Evolution of root nodule symbiosis: Focusing on the transcriptional regulation from the genomic point of view

Since molecular phylogenetics recognized root nodule symbiosis (RNS) of all lineages as potentially homologous, scientists have tried to understand the “when” and the “how” of RNS evolution. Initial progress was made on understanding the timing of RNS evolution, facilitating our progress on understanding the underlying genomic changes leading to RNS. Here, we will first cover the different hypotheses on the timings of gains/losses of RNS and show how this has helped us understand how RNS has evolved. Finally, we will discuss how our improved understanding of the genetic changes that led to RNS is now helping us refine our understanding on when RNS has evolved.     

A method to the madness: Disentangling the individual forces that shape the rumen microbiome

The rumen microbiome ‐ a remarkable example of obligatory symbiosis with high ecological and social relevance Subject Categories: Digestive System, Ecology, Microbiology, Virology & Host Pathogen Interaction

Ruminants are intimately linked to mankind since their domestication some 8,000 years ago, and their close relationship may have well been one of the main drivers of human civilization (Diamond, 1997). Ruminants—cattle, sheep, goats, deer, gazelles, and so on—also embody the close link between solar energy transformed via photosynthesis and digestion into consumable products, such as meat, milk, leather, or wool, that have sustained humanity for millennia. Throughout this shared history, constant improvements through breeding, husbandry, and industrial livestock farming have greatly increased the production of milk, meat, and other animal‐based products.Ruminants, more so than any other mammalian group also represent the epitome of mammalian‐microbe symbiosis, as they rely completely on microbial fermentation to sustain their lives. In the rumen, the fermentative organ situated in the upper gastrointestinal tract resides a vast microbial community from all domains of life—bacteria, archaea, and eukarya—that turn indigestible plant feed into food for the animal. The rumen microbiome produces up to 70% of the energy the animal needs for growth and maintenance, and, from mankind''s perspective, for the production of food and other consumables.

Ruminants, more so than any other mammalian group, also represent the epitome of mammalian‐microbe symbiosis, as they rely completely on microbial fermentation to sustain their lives.

With growing understanding that these microorganisms are responsible for degrading plant material and supplying nutrients for the animals, a new research discipline emerged along with aspirations to improve the yield of livestock farming. While most research had understandably focused on production efficiency, it also showed that the rumen microbiome is intricately linked to many other phenotypes of the animal. This understanding comes at a time when we increasingly realize that mankind''s actions have a detrimental effect on the environment. The microbial fermentation in the rumen produces large amounts of methane, a potent greenhouse gas that has been demonstrated to contribute to global climate change. We therefore need to consider both our increased demand for meat and milk products and aim to mitigate the negative environmental impact of intensive livestock farming. Modulating the microbial community to sustain or further increase productivity while decreasing methane emissions has indeed become a major goal for microbial ecologists studying the rumen microbiome and its interactions with the host animal. In this article, we discuss the driving forces that affect the establishment and composition of the rumen microbiome and its plasticity, and potential avenues for harnessing these forces for a more sustainable production of animal products.     

Do we still need animals? Surveying the role of animal‐free models in Alzheimer’s and Parkinson’s disease research

The use of animals in neuroscience and biomedical research remains controversial. Policy is built around the “3R” principle of “Refining, Reducing and Replacing” animal experiments, and across the globe, different initiatives stimulate the use of animal‐free methods. Based on an extensive literature screen to map the development and adoption of animal‐free methods in Alzheimer''s and Parkinson''s disease research, we find that at least two in three examined studies rely on animals or on animal‐derived models. Among the animal‐free studies, the relative contribution of innovative models that may replace animal experiments is limited. We argue that the distinction between animal research and alternative models presents a false dichotomy, as the role and scientific value of both animal and animal‐free approaches are intertwined. Calls to halt all animal experiments appear premature, as insufficient non‐animal‐based alternatives are available and their development lags behind. In light of this, we highlight the need for objective, unprejudiced monitoring, and more robust performance indicators of animal‐free approaches.     

A model of naturalistic decision making in preference tests

Decisions as to whether to continue with an ongoing activity or to switch to an alternative are a constant in an animal’s natural world, and in particular underlie foraging behavior and performance in food preference tests. Stimuli experienced by the animal both impact the choice and are themselves impacted by the choice, in a dynamic back and forth. Here, we present model neural circuits, based on spiking neurons, in which the choice to switch away from ongoing behavior instantiates this back and forth, arising as a state transition in neural activity. We analyze two classes of circuit, which differ in whether state transitions result from a loss of hedonic input from the stimulus (an “entice to stay” model) or from aversive stimulus-input (a “repel to leave” model). In both classes of model, we find that the mean time spent sampling a stimulus decreases with increasing value of the alternative stimulus, a fact that we linked to the inclusion of depressing synapses in our model. The competitive interaction is much greater in “entice to stay” model networks, which has qualitative features of the marginal value theorem, and thereby provides a framework for optimal foraging behavior. We offer suggestions as to how our models could be discriminatively tested through the analysis of electrophysiological and behavioral data.     

Characterization and Function of the First Antibiotic Isolated from a Vent Organism: The Extremophile Metazoan Alvinella pompejana

The emblematic hydrothermal worm Alvinella pompejana is one of the most thermo tolerant animal known on Earth. It relies on a symbiotic association offering a unique opportunity to discover biochemical adaptations that allow animals to thrive in such a hostile habitat. Here, by studying the Pompeii worm, we report on the discovery of the first antibiotic peptide from a deep-sea organism, namely alvinellacin. After purification and peptide sequencing, both the gene and the peptide tertiary structures were elucidated. As epibionts are not cultivated so far and because of lethal decompression effects upon Alvinella sampling, we developed shipboard biological assays to demonstrate that in addition to act in the first line of defense against microbial invasion, alvinellacin shapes and controls the worm''s epibiotic microflora. Our results provide insights into the nature of an abyssal antimicrobial peptide (AMP) and into the manner in which an extremophile eukaryote uses it to interact with the particular microbial community of the hydrothermal vent ecosystem. Unlike earlier studies done on hydrothermal vents that all focused on the microbial side of the symbiosis, our work gives a view of this interaction from the host side.     

Natural experiments and long-term monitoring are critical to understand and predict marine host–microbe ecology and evolution

Marine multicellular organisms host a diverse collection of bacteria, archaea, microbial eukaryotes, and viruses that form their microbiome. Such host-associated microbes can significantly influence the host’s physiological capacities; however, the identity and functional role(s) of key members of the microbiome (“core microbiome”) in most marine hosts coexisting in natural settings remain obscure. Also unclear is how dynamic interactions between hosts and the immense standing pool of microbial genetic variation will affect marine ecosystems’ capacity to adjust to environmental changes. Here, we argue that significantly advancing our understanding of how host-associated microbes shape marine hosts’ plastic and adaptive responses to environmental change requires (i) recognizing that individual host–microbe systems do not exist in an ecological or evolutionary vacuum and (ii) expanding the field toward long-term, multidisciplinary research on entire communities of hosts and microbes. Natural experiments, such as time-calibrated geological events associated with well-characterized environmental gradients, provide unique ecological and evolutionary contexts to address this challenge. We focus here particularly on mutualistic interactions between hosts and microbes, but note that many of the same lessons and approaches would apply to other types of interactions.

This Essay argues that in order to truly understand how marine hosts benefit from the immense diversity of microbes, we need to expand towards long-term, multi-disciplinary research focussing on few areas of the world’s ocean that we refer to as “natural experiments,” where processes can be studied at scales that far exceed those captured in laboratory experiments.     

Are British urban foxes (Vulpes vulpes) “bold”? The importance of understanding human–wildlife interactions in urban areas

Human–wildlife interactions are believed to be increasing in urban areas. In Britain, numerous media reports have stated that urban foxes (Vulpes vulpes) are becoming “bolder,” thereby posing a risk to public safety. However, such claims overlook how an individual''s personality might influence urban fox behavior. Personality determines multiple aspects of an animal''s interactions with both conspecifics and its environment, and can have a significant impact on how people perceive wildlife. Furthermore, describing urban foxes as “bold” confounds two different but inter‐related behaviors, both of which influence an animal''s propensity to take risks. Neophobia affects an animal''s reaction to novelty, wariness its reaction to potential threats. Since urban wildlife frequently encounters both novel and threatening stimuli, a highly adaptable species such as the red fox might be predicted to exhibit reduced neophobia and wariness. We investigated how social status influenced both behaviors in Bristol''s fox population. Dominant foxes were significantly more neophobic and warier than subordinates, which adopt a more exploratory and risk‐taking lifestyle to meet their energetic and other needs. We found no seasonal effect on neophobia and wariness, although this may be due to sample size. The presence of conspecifics decreased neophobia for dominants, and wariness for both dominants and subordinates. We highlight the importance of considering animal social status and personality when planning management protocols, since interventions that destabilize fox social groups are likely to increase the number of subordinate foxes in the population, thereby increasing rather than decreasing the number of interactions between humans and urban foxes.     

Revisiting Metchnikoff's work in light of the COVID-19 pandemic

Revisiting Metchnikoff''s work in light of the COVID-19 pandemic illustrates how much this amazing scientist was a polymath, and one could speculate how much he would have been fascinated and most interested in following the course of the pandemic. Since he coined the word “gerontology”, he would have been intrigued by the high mortality among the elderly, and by the concepts of immunosenescence and inflammaging that characterize the SARS-CoV-2 infection. While Metchnikoff''s work is mainly associated with the discovery of the phagocytes and the birth of cellular innate immunity, he regularly invited his closest collaborators to investigate humoral immunity, and it was in his laboratory that Jules Bordet made his major discovery of the complement system. While Metchnikoff and his team investigated many infectious diseases, he also contributed to studies linked to vaccination, such as those on typhoid fever performed in chimpanzees, illustrating that non-human primates can provide animal models which are potentially helpful for understanding the pathophysiology of the COVID-19 virus. In the present review, we illustrate how much his own work and the investigations of his trainees were pertinent to this new disease.     

Where Next for Microbiome Research?

The development of high-throughput sequencing technologies has transformed our capacity to investigate the composition and dynamics of the microbial communities that populate diverse habitats. Over the past decade, these advances have yielded an avalanche of metagenomic data. The current stage of “van Leeuwenhoek”–like cataloguing, as well as functional analyses, will likely accelerate as DNA and RNA sequencing, plus protein and metabolic profiling capacities and computational tools, continue to improve. However, it is time to consider: what’s next for microbiome research? The short pieces included here briefly consider the challenges and opportunities awaiting microbiome research.

This Perspective is part of the “Where next?” Series.

Soon, we will enter an era when “the number of population genomes deposited in public databases will dwarf those from isolates and single cells” (Gene Tyson). Clearly, as all authors noted in the following, our focus will move from describing the composition of microbial communities to elucidating the principles that govern their assembly, dynamics, and functions. How will such principles be discovered? Elhanan Borenstein proposes that a systems biology–based approach, particularly the development of mathematical and computational models of the interactions between the specific community components, will be critical for understanding the function and dynamics of microbiomes. Evolutionary biologists Howard Ochman and Andrew Moeller want to decipher how microbial assemblies evolve but challenge us to also consider the role of microbial communities in organismal evolution, and they make the exciting prediction that microbes will be implicated in the evolution of eusociality and cooperation. Brett Finlay underscores the need for deciphering the mechanistic bases—particularly the chemical/metabolite signals—for interactions between members of microbial communities and their hosts. He emphasizes how this knowledge will enable creation of new tools to manipulate the microbiota, a key challenge for future investigation. Heidi Kong also encourages deciphering the mechanisms that underlie associations between particular skin surfaces and disorders and their respective microbiota. Jeffrey Gordon considers several intriguing opportunities as well as challenges that manipulation of the gut microbiota presents for improved human nutrition and health. Finally, Karen Nelson, Karim Dabbagh and Hamilton Smith suggest that using synthetic genomes to create novel microbes or even synthetic microbiomes offers a new way to engineer the microbiota. Overall, future microbiome research regarding the molecules and mechanisms mediating interactions between members of microbial communities and their hosts should lead to discovery of exciting new biology and transformative therapeutics.     

Identifying Keystone Species in the Human Gut Microbiome from Metagenomic Timeseries Using Sparse Linear Regression

Human associated microbial communities exert tremendous influence over human health and disease. With modern metagenomic sequencing methods it is now possible to follow the relative abundance of microbes in a community over time. These microbial communities exhibit rich ecological dynamics and an important goal of microbial ecology is to infer the ecological interactions between species directly from sequence data. Any algorithm for inferring ecological interactions must overcome three major obstacles: 1) a correlation between the abundances of two species does not imply that those species are interacting, 2) the sum constraint on the relative abundances obtained from metagenomic studies makes it difficult to infer the parameters in timeseries models, and 3) errors due to experimental uncertainty, or mis-assignment of sequencing reads into operational taxonomic units, bias inferences of species interactions due to a statistical problem called “errors-in-variables”. Here we introduce an approach, Learning Interactions from MIcrobial Time Series (LIMITS), that overcomes these obstacles. LIMITS uses sparse linear regression with boostrap aggregation to infer a discrete-time Lotka-Volterra model for microbial dynamics. We tested LIMITS on synthetic data and showed that it could reliably infer the topology of the inter-species ecological interactions. We then used LIMITS to characterize the species interactions in the gut microbiomes of two individuals and found that the interaction networks varied significantly between individuals. Furthermore, we found that the interaction networks of the two individuals are dominated by distinct “keystone species”, Bacteroides fragilis and Bacteroided stercosis, that have a disproportionate influence on the structure of the gut microbiome even though they are only found in moderate abundance. Based on our results, we hypothesize that the abundances of certain keystone species may be responsible for individuality in the human gut microbiome.     

Impediment to Symbiosis Establishment between Giant Clams and Symbiodinium Algae Due to Sterilization of Seawater

To survive the juvenile stage, giant clam juveniles need to establish a symbiotic relationship with the microalgae Symbiodinium occurring in the environment. The percentage of giant clam juveniles succeeding in symbiosis establishment (“symbiosis rate”) is often low, which is problematic for seed producers. We investigated how and why symbiosis rates vary, depending on whether giant clam seeds are continuously reared in UV treated or non treated seawater. Results repeatedly demonstrated that symbiosis rates were lower for UV treated seawater than for non treated seawater. Symbiosis rates were also lower for autoclaved seawater and 0.2-µm filtered seawater than for non treated seawater. The decreased symbiosis rates in various sterilized seawater suggest the possibility that some factors helping symbiosis establishment in natural seawater are weakened owing to sterilization. The possible factors include vitality of giant clam seeds, since additional experiments revealed that survival rates of seeds reared alone without Symbiodinium were lower in sterilized seawater than in non treated seawater. In conclusion, UV treatment of seawater was found to lead to decreased symbiosis rates, which is due possibly to some adverse effects common to the various sterilization techniques and relates to the vitality of the giant clam seeds.     

When a Text Is Translated Does the Complexity of Its Vocabulary Change? Translations and Target Readerships

In linguistic studies, the academic level of the vocabulary in a text can be described in terms of statistical physics by using a “temperature” concept related to the text''s word-frequency distribution. We propose a “comparative thermo-linguistic” technique to analyze the vocabulary of a text to determine its academic level and its target readership in any given language. We apply this technique to a large number of books by several authors and examine how the vocabulary of a text changes when it is translated from one language to another. Unlike the uniform results produced using the Zipf law, using our “word energy” distribution technique we find variations in the power-law behavior. We also examine some common features that span across languages and identify some intriguing questions concerning how to determine when a text is suitable for its intended readership.     

Novel method for evaluating the health condition of mice in space through a video downlink

Clarification of the criteria for managing animal health is essential to increase the reliability of experiments and ensure transparency in animal welfare. For experiments performed in space, there is no consensus on how to care for animals owing to technical issues, launch mass limitation, and human resources. Some biological processes in mammals, such as musculoskeletal or immune processes, are altered in the space environment, and mice in space can be used to simulate morbid states, such as senescence acceleration. Thus, there is a need to establish a novel evaluation method and evaluation criteria to monitor animal health. Here, we report a novel method to evaluate the health of mice in space through a video downlink in a series of space experiments using the Multiple Artificial-gravity Research System (MARS). This method was found to be more useful in evaluating animal health in space than observations and body weight changes of the same live mice following their return to Earth. We also developed criteria to evaluate health status via a video downlink. These criteria, with “Fur condition” and “Respiratory” as key items, provided information on the daily changes in the health status of mice and helped to identify malfunctions at an early stage. Our method and criteria led to the success of our missions, and they will help establish appropriate rules for space experiments in the future.     

Detailed Analysis of the Microbial Population in Malaysian Spontaneous Cocoa Pulp Fermentations Reveals a Core and Variable Microbiota

The fermentation of cocoa pulp is one of the few remaining large-scale spontaneous microbial processes in today''s food industry. The microbiota involved in cocoa pulp fermentations is complex and variable, which leads to inconsistent production efficiency and cocoa quality. Despite intensive research in the field, a detailed and comprehensive analysis of the microbiota is still lacking, especially for the expanding Asian production region. Here, we report a large-scale, comprehensive analysis of four spontaneous Malaysian cocoa pulp fermentations across two time points in the harvest season and two fermentation methods. Our results show that the cocoa microbiota consists of a “core” and a “variable” part. The bacterial populations show a remarkable consistency, with only two dominant species, Lactobacillus fermentum and Acetobacter pasteurianus. The fungal diversity is much larger, with four dominant species occurring in all fermentations (“core” yeasts), and a large number of yeasts that only occur in lower numbers and specific fermentations (“variable” yeasts). Despite this diversity, a clear pattern emerges, with early dominance of apiculate yeasts and late dominance of Saccharomyces cerevisiae. Our results provide new insights into the microbial diversity in Malaysian cocoa pulp fermentations and pave the way for the selection of starter cultures to increase efficiency and consistency.     

Is the Mutation Rate Lower in Genomic Regions of Stronger Selective Constraints?

A study of the plant Arabidopsis thaliana detected lower mutation rates in genomic regions where mutations are more likely to be deleterious, challenging the principle that mutagenesis is blind to its consequence. To examine the generality of this finding, we analyze large mutational data from baker''s yeast and humans. The yeast data do not exhibit this trend, whereas the human data show an opposite trend that disappears upon the control of potential confounders. We find that the Arabidopsis study identified substantially more mutations than reported in the original data-generating studies and expected from Arabidopsis'' mutation rate. These extra mutations are enriched in polynucleotide tracts and have relatively low sequencing qualities so are likely sequencing errors. Furthermore, the polynucleotide “mutations” can produce the purported mutational trend in Arabidopsis. Together, our results do not support lower mutagenesis of genomic regions of stronger selective constraints in the plant, fungal, and animal models examined.     

Mycobacterium tuberculosis: the honey badger of pathogens

Mycobacterium tuberculosis is a fascinating object of study: it is one of the deadliest pathogens of humankind, able to fend off persistent attacks by the immune system or drugs Subject Categories: Immunology, Microbiology, Virology & Host Pathogen Interaction, Chemical Biology

I have always been interested in infectious diseases since I began to study biology. As a graduate student, my pathogen of choice was Salmonella typhimurium, which typically causes diarrhea that can potentially lead to death. Salmonella''s rapid doubling time, and the availability of elegant genetic tools, a wealth of reagents, and a robust animal infection model put this bug at the apex of ideal host–pathogen systems to study. After I finished my PhD studies—and for reasons to be told another day—my career took an unexpected detour into an area of research I never thought I would be interested in: I went from the sublime to the ridiculous, from Salmonella to Mycobacterium tuberculosis (Mtb), an excruciatingly slow‐growing bacillus with few genetic tools, a paucity of reagents, and an animal model in which an experiment can take a year or longer. Having said all of that, I love working on this pathogen.For those of you who do not know much about Mtb, it is the world''s deadliest bacterium that causes the disease tuberculosis (TB). As Mtb is spread in aerosol droplets coughed up by infected individuals, TB is highly contagious, and about one‐third of the world''s population may be infected with Mtb, although this number has been reasonably challenged (Behr et al, 2021). Even if the numbers of latent or asymptomatic infections are debated, there are some back‐of‐the‐envelope estimates that Mtb has killed more than a billion humans over the millennia. Although TB is often treatable with antibiotics and most Mtb‐infected healthy individuals are asymptomatic, the appearance of multi‐drug‐resistant Mtb and HIV/AIDS has further increased the number of deaths caused by this pathogen.How has Mtb become such a successful pathogen? For one, we lack an effective vaccine to prevent infection. Many readers may point out that they have themselves been given a TB vaccine; known as “BCG” for bacille Calmette–Guérin, this is a laboratory‐attenuated strain of a species related to Mtb called Mycobacterium bovis. While BCG does provide some protection for children against TB, BCG is essentially ineffective against pulmonary TB in adults. For this reason, it is not used in the USA and many other countries.Another major challenge to treating TB has been a lack of antimicrobials that can access Mtb bacilli in privileged sites known as granulomas, which are cell‐fortified structures our immune system builds to contain microbial growth. In addition to the granuloma walls, Mtb has a highly complex cell envelope that protects it from many small molecules. I imagine that antimicrobial molecules have the challenging task of reaching an enemy shielded in armor, hiding deep inside a castle keep, and surrounded by a vast moat, and an army of orcs.On top of these therapeutic barriers, most antimicrobials work on metabolically active or growing bacteria. Mtb, however, grows very slowly, with a doubling time under optimal laboratory conditions of about 20 h—compared with 20 min for Salmonella. Moreover, Mtb is believed to enter a “persistent” or “latent” state in its natural host with limited cell divisions. This extremely slow growth makes treatment a long and tedious prospect: 6–12 months of antibiotic treatment are generally required, during which time one cannot drink alcohol due to the potential liver toxicity of the drugs. Believe it or not, there are people who would rather refuse TB treatment than give up alcohol for a few months. Additionally, the perception of “feeling cured” after a few weeks of TB therapy can also lead to a lapse in compliance. The consequence of failing to clear a partially treated infection is the emergence of drug resistance, which has created strains that are extensively resistant to most frontline TB drugs.When thinking about the difficulty of curing Mtb infections, I am reminded of the fierce and fearless honey badger, which came to fame through a viral YouTube video. The narrator points out how honey badgers “don''t care” about battling vicious predators in order to get food: venomous snakes, stinging bees—you name it. I once saw a photo of a honey badger that looked more like a pin cushion, harpooned with numerous porcupine quills. This battle royale of the wilderness is a perfect analogy of Mtb versus the immune system: Like the honey badger, Mtb really don''t care.Vaccines primarily work by coaxing our immune system to make antibodies that neutralize foreign invaders, most typically viruses, but also bacteria, some of which have evolved mechanisms to evade detection by antibodies or otherwise render them useless. In most cases, phagocytes then gobble up and kill invading bacteria. While phagocytes are critical in controlling Mtb infections, it is unclear which of their molecules or “effectors” act as executioners of Mtb. For example, nitric oxide and copper play roles in controlling Mtb in a mouse model, but it is unknown how these molecules exert their host‐protective activity, and whether or not they play a similar role in humans. Furthermore, despite the production of these antibacterial effectors—the “porcupine quills”—Mtb often persists due to intrinsic resistance mechanisms. Thus, while our immune system may have the tools to keep Mtb under control, it falls short of eradicating it from our bodies and, in many cases, fails to prevent the progression of the disease. Perhaps a most worrying observation is that prior infection, which is generally considered the most effective path to immunity for many infectious diseases, does not consistently protect against reinfection with Mtb.The above facts have left the TB field scrambling to identify new ways to fight this disease. Much of this work requires that researchers understand both the fundamental processes of the bacterium and its host. Studies of human populations around the globe have revealed differences in susceptibility to infection, the genetic and immunological bases of which are being investigated (Bellamy et al, 2000; Berry et al, 2010; Möller et al, 2010). These studies have made researchers increasingly aware that how the immune system responds to Mtb may play a critical role in disease control. For example, understanding why some individuals or populations are more or less susceptible to TB may help in the development of better vaccines. Also, the more we understand what makes this pathogen so resilient to the immune system could facilitate the development of new antibacterial drugs or host‐directed therapies. These questions can only be answered once we fully understand how the host combats Mtb infections, and how the bacteria counteract these host defenses. While it is a daunting endeavor, my hope is that the efforts of many laboratories around the world will get a better understanding of the host–Mtb interface and ultimately help to eradicate this disease for good.     

Modeling the impact of single-cell stochasticity and size control on the population growth rate in asymmetrically dividing cells

Microbial populations show striking diversity in cell growth morphology and lifecycle; however, our understanding of how these factors influence the growth rate of cell populations remains limited. We use theory and simulations to predict the impact of asymmetric cell division, cell size regulation and single-cell stochasticity on the population growth rate. Our model predicts that coarse-grained noise in the single-cell growth rate λ decreases the population growth rate, as previously seen for symmetrically dividing cells. However, for a given noise in λ we find that dividing asymmetrically can enhance the population growth rate for cells with strong size control (between a “sizer” and an “adder”). To reconcile this finding with the abundance of symmetrically dividing organisms in nature, we propose that additional constraints on cell growth and division must be present which are not included in our model, and we explore the effects of selected extensions thereof. Further, we find that within our model, epigenetically inherited generation times may arise due to size control in asymmetrically dividing cells, providing a possible explanation for recent experimental observations in budding yeast. Taken together, our findings provide insight into the complex effects generated by non-canonical growth morphologies.     

Dogs (Canis familiaris) Evaluate Humans on the Basis of Direct Experiences Only

Reputation formation is a key component in the social interactions of many animal species. An evaluation of reputation is drawn from two principal sources: direct experience of an individual and indirect experience from observing that individual interacting with a third party. In the current study we investigated whether dogs use direct and/or indirect experience to choose between two human interactants. In the first experiment, subjects had direct interaction either with a “nice” human (who played with, talked to and stroked the dog) or with an “ignoring” experimenter who ignored the dog completely. Results showed that the dogs stayed longer close to the “nice” human. In a second experiment the dogs observed a “nice” or “ignoring” human interacting with another dog. This indirect experience, however, did not lead to a preference between the two humans. These results suggest that the dogs in our study evaluated humans solely on the basis of direct experience.