One cell, two cell, red cell, blue cell: The persistence of a unicellular stage in multicellular life histories

As developmental biologists come closer to understanding at the molecular and genetic levels how a zygote becomes an adult, it is easy to forget that the very phenomenon that gives them an occupation remains a vexing problem to evolutionary biologists: why do unicellular stages persist in life histories of multicellular organisms? There are two explanatory hypotheses. One is that a unicellular stage purges multicellular organisms of deleterious mutations by exposing offspring that are each uniformly of one genotype to selection. Another is that a one-cell stage reduces conflicts of interest among genetically different replicators within an organism.     

Risk management in biological evolution

I present a framework to study the evolution of traits that allow an organism to survive life-threatening but rare risks. Specifically, I am concerned with risks so rare that any one individual in a population may not experience the risk-causing event in its lifetime. A theory of rare risk management is virtually absent in evolutionary biology, although it is well developed in economics. This is surprising because of the great influence economics had on evolutionary biology, and because biology is full of examples for evolved risk management traits. They include the ability of bacteria to sporulate, of pathogens to survive antibiotic treatment, of temperate bacteriophages to enter a lytic life cycle, as well as traits that allow higher organisms to survive rare environmental disasters, such as sporadic wildfires and irregular flooding. I make predictions about the sustenance of risk management traits under two scenarios, one where the catastrophic events cause individual deaths, and another one where catastrophic events cause population extinction. A well-developed theory of risk management will not only predict the distribution of risk management traits, but may also serve other purposes, such as to reconstruct the spectrum of environments that an organism encountered in its evolutionary history from the record stored in its genome's memory.     

Planetary Interchange of Bioactive Material: Probability Factors and Implications

It is now well-accepted that both lunar andmartian materials are represented in the meteoritecollections. Early suggestions that viable organismsmight survive natural transport between planets havenot yet been thoroughly examined. The concept ofPlanetary Interchange of Bioactive Material (PIBM) ispotentially relevant to the conditions under whichlife originated. PIBM has been also invoked to inferthat the potential danger to Earth from martianmaterials is non-existent, an inference with, however,many pitfalls. Numerous impediments to efficienttransfer of viable organisms exist. In this work, thelethality of space radiation during long transientsand the biasing of launched objects toward materialsunlikely to host abundant organisms are examined andshown to reduce the likelihood of successful transferby orders of magnitude. It is also shown that martianmeteorites studied to date assuredly have beensubjected to sterilizing levels of ionizing radiationin space. PIBM considerations apply to both the solarsystem locale(s) of the origin of life and to theapplicability of planetary protection protocols topreserve the biospheres of planetary bodies, including our own.     

Complex patterns of local adaptation in heat tolerance in Drosophila simulans from eastern Australia

Latitudinal clines are considered a powerful means of investigating evolutionary responses to climatic selection in nature. However, most clinal studies of climatic adaptation in Drosophila have involved species that contain cosmopolitan inversion polymorphisms that show clinal patterns themselves, making it difficult to determine whether the traits or inversions are under selection. Further, although climatic selection is unlikely to act on only one life stage in metamorphic organisms, a few studies have examined clinal patterns across life stages. Finally, clinal patterns of heat tolerance may also depend on the assay used. To unravel these potentially confounding effects on clinal patterns of thermal tolerance, we examined adult and larval heat tolerance traits in populations of Drosophila simulans from eastern Australia using static and dynamic (ramping 0.06 °C min^?1) assays. We also used microsatellites markers to clarify whether demographic factors or selection are responsible for population differentiation along clines. Significant cubic clinal patterns were observed for adult static basal, hardened and dynamic heat knockdown time and static basal heat survival in larvae. In contrast, static, hardened larval heat survival increased linearly with latitude whereas no clinal association was found for larval ramping survival. Significant associations between adult and larval traits and climatic variables, and low population differentiation at microsatellite loci, suggest a role for climatic selection, rather than demographic processes, in generating these clinal patterns. Our results suggest that adaptation to thermal stress may be species and life‐stage specific, complicating our efforts to understand the evolutionary responses to selection for increasing thermotolerance.     

The evolutionary pattern of early life history in water currents

Explanation of the characteristics of the early developmental stage of organisms is an important problem in evolutionary biology. In studies to date, evolutionary biologists have proposed some theories that successfully explain egg size variation. Mesoscale water movements may transport early life stage organisms in the aquatic biosphere. We propose a novel biological view to explain the duration of the retention period at the spawning ground and egg size variations in aquatic organisms with a planktonic stage at least during the early part of their life history. We develop a life history model of the early life stage of such aquatic organisms that takes into account their adaptations to water currents and biotic environmental gradients in the currents. We hypothesize that the distance from the spawning grounds to the nursery grounds and the biological richness of the currents affect the adaptive life history design of these aquatic organisms, including adaptive retention time at the spawning ground and egg size. Various studies of fish biology describe in passing phenomena that suggest the validity of our deductions, but explicit empirical attempts to evaluate our predictions in the field of evolutional biology are needed.     

Evolution of stress response in the face of unreliable environmental signals

Most organisms live in ever-changing environments, and have to cope with a range of different conditions. Often, the set of biological traits that are needed to grow, reproduce, and survive varies between conditions. As a consequence, organisms have evolved sensory systems to detect environmental signals, and to modify the expression of biological traits in response. However, there are limits to the ability of such plastic responses to cope with changing environments. Sometimes, environmental shifts might occur suddenly, and without preceding signals, so that organisms might not have time to react. Other times, signals might be unreliable, causing organisms to prepare themselves for changes that then do not occur. Here, we focus on such unreliable signals that indicate the onset of adverse conditions. We use analytical and individual-based models to investigate the evolution of simple rules that organisms use to decide whether or not to switch to a protective state. We find evolutionary transitions towards organisms that use a combination of random switching and switching in response to the signal. We also observe that, in spatially heterogeneous environments, selection on the switching strategy depends on the composition of the population, and on population size. These results are in line with recent experiments that showed that many unicellular organisms can attain different phenotypic states in a probabilistic manner, and lead to testable predictions about how this could help organisms cope with unreliable signals.     

Hexose transport in asexual stages of Plasmodium falciparum and kinetoplastidae

The hexose sugar, glucose, is a vital energy source for most organisms and an essential nutrient for asexual stages of Plasmodium falciparum. Kinetoplastid organisms (e.g. Trypanosoma and Leishmania spp) also require glucose at certain critical stages of their life cycles. Although phylogenetically unrelated, these organisms share many common challenges during the mammalian stages of a parasitic life cycle, and possess hexose uptake mechanisms that are amenable to study using similar methods. Defining hexose permeation pathways into parasites might expose an Achilles' heel at which both antidisease and antiparasite measures can be aimed. Understanding the mode of entry of glucose also presents a good general model for substrate acquisition in multicompartment systems. In this review, Sanjeev Krishna and colleagues summarize current understanding of hexose transport processes in P. falciparum and provide a comparison with data obtained from kinetoplastids.     

The solar UV environment and bacterial spore UV resistance: considerations for Earth-to-Mars transport by natural processes and human spaceflight

The environment in space and on planets such as Mars can be lethal to microorganisms because of the high vacuum and high solar radiation flux, in particular UV radiation, in such environments. Spores of various Bacillus species are among the organisms most resistant to the lethal effects of high vacuum and UV radiation, and as a consequence are of major concern for planetary contamination via unmanned spacecraft or even natural processes. This review focuses on the spores of various Bacillus species: (i) their mechanisms of UV resistance; (ii) their survival in unmanned spacecraft, space flight and simulated space flight and Martian conditions; (iii) the UV flux in space and on Mars; (iv) factors affecting spore survival in such high UV flux environments.     

Local adaptation of reproductive performance during thermal stress

Considerable evidence exists for local adaptation of critical thermal limits in ectotherms following adult temperature stress, but fewer studies have tested for local adaptation of sublethal heat stress effects across life‐history stages. In organisms with complex life cycles, such as holometabolous insects, heat stress during juvenile stages may severely impact gametogenesis, having downstream consequences on reproductive performance that may be mediated by local adaptation, although this is rarely studied. Here, we tested how exposure to either benign or heat stress temperature during juvenile and adult stages, either independently or combined, influences egg‐to‐adult viability, adult sperm motility and fertility in high‐ and low‐latitude populations of Drosophila subobscura. We found both population‐ and temperature‐specific effects on survival and sperm motility; juvenile heat stress decreased survival and subsequent sperm motility and each trait was lower in the northern population. We found an interaction between population and temperature on fertility following application of juvenile heat stress; although fertility was negatively impacted in both populations, the southern population was less affected. When the adult stage was also subject to heat stress, the southern population exhibited positive carry‐over effects whereas the northern population's fertility remained low. Thus, the northern population is more susceptible to sublethal reproductive consequences following exposure to juvenile heat stress. This may be common in other organisms with complex life cycles and current models predicting population responses to climate change, which do not take into account the impact of juvenile heat stress on reproductive performance, may be too conservative.     

The morphological and ontogenetic changes in the Protozoa during the transition to a sessile mode of life

The morphological adaptations of protozoans to sessile mode life and evolutionary changes in ontogeny are considered. There are main morphotypes of sessile protists: stalked organisms that attached to substrate by the extended base of body (basal disk), and unstalked organisms that are flatted on substrate. The origin of the morphotypes was independent in different taxa and involved nonhomologous structures. Adaptation to the sessile mode of life in the protists was connected with the progressive increase in the body size and intensity of organelle functions by polymerisation, subsequent division of function and change of functions. Evolution of adhesive organelles is characterised by growing intensity of their functions by allometric growth (usually without polymerisation), and in some cases with the subsequent division of functions and change of functions. The evolution manifests itself primarily in the organelles that provide interaction of cell with environment. The organelles that ensuring functioning of cell change due to correlations with the organelles of the first group. These two groups of organelles are similar to A.N. Sewertsoff's ecto- and endosomatic organs in multicellular organisms. The ontogeny of the sessile protists included three stages: formation of the migratory stage, distribution and choice of substrate and metamorphosis of the migratory stage after adhesion. As a rule there are no recapitulations on the first stage. The majority of structures tomotes or zoospores are inherited from the parent cell. Thus the present of some ancestral characteristics at the earlier stages of protistean ontogeny is display of the Baer's law. The main features of ontogeny evolution in sessile protists are the anaboly of the additional stages of life cycle, the displays of archallaxis or deviation during the migratory stage formation, and anaboly at the stage of buds morphogenesis after adhesion. At the last stage, the study of recapitulations is most perspective with the decision of phylogenetic problems in sessile protists.     

Tardigrades survive exposure to space in low Earth orbit

Vacuum (imposing extreme dehydration) and solar/galactic cosmic radiation prevent survival of most organisms in space [1]. Only anhydrobiotic organisms, which have evolved adaptations to survive more or less complete desiccation, have a potential to survive space vacuum, and few organisms can stand the unfiltered solar radiation in space. Tardigrades, commonly known as water-bears, are among the most desiccation and radiation-tolerant animals and have been shown to survive extreme levels of ionizing radiation [2-4]. Here, we show that tardigrades are also able to survive space vacuum without loss in survival, and that some specimens even recovered after combined exposure to space vacuum and solar radiation. These results add the first animal to the exclusive and short list of organisms that have survived such exposure.     

Using functional morphology to examine the ecology and evolution of specialization

Researchers strive to understand what makes species different,and what allows them to survive in the time and space that theydo. Many models have been advanced which encompass an arrayof ecological, evolutionary, mathematical, and logical principles.The goal has been to develop ecological theories that can, amongother things, make specific and robust predictions about howand where organisms should live and what organisms should utilize.The role of functional morphology is often an under-appreciatedparameter of these models. A more complete understanding ofhow anatomical features work to allow the organism to accomplishcertain tasks has allowed us to revisit some of these ideaswith a new perspective. We illustrate our view of this rolefor functional morphology in ecology by considering the issueof specialization: we attempt to align several definitions ofspecialization based upon shared ecological and evolutionaryprinciples, and we summarize theoretical predictions regardingwhy an organism might specialize. Kinematic studies of preycapture in several types of fishes are explored with regardto the potential ecological and evolutionary consequences ofspecialization, most notably in the area of trade-offs. We suggestthat a functional morphological perspective can increase ourunderstanding of the ecological concepts of specialization andit consequences. The kinds of data that functional morphologistscollect can help us to quantify organismal performance associatedwith specialization and the union of functional morphology withecology can help us to better understand not just how but whyorganisms interact in the manner that they do.     

Optimal life histories with age dependent tradeoff curves

Following the approach of Schaffer (1974, Ecology 55, 291-303.) and Charlesworth & Leon (1976, Am. Nat. 110, 449-459.) the tradeoff between fecundity and survival/growth is investigated in an age-structured population with density independent life history parameters. The results of the above authors are generalized by allowing the tradeoff curve to vary with age; the life cycle is assumed to have two stages: an initial stage during which the organism generally improves in her capacity to reproduce, grow and survive, and a final stage during which her general performance remains constant or declines. The principal result is that, during the final stage, RV per unit size should decrease and over the course of the entire life, should either decrease, or increase at first and then decrease. With the additional assumption that the tradeoff curves at different ages are similar in shape, it is shown that unit fecundity should increase throughout the reproductive life of the organism.     

Potential effects of recent findings on spacecraft sterilization requirements

An important task related to the formulation of planetary quarantine standards is the achievement of an acceptable compromise between (1) the prevention of planetary contamination and (2) the impact of quarantine requirements on the conduct of planetary missions. Such a task is a continuing effort, which must take all pertinent new information into account as it becomes available. This paper provides an analytical framework for the assessment of data which have become available during the past year or which are currently being evolved. In particular an evaluation is made of the probability of release of viable organisms from the spacecraft as a function of: (1) impact velocity magnitudes and the probability of their occurrence; (2) the degree of equipment fracturing at impact velocities; and (3) the number of viable organisms in spacecraft materials. Work being done to quantify each of three types of contamination, i.e. that on open surfaces, mated surfaces and buried contamination, is described in the context of seeking an approach to spacecraft sterilization that would be most compatible with the implementation of planetary missions. It is concluded that the results of work now in progress on spacecraft-material fracturing, on the estimation of buried contamination loads, and on microbial resistance on mated surfaces, may lead to less severe dry-heat sterilization of planetary spacecraft than had been considered necessary in the past.Work reported herein by Exotech Inc. authors has been supported under contract NASw-1558 with the NASA Office of Planetary Program and under contract NASw-1666 with the NASA Office of Biosciences.     

Hatching fraction and timing of resting stage production in seasonal environments: effects of density dependence and uncertain season length

Many organisms survive unfavourable seasons as resting stages, some of which hatch each favourable season. Hatching fraction and timing of resting stage production are important life history variables. We model life cycles of freshwater invertebrates in temporary pools, with various combinations of uncertain season length and density‐dependent fecundity. In deterministic density‐independent conditions, resting stage production begins suddenly. With uncertain season length and density independence, resting stage production begins earlier and gradually. A high energetic cost of resting stages favours later resting stage production and a lower hatching fraction. Deterministic environments with density dependence allow sets of coexisting strategies, dominated by pairs, each switching suddenly to resting stage production on a different date, usually earlier than without density dependence. Uncertain season length and density dependence allow a single evolutionarily stable strategy, around which we observe many mixed strategies with negatively associated yield (resting stages per initial active stage) and optimal hatching fraction.     

The ghosts of selection past reduces the probability of plastic rescue but increases the likelihood of evolutionary rescue to novel stressors in experimental populations of wild yeast

Persistence by adaptation is called evolutionary rescue. Evolutionary rescue is more likely in populations that have been previously exposed to lower doses of the same stressor. Environmental fluctuations might also reduce the possibility of rescue, but little is known about the effect of evolutionary history on the likelihood of rescue. In this study, we hypothesised that the ubiquitous operation of generalised stress responses in many organisms increases the likelihood of rescue after exposure to other stressors. We tested this hypothesis with experimental populations that had been exposed to long‐term starvation and were then selected on different, unrelated stressors. We found that prior adaptation to starvation imposes contrary effects on the plastic and evolutionary responses of populations to subsequent stressors. When first exposed to new stressors, such populations become extinct more often. If they survive the initial exposure to the new stressors, however, they are more likely to undergo evolutionary rescue.     

Niche construction through phenological plasticity: life history dynamics and ecological consequences

The ability of an organism to alter the environment that it experiences has been termed 'niche construction'. Plants have several ways whereby they can determine the environment to which they are exposed at different life stages. This paper discusses three of these: plasticity in dispersal, flowering timing and germination timing. It reviews pathways through which niche construction alters evolutionary and ecological trajectories by altering the selective environment to which organisms are exposed, the phenotypic expression of plastic characters, and the expression of genetic variation. It provides examples whereby niche construction creates positive or negative feedbacks between phenotypes and environments, which in turn cause novel evolutionary constraints and novel life-history expression.     

Migration-Induced Architectures of Planetary Systems

The recent increase in number of known multi-planet systems gives a unique opportunity to study the processes responsible for planetary formation and evolution. Special attention is given to the occurrence of mean-motion resonances, because they carry important information about the history of the planetary systems. At the early stages of the evolution, when planets are still embedded in a gaseous disc, the tidal interactions between the disc and planets cause the planetary orbital migration. The convergent differential migration of two planets embedded in a gaseous disc may result in the capture into a mean-motion resonance. The orbital migration taking place during the early phases of the planetary system formation may play an important role in shaping stable planetary configurations. An understanding of this stage of the evolution will provide insight on the most frequently formed architectures, which in turn are relevant for determining the planet habitability. The aim of this paper is to present the observational properties of these planetary systems which contain confirmed or suspected resonant configurations. A complete list of known systems with such configurations is given. This list will be kept by us updated from now on and it will be a valuable reference for studying the dynamics of extrasolar systems and testing theoretical predictions concerned with the origin and the evolution of planets, which are the most plausible places for existence and development of life.     

Terre et Vie : des histoires imbriquées

The Earth's history consists in recurrent flashbacks of similar events and scenarios: redistribution of continents, orogenic cycles, glaciations, marine transgressions and regressions, etc. In contrast, Life's history evolves according to a succession of stages: prokaryotic stage, eukaryotic cell stage, pluricellular organism stage, terrestrialization, development of animal societies, hominization. With each successive stage the biosphere rises to a higher level of organization and complexity. This evolution results from the natural trend of living organisms to extend their control over the entire planet while they progressively escape the constraints of the aquatic environments and climates. During the last four billion years close and complex interactions prevailed between the history of both the Earth and Life. Living organisms have a profound effect on their environment and on the processes of the Earth dynamics, while the planetary environment controls the evolution of living species. Nevertheless, from time to time, the fragile equilibrium established between Earth's and Life's dynamics breaks down and triggers mass extinctions. It is presently the case of the increasing impact of human activities on the integrity of our planet, a major challenge for humankind during the 21st century.     

Portals of entry and systemic localization of proliferative gill disease organisms in channel catfish Ictalurus punctatus

Immunized rabbit serum adsorbed in live catfish was used in indirect fluorescent antibody test (IFAT) to detect developmental life stages of Henneguya ictaluri n. sp. This myxozoan parasite is associated with proliferative gill disease in channel catfish Ictalurus punctatus (Rafinesque) in the USA. Specific pathogen free fingerlings were experimentally infected with the actinosporean stage of H. ictaluri and necropsied 24, 48, 72, and 96 h post-infection. At 24 h post-infection parasite stages were observed primarily in the gastric mucosa and submucosa but were also observed in the skin and buccal cavity. Ovoid organisms were detected in heart and blood vessels of the liver. From 48 to 72 h after exposure, fewer fluorescent organisms were located in all organs, with the exception of the gills, than were observed at 24 h. These organisms appeared to be degenerating except for those in the gills, which appeared to be multinucleated. By 96 h post-infection, the organisms could not be detected in fish tissues with the exception of the stages in the gills, which appeared to be a preferred site of development. Throughout the entire 96 h period of study, no stage of the organism was detected in the brain. Infected tissue sections treated with non-immune rabbit serum and non-infected tissue sections treated with immune rabbit sera all showed negative results by IFAT.