Programmed cell death in trypanosomatids and other unicellular organisms

In multicellular organisms, cellular growth and development can be controlled by programmed cell death (PCD), which is defined by a sequence of regulated events. However, PCD is thought to have evolved not only to regulate growth and development in multicellular organisms but also to have a functional role in the biology of unicellular organisms. In protozoan parasites and in other unicellular organisms, features of PCD similar to those in multicellular organisms have been reported, suggesting some commonality in the PCD pathway between unicellular and multicellular organisms. However, more extensive studies are needed to fully characterise the PCD pathway and to define the factors that control PCD in the unicellular organisms. The understanding of the PCD pathway in unicellular organisms could delineate the evolutionary origin of this pathway. Further characterisation of the PCD pathway in the unicellular parasites could provide information regarding their pathogenesis, which could be exploited to target new drugs to limit their growth and treat the disease they cause.     

Apoptosis in a unicellular eukaryote (Trypanosoma cruzi): implications for the evolutionary origin and role of programmed cell death in the control of cell proliferation, differentiation and survival

The origin of programmed cell death (PCD) has been linked to the emergence of multicellular organisms. Trypanosoma cruzi, a member of one of the earliest diverging eukaryotes, is a protozoan unicellular parasite that undergoes three major differentiation changes and requires two different hosts. We report that the in vitro differentiation of the proliferating epimastigote stage into the G0/G1 arrested trypomastigote stage is associated with massive epimastigote death that shows the cytoplasmic and nuclear morphological features and DNA fragmentation pattern of apoptosis, the most frequent phenotype of PCD in multicellular organisms. Apoptosis could be accelerated or prevented by modifying culture conditions or cell density, indicating that extracellular signals influenced the epimastigote decision between life and death. Epimastigotes responded to complement-mediated immunological agression by undergoing apoptosis, while undergoing necrosis in response to nonphysiological saponin-mediated damage. PCD may participate into the optimal adaptation of T. cruzi to its different hosts, and the avoidance of a local competition between a G0/G1 arrested stage and its proliferating progenitor. The existence of a regulated cell death programme inducing an apoptotic phenotype in a unicellular eukaryote provides a paradigm for a widespread role for PCD in the control of cell survival, which extends beyond the evolutionary constraints that may be specific to multicellular organisms and raises the question of the origin and nature of the genes involved. Another implication is that PCD induction could represent a target for therapeutic strategies against unicellular pathogens.     

Autophagy in Dictyostelium: Genes and pathways,cell death and infection

The use of simple organisms to understand the molecular and cellular function of complex processes is instrumental for the rapid development of biomedical research. A remarkable example has been the discovery in S. cerevisiae of a group of proteins involved in the pathways of autophagy. Orthologues of these proteins have been identified in humans and experimental model organisms. Interestingly, some mammalian autophagy proteins do not seem to have homologues in yeast but are present in Dictyostelium, a social amoeba with two distinctive life styles, a unicellular stage in nutrient-rich conditions that differentiates upon starvation into a multicellular stage that depends on autophagy. This review focuses on the identification and annotation of the putative Dictyostelium autophagy genes and on the role of autophagy in development, cell death and infection by bacterial pathogens.     

Protozoan programmed cell death--insights from Blastocystis deathstyles

Programmed cell death (PCD) is an essential process in the growth and development of multicellular organisms. However, accumulating evidence indicates that unicellular eukaryotes can also undergo PCD with apoptosis-like features. The protozoan parasite Blastocystis hominis has been reported to exhibit both apoptotic and non-apoptotic features of PCD when exposed to a variety of stimuli. Recent observations of PCD pathways in Blastocystis suggest that this protozoan, as is the case with its multicellular counterparts, possesses complex cell-death mechanisms.     

On the transfer of fitness from the cell to the multicellular organism

The fitness of any evolutionary unit can be understood in terms of its two basic components: fecundity (reproduction) and viability (survival). Trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. We argue that these trade-offs gain special significance during the transition from unicellular to multicellular life. In particular, the evolution of germ–soma specialization and the emergence of individuality at the cell group (or organism) level are also consequences of trade-offs between the two basic fitness components, or so we argue using a multilevel selection approach. During the origin of multicellularity, we study how the group trade-offs between viability and fecundity are initially determined by the cell level trade-offs, but as the transition proceeds, the fitness trade-offs at the group level depart from those at the cell level. We predict that these trade-offs begin with concave curvature in single-celled organisms but become increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the cost of reproduction which increases as group size increases. We consider aspects of the biology of the volvocine green algae – which contain both unicellular and multicellular members – to illustrate the principles and conclusions discussed.     

Apoptosis pathways in fungal growth, development and ageing

Apoptosis is one type of programmed cell death with great importance for development and homeostasis of multicellular organisms. Unexpectedly, during the past decade, evidence has been obtained for the existence of a basal apoptosis machinery in yeast, as unicellular fungus, and in some filamentous fungi, a group of microorganisms that are neither true unicellular nor true multicellular biological systems but something in between. Here, we review evidence for a role of apoptotic processes in fungal pathogenicity, competitiveness, propagation, ageing and lifespan control.     

Which functional processes control the short-term effect of grazing on net primary production in grasslands?

For unicellular organisms, a lack of effects of local species richness on ecosystem function has been proposed due to their locally high species richness and their ubiquitous distribution. High dispersal ability and high individual numbers may enable unicellular taxa to occur everywhere. Using our own and published data sets on uni- and multicellular organisms, we conducted thorough statistical analyses to test whether (1) unicellular taxa show higher relative local species richness compared to multicellular taxa, (2) unicellular taxa show lower slopes of the species:area relationships and species:individuals relationships, and (3) the species composition of unicellular taxa is less influenced by geographic distance compared to multicellular taxa. We found higher local species richness compared to the global species pool for unicellular organisms than for metazoan taxa. The difference was significant if global species richness was conservatively estimated but not if extrapolated, and therefore higher richness estimates were used. Both microalgae and protozoans showed lower slopes between species richness and sample size (area or individuals) compared to macrozoobenthos, also indicating higher local species richness for unicellular taxa. The similarity of species composition of both benthic diatoms and ciliates decreased with increasing geographic distance. This indicated restricted dispersal ability of protists and the absence of ubiquity. However, a steeper slope between similarity and distance was found for polychaetes and corals, suggesting a stronger effect of distance on the dispersal of metazoans compared to unicellular taxa. In conclusion, we found partly different species richness patterns among uni- and multicellular eukaryotes, but no strict ubiquity of unicellular taxa. Therefore, the effect of local unicellular species richness on ecosystem function has to be reanalyzed. Macroecological patterns suggested for multicellular organisms may differ in unicellular communities.     

The evolutionary context of robust and redundant cell biological mechanisms

The robustness of biological processes to perturbations has so far been mainly explored in unicellular organisms; multicellular organisms have been studied for developmental processes or in the special case of redundancy between gene duplicates. Here we explore the robustness of cell biological mechanisms of multicellular organisms in an evolutionary context. We propose that the reuse of similar cell biological mechanisms in different cell types of the same organism has evolutionary implications: (1) the maintenance of apparently redundant mechanisms over evolutionary time may in part be explained by their differential requirement in various cell types; (2) the relative requirement for two alternative mechanisms may evolve among homologous cells in different organisms. We present examples of cell biological processes, such as centrosome separation in prophase, spindle formation or cleavage furrow positioning, that support the first proposition. We propose experimental tests of these hypotheses.     

Regional occupancy in unicellular eukaryotes: a reflection of niche breadth,habitat availability or size‐related dispersal capacity?

1. The distribution patterns of unicellular and multicellular organisms have recently been shown to differ profoundly, with the former probably being mostly cosmopolitan, whereas the latter are mostly restricted to certain regions. However, the within‐region distribution patterns of these two organism groups may be rather similar. 2. We predicted that the degree of regional occupancy in unicellular eukaryotes would be related to niche characteristics, dispersal ability and size, as has been found previously for multicellular organisms. The niche characteristics we considered were niche position, that measures marginality in species habitat distribution, and niche breadth, that measures amplitude in species habitat distribution. Niche characteristics were determined using Outlying Mean Index (OMI) analysis. 3. We found that the regional occupancy in our model group of unicellular eukaryotes, stream diatoms, was primarily a reflection of the niche position of a species or, more generally, habitat availability. Thus, non‐marginal species (i.e. species that occupied common habitat conditions across the region) tended to be more widely distributed than marginal species (i.e. species that were restricted to a limited range of rare habitat conditions). This finding was further supported by the general linear model, with niche position, niche breadth, maximum size and attachment mode as explanatory variables: niche position was by far the most important variable accounting for variability in regional occupancy, with significant amounts of additional variation related to niche breadth and maximum size of diatoms. 4. Thus, the degree of regional occupancy among unicellular eukaryotes may be primarily governed by habitat availability, supporting former findings for multicellular organisms.     

The second cell membrane: the basal lamina and the tissue cell theory of metazoa.

We suggest that the basal lamina is essentially a second plasma or cell membrane appearing at the next higher level of biological organization; that together with associated cell monolayers it creates a tissue level membrane which is used to form multicellular cells and that collections of these provide the essential structure of metazoa. Thus when the histological structure of multicellular organisms is viewed in a topologically simplified form such organisms appear to be sets of multicellular cells (m-cells) formed by a unit tissue membrane built around the basal lamina. Not only are m-cells in this way structurally isomorphous (homeomorphic) to unit or classical biological cells (u-cells) but the two cellular levels are also functionally isomorphous. This suggests a “General Principle of Hierarchical Isomorphism or Iteration”, i.e. that multicellular evolution recapitulates unicellular evolution. This principle of structural and functional isomorphic mappability of unicellular onto multicellular organisms then governs the organization of matter all the way from molecules to man. Just as cytoplasm precipitates the bimolecular plasma membrane to form u-cells for the purpose of achieving reaction sequestration, in turn, these u-cells precipitate a common basal lamina to form m-cells, the histologist's acini, to produce sequestered “tissue plasms”. Thus, the “generalized acinus” with its basal laminar complex seem to constitute a second level (multicellular) cell and cell membrane, respectively.Four operators, ultimately under genetic control, can generate both u and m-cells from planar configurations of their respective unit membranes therewith providing the essential structure of all cells, tissues, organs and organisms. These are the ply, permeability vector, topological and stratificational operators. They are collected into a set of “organ formulae”. Both the plasma membrane and the basal lamina act as covering membranes and, again, as membranes for subcells so that a complete multicellular organism is a tetrahierarchical cell in which the molecule is the element of the first two cellular domains and the cell is the element of the last two. The analysis identifies a new transport organ group which together with the classical endocrine and exocrine groups comprises nearly the whole of the soft tissue organs. In a major reduction, all these organs are continuously (topologically) transformable into each and into hollow spheres, cells or acini thus greatly simplifying the histology of metazoa. Given this emphasis on cellularization it would seem that life, i.e. the autonomous chemoservo, results from the cooperation of cellularization and replication operations on the catalyzation process. Through cellularization, the lipid bilayer and basal laminar membranes provide the essential catalytic reaction sequestration demanded by chemical reaction theory while through complementary base pairing the DNA double helix provides the essential memory which stores the patterns of the variations of the sequestered reactions.     

Positional information in cells and organisms

The processes of pattern formation are usually considered to be quite different in unicellular and multicellular organisms. The only unifying ideas have been very general, such as those concerning regional differences and organization along a polar axis. Concepts like induction, fields and gradients have generally been applied only to the development of multicellular organisms. Here, Joseph Frankel suggests that pattern formation by multicellular organisms evolved in their progenitors in response to multiplication of cytoplasmic structural units rather than of nuclei. Ciliates provide a living example of complex patterning in a compound uninucleate organism.     

The evolution of cell death programs as prerequisites of multicellularity

One of the hallmarks of multicellularity is that the individual cellular fate is sacrificed for the benefit of a higher order of life-the organism. The accidental death of cells in a multicellular organism results in swelling and membrane-rupture and inevitably spills cell contents into the surrounding tissue with deleterious effects for the organism. To avoid this form of necrotic death the cells of metazoans have developed complex self-destruction mechanisms, collectively called programmed cell death, which see to an orderly removal of superfluous cells. Since evolution never invents new genes but plays variations on old themes by DNA mutations, it is not surprising, that some of the genes involved in metazoan death pathways apparently have evolved from homologues in unicellular organisms, where they originally had different functions. Interestingly some unicellular protozoans have developed a primitive form of non-necrotic cell death themselves, which could mean that the idea of an altruistic death for the benefit of genetically identical cells predated the invention of multicellularity. The cell death pathways of protozoans, however, show no homology to those in metazoans, where several death pathways seem to have evolved in parallel. Mitochondria stands at the beginning of several death pathways and also determines, whether a cell has sufficient energy to complete a death program. However, the endosymbiotic bacterial ancestors of mitochondria are unlikely to have contributed to the recent mitochondrial death machinery and therefore, these components may derive from mutated eukaryotic precursors and might have invaded the respective mitochondrial compartments. Although there is no direct evidence, it seems that the prokaryotic-eukaryotic symbiosis created the space necessary for sophisticated death mechanisms on command, which in their distinct forms are major factors for the evolution of multicellular organisms.     

Does Formation of Multicellular Colonies by Choanoflagellates Affect Their Susceptibility to Capture by Passive Protozoan Predators?

Microbial eukaryotes, critical links in aquatic food webs, are unicellular, but some, such as choanoflagellates, form multicellular colonies. Are there consequences to predator avoidance of being unicellular vs. forming larger colonies? Choanoflagellates share a common ancestor with animals and are used as model organisms to study the evolution of multicellularity. Escape in size from protozoan predators is suggested as a selective factor favoring evolution of multicellularity. Heterotrophic protozoans are categorized as suspension feeders, motile raptors, or passive predators that eat swimming prey which bump into them. We focused on passive predation and measured the mechanisms responsible for the susceptibility of unicellular vs. multicellular choanoflagellates, Salpingoeca helianthica, to capture by passive heliozoan predators, Actinosphaerium nucleofilum, which trap prey on axopodia radiating from the cell body. Microvideography showed that unicellular and colonial choanoflagellates entered the predator's capture zone at similar frequencies, but a greater proportion of colonies contacted axopodia. However, more colonies than single cells were lost during transport by axopodia to the cell body. Thus, feeding efficiency (proportion of prey entering the capture zone that were engulfed in phagosomes) was the same for unicellular and multicellular prey, suggesting that colony formation is not an effective defense against such passive predators.     

The origin of Metazoa and Weismann's germ line theory

Weismann's theory asserts that the continuity of germ cells throughout the entire life cycle is insured by the protection of the somatic cells which represent a distinct lineage. On the basis of embryological data and the organization pattern displayed by some highly differentiated unicellular organisms, it is postulated that the soma of multicellular animals has probably arisen as an accessory structure for an initially unicellular system.     

ON THE PARADIGM OF ALTRUISTIC SUICIDE IN THE UNICELLULAR WORLD

Altruistic suicide is best known in the context of programmed cell death (PCD) in multicellular individuals, which is understood as an adaptive process that contributes to the development and functionality of the organism. After the realization that PCD‐like processes can also be induced in single‐celled lineages, the paradigm of altruistic cell death has been extended to include these active cell death processes in unicellular organisms. Here, we critically evaluate the current conceptual framework and the experimental data used to support the notion of altruistic suicide in unicellular lineages, and propose new perspectives. We argue that importing the paradigm of altruistic cell death from multicellular organisms to explain active death in unicellular lineages has the potential to limit the types of questions we ask, thus biasing our understanding of the nature, origin, and maintenance of this trait. We also emphasize the need to distinguish between the benefits and the adaptive role of a trait. Lastly, we provide an alternative framework that allows for the possibility that active death in single‐celled organisms is a maladaptive trait maintained as a byproduct of selection on pro‐survival functions, but that could—under conditions in which kin/group selection can act—be co‐opted into an altruistic trait.     

Life-history evolution and the origin of multicellularity

The fitness of an evolutionary individual can be understood in terms of its two basic components: survival and reproduction. As embodied in current theory, trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. Here, we argue that the evolution of germ-soma specialization and the emergence of individuality at a new higher level during the transition from unicellular to multicellular organisms are also consequences of trade-offs between the two components of fitness-survival and reproduction. The models presented here explore fitness trade-offs at both the cell and group levels during the unicellular-multicellular transition. When the two components of fitness negatively covary at the lower level there is an enhanced fitness at the group level equal to the covariance of components at the lower level. We show that the group fitness trade-offs are initially determined by the cell level trade-offs. However, as the transition proceeds to multicellularity, the group level trade-offs depart from the cell level ones, because certain fitness advantages of cell specialization may be realized only by the group. The curvature of the trade-off between fitness components is a basic issue in life-history theory and we predict that this curvature is concave in single-celled organisms but becomes increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the initial cost of reproduction to survival which increases as group size increases. To illustrate the principles and conclusions of the model, we consider aspects of the biology of the volvocine green algae, which contain both unicellular and multicellular members.     

Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells.

Apoptosis as a form of programmed cell death (PCD) in multicellular organisms is a well-established genetically controlled process that leads to elimination of unnecessary or damaged cells. Recently, PCD has also been described for unicellular organisms as a process for the socially advantageous regulation of cell survival. The human Bcl-2 family member Bak induces apoptosis in mammalian cells which is counteracted by the Bcl-x(L) protein. We show that Bak also kills the unicellular fission yeast Schizosaccharomyces pombe and that this is inhibited by coexpression of human Bcl-x(L). Moreover, the same critical BH3 domain of Bak that is required for induction of apoptosis in mammalian cells is also required for inducing death in yeast. This suggests that Bak kills mammalian and yeast cells by similar mechanisms. The phenotype of the Bak-induced death in yeast involves condensation and fragmentation of the chromatin as well as dissolution of the nuclear envelope, all of which are features of mammalian apoptosis. These data suggest that the evolutionarily conserved metazoan PCD pathway is also present in unicellular yeast.     

Fundamental taboos of biology

This paper formulates some taboos relating to living systems and cognition of these systems: in nature, there exist no two identical living complex multicellular organisms; there is no way to create an exact copy of a multicellular organism; there is no way to obtain two identical clones of a unicellular organism if they contain a sufficiently large number of cells; based on comparing present-day organisms, it is impossible to restore the structure of the first living cell and the processes that have led to its emergence; it is impossible to create a living cell from its separate simple constituents; the mechanisms determining cell vitality are essentially incognizable.     

Mitochondrial DNA,chloroplast DNA and the origins of development in eukaryotic organisms

Background  

Several proposals have been made to explain the rise of multicellular life forms. An internal environment can be created and controlled, germ cells can be protected in novel structures, and increased organismal size allows a "division of labor" among cell types. These proposals describe advantages of multicellular versus unicellular organisms at levels of organization at or above the individual cell. I focus on a subsequent phase of evolution, when multicellular organisms initiated the process of development that later became the more complex embryonic development found in animals and plants. The advantage here is realized at the level of the mitochondrion and chloroplast.