The emergence and maintenance of diversity: insights from experimental bacterial populations

Mechanisms maintaining genetic and phenotypic variation in natural populations are central issues in ecology and evolution. However, the long generation times of most organisms and the complexity of natural environments have made elucidation of ecological and evolutionary mechanisms difficult. Experiments using bacterial populations propagated in controlled environments reduce ecosystem complexity to the point where understanding simple processes in isolation becomes possible. Recent studies reveal the circumstances and mechanisms that promote the emergence of stable polymorphisms.     

Substrate degradation and pressure tolerance of free-living and attached bacterial populations in the intestines of shallow-water fish

This report compares recovery of non-O1 Vibrio cholerae strains from seven California coastal sites during the winter and summer of 1983. A total of 41 identified and 27 presumptive nn-O1 V. cholerae strains were recovered from six of seven coastal sites in the summer. A 5-to 56-fold increase in the numbers of organisms isolated from different sites occurred in the summer months, when water temperatures were 1.9 to 5.1 degrees C higher. At the three sites where the highest levels of non-O1 V. cholerae were found, pollution, as measured by the total number of coliforms, exceeded the legal limit (less than 1,000 coliforms per 100 ml.).     

Environmental variability and the persistence of multiple emergence strategies

Studies of plant and animal populations have demonstrated the occurrence of multiple and mixed life history strategies such as polymodal timing of germination and emergence from dormancy. We present the results of a simulation model used to test whether between-year variance in mortality can lead to the persistence of multiple hatching strategies considered over an ecological time scale (50 years). The model is based on the general life history characteristics of a population of planktonic copepods (Diaptomus sanguineus) in Bullhead Pond, Rhode Island. Our model results demonstrate that, given a range of between-year variance in mortality, multiple strategies for timing of emergence can persist in a common environment for ecologically relevant periods of time. A qualitative test of the model comparing field estimates of mean and variance of mortality in Bullhead Pond with the region of persistence indicates that the model results are in approximate agreement with field estimates. The results suggest that variability in year-to-year selection pressures, such as predation or harsh winters, may play an important role in determining the evolution of life histories.     

Mathematical modeling of the evolution of bacterial populations in continuous cultures with regard to the nonmutagenic variability of genome

A mathematical model of the evolution of the genetic structure of the bacterial population during prolonged cultivation in a chemostat has been constructed. In addition to genetic mutations, some factors of the nonmutagenic variability of genome were taken into account, namely, the structural reorganization of plasmid and virus DNA, the DNA loss due to cellular division, the conjugative transfer of plasmids, and the plasmid replication. The general model also takes into account the formation of cellular aggregates during conjugation. The results of numerical and analytical investigation of the special cases of the general model were treated. Simplified mathematical models are considered, which can be used to explain the experimentally observed evolutionary variations resulting from the plurality of evolution attractors, multi-stage microevolutionary transitions, the semi-stable states of the bacterial population genome, and the undamped oscillations of the genetic structure of the population during prolonged cultivation in the chemostat.     

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolerance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly G_iα inhibitory to G_βγ stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.     

Dietary variability in primate populations

Dietary variability among primates is examined based on a review of 46 long-term studies of wild populations. Results suggest that primates do not consistently combine the same kinds of foods in their diets, as many past categorizations would suggest, but rather, that they often switch between diet categories (e.g., fruit, insects, etc.). Dietary variability, as quantified in our review, did not appear to be constrained by phylogeny or to differ between species placed in different diet categories (e.g., frugivores, insectivores, etc.). In addition, dietary variability was not related to body size, habitat productivity, seasonality, population density, or the number of sympatric primate species.     

Enzyme variability in populations of aphids

Summary From 1976 to 1978 nine aphid species were investigated electrophoretically. A low degree of polymorphism was found. Only 5 of the species were polymorphic, and only at most in 3 enzyme systems out of 19.     

Measures of variability in age-structured populations

An expression for the entropy of a population was derived in Demetrius (1974) by using a variational principle argument. This entropy measure is precisely the information content of the distribution in the ages of reproducing individuals in a stationary population. This paper introduces another expression for the entropy by considering the variation in the ages at which offspring will be produced by newborn individuals.The relation between these two measures of entropy and their biological significance are discussed.     

Frequency-dependent selection in bacterial populations

There are many situations in which the direction and intensity of natural selection in bacterial populations will depend on the relative frequencies of genotypes. In some cases, this selection will favour rare genotypes and result in the maintenance of genetic variability; this is termed stabilizing frequency-dependent selection. In other cases, selection will only favour genotypes when they are common. Rare types cannot invade and genetic variability will not be maintained; this is known as disruptive frequency-dependent selection. Phage-mediated selection for bacteria with novel restriction-modification systems is frequency-dependent and stabilizing. In mass culture, selection for the production of toxins and allelopathic agents is likely to be frequency-dependent but disruptive. This also occurs in selection favouring genes and transposable elements that cause mutations. Here I review the results of theoretical and experimental studies of stabilizing and disruptive frequency-dependent selection in bacterial populations, and speculate on the importance of this kind of selection in the adaptation and evolution of these organisms and their accessory elements (plasmid, phage and transposons).     

Chiasmata and variability in Lolium and Festuca populations

There are significant differences in mean pollen mother cell chiasma frequencies between populations within Lolium perenne, L. multiflorum and Festuca pratensis. The differences are genotypically controlled. With low chiasma frequencies the chiasmata are distally located. With increasing chiasma frequency the frequency of chiasmata in interstitial segments increases. Shorter lived populations have higher chiasma frequencies than the more perennial. — The higher the chiasma frequency of a population the lower the phenotypic and genetic variance for characters under polygenic control, such as flowering time, and the less effective also is the response to selection for such characters. These observations are interpreted on the premise that high chiasma frequencies are instrumental in the breaking up of supergene sequences in interstitial chromosome segments.     

Climate-induced variability in Calanus marshallae populations

Calanus marshallae is the dominant mesozooplankton copepod speciesover the south-eastern Bering Sea middle shelf. Climate-inducedchanges in the magnitude and timing of production by C. marshallaemay affect the living marine resources of the Bering Sea shelfecosystem. We examined springtime abundance, gonadal maturityand stage distributions of C. marshallae copepodites duringfive consecutive years (1995–1999) that spanned the rangeof variability observed over the past 34 years in terms of watertemperature and ice cover. We compared our results with previouswork conducted during cool (1980) and warm (1981) years [ Smith,S. L. and Vidal, J. (1986) Cont. Shelf Res., 5, 215–239].The spring phytoplankton bloom began relatively early in associationwith ice (1995, 1997, 1999), but began late when ice was absentor retreated early (1996, 1998). Egg production began well beforethe bloom and continued over a long duration. Copepodites, however,were recruited during a relatively short period, coincidentwith the spring phytoplankton bloom. The relationship betweenbrood stock and spring-generation copepodite abundances wasweak. Copepodite concentrations during May were greatest inyears of most southerly ice extent. Copepodite populations werehighly variable among years, reflecting interannual variabilityin the atmosphere–ice–ocean system.     

Climatic variability and the evolution of insect freeze tolerance

Insects may survive subzero temperatures by two general strategies: Freeze-tolerant insects withstand the formation of internal ice, while freeze-avoiding insects die upon freezing. While it is widely recognized that these represent alternative strategies to survive low temperatures, and mechanistic understanding of the physical and molecular process of cold tolerance are becoming well elucidated, the reasons why one strategy or the other is adopted remain unclear. Freeze avoidance is clearly basal within the arthropod lineages, and it seems that freeze tolerance has evolved convergently at least six times among the insects (in the Blattaria, Orthoptera, Coleoptera, Hymenoptera, Diptera and Lepidoptera). Of the pterygote insect species whose cold-tolerance strategy has been reported in the literature, 29% (69 of 241 species studied) of those in the Northern Hemisphere, whereas 85 % (11 of 13 species) in the Southern Hemisphere exhibit freeze tolerance. A randomization test indicates that this predominance of freeze tolerance in the Southern Hemisphere is too great to be due to chance, and there is no evidence of a recent publication bias in favour of new reports of freeze-tolerant species. We conclude from this that the specific nature of cold insect habitats in the Southern Hemisphere, which are characterized by oceanic influence and climate variability must lead to strong selection in favour of freeze tolerance in this hemisphere. We envisage two main scenarios where it would prove advantageous for insects to be freeze tolerant. In the first, characteristic of cold continental habitats of the Northern Hemisphere, freeze tolerance allows insects to survive very low temperatures for long periods of time, and to avoid desiccation. These responses tend to be strongly seasonal, and insects in these habitats are only freeze tolerant for the overwintering period. By contrast, in mild and unpredictable environments, characteristic of habitats influenced by the Southern Ocean, freeze tolerance allows insects which habitually have ice nucleators in their guts to survive summer cold snaps, and to take advantage of mild winter periods without the need for extensive seasonal cold hardening. Thus, we conclude that the climates of the two hemispheres have led to the parallel evolution of freeze tolerance for very different reasons, and that this hemispheric difference is symptomatic of many wide-scale disparities in Northern and Southern ecological processes.     

Genetic variability of NaCl tolerance in tomato

Cultivation of crops in soils with high salt (NaCl) content can affect plant development. We examined the morphological and physiological mechanisms of salt tolerance in tomato. The responses of 72 accessions of tomato (Solanum lycopersicum) to salinity were compared by measuring shoot and root lengths, and fresh shoot and root weights relative to those of controls (plants grown in normal salt levels). All traits were reduced at the seedling stage when salinity levels were increased. The accession x salinity interaction was significant for all traits. Root length had higher heritability than other traits and was used as a selection criterion to identify salt-tolerant and -non-tolerant accessions. On the basis of root length, accessions LA2661, CLN2498A, CLN1621L, BL1176, 6233, and 17870 were considered to be more tolerant than accessions 17902, LO2875 and LO4360. The degree of salt tolerance was checked by analyzing K+ and Na+ concentrations and K+/Na+ ratio in tissues of plants treated with 10 and 15 dS/m salinity levels. Tolerance of these accessions to salinity was most associated with low accumulation of Na+ and higher K+/Na+ ratios.     

Clinical significance of the emergence of bacterial resistance in the hospital environment

Antibiotic resistance is an increasing threat in hospitals and both morbidity and mortality from infections are greater when caused by drug-resistant organisms. Whilst hospitals are universally blamed for this increase, there is an insufficient appreciation of external sources of resistance, such as when patients are admitted to hospitals from long-term care facilities in the community. The use of antibiotics in family practice and animal husbandry has also been linked to drug resistance being encountered in the hospital setting. Justifiable hospital antibiotic use, which can be life saving, may lead to 'collateral damage' with the emergence of resistance in non-target bacteria in the bowel, for example, with subsequent spread by cross-infection. At a management level, antibiotic resistance can have a significant impact on the ability of hospitals to maintain services since cohorting of patients and ward closures from outbreaks add to continuing bed shortages and waiting lists. Hospital laboratories must review their standard operating procedures since some resistance mechanisms may be missed by current methods of antibiotic susceptibility testing. With increasing public concern from press reports of 'multiresistant Staphylococcus aureus killer virus' and other drug-resistant organisms, there will inevitably be a push by national authorities for more surveillance data on antibiotic resistance; however, the cost-effectiveness of different surveillance strategies should be considered. Clinical governance and risk management are dominant themes in the National Health Service and hospital hygiene and antibiotic resistance are likely to feature prominently in audits related to these themes in the near future.     

Concepts in disinfection of bacterial populations

Bacteria are responsible for a range of human diseases that are difficult to clear for a variety of reasons. Understanding the multilayered mechanisms that protect pathogens is of primary importance for developing effective treatments. Research over the past several decades has shown that bacteria can seek protected environments passively by avoiding deadly environments or actively by manipulating their phenotypic expression and gathering in structured biofilm communities. This article outlines the main tolerance mechanisms that have been studied and describes many of the mathematical models that indicate directions for new treatments. It is essential to understand that bacterial populations exploit a range of mechanisms to survive challenges and the mechanisms are often intertwined. This provides an opportunity for mathematical modeling to help provide insights into experiments that may uncouple particular mechanisms or optimize treatments.     

Drivers for the emergence and re-emergence of vector-borne protozoal and bacterial diseases

In recent years, vector-borne parasitic and bacterial diseases have emerged or re-emerged in many geographical regions causing global health and economic problems that involve humans, livestock, companion animals and wild life. The ecology and epidemiology of vector-borne diseases are affected by the interrelations between three major factors comprising the pathogen, the host (human, animal or vector) and the environment. Important drivers for the emergence and spread of vector-borne parasites include habitat changes, alterations in water storage and irrigation habits, atmospheric and climate changes, immunosuppression by HIV, pollution, development of insecticide and drug resistance, globalization and the significant increase in international trade, tourism and travel. War and civil unrest, and governmental or global management failure are also major contributors to the spread of infectious diseases. The improvement of epidemic understanding and planning together with the development of new diagnostic molecular techniques in the last few decades have allowed researchers to better diagnose and trace pathogens, their origin and routes of infection, and to develop preventive public health and intervention programs. Health care workers, physicians, veterinarians and biosecurity officers should play a key role in future prevention of vector-borne diseases. A coordinated global approach for the prevention of vector-borne diseases should be implemented by international organizations and governmental agencies in collaboration with research institutions.