Division of labour and the evolution of multicellularity

Understanding the emergence and evolution of multicellularity and cellular differentiation is a core problem in biology. We develop a quantitative model that shows that a multicellular form emerges from genetically identical unicellular ancestors when the compartmentalization of poorly compatible physiological processes into component cells of an aggregate produces a fitness advantage. This division of labour between the cells in the aggregate occurs spontaneously at the regulatory level owing to mechanisms present in unicellular ancestors and does not require any genetic predisposition for a particular role in the aggregate or any orchestrated cooperative behaviour of aggregate cells. Mathematically, aggregation implies an increase in the dimensionality of phenotype space that generates a fitness landscape with new fitness maxima, in which the unicellular states of optimized metabolism become fitness saddle points. Evolution of multicellularity is modelled as evolution of a hereditary parameter: the propensity of cells to stick together, which determines the fraction of time a cell spends in the aggregate form. Stickiness can increase evolutionarily owing to the fitness advantage generated by the division of labour between cells in an aggregate.     

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.     

Apoptosis in yeast--a monocellular organism exhibits altruistic behaviour

Apoptosis is a highly regulated form of programmed cell death crucial for life and health in metazoan animals. Apoptosis is defined by a set of cytological alterations. The recent discovery of these markers in yeast indicates the presence of the basic mechanisms of apoptosis already in unicellular eukaryotes. Oxygen radicals regulate both mammalian and yeast apoptosis. We suggest that apoptosis originated in unicellular organisms as an altruistic response to severe oxidative damage. Later, cells developed mechanisms to purposely produce reactive oxygen species as a regulator of apoptosis. Yeast may become an important model to investigate the conserved steps of apoptosis.     

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.     

Establishment of a pure culture of the hitherto uncultured unicellular cyanobacterium Aphanothece sacrum, and phylogenetic position of the organism

Aphanothece sacrum, an edible freshwater unicellular cyanobacterium, was isolated by using novel synthetic media (designated AST and AST-5xNP). The media were designed on the basis of the ratio of inorganic elements contained in A. sacrum cells cultured in a natural pond. The isolated strain exhibits unicellular rod-shaped cells approximately 6 microm in length that are scattered in an exopolysaccharide matrix, a feature similar to that of natural A. sacrum. DNA analysis of the isolated strain revealed that it carried two ferredoxin genes whose deduced amino acid sequences were almost identical to previously published sequences of ferredoxins from natural A. sacrum. Analysis of the 16S rRNA gene and ferredoxin genes revealed that A. sacrum occupies a phylogenetically unique position among the cyanobacteria.     

Relationship between unicellular cyanobacteria established by indirect immunofluorescence of intact cells

Antisera prepared against intact, viable cells were used to show the applicability of a serological approach to detect relationships between unicellular cyanobacteria. Antisera were raised against eight unicellular cyanobacteria and two chlorophycean unicellular organisms. The staining reactivity of each antiserum was tested by the fixed indirect immunofluorescence assay against the different organisms, and each organism was tested for its reactivity with all of the different antisera. Absorption of antisera with the appropriate heterologous antigens was used to further characterize the relationship betweenAnacystis nidulans andSynechococcus cedrorum, and also the relationship between two subcultures of an isolate distinguished by morphological features. Absorption of antiserum was also used for the removal of antibodies to contaminating bacteria. The approaches used are suggested as a useful tool for determining relationships between unicellular cyanobacteria.     

Histological study of the teleost liver. III. The system of biliary pathways]

In the liver of Haplochromis burtoni four divisions of the intrahepatic biliary pathways can be distinguished: canaliculus, canaliculus ductulus transition, ductulus biliferus, ductus biliferus. There are no transitional stages between the hepatocytes and biliary epithelial cells. The results of the investigation suggest that the ductules open into the ducts in two ways, directly and by graded transition of structure. The first part of the ductulus has a very narrow lumen and therefore is similar to the "Schaltstück" of salivary glands. Terminal ductulus cells can protrude into the canaliculus (= lumen of the liver tubulus), giving the appearance of centrotubular cells in cross-section. In the liver of most of the teleosts investigated here the canaliculi are intercellular. Tangentially cut diverticuli of the canaliculi appear to be unicellular ("intracellular") canaliculi, but are, however, merely unicellular ("intracellular") protrusions of the intercellular canaliculi. On the other hand the three cyprinid species (Barbus tetrazona, Idus idus, Carassius auratus) possess true unicellular ("intracellular") canaliculi. These can definitely be distinguished from the diverticuli of intercellular canaliculi by several criteria. Phylogenetically the unicellular ("intracellular") canaliculus must be regarded as derivative, and the intercellular canaliculus as the original form. The morphology of the wall of the gall-bladder of Haplochromis burtoni indicates that transfer of substances is increased together with transportation and drainage of fluids and variations in volume. The ultrastructure of the epithelial cells reveals a zonal arrangement.     

Comparative studies on physiological responses to phosphorus in two phenotypes of bloom-forming <Emphasis Type="Italic">Microcystis</Emphasis>

Toxic Microcystis blooms frequently occur in eutrophic water bodies and exist in the form of colonial and unicellular cells. In order to understand the mechanism of Microcystis dominance in freshwater bodies, the physiological and biochemical responses of unicellular (4 strains) and colonial (4 strains) Microcystis strains to phosphorus (P) were comparatively studied. The two phenotype strains exhibit physiological differences mainly in terms of their response to low P concentrations. The growth of four unicellular and one small colonial Microcystis strain was significantly inhibited at a P concentration of 0.2 mg l⁻¹; however, that of the large colonial Microcystis strains was not inhibited. The results of phosphate uptake experiments conducted using P-starved cells indicated that the colonial strains had a higher affinity for low levels of P. The unicellular strains consumed more P than the colonial strains. Alkaline phosphatase activity in the unicellular strains was significantly induced by low P concentrations. Under P-limited conditions, the oxygen evolution rate, F _v/F _m, and ETR _max were lower in unicellular strains than in colonial strains. These findings may shed light on the mechanism by which colonial Microcystis strains have an advantage with regard to dominance and persistence in fluctuating P conditions. Handling editor: L. Naselli-Flores     

Comparative analysis of ultrastructure of glandular trichomes in two Nepeta cataria chemotypes (N. cataria and N. catena var. citriodora)

Glandular trichomes from the leaf surface of Nepeta cataria and N. cataria vai.citriodora have been examined using transmission and scanning electron microscopy. Peltate glands and capitate hairs type I were found on leaves of N. cataria. Both types had single stalk cells. Leaves of N. cataria var. citriodora bore peltate glands with unicellular or bicellular stalk, capitate hairs type I (with unicellular stalk) and capitate hairs type II (with unicellular or bicellular stalk). Peltate glands of N. cataria and of N. cataria var. citriodora were characterized by numerous leucoplasts sheathed by smooth reticular tubules and smooth endoplasmic reticulum; they are proposed to synthesize terpenes. The secretory cells of capitate hairs type I of N. cataria and those of N. cataria var. citriodora had well developed rough endoplasmic reticulum and dictyosomes. They had plastids with protein inclusions. These glands are supposed to produce slime. Capitate hairs type II of N. cataria var. citriodora had no analogs in N. cataria. Their secretory cells exhibited abundance of tubular endoplasmic reticulum and had unsheathed plastids with starch grains. Probably, these glands synthesize terpenes. The results of the study indicate that there is an obvious difference both in morphology and in ultrastructure of glandular trichomes in different chemotypes of N. cataria.     

Induction of steroid binding sites (receptors) and presence of steroid hormones in the unicellular Tetrahymena pyriformis

The unicellular Tetrahymena does not normally possess a steroid hormone (dehydroepiandrosterone, DHEA) or a glucocorticoid (dexamethasone) receptor, but both kinds of receptor can be induced in it by pretreatment (imprinting) with the adequate hormone. The specific receptors which arise are demonstrable experimentally. Examination of Tetrahymena cells for endogenous steroids by the radioimmunoassay (RIA) technique detected an appreciable concentration of DHEA and DHEA sulphate, and lesser concentrations of testosterone and estradiol in this unicellular organism.     

Diversity and biomass dynamics of unicellular marine fungi during a spring phytoplankton bloom

Microbial communities have important functions during spring phytoplankton blooms, regulating bloom dynamics and processing organic matter. Despite extensive research into such processes, an in-depth assessment of the fungal component is missing, especially for the smaller size fractions. We investigated the dynamics of unicellular mycoplankton during a spring phytoplankton bloom in the North Sea by 18S rRNA gene tag sequencing and a modified CARD-FISH protocol. Visualization and enumeration of dominant taxa revealed unique cell count patterns that varied considerably over short time scales. The Rozellomycota sensu lato (s.l.) reached a maximum of 10⁵ cells L⁻¹, being comparable to freshwater counts. The abundance of Dikarya surpassed previous values by two orders of magnitude (10⁵ cells L⁻¹) and the corresponding biomass (maximum of 8.9 mg C m⁻³) was comparable to one reported for filamentous fungi with assigned ecological importance. Our results show that unicellular fungi are an abundant and, based on high cellular ribosome content and fast dynamics, active part of coastal microbial communities. The known ecology of the visualized taxa and the observed dynamics suggest the existence of different ecological niches that link primary and secondary food chains, highlighting the importance of unicellular fungi in food web structures and carbon transfer.     

Establishment of a Pure Culture of the Hitherto Uncultured Unicellular Cyanobacterium Aphanothece sacrum, and Phylogenetic Position of the Organism

Aphanothece sacrum, an edible freshwater unicellular cyanobacterium, was isolated by using novel synthetic media (designated AST and AST-5xNP). The media were designed on the basis of the ratio of inorganic elements contained in A. sacrum cells cultured in a natural pond. The isolated strain exhibits unicellular rod-shaped cells ~6 μm in length that are scattered in an exopolysaccharide matrix, a feature similar to that of natural A. sacrum. DNA analysis of the isolated strain revealed that it carried two ferredoxin genes whose deduced amino acid sequences were almost identical to previously published sequences of ferredoxins from natural A. sacrum. Analysis of the 16S rRNA gene and ferredoxin genes revealed that A. sacrum occupies a phylogenetically unique position among the cyanobacteria.     

New unicellular red alga,Bulboplastis apyrenoidosa gen. et sp. nov. (Rhodellophyceae,Rhodophyta) from the mangroves of Japan: Phylogenetic and ultrastructural observations

A novel unicellular red alga collected from a mangrove area on Iriomote Island in southwest Japan is described as Bulboplastis apyrenoidosa gen. et sp. nov. The cells are spherical, mean 11.2 µm in diameter, and surrounded by a thick mucilaginous sheath. The grayish‐green chloroplast has many lobes extending throughout the cell and lacks a pyrenoid. This chloroplast type is similar to Glaucosphaera vacuolata, but differs from other unicellular red algae. Plastoglobuli clusters occur beneath the chloroplast envelope but only at the cell periphery. A peripheral encircling thylakoid is absent. Golgi bodies surround the central nucleus, which is an arrangement shared with all members of the Dixoniellales. The subcellular features of some mitotic phases are quite similar to those of other unicellular red algae. A pair of ring‐shaped structures located within electron‐dense material can be seen in cells undergoing telophase. The size of the polar rings ranged within those reported from the Dixoniellales. A phylogenetic analysis based on small subunit rDNA indicates that B. apyrenoidosa is a member of the Dixoniellales and a sister lineage to Neorhodellaand Dixoniella.     

Epidermal structure in the ligules of four species of the genus Poa L. (Poaceae)

The structure of the abaxial epidermis of the ligules of mid- and upper-culm leaves of the grasses Poa annua, P. nemoralis, P. pratensis and P. trivialis as seen by light microscopy is described. Amongst the structural elements found were long cells, short cells and unicellular prickle hairs over the surface, with unicellular hairs and papillate cells on the free edges of the ligules. Ligules of upper-culm leaves of some plants of P. trivialis L. contained vascular tissue with associated green (chlorenchyma) regions and stomata-like structures. The value of ligule characters in grass identification and classification is briefly discussed.     

Modelling population dynamics in a unicellular social organism community using a minimal model and evolutionary game theory

Most unicellular organisms live in communities and express different phenotypes. Many efforts have been made to study the population dynamics of such complex communities of cells, coexisting as well-coordinated units. Minimal models based on ordinary differential equations are powerful tools that can help us understand complex phenomena. They represent an appropriate compromise between complexity and tractability; they allow a profound and comprehensive analysis, which is still easy to understand. Evolutionary game theory is another powerful tool that can help us understand the costs and benefits of the decision a particular cell of a unicellular social organism takes when faced with the challenges of the biotic and abiotic environment. This work is a binocular view at the population dynamics of such a community through the objectives of minimal modelling and evolutionary game theory. We test the behaviour of the community of a unicellular social organism at three levels of antibiotic stress. Even in the absence of the antibiotic, spikes in the fraction of resistant cells can be observed indicating the importance of bet hedging. At moderate level of antibiotic stress, we witness cyclic dynamics reminiscent of the renowned rock–paper–scissors game. At a very high level, the resistant type of strategy is the most favourable.     

Prey Capture and Phagocytosis in the Choanoflagellate Salpingoeca rosetta

Choanoflagellates are unicellular and colonial aquatic microeukaryotes that capture bacteria using an apical flagellum surrounded by a feeding collar composed of actin-filled microvilli. Flow produced by the apical flagellum drives prey bacteria to the feeding collar for phagocytosis. We report here on the cell biology of prey capture in rosette-shaped colonies and unicellular “thecate” or substrate attached cells from the choanoflagellate S. rosetta. In thecate cells and rosette colonies, phagocytosis initially involves fusion of multiple microvilli, followed by remodeling of the collar membrane to engulf the prey, and transport of engulfed bacteria into the cell. Although both thecate cells and rosette colony cells produce ∼70 nm “collar links” that connect and potentially stabilize adjacent microvilli, only thecate cells were observed to produce a lamellipod-like “collar skirt” that encircles the base of the collar. This study offers insight into the process of prey ingestion by S. rosetta, and provides a context within which to consider potential ecological differences between solitary cells and colonies in choanoflagellates.     

Lipocalin signaling controls unicellular tube development in the Caenorhabditis elegans excretory system

Unicellular tubes or capillaries composed of individual cells with a hollow lumen perform important physiological functions including fluid or gas transport and exchange. These tubes are thought to build intracellular lumina by polarized trafficking of apical membrane components, but the molecular signals that promote luminal growth and luminal connectivity between cells are poorly understood. Here we show that the lipocalin LPR-1 is required for luminal connectivity between two unicellular tubes in the Caenorhabditis elegans excretory (renal) system, the excretory duct cell and pore cell. Lipocalins are a large family of secreted proteins that transport lipophilic cargos and participate in intercellular signaling. lpr-1 is required at a time of rapid luminal growth, it is expressed by the duct, pore and surrounding cells, and it can function cell non-autonomously. These results reveal a novel signaling mechanism that controls unicellular tube formation, and provide a genetic model system for dissecting lipocalin signaling pathways.     

ULTRASTRUCTURE OF GLANDULAR TRICHOMES OF LEAVES OF NICOTIANA TABACUM L., cv XANTHI

Electron microscopy confirms previous light microscope observations that tobacco leaf trichomes are glandular and that there are two different types. Both the tall trichome (multicellular stalk, unicellular or multicellular head) and the short trichome (unicellular stalk; multicellular head) exhibit characteristics common to gland cells—a dense cytoplasm, numerous mitochondria, and little vacuolation. The tall trichome contains structurally well developed chloroplasts and an elaborate network of endoplasmic reticulum. The short trichome contains undifferentiated plastids and endoplasmic reticulum which parallels the nucleus and plasmalemma. Few dictyosomes are seen either in the short trichome or in the tall trichome. The short trichome appears to undergo structural changes concurrently with the appearance of secretory product within the cells. The most noticeable change is the formation of the extraplasmic space between the cell wall and the plasmalemma. Electron dense secretory product is observed between the plasmalemma and the cell wall and within the intercellular spaces.     

Light and acetate regulate a mitochondrial malate dehydrogenase

A malate dehydrogenase was purified from the unicellular green alga Chlorogonium elongatum Dangeard. The enzyme was localized in the mitochondria by immunogold electron microscopy and was found to be present on the cristae. The concentration of the enzyme is regulated by acetate and light. In cells cultured heterotrophically with acetate as carbon source the activity and the concentration of the enzyme is 5- to 6-fold higher than in autotrophic cells. In mixotrophically cultured cells (light and acetate) the enzyme level attains only half of the value of that in heterotrophic cells. Acetate induces an increase of the enzyme concentration while light has an inhibitory effect on this process.     

Fixation of phycobiliproteins to photosynthetic membranes by glutaraldehyde

Summary Treatment of intact cells of two unicellular red algae (Porphyridium spp.) and of an unicellular blue-green alga (Gloeocapsa alpicola) with glutaraldehyde (3% v/v) has only a slight effect on the absorption spectra of their photopigment systems, but stabilizes the normally labile physical association between phycobili-proteins and the chlorophyll-bearing thylacoids. On subsequent breakage of such cells, a substantial fraction of the total phycobiliprotein (30–70%) sediments in a sucrose gradient, in association with the chlorophyll-containing membrane fragments, from which it cannot be dissociated by treatment with detergents.