Competition of two microbial populations for a single resource in a chemostat when one of them exhibits wall attachment

It is known that two microbial populations competing for a single resource in a homogeneous environment with time-invariant inputs cannot coexist in a steady state. The case where two microbial populations compete for a single resource in a chemostat but one of them exhibits attachment to the chemostat walls is studied theoretically. Because of the cells' attachment to the walls, the environment is no longer homogeneous. The present article considers the case where the attached cells form no more than a monolayer. Other situations occur, often frequently, but we do not consider them here. Two models are used to represent the attachment to the walls: the Topiwala-Hamer model and a model which assumes that the attachment of microbial cells to the solid surfaces is a reversible process. The first model does not allow the population that exhibits wall attachment to wash out from the chemostat, in contrast to the second model (which nevertheless reduces to the first one in the limit). It has been found that in most of the possible cases for both models, the two competitors can coexist in a stable steady state for a wide range of the operating parameters space. The results of the stability analysis are discussed and analytical expressions for the conditions and the boundaries of the domains of stable coexistence are given for all the possible situations that may arise.     

Hybrid ‘superswarm’ leads to rapid divergence and establishment of populations during a biological invasion

Understanding the genetic background of invading species can be crucial information clarifying why they become invasive. Intraspecific genetic admixture among lineages separated in the native ranges may promote the rate and extent of an invasion by substantially increasing standing genetic variation. Here, we examined the genetic relationships among threespine stickleback that recently colonized Switzerland. This invasion results from several distinct genetic lineages that colonized multiple locations and have since undergone range expansions, where they coexist and admix in parts of their range. Using 17 microsatellites genotyped for 634 individuals collected from 17 Swiss and two non‐Swiss European sites, we reconstruct the invasion of stickleback and investigate the potential and extent of admixture and hybridization among the colonizing lineages from a population genetic perspective. Specifically, we test for an increase in standing genetic variation in populations where multiple lineages coexist. We find strong evidence of massive hybridization early on, followed by what appears to be recent increased genetic isolation and the formation of several new genetically distinguishable populations, consistent with a hybrid ‘superswarm’. This massive hybridization and population formation event(s) occurred over approximately 140 years and likely fuelled the successful invasion of a diverse range of habitats. The implications are that multiple colonizations coupled with hybridization can lead to the formation of new stable genetic populations potentially kick‐starting speciation and adaptive radiation over a very short timescale.     

Effect of bacterial chemotaxis on dynamics of microbial competition

Although the dynamic behavior of microbial populations in nonmixed systems is a central aspect of many problems in biochemical engineering and microbiology, the factors that govern this behavior are not well understood. In particular, the effects of bacterial chemotaxis (biased migration of cells in the direction of chemical concentration gradients) have been the subject of much speculation but very little quantitative investigation. In this paper, we provide the first theoretical analysis of the effects of bacterial chemotaxis on the dynamics of competition between two microbial populations for a single rate-limiting nutrient in a confined nonmixed system. We use a simple unstructured model for cell growth and death, and the most soundly based current model for cell population migration. Using numerical finite element techniques, we examine both transient and steady-state behavior of the competing populations, focusing primarily on the influence of the cell random motility coefficient,, and the cell chemotaxis coefficient, . We find that, in general, there are four possible steady-state outcomes: both populations die out, population 1 exists alone, population 2 exists alone, and the two populations coexist. We find that, in contrast to well-mixed systems, the slower-growing population can coexist and even exist alone if it possesses sufficiently superior motility and chemotaxis properties. Our results allow estimation of the value of necessary to allow coexistence and predominance for reasonable values of growth and random motility parameters in common systems. An especially intriguing finding is that there is a minimum value of necessary for a chemotactic population to have a competitive advantage over an immotile population in a confined nonmixed system. Further, for typical system parameter values, this minimum value of is the range of values that can be estimated from independent experimental assays for chemotaxis.Thus, in typical nonmixed systems, cell motility and chemotaxis properties can be the determining factors in governing population dynamics.     

Population Dynamics of Metastable Growth-Rate Phenotypes

Neo-Darwinian evolution has presented a paradigm for population dynamics built on random mutations and selection with a clear separation of time-scales between single-cell mutation rates and the rate of reproduction. Laboratory experiments on evolving populations until now have concentrated on the fixation of beneficial mutations. Following the Darwinian paradigm, these experiments probed populations at low temporal resolution dictated by the rate of rare mutations, ignoring the intermediate evolving phenotypes. Selection however, works on phenotypes rather than genotypes. Research in recent years has uncovered the complexity of genotype-to-phenotype transformation and a wealth of intracellular processes including epigenetic inheritance, which operate on a wide range of time-scales. Here, by studying the adaptation dynamics of genetically rewired yeast cells, we show a novel type of population dynamics in which the intracellular processes intervene in shaping the population structure. Under constant environmental conditions, we measure a wide distribution of growth rates that coexist in the population for very long durations (>100 generations). Remarkably, the fastest growing cells do not take over the population on the time-scale dictated by the width of the growth-rate distributions and simple selection. Additionally, we measure significant fluctuations in the population distribution of various phenotypes: the fraction of exponentially-growing cells, the distributions of single-cell growth-rates and protein content. The observed fluctuations relax on time-scales of many generations and thus do not reflect noisy processes. Rather, our data show that the phenotypic state of the cells, including the growth-rate, for large populations in a constant environment is metastable and varies on time-scales that reflect the importance of long-term intracellular processes in shaping the population structure. This lack of time-scale separation between the intracellular and population processes calls for a new framework for population dynamics which is likely to be significant in a wide range of biological contexts, from evolution to cancer.     

Cannibalism in discrete-time predator-prey systems

In this study, we propose and investigate a two-stage population model with cannibalism. It is shown that cannibalism can destabilize and lower the magnitude of the interior steady state. However, it is proved that cannibalism has no effect on the persistence of the population. Based on this model, we study two systems of predator-prey interactions where the prey population is cannibalistic. A sufficient condition based on the nontrivial boundary steady state for which both populations can coexist is derived. It is found via numerical simulations that introduction of the predator population may either stabilize or destabilize the prey dynamics, depending on cannibalism coefficients and other vital parameters.     

Quantitation of total DNA per cell in an exponentially growing population using the diphenylamine reaction and flow cytometry

The diphenylamine assay used to estimate the absolute mass of DNA/cell as well as absolute differences in DNA content between cell populations is based upon the assumption that all of the cells are in the G0 or G1 phase of the DNA synthetic cycle. However, if cells are in exponential growth and synthesizing DNA, portions of the population will be in S or G2 phases and the diphenylamine assay will overestimate the total mass of DNA/cell. Conversely, flow cytometry (FCM) can estimate relative differences in total DNA/cell and the proportions of an exponentially growing population in G1, S, and G2 but cannot estimate absolute mass or differences in DNA/cell. In this report, we describe a methodology of combined diphenylamine and FCM assays of total DNA/cell which is applicable to any eukaryotic cell population. The method involves using the two assay methods concurrently and correcting the diphenylamine data for the FCM-derived distribution of the cells within the DNA synthetic cycle. The methodology was tested on single-cell-derived stocks of the obligate intracellular protozoan parasite Trypanosoma cruzi which displays marked but stable intraspecific heterogeneity.     

Scrounging behavior regulates population dynamics

The integration of behavioral and population ecology is necessary when behavior both feeds into demographic parameters and depends on population parameters. We show that scrounging behavior, the exploitation of others' resources, can affect both demographic parameters and population dynamics, including the stability of interactions with prey. Scrounging is a common tactic and its pay-offs exhibit both density- and frequency-dependence. We demonstrate that scrounging can act as a population regulator through its effects on individuals' reproductive rate and mortality. We also explore its effects on predator-prey population dynamics and show that the presence of scrounging predators allows an increased predator population size and contributes to the regulation of both predator and prey populations. Behavioral ecologists will appreciate that although scrounging is often pictured as imposing a social foraging cost to group membership, at the population level it also allows higher numbers of both prey and predators to coexist at equilibrium.     

Cell volume distributions reveal cell growth rates and division times

A population of cells in culture displays a range of phenotypic responses even when those cells are derived from a single cell and are exposed to a homogeneous environment. Phenotypic variability can have a number of sources including the variable rates at which individual cells within the population grow and divide. We have examined how such variations contribute to population responses by measuring cell volumes within genetically identical populations of cells where individual members of the population are continuously growing and dividing, and we have derived a function describing the stationary distribution of cell volumes that arises from these dynamics. The model includes stochastic parameters for the variability in cell cycle times and growth rates for individual cells in a proliferating cell line. We used the model to analyze the volume distributions obtained for two different cell lines and one cell line in the absence and presence of aphidicolin, a DNA polymerase inhibitor. The derivation and application of the model allows one to relate the stationary population distribution of cell volumes to extrinsic biological noise present in growing and dividing cell cultures.     

季节变化对沙漠生态系统土壤中自由生活线虫和原生动物间营养关系的影响

选择沙漠常绿灌木丛枝霸王(Zygophyllum dumosum)群落及群落间的开阔地带采样,研究土壤线虫和原生动物的种群大小、物种组成及营养结构.本研究共获线虫29属,裸变形虫33属90种;在所获各类原生动物中,鞭毛虫和纤毛虫均为食细菌类群,而鞭毛虫个体数量最多;在沙漠生态系统中,土壤湿度被认为是影响生物活性的最重要因素之一.然而,本研究显示:土壤湿度对线虫和原生动物是否重要取决于这些动物对土壤湿度的生理生态适应能力;土壤线虫和原生动物的体型大小、生活史策略和活动能力与土壤湿度的有效性相关不显著,但它们对微生物和微小土壤动物区系间的相互作用有重要影响,在干旱生态系统中,这种影响能够显著改变土壤生态系统短期的养分循环.本研究还表明,土壤线虫和原生动物种群间的关系主要取决于两者营养类群组成的变化.     

Effects of the inability of suspension-feeding protozoa to collect all cell sizes of a bacterial population

The effects of the inability of suspension-feeding protozoa to collect bacteria over the whole range of sizes in the bacterial size distribution were examined by constructing mathematical models based on this assumption. Systems of suspension-feeding protozoa grown on both growing and nongrowing bacteria were examined in both batch and continuous culture. The models were able to predict three experimental observations common in such systems. Some additional features of the systems which should be useful in interpreting results of experiments with suspension feeding protozoa and in designing new experiments were predicted, also.     

A lotka-volterra model of coexistence between a sexual population and multiple asexual clones

At carrying capacity, small advantages in competitive ability can compensate a sexual population for its two-fold disadvantage in growth capacity when facing invasion by asexual mutants. In this paper, we develop a generic analytical model to consider the ecology of a sexual population comprising equal numbers of males and females, competing for shared prey resources with multiple female-only clones. We assume that the clones arise from the sexual population and are distinguished from it only by having narrower resource niches and twice the growth capacity. For sexual populations, at density-dependent carrying capacity, intra-specific competition between clonal individuals prevents them from realizing their two-fold advantage in intrinsic growth. This prediction leads to three novel outcomes: (i) a sexual population can coexist with any number of clones, provided their combined competitive impact remains less than the impact of the clones on each other; (ii) a sexual species can immediately exclude asexual invaders if it is a fast growing and strong competitor of shared resources and also has refuge in an abundant alternative resource; (iii) the rate of accumulation of clones in a sexual population will be slowed by intra and inter-specific competition amongst the clones themselves, in addition to the competitive impact from the original sexual population.     

Some physiological aspects of the autecology of the suspension-feeding protozoanTetrahymena pyriformis

Feeding, growth, and reproductive responses of the suspension-feeding protozoanTetrahymena pyriformis to shifts up or down of the density of its bacterial food were observed. The rates of feeding, growth, and reproduction were determined by measuring the rates of uptake of viable bacterial cells, of change of mean volume of the protozoan cells, and of change of number of protozoan cells, respectively. The effects of the nutritional status of the protozoans at the time of shifting were observed also. Results are interpreted in terms of the limited polymorphism exhibited in the life cycle of this organism. Responses in all cases seem to reflect a strategy for exploiting a patchy, transient environment, a conclusion already reached by several earlier investigators.     

Sex ratio, reproductive mode and genetic diversity in Triops cancriformis

1. Aquatic invertebrates display a wide array of alternative reproductive modes from apomixis to hermaphroditism and cyclical parthenogenesis. These have important effects on genetic diversity and population structure. Populations of the 'living fossil' Triops cancriformis display a range of sex ratios, and various reproductive modes are thought to underlie this variation. Using sex ratio information and histological analyses European populations have been inferred to be gonochoric (with separate males and females), selfing hermaphroditic and androdioecious, a rare reproductive mode in which selfing hermaphrodites coexist with variable proportions of males. In addition, some populations have been described as meiotic parthenogens.
2. Here we use population genetic analysis using microsatellite loci in populations with a range of sex ratios including a gonochoric population, and marker segregation patterns in heterozygote individuals reared in isolation, to clarify the reproductive mode in this species.
3. Our data show that populations in general have very low levels of genetic diversity. Non-gonochoric populations show lower genetic diversity, more heterozygote deficiencies, higher inbreeding coefficients and stronger linkage disequilibria than the gonochoric population. The maintenance of some heterozygosity in populations is consistent with some male influence in T. cancriformis populations, as would be expected from an androdioecious reproductive system. Results of marker segregation in eggs produced in isolation from non-gonochoric populations indicate that meiosis occurs and are consistent with two reproductive modes: selfing hermaphroditism and a type of ameiotic parthenogenesis.
4. Overall, our data indicate that androdioecy and selfing hermaphroditism are the most likely reproductive modes of non-gonochoric European Triops populations. Triops populations are strongly structured, suggesting high genetic drift and low levels of gene flow.     

Predation on Protozoa: its importance to zooplankton

Protozoa are an important component of both the nano- and microplanktonin marine and freshwater environments and are preyed upon byzooplankton, including suspension-feeding cope pods, some gelatinouszoopiankters and some first-feeding fish larvae. The clearancerates of suspension-feeding zooplankton for ciliates, in particular,are higher than for most phytoplankton. For at least some suspension-feedingzooplankton, protozoans are calculated to be quantitativelyan important component of the diet during certain seasons. Inlaboratory studies, protozoan components in the diet appearto enhance growth and survival of certain life-history stagesor enhance fecundity. These data suggest that protozoans arequalitatively as well as quantitatively important in the dietsof marine zooplankton. Most studies of predation on Protozoahave focused on the euphotic zone in nearshore waters. Predationon Protozoa is expected, however, to be particularly importantboth quantitatively and qualitatively in marine environmentsand seasons in which primary production is dominated by cells<5 µm in size, such as nearshore environments afterthe spring phytoplankton bloom, in oligotrophic waters, andin environments dominated by detritus-dominated food webs, suchas the deep sea. In detritus-dominated food webs, Protozoa maybe a source of essential nutrients and may thus facilitate utilizationof bacterial and detrital carbon by metazoan plankton.     

Size-structure-based models of forest dynamics to interpret population- and community-level mechanisms

Functional size-structure-based models of forest tree population dynamics present a unifying explanation for population-level patterns and tree community organization. Density-dependent regulation can be explicitly replaced by the effect of size-structure-dependent suppression on demographic processes in functional size-structure models. This suppression effect sufficiently explains various patterns reported for crowded evenaged populations. Further, it stabilizes natural forest populations of overlapping generations at a stationary state with balanced recruitment and mortality. The spatial heterogeneity of light resources created by tree size structure offers an opportunity for multiple species to coexist by means of trade-offs between demographic parameters. The energy correlation of tree species diversity at a geographic scale is also attributable to the architectural feature of forests. Recipient of the Botanical Society Award of Young Scientists, 1992.     

Parthenogenetic flatworms have more symbionts than their coexisting,sexual conspecifics,but does this support the Red Queen?

The Red Queen hypothesis predicts that sexuality is favoured when virulent parasites adapt quickly to host genotypes. We studied a population of the flatworm Schmidtea polychroa in which obligate sexual and parthenogenetic individuals coexist. Infection rates by an amoeboid protozoan were consistently higher in parthenogens than in sexuals. Allozyme analysis showed that infection was genotype specific, with the second most common clone most infected. A laboratory measurement of fitness components failed to reveal high infection costs as required for the Red Queen. Although fertility was lower in more infected parthenogens, this effect can also be explained by the accumulation of mutations. We discuss these and other characteristics of our model system that may explain how a parasite with low virulence can show this pattern.     

Coexistence of bacteria and feeding ciliates: Growth of bacteria on autochthonous substrates as a stabilizing factor for coexistence

There is good evidence that the specific feeding and growth rates of a population of suspension-feeding ciliates on a population of bacteria is described by the classic Monod model or something very like it. However, use of this model combined with the assumption that feeding of the ciliates is the only interaction between the populations leads to predictions grossly at variance with experimental observations. A second interaction between the populations, growth of the bacteria on products of lysis, and perhaps metabolism of the ciliates is known to occur and might be an important factor in determining the dynamics of ciliate-bacterial systems. Whether or not this is the case is examined in the present article. It is concluded that accounting for the second interaction does not remove the discrepancies between model predictions and experimental observations. It is concluded also that the second interaction is likely not important except in batchtype situations where the supply of available energy for supporting biological activity is not replenished.     

Experimental evolution of protozoan traits in response to interspecific competition

Decades of experiments have demonstrated the ecological effect of competition, but experimental evidence for competitive effects on trait evolution is rare. I measured the evolution of six protozoan traits in response to competitors from the inquiline community of pitcher plants. Replicate populations of Colpoda, a ciliated protozoan, were allowed to evolve in response to intra- and interspecific competition for 20 days (approximately 100 generations), before traits were measured in two common garden environments. Populations that evolved with interspecific competition had smaller cell sizes, produced fewer cysts and had higher population growth rates relative to populations grown in monoculture. The presence of interspecific competitors led to differential lineage sorting, most likely by increasing the strength of selection. These results are the first to demonstrate protozoan evolution in response to competition and may have implications for species coexistence in this system.     

Space mediates coexistence of females and hermaphrodites

In gynodioecious populations of flowering plants females and hermaphrodites coexist. Gynodioecy is widespread and occurs in both asexual and sexual species but does not admit a satisfactory explanation from classical sex ratio theory. In sexual populations male fertility restoring genes have evolved to counter non-nuclear male sterility mutations. In pseudogamous asexual populations pollen retention and increased self-fertilization can make male sterility costly. Both of these mechanisms can promote coexistence. However, it remains unclear how either of these mechanisms could evolve if coexistence was not initially possible. In the absence of these adaptations non-spatial models predict that females either fail to invade hermaphrodite populations or else displace them until pollen shortage drives the population to extinction. We develop a pair approximation to a probabilistic cellular automata model in which females and hermaphrodites interact on a regular lattice. The model features independent pollination and colonization processes which take place on different timescales. The timescale separation is exploited to obtain, with perturbation methods, a more manageable aggregated pair approximation. We present both the mean field model which recreates the classical non-spatial predictions and the pair approximation, which strikingly predicts different invasion criteria and coexistence under a wide range of parameters. The pair approximation is shown to correspond well qualitatively with simulation behaviour.     

The ecology and function of Protozoa in sewage purification

Seasonal recording of the Protozoa in various sewage disposal units has shown that, together with changes correlated with the degree of purification of the sewage, fluctuations occur as a result of the interplay of seasonal non-specific factors and local specific factors. The incidence and abundance of Protozoa in each disposal plant would seem to be controlled by sewage 'strength' and characteristic local drainage. Fluctuations of the population, however, are limited by different factors in different seasons.
In the bacteria bed investigations a 'weak' sewage shows a decrease of a generally rich fauna during the periods of sloughing, a 'strong' acid sewage during sloughing shows an increase in an otherwise restricted fauna and where the sewage is medium in 'strength' and supports an abundant insect population the fluctuations of the Protozoa are dominated by the more active insect populations of summer and by temperature in winter. In this last-named environment a significant inverse correlation between ciliate and insect frequency throughout the summer indicates the thoroughness with which the insect larvae devour the film; this may explain the summer reductions in certain groups of Protozoa.
In activated sludge channels conditions favour a uniform population which may vary in composition according to the purity of the sewage. Flocculation is most pronounced when protozoan populations comprise a restricted variety of ciliates and would seem to depend on a protozoan fauna since cultures of bacteria obtained from sewage did not flocculate unless Protozoa, free from or contaminated with bacteria, were added.
The protozoan populations of the two systems of disposal are compared in discussion.