Why do some coronaviruses become pandemic threats when others do not?

Despite multiple spillover events and short chains of transmission on at least 4 continents, Middle East Respiratory Syndrome Coronavirus (MERS-CoV) has never triggered a pandemic. By contrast, its relative, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has, despite apparently little, if any, previous circulation in humans. Resolving the unsolved mystery of the failure of MERS-CoV to trigger a pandemic could help inform how we understand the pandemic potential of pathogens, and probing it underscores a need for a more holistic understanding of the ways in which viral genetic changes scale up to population-level transmission.

What does it take for a coronavirus to become a pandemic threat? And why has MERS-CoV repeatedly failed to emerge as a pandemic pathogen when SARS-CoV-2 has successfully spread around the world?     

Why are some snails visibly polymorphic,and others not?

Work on Cepaea land-snails since 1950 is surveyed, and various explanations for their visible polymorphism, including predator selection, the influence of sunlight and temperature, co-adaptation and linkage disequilibrium, and "area effects", are discussed. All of these can modify the genetic make-up of natural populations in particular circumstances, but none provide a satisfactory answer to the question of why some species are visibly polymorphic whereas others present a uniform external appearance. The likely explanation is that probably all species are genetically heterogeneous at numerous loci, as a result of heterozygote advantage, co-adaptation and other selective factors maintaining the different alleles in equilibrium, which may sometimes have visible effects on the phenotype. If these are positively disadvantageous, selection for epistatic genes will suppress the visible polymorphism, without affecting the underlying genetical heterogeneity, preserved by selection for other non-visible pleiotropic effects of the alleles involved. But this will not happen if the visibly distinct effects of these different allelomorphs are selectively more or less neutral. Many examples of polymorphism, including the so-called 'pseudo'-polymorphism, are therefore essentially non-adaptive so far as their visible manifestation is concerned, being maintained as balanced polymorphism by selection for non-visible pleiotropic effects of the genes involved.     

Why do some nectar foragers perch and others hover while probing flowers?

Diurnal hawkmoths, Hemaris fuciformis, and bumblebees, Bombus pasquorum, were observed foraging for nectar in flowers of Viscaria vulgaris. The hawkmoths hovered in front of the flowers, while the bees perched on them. The hawkmoths had a faster probing rate than the bees, and consequently also had higher gross and net rates of energy gain. A model is presented that shows that hovering only yields a higher net rate of energy gain (NREG) than perching when nectar volumes are high due to low competition for the resource. The difference in NREG of perchers and hoverers decreases with an increase of competition, and eventually perching yields the highest NREG. This is an effect of the higher cost of hovering. The results suggest that hovering can only evolve as a pure evolutionarily stable strategy (ESS) if competition is reduced, for example by co-evolutionary specializations with plants. The possibility that it has evolved as a mixed ESS (i.e. individuals can both hover and perch depending on the resource level) is discussed. The evolution of optimal foraging strategies is discussed, and it is pointed out that the rate of gain of an animal is independent of the strategy used when all competing foragers use the same strategy, but competitively superior strategies will nevertheless evolve because they are ESSs. Competition between strategies with different energy costs are special, because resource availability determines which strategy is competitively superior. A high-cost strategy can only evolve as a pure ESS at high resource levels, or as a mixed ESS at intermediate levels.     

Why do some fish do it younger than others? learning from experiments

Atlantic salmon in Maine were once abundant but have become depleted, and are listed as endangered under the federal Endangered Species Act. Historically, salmon numbers in Maine may have been as high as 100 000 adults, but habitat loss, pollution and overfishing have contributed to the decline of the species. In 2000, only 110 adults returned to spawn in Maine rivers. Maine produces c. 15 000 metric tons/year of aquacultured Atlantic salmon from a total of nearly 600 coastal net pens. Escapees from these pens may interact with the wild salmon. The dynamics of salmon populations under such conditions are poorly understood. In order to illuminate the role aquaculture may play in such a system, we have developed a model for simulating population trajectories for both wild salmon and competing populations derived from aquaculture escapes. The model simulates a small population of wild salmon based in a stream/estuary system, into which an aquaculture facility is losing fish to escapes. Biological parameters in the model were estimated as much as possible from data in the USFWS report on Maine salmon. We used the model to investigate the consequences of a variety of ecological interactions between the wild and cultured fish including competitive, genetic and disease effects. Initial results indicate that many of these effects allow the aquaculture‐derived population to supplant the wild fish, but that wild populations may still persist under some conditions.     

Why dengue and yellow fever coexist in some areas of the world and not in others?

Urban yellow fever and dengue coexist in Africa but not in Asia and South America. In this paper, we examine four hypotheses (and various combinations thereof) to explain the absence of yellow fever in urban areas of Asia and South America. In addition, we examine an additional hypothesis that offers an explanation of the coexistence of the infections in Africa while at the same time explaining their lack of coexistence in Asia. The hypotheses we tested to explain the nonexistence of yellow fever in Asia are the following: (1) the Asian Aedes aegypti is relatively incompetent to transmit yellow fever; (2) there would exist a competition between dengue and yellow fever viruses within the mosquitoes, as suggested by in vitro studies in which the dengue virus always wins; (3) when an A. aegypti mosquito that is infected by or latent for yellow fever acquires dengue, it becomes latent for dengue due to internal competition within the mosquito between the two viruses; (4) there is an important cross-immunity between yellow fever and other flaviviruses, dengue in particular, such that a person recovered from a bout of dengue exhibits a diminished susceptibility to yellow fever. This latter hypothesis is referred to below as the "Asian hypothesis." Finally, we hypothesize that: (5) the coexistence of the infections in Africa is due to the low prevalence of the mosquito Aedes albopictus in Africa, as it competes with A. aegypti. We will refer to this latter hypothesis as the "African hypothesis." We construct a model of transmission that allows all of the above hypotheses to be tested. We conclude that the Asian and the African hypotheses can explain the observed phenomena, whereas other hypotheses fail to do so.     

Why are some people bitten more than others?

Much progress has been made in describing how it is, in a mechanistic sense, that some vertebrate hosts (species or individuals) are bitten more than others, principally because of their odour or appearance. Little attention has been paid to why, in an evolutionary sense, these particular animals are bitten. Irrespective of the proximate mechanisms of host choice, there must be an intense selection pressure on insects to feed on those hosts that are most amenable to being bitten. We should be better able to predict host choice by understanding the evolutionary processes at work.     

Why some parasites are widespread and abundant while others are local and rare?

Abundances and distributions of species are usually associated. This implies that as a species declines in abundance so does the number of sites it occupies. Conversely, when there is an increase in a species' range size, it is usually followed by an increase in population size (Gaston et al. 2000 ). This ecological phenomenon, also known as the abundance–occupancy relationship (AOR), is well documented in several species of animals and plants (Gaston et al. 2000 ) but has been little investigated in parasites. In this issue of Molecular Ecology, Drovetski et al. ( 2014 ) investigated the AOR in avian haemosporidians (vector‐borne blood parasites) using data from four well‐sampled bird communities. In support of the AOR, the research group found that the abundance of parasite cytochrome b lineages (a commonly used proxy for species identification within this group of parasites) was positively linked with the abundance of susceptible avian host species and that the most abundant haemospordian lineages were those with larger ranges. Drovetski et al. ( 2014 ) also found evidence for both hypotheses proposed to explain the AOR in parasites: the trade‐off hypothesis (TOH) and the niche‐breadth hypothesis (NBH). Interestingly, the main predictor of the AOR was the number of susceptible hosts (i.e. number of infected birds) and not the number of host species the parasites were able to exploit.     

Why do adult insects not moult?

Hypotheses are presented concerning why mayflies moult after functional wings develop and why most insects cease to moult at this time. The pattern of retention or loss of the subimaginal moult in extant mayflies suggests that this moult may be necessary to complete elongation of caudal filaments and forelegs of adults. It is then analogous to the pupal moult of holometabolous insects. I propose that selection for wing efficiency has normally confined the presence of functional wings to one instar only, which is also the last and reproductive stage. Selection for light wings has generally caused the epidermis of membranous insect wings to degenerate, thereby precluding otherwise advantageous adult moults.     

Why are some people more jealous than others? Genetic and environmental factors

Research on romantic jealousy has traditionally focused on sex differences. We investigated why individuals vary in romantic jealousy, even within the sexes, using a genetically informed design of ~7700 Finnish twins and their siblings. First, we estimated genetic, shared environmental and nonshared environmental influences on jealousy, Second, we examined relations between jealousy and several variables that have been hypothesized to relate to jealousy because they increase the risk (e.g., mate-value discrepancy) or costs (e.g., restricted sociosexuality) of infidelity. Jealousy was 29% heritable, and non-shared environmental influences explained the remaining variance. The magnitude and sources of genetic influences did not differ between the sexes. Jealousy was associated with: having a lower mate value relative to one's partner; having less trust in one's current partner; having been cheated by a previous or current partner; and having more restricted sociosexual attitude and desire. Within monozygotic twin pairs, the twin with more restricted sociosexual desire and less trust in their partner than his or her co-twin experienced significantly more jealousy, showing that these associations were not merely due to the same genes or family environment giving rise to both sociosexual desire or trust and jealousy. The association between sociosexual attitude and jealousy was predominantly explained by genetic factors (74%), whereas all other associations with jealousy were mostly influenced by nonshared environmental (non-familial) factors (estimates >71%). Overall, our findings provide some of the most robust support to date on the importance of variables predicted by mate-guarding accounts to explain why people vary in jealousy.     

When and why do some social cleavages become politically salient rather than others?

ABSTRACT

Building on Posner (Posner, Daniel N. 2005. Institutions and Ethnic Politics in Africa. New York: Cambridge University Press), this article describes a framework for organizing the information about a community’s social cleavage structure so as to identify the incentives that individuals face to adopt particular social identities. The framework is parsimonious but powerful: it generates predictions about the social cleavages that will emerge as salient in politics, the lobbying we can expect to see regarding the social categories with which community members should identify, and the attempts that will be made to assimilate or engage in “identity entrepreneurship” to fashion entirely new social identities. The framework also clarifies why partition is unlikely to be a remedy for intractable ethnic conflicts.     

Why do some thorny plants resemble green zebras?

The rosette and cauline leaves of the highly thorny winter annual plant species of the Asteraceae in Israel (Silybum marianum) resemble green zebras. The widths of typical variegation bands were measured and found to be highly correlated with leaf length, length of the longest spines at leaf margins and the number of spines along leaf circumference. Thus, there is a significant correlation between the spinyness and strength of variegation. I propose that this is a special case of aposematic (warning) coloration.     

Why do anemonefishes inhabit only some host actinians?

Synopsis The 25 species ofAmphiprion and one ofPremnas (family Pomacentridae) are obligate symbionts of 10 species of facultatively symbiotic sea anemones. Throughout the tropical Indo-West Pacific range of the relationship, a fish species inhabits only certain of the hosts potentially available to it. This specificity is due to the fishes. Five fishes occupy six sea anemone species at Lizard Island, Great Barrier Reef, Australia.Entacmaea quadricolor harborsP. biaculeatus, A. melanopus andA. akindynos. Adults ofPremnas occur deeper than about 3 m in large, primarily solitary actinians; juveniles may occupy peripheral members ofEntacmaea clones in shallow water. Specimens ofA. melanopus live exclusively in clonal anemones, which are found no deeper than 3 m. Most individuals ofA. akindynos inEntacmaea are juveniles, occurring shallow and deep, in solitary anemones or at the margins of clones. Interspecific as well as intraspecific social control of growth may be responsible for keeping fish small at clone fringes. Conspicuous specimens ofE. quadricolor depend upon their anemonefish to survive. Actinians cleared of symbionts disappeared within 24 h, probably having been eaten by reef fishes.Entacmaea, the most abundant and widespread host actinian at Lizard Island and throughout the range of the association, is also arguably the most attractive to anemonefishes. I believe its vulnerability to predation was a factor in its evolving whatever makes it desirable to fishes. Experimental transfers pitted fish of one species against those of another, controlling for ecophenotype of host, and sex, size and number of fish. Competitive superiority was in the same order as abundance and over-all host specificity:P. biaculeatus, A. melanopus, A. akindynos. At least three factors are necessary to explain patterns of species specificity - innate or learned host preference, competition, and stochastic processes.     

Why do some African children develop severe malaria?

Malaria is still a major cause of death and severe illness among children in many parts of tropical Africa, but only a small proportion of children, perhaps 1-2%, who become ill with malaria develop severe disease. Why only, some children experience a severe or fatal attack is not understood clearly. In this article, Brian Greenwood, Kevin Marsh and Robert Snow review some of those characteristics of the parasite and the host that may influence the outcome of a malaria infection. Identification of the relative importance of the many factors likely to be involved is needed in order to develop rational strategies for the prevention of deaths from malaria among children in malaria-endemic areas.