distance mass transport, controlling mutational load, cell-to-cell communication, differentiation and adhesion. Some of these are characteristic of all multicellular transitions, whereas others are unique to fungi. We hope this review will facilitate the interpretation of fungal multicellularity in comparison with that of other multicellular lineages and will prompt further research into how fungi solved fundamental challenges in one of the major transitions in their evolutionary history.     

The phylogenetic analysis of tetraspanins projects the evolution of cell-cell interactions from unicellular to multicellular organisms

In animals, the tetraspanins are a large superfamily of membrane proteins that play important roles in organizing various cell-cell and matrix-cell interactions and signal pathways based on such interactions. However, their origin and evolution largely remain elusive and most of the family's members are functionally unknown or less known due to difficulties of study, such as functional redundancy. In this study, we rebuilt the family's phylogeny with sequences retrieved from online databases and our cDNA library of amphioxus. We reveal that, in addition to in metazoans, various tetraspanins are extensively expressed in protozoan amoebae, fungi, and plants. We also discuss the structural evolution of tetraspanin's major extracellular domain and the relation between tetraspanin's duplication and functional redundancy. Finally, we elucidate the coevolution of tetraspanins and eukaryotes and suggest that tetraspanins play important roles in the unicell-to-multicell transition. In short, the study of tetraspanin in a phylogenetic context helps us understand the evolution of intercellular interactions.     

Size doesn’t matter: towards a more inclusive philosophy of biology

Philosophers of biology, along with everyone else, generally perceive life to fall into two broad categories, the microbes and macrobes, and then pay most of their attention to the latter. ‘Macrobe’ is the word we propose for larger life forms, and we use it as part of an argument for microbial equality. We suggest that taking more notice of microbes – the dominant life form on the planet, both now and throughout evolutionary history – will transform some of the philosophy of biology’s standard ideas on ontology, evolution, taxonomy and biodiversity. We set out a number of recent developments in microbiology – including biofilm formation, chemotaxis, quorum sensing and gene transfer – that highlight microbial capacities for cooperation and communication and break down conventional thinking that microbes are solely or primarily single-celled organisms. These insights also bring new perspectives to the levels of selection debate, as well as to discussions of the evolution and nature of multicellularity, and to neo-Darwinian understandings of evolutionary mechanisms. We show how these revisions lead to further complications for microbial classification and the philosophies of systematics and biodiversity. Incorporating microbial insights into the philosophy of biology will challenge many of its assumptions, but also give greater scope and depth to its investigations.     

A New Aspect to the Origin and Evolution of Eukaryotes

One of the most important omissions in recent evolutionary theory concerns how eukaryotes could emerge and evolve. According to the currently accepted views, the first eukaryotic cell possessed a nucleus, an endomembrane system, and a cytoskeleton but had an inefficient prokaryotic-like metabolism. In contrast, one of the most ancient eukaryotes, the metamonada Giardia lamblia, was found to have formerly possessed mitochondria. In sharp contrast with the traditional views, this paper suggests, based on the energetic aspect of genome organization, that the emergence of eukaryotes was promoted by the establishment of an efficient energy-converting organelle, such as the mitochondrion. Mitochondria were acquired by the endosymbiosis of ancient α-purple photosynthetic Gram-negative eubacteria that reorganized the prokaryotic metabolism of the archaebacterial-like ancestral host cells. The presence of an ATP pool in the cytoplasm provided by this cell organelle allowed a major increase in genome size. This evolutionary change, the remarkable increase both in genome size and complexity, explains the origin of the eukaryotic cell itself. The loss of cell wall and the appearance of multicellularity can also be explained by the acquisition of mitochondria. All bacteria use chemiosmotic mechanisms to harness energy; therefore the periplasm bounded by the cell wall is an essential part of prokaryotic cells. Following the establishment of mitochondria, the original plasma membrane-bound metabolism of prokaryotes, as well as the funcion of the periplasm providing a compartment for the formation of different ion gradients, has been transferred into the inner mitochondrial membrane and intermembrane space. After the loss of the essential function of periplasm, the bacterial cell wall could also be lost, which enabled the naked cells to establish direct connections among themselves. The relatively late emergence of mitochondria may be the reason why multicellularity evolved so slowly. Received: 29 May 1997 / Accepted: 9 October 1997     

Metagenomics and biological ontology

Metagenomics is an emerging microbial systems science that is based on the large-scale analysis of the DNA of microbial communities in their natural environments. Studies of metagenomes are revealing the vast scope of biodiversity in a wide range of environments, as well as new functional capacities of individual cells and communities, and the complex evolutionary relationships between them. Our examination of this science focuses on the ontological implications of these studies of metagenomes and metaorganisms, and what they mean for common sense and philosophical understandings of multicellularity, individuality and organism. We show how metagenomics requires us to think in different ways about what human beings are and what their relation to the microbial world is. Metagenomics could also transform the way in which evolutionary processes are understood, with the most basic relationship between cells from both similar and different organisms being far more cooperative and less antagonistic than is widely assumed. In addition to raising fundamental questions about biological ontology, metagenomics generates possibilities for powerful technologies addressed to issues of climate, health and conservation. We conclude with reflections about process-oriented versus entity-oriented analysis in light of current trends towards systems approaches.