When does division of labor lead to increased system output?

This paper develops a set of simplified dynamical models with which to explore the conditions under which division of labor leads to optimized system output, as measured by the rate of production of a given product. We consider two models: in the first model, we consider the flow of some resource into a compartment, and the conversion of this resource into some product. In the second model, we consider the growth of autoreplicating systems. In this case, we divide the replication and metabolic tasks among different agents. The general features that emerge from our models is that division of labor is favored when the resource to agent ratio is at intermediate values, and when the time cost associated with transporting intermediate products is small compared to characteristic process times. The results of our model are consistent with the behavior of the cellular slime mold Dictyostelium discodeum, which switches from a single-celled to a multi-celled state when resources become limited. We also argue that division of labor in the context of our replication model suggests an evolutionary basis for the emergence of the stem-cell-based tissue architecture in complex organisms. Finally, the results of this paper may be useful for understanding how, in an economic context, firm productivity is maximized at intermediate firm sizes.     

Where and how does phototropin transduce light signals in the cell?

Light plays pivotal roles as an important environmental signal in plant growth and development. In Arabidopsis, phototropin 1 (phot1) and 2 (phot2) are the photoreceptors that mediate phototropism, chloroplast relocation, stomatal opening and leaf flattening, in response to blue light. However, little is known about how phototropins transduce the signals after the light is perceived. Changes induced by blue light in terms of intracellular localization patterns of phot2 in Arabidopsis were examined. Phot2 distributed uniformly in the plasma membrane under dark conditions. Upon irradiation with blue light, some of the phot2 associated with the Golgi apparatus. It was also shown that the kinase domain, but not the photosensory domain, is required for a plasma membrane and Golgi localization. Furthermore a kinase fragment, lacking the photosensory domain, constitutively triggered physiological responses in planta. Thus, the plasma membrane and the Golgi apparatus appear to be the most likely sites for the initial step of phot2 signal transduction. The Golgi apparatus facilitates vesicle trafficking and delivery of membrane proteins to the required locations in the cell. Therefore, this study implicates the regulation of vesicle trafficking by the Golgi apparatus as a mechanism by which phot2 elicits its cellular responses.Key words: Golgi apparatus, kinase, light signal transduction, photoreceptor, phototropin, vesicle traffickingA range of physiological responses in plants is brought about by blue (390–500 nm) and ultraviolet-A (320–390 nm) light. Phototropin, one of major classes of blue light photoreceptors in plants, mediates responses such as phototropism, chloroplast relocation, light-induced stomatal opening and leaf flattening.^1–6 The dicotyledon Arabidopsis, possesses two phototropins, termed phot1 and phot2, which have both overlapping and distinct functions.^5,7 Phototropins consist of two functional domains, a N-terminal photosensory domain, containing two LOV (Light, Oxygen, Voltage) domains (LOV1 and LOV2) and a flavin-mononucleotide (FMN) chromophore and a regulatory serine/threonine kinase domain at the C-terminus.⁸To understand the mechanism of phototropin signal transduction, we expressed phot2 derivatives with translationally-fused green fluorescent protein (GFP) in a phot1phot2 double mutant in a wild type background in Arabidopsis.^9,10 Phototropin is a membrane- associated protein lacking a membrane spanning domain.⁸ Phot1 fused to GFP (P1G) is mainly localized to the plasma membrane, regardless of the light conditions.⁶ This property was retained when phot2 was fused to GFP (P2G).⁹ A part of P2G associates with punctate structures in the cytoplasm in response to blue light. The punctate P2G colocalized with KAM1ΔC:mRFP, a Golgi marker, we therefore conclude that phot2 associated with the Golgi apparatus in a blue light-dependent manner.⁹ This association was observed even in the presence of brefeldin A (BFA), an inhibitor of the vesicle trafficking.⁹To determine which domain of phot2 is responsible for the Golgi association, fragments of phot2 were fused to GFP and expressed in protoplasts.⁹ The N-terminal fragment fused to GFP (P2NG) was distributed uniformly in the cytoplasm. By contrast, the C-terminal fragment fused to GFP (P2CG) localized to both plasma membrane and punctate structures. The latter was shown to be the Golgi apparatus with the aid of the Golgi marker, KAM1ΔC:mRFP.⁹ These observations were corroborated from data using transgenic plants.¹⁰ Hence the C-terminal kinase domain, but not the N-terminal photo-sensory domain, is essential for the association of phot2 with the plasma membrane and the Golgi apparatus.The Golgi network is a key player in vesicle trafficking, to and from ER, vacuoles, trans-Golgi network, endosome and the plasma membrane.¹¹ Membrane spanning proteins are delivered and recycled through the Golgi apparatus. Among the membrane spanning proteins that are especially interesting, with respect to phototropin function, are auxin carriers such as PIN proteins. Phototropic curvature, which is under the control of phototropin, is believed to be caused by an uneven distribution of auxin.¹² The intracellular distribution of PIN proteins is maintained and regulated by vesicle trafficking.¹³ Indeed, factors such as ADP-ribosylation factor1 (ARF1) and guanine-nucleotide exchange factors (GEFs), which are involved in vesicle trafficking, are indispensable for the proper distribution of PIN proteins.^14–17 It is intriguing that a light stimulus alters the distribution pattern of PIN proteins.¹⁸ Hence, a fascinating possibility arises that phot2 alters the intracellular distribution of PIN proteins by regulating vesicle trafficking at the level of the Golgi apparatus.Phototropins are members of the subfamily VIII of AGC kinases.¹⁹ Interestingly, PINOID, another member of the subfamily, is localized at the cell periphery and regulates the apical-basal polar distribution of PIN proteins.^20–22 Accordingly, overexpression of PINOID disturbs the auxin distribution in transgenic plants.^23,24 The kinase fragment of phototropin exhibits constitutive kinase activity in vitro.²⁵ Interestingly, the auxin distribution is disturbed in plants expressing P2CG, as is the case with PINOID.¹⁰ Hence, both PINOID and phot2 might alter the PIN protein distribution in the cell through a common mechanism, in response to distinct stimuli.To date, no authentic substrate has been described for any of the AGC VIII kinases.¹⁹ Considering the localization pattern of phototropins, the substrates are most likely to reside in the plasma membrane and/or the Golgi apparatus. NPH3, RPT2 and PKS1 are downstream factors for phototropic responses,^26–28 all associating with the plasma membrane. Although they interact preferentially with the N-terminal rather than the C-terminal domain of phot1,^26,29 it is also possible that the C-terminal kinase domain interacts transiently with these factors leading to their phosphorylation. However, at present the molecular functions of NPH3, RPT2 and PKS1 remain unclear and await future investigation.Although both phot1 and phot2 are localized to the plasma membrane, punctate structures are yet to be described for P1G. Instead, a part of phot1-GFP is released from the plasma membrane to the cytosol in response to a light stimulus.⁶ We recently reexamined the intracellular localization of P1G. A specific network-like structure in the cytoplasm in addition to intense plasma membrane staining was observed (Fig. 1). A similar pattern was observed for P2G although it is less clear.⁹ Hence, both phot1 and phot2 might be associating with a structure in the cytoplasm that has yet to be described, and which might be another site of phototropin signaling in the cell.Open in a separate windowFigure 1A light-induced network-like distribution pattern of P1G in the cytoplasm. The P1G seedlings grown under dark conditions⁶ were incubated in MS solution (diluted 50%) without (upper panels) or with (lower panels) 100 µM BFA. The cells were inspected with a confocal laser scanning microscope. Images taken before (left) or after (right) blue light illumination at 48 µmol m⁻² sec⁻¹ are shown. Bar = 10 µm.P2CG elicits some phototropin responses without a light stimulus.¹⁰ That is, chloroplasts were in the avoidance position and stomata opened without a blue light stimulus in the P2CG overexpressing plants. It is a fascinating possibility that phototropin elicits those responses through the regulation of vesicle trafficking, although other possibilities exist. Stomata open as the result of phosphorylation of the plasma membrane H⁺-ATPase³⁰ and it is unlikely that the vesicle trafficking is directly involved in this regulatory process. It is possible to conjecture that vesicle trafficking affects chloroplast positioning but how this would work remains to be determined. Overall how a single photoreceptor such as phototoropin might regulate diverse physiological responses awaits future study.     

Fragmentation of clones: how does it influence dispersal and competitive ability?

We applied individual-based simulations to study the effect of physiological integration among ramets in clonal species that live in patchy habitats. Three strategies were compared: (1) Splitter, in which the genet was fragmented into independent ramets; (2) Transient Integrator, where only groups of ramets were connected; and (3) Permanent Integrator, in which fragmentation did not occur, and the whole genet was integrated. We studied the dynamics of spatial spreading and population growth in these strategies separately and in competition. Various habitat types were modeled by changing the density of favorable habitat patches. We found that the spatial pattern of good patches significantly influenced the growth of the populations. When the resource patches were scarce, a large proportion of the carrying capacity of the habitat was not utilized by any of the strategies. It was the Splitter that proved to be the most severely dispersal-limited. But at the same time, it could compete for the good patches most efficiently. The balance between these two contradictory effects was largely determined by the proportion of favorable to unfavorable areas. When this proportion was low or intermediate (up to ca. 50% good), integration was more advantageous. At higher proportions, fragmentation became beneficial. Fragmentation into groups of ramets (Transient Integration) was not sufficient, only radical splitting could ensure a significant selective advantage. Transient Integrators got fragmented according to the spatial pattern of ramet mortality. It was interesting that the enrichment of the area in good sites did not lead to larger fragment sizes. It merely raised the number of fragments. Nevertheless, these small fragments were more similar to integrated genets (in the Permanent Integrator) than to solitary ramets (in the Splitter) in terms of dispersal and competitive ability. This suggests that even a slightly integrated clonal species can be ecologically considered as an integrator.     

Symmetry breakage in the vertebrate embryo: When does it happen and how does it work?

Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.     

The influence of birth order on birth weight: does the sex of preceding siblings matter?

The purpose of this study was to investigate whether the well-established relationship between parity and birth weight is affected by the sex composition of siblings, especially for male newborns. Subjects were 856 male and 862 female newborns who weighed at least 2500 g at birth, who were born after 37 completed weeks of gestation, who obtained an Apgar score of 7 or higher, who had the same biological parents as all other children in the sibship, and who lived in the same household. Information on birth weight was collected from hospital records. Results showed that male newborns with older brothers weighed less than male newborns with older sisters. In contrast, the weight of female newborns with older brothers did not differ from the weight of female newborns with older sisters. One explanation of these results is that maternal immunoreactivity to some male-specific feature of the fetus affects prenatal development and consequently reduces birth weight in males. The relation between older brothers and birth weight may have theoretical significance for behavioural variables.     

When does the variance of replicator fitness decrease?

For the general replicator dynamics with regular relative advantage functions (having non-singular Jacobian and symmetrized Jacobian), it will be shown that the variance of marginal fitness of the replicators strictly decreases along each trajectory of the replicator dynamics near an interior rest point if and only if this rest point is a regular evolutionarily stable state.     

Can elastic stresses in gels cause the elongation and division of a cell?

Some metabolites present in the cell may affect its colloidal structure. If the cytoplasm is in the state of a gel, elastic stresses may appear as a result of non-uniform concentration of some metabolites which affect mechanical properties of gels. This paper investigates the question whether mechanical stresses, thus produced, can result in such a deformation of the cell which may lead to a division. The result of this study indicates that this is not possible. Elastic stresses, produced by non-uniformities of concentration of metabolites which affect the colloidal structures of the cytoplasm, may produce only a general dilation or contraction of the system, without change in shape.     

Cell division and DNA segregation in Streptomyces: how to build a septum in the middle of nowhere?

Streptomycetes are antibiotic-producing filamentous microorganisms that have a mycelial life style. In many ways streptomycetes are the odd ones out in terms of cell division. While the basic components of the cell division machinery are similar to those found in rod-shaped bacteria such as Escherichia coli and Bacillus subtilis, many aspects of the control of cell division and its co-ordination with chromosome segregation are remarkably different. The rather astonishing fact that cell division is not essential for growth makes these bacteria unique. The fundamental difference between the cross-walls produced during normal growth and sporulation septa formed in aerial hyphae, and the role of the divisome in their formation are discussed. We then take a closer look at the way septum site localization is regulated in the long and multinucleoid Streptomyces hyphae, with particular focus on actinomycete-specific proteins and the role of nucleoid segregation and condensation.     

What is the role of cell division in leaf development?

An idea underlying a great deal of research and discussion in plant cell and developmental biology is that the spatial regulation of cell division plays a key role in plant development. In this article, the role of cell division in two aspects of leaf development is analysed: morphogenesis (leaf initiation, growth, and the generation of leaf shape) and histogenesis (the differentiation of leaf cells to form the various cell types that make up a functional leaf). The point of view that emerges from this analysis is that the rate and pattern of cell division is important for leaf development, but does not dictate leaf size, shape, or cell fate.     

When and how did the terrestrial mid-Permian mass extinction occur? Evidence from the tetrapod record of the Karoo Basin,South Africa

A mid-Permian (Guadalupian epoch) extinction event at approximately 260 Ma has been mooted for two decades. This is based primarily on invertebrate biostratigraphy of Guadalupian–Lopingian marine carbonate platforms in southern China, which are temporally constrained by correlation to the associated Emeishan Large Igneous Province (LIP). Despite attempts to identify a similar biodiversity crisis in the terrestrial realm, the low resolution of mid-Permian tetrapod biostratigraphy and a lack of robust geochronological constraints have until now hampered both the correlation and quantification of terrestrial extinctions. Here we present an extensive compilation of tetrapod-stratigraphic data analysed by the constrained optimization (CONOP) algorithm that reveals a significant extinction event among tetrapods within the lower Beaufort Group of the Karoo Basin, South Africa, in the latest Capitanian. Our fossil dataset reveals a 74–80% loss of generic richness between the upper Tapinocephalus Assemblage Zone (AZ) and the mid-Pristerognathus AZ that is temporally constrained by a U–Pb zircon date (CA-TIMS method) of 260.259 ± 0.081 Ma from a tuff near the top of the Tapinocephalus AZ. This strengthens the biochronology of the Permian Beaufort Group and supports the existence of a mid-Permian mass extinction event on land near the end of the Guadalupian. Our results permit a temporal association between the extinction of dinocephalian therapsids and the LIP volcanism at Emeishan, as well as the marine end-Guadalupian extinctions.     

How does transferrin overcome the in vitro block to development of the mouse preimplantation embryo?

Transferrin promotes development of mouse embryos through the two-cell block in vitro. Uptake of transferrin into blastocysts was shown to occur by both receptor-mediated and nonspecific pathways, but neither pathway was used to a detectable extent by embryos before the eight-cell stage. Conversely, the dialysis of culture medium, non-permissive for development through the two-cell block, against a solution of transferrin rendered it capable of supporting development. It was therefore concluded that transferrin exerts its supportive effect on development in vitro via its chelating effects.     

How does parental role influence the activity and movements of breeding wolves?

We studied the activity and movements of a male and a female wolf during breeding in the Bieszczady Mountains, Poland. The female was less active and mobile and on average closer to the den than the male. The male was less active and mobile when the pack size was seven compared to a year when the pack consisted only of the breeding pair. We conclude that the roles played by breeding males and females rearing pups influence their activity, and that breeding males in larger packs move less during the nursing period because of help by other pack members.     

How do tenascins influence the birth and life of a malignant cell?

Tenascins are large glycoproteins found in embryonic and adult extracellular matrices. Of the four family members, two have been shown to be overexpressed in the microenvironment of solid tumours: tenascin-C and tenascin-W. The regular presence of these proteins in tumours suggests a role in tumourigenesis, which has been investigated intensively for tenascin-C and recently for tenascin-W as well. In this review, we follow a malignant cell starting from its birth through its potential metastatic journey and describe how tenascin-C and tenascin-W contribute to these successive steps of tumourigenesis. We consider the importance of the mechanical aspect in tenascin signalling. Furthermore, we discuss studies describing tenascin-C as an important component of stem cell niches and present examples reporting its role in cancer therapy resistance.     

How does the presence of predators influence the persistence of antipredator behavior?

We developed a virtual world to study the effect of predators on predator recognition. We trained a neural network to discriminate between the shapes of simulated aerial and terrestrial predators and non-predators. Then, the network's weighting values were fixed into the genomes of a set of autonomous agents. These animats were required to eat, avoid death due to starvation, and avoid predation, by fleeing from approaching predators. We systematically varied the predator's lethality, the mutation rate, the cost of fleeing a predator, and the presence or absence of aerial and terrestrial predators. We used ANOVA to analyse the average recognition ability (a measure of directional selection) and the standard deviation of recognition ability (a measure of relaxed selection) after 500 generations of selection. Mutation rate and the cost of flight had the greatest effect on both the average and standard deviation of recognition abilities. The loss of all predators relaxed selection on predator recognition abilities. The loss of specific predators had complex effects on recognition abilities. Persistence is largely influenced by escape costs.     

When does a functional correctly describe both the structure and the energy of the transition state?

Requiring that several properties are well reproduced is a severe test on density functional approximations. This can be assessed through the estimation of joint and conditional success probabilities. An example is provided for a small set of molecules, for properties characterizing the transition states (geometries and energies).     

When should a female avoid adding eggs to the clutch of another female? A simultaneous oviposition and sex allocation game

Summary Avoidance of double oviposition (ADO) is the strategy not to oviposit on food patches where another female has oviposited before. If two females oviposit on the same patch, competitive and mating interactions within and between broods may lead to both a clutch size game and a sex allocation game between the two visitors. Though the two games interact, they are usually considered separately. Here, the ESS conditions for ADO are investigated in an analysis that combines the two games into one. The analysis strengthens the notion that it is really ADO that needs to be explained, because role-dependent net pay-off from an additional egg is most likely to favour double oviposition (DO). To a first female, the net payoff includes the effect on the eggs already present, whereas to a second female only the egg's gross pay-off matters. ADO is the evolutionary stable strategy (ESS) if there are enough patches still without eggs and either (1) the fitness of an additional egg is so low that the first female would not lay it even in the absence of detrimental effects on earlier offspring, so neither would a second female, or (2) differences in either the survival probability of the offspring or their reproductive success are sufficient to counterbalance the differential interest in the eggs already present. The first condition requires that eggs are relatively large, because then the decrease in pay-off between two successive eggs can be large. The second condition may be met when there is a time interval between ovipositions of subsequent females. The resulting developmental lag of the second clutch will (1) diminish its ability to compete for food and (2) lower its reproductive success when there is local mate competition and sons are too late to mate with daughters of the first female. If sons of first and second females compete on equal terms, however, ADO is unlikely. Male migration between patches reduces the influence of sex allocation strategies on clutch size decisions; the same holds for small clutch sizes. To illustrate the importance of considering sex allocation and clutch size decisions in an integrated way, oviposition strategies of plant-inhabiting predatory mites (Acari: Phytoseiidae) are discussed.