Salmonella typhimurium enterotoxin epitopes shared among bacteria

Abstract Selection-induced mutations (SIMS) are mutations that occur as specific and direct responses to environmental challenges, and that occur more often when they are selectively advantageous than when they are selectively neutral. This review includes discussions of how the occurrence of SIMS is measured, acquisitive evolution by SIMS, some of the controversies surrounding SIMS and models that have been advanced to explain the specificity of SIMS, and the requirement of a functional recA gene product for SIMS to occur.     

The role of single-mutant intermediates in the generation of trpAB double revertants during prolonged selection.

Selection-induced mutations are nonrandom mutations that occur as specific, direct responses to environmental challenges and that occur more often when they are selectively advantageous than when they are selectively neutral. One of the most puzzling examples of selection-induced mutations involved the simultaneous reversions of two mutations, one in trpA and the other in trpB, at rates that were several orders of magnitude greater than would have been predicted if the two mutations had occurred as independent events (B. G. Hall, Proc. Natl. Acad. Sci. USA 88:5882-5886, 1991). Here I examine the possibility that the double mutations might be accounted for by sequential mutations with intervening growth.     

Japanese quail (Coturnix japonica) inseminations are more likely to fertilize eggs in a context predicting mating opportunities

Theoretical developments in behavioural ecology have generated increased interest in the proximate mechanisms underlying fertilization, but little is known about how fertilization success is regulated by cues from the external or social environment in males and females. Here, we use a Pavlovian conditioning paradigm to show that inseminations resulting from mating male and female Japanese quail (Coturnix japonica) are more likely to fertilize eggs when they occur in a context predicting that an opposite-sex bird will appear than when they occur in a context predicting that an opposite-sex bird will not appear. This effect occurs when either the male or the female is the target of the conditioning. Thus, processes occurring during or after mating that contribute to fertilization success are subject to the influence of distal cues, confirming control by brain-level mechanisms. Conditioning is a widespread property of the nervous system and the demonstration that context conditioning can influence male and female reproductive success, and not simply mating success, has widespread implications for the fertilization successes of different types of copulation in natural mating systems.     

Pollination failure in plants: why it happens and when it matters

Pollination is the primary step in seed formation. Pollination biologists have shown that pollination failure can occur at all steps in the dispersal process and at several different levels. Increased risk of pollination failure is associated with pollen if it is delivered to a stigma too little, too much, too late, too mixed in composition or too poor in quality. It is associated with pollinators when they are too few or too inconstant, and with plants when they are too specialized or too selective. It is associated with populations when they are too sparse, too small in number or too uniform genetically, and with communities when they are too fragmented, genetically impoverished or under rapid modification. Understanding the causes of pollination failure in plants can aid the successful conservation and recovery of rare plants, maintenance of crop yields, and sustainable use of wild plant resources such as forest timber.     

Virally induced alterations in cellular permeability: A basis of cellular and physiological damage?

Virally induced permeability changes occur when haemolytic paramyxoviruses are added to cells; similar (though not identical) changes take place during infection of ceils with viruses from several families (including paramyxoviruses). These changes occur in intact) viable cells) and precede subsequent cytopathic effects) to which they are likely to contribute. There is accumulating evidence to suggest that virally induced permeability changes may also underlie the physiological and clinical consequences of viral infection in certain situations.     

SOME HISTOLOGICAL OBSERVATIONS ON VIRUS-INFECTED THEOBROMA CACAO L.

Although the five cacao viruses studied produce different external symptoms in infected plants, they affect internal anatomy similarly. Symptoms on leaves occur only if these are still developing when they become infected, and the viruses seem to produce their effects usually by preventing differentiation of the cells. The tissues of chlorotic areas of infected leaves are undifferentiated and similar in structure to young unexpanded leaves. In stem and root swellings xylem and phloem are both increased, but they occur in the same proportions as in normal secondary thickening. The anatomical effects of infection seem insufficient alone to account for the death of cacao trees, but they may well be complementary to the serious necrosis of the root system which results from virus infections.     

Frequency-dependent selection on aposematic prey: some experiments

Experiments to measure the strength of frequency-dependent selection on dimorphic populations of distasteful, brightly coloured prey were carried out, using pastry cylinders as prey and wild birds as predators. When the two forms were haphazardly intermingled, the birds took a relative excess of whichever form happened to be the rarer if all prey in the population were distasteful but selected the forms independently of frequency if they were all neutral in flavour. They did not take an excess of the rarer form from populations of distasteful prey when that form was distributed in a single clump rather than evenly dispersed among the commoner form. This confirms the expectation that rarer forms of aposematic animals are at a frequency-dependent disadvantage when they are intermingled with commoner forms but not when they occur in clusters.     

Synchronization of spontaneous eyeblinks while viewing video stories

Blinks are generally suppressed during a task that requires visual attention and tend to occur immediately before or after the task when the timing of its onset and offset are explicitly given. During the viewing of video stories, blinks are expected to occur at explicit breaks such as scene changes. However, given that the scene length is unpredictable, there should also be appropriate timing for blinking within a scene to prevent temporal loss of critical visual information. Here, we show that spontaneous blinks were highly synchronized between and within subjects when they viewed the same short video stories, but were not explicitly tied to the scene breaks. Synchronized blinks occurred during scenes that required less attention such as at the conclusion of an action, during the absence of the main character, during a long shot and during repeated presentations of a similar scene. In contrast, blink synchronization was not observed when subjects viewed a background video or when they listened to a story read aloud. The results suggest that humans share a mechanism for controlling the timing of blinks that searches for an implicit timing that is appropriate to minimize the chance of losing critical information while viewing a stream of visual events.     

Copy correction and concerted evolution in the conservation of yeast genes

The yeast Saccharomyces cerevisiae and other members of the genus Saccharomyces are descendants of an ancient whole-genome duplication event. Although most of the duplicate genes have since been deleted, many remain, and so there are many pairs of related genes. We have found that poorly expressed genes diverge rapidly from their paralog, while highly expressed genes diverge little, if at all. This lack of divergence of highly expressed paralogous gene pairs seems to involve gene correction: one member of the pair “corrects” the sequence of its twin, and so the gene pair evolves as a unit. This correction presumably involves gene conversion and could occur via a reverse-transcribed cDNA intermediate. Such correction events may also occur in other organisms. These results support the idea that copies of poorly expressed genes are preserved when they diverge to take on new functions, while copies of highly expressed genes are preserved when they are needed to provide additional gene product for the original function.     

Integumentary appendages of chelonians

The head and neck of four families of turtles, the Chelydridae, Kinosternidae, Pelomedusidae, and Chelidae, possess a diverse assemblage of skin appendages. Appendages are termed barbels when they occur in the gular region and tubercles when they occur other places. The appendages consist of protrusions of the dermis and epidermis and are devoid of such specializations as taste buds or neuromasts. They lack skeletal tissue, muscle, or erectile tissue. Methylene blue and silver staining techniques reveal a high density of nerves. The occurrence and morphology of barbels and tubercles suggest that they function as mechanoreceptors. Skin appendages are most elaborate in carnivorous species and reach maximum development in two distantly related convergent species: Macroclemys temmincki (Chelydridae) and Chelus fimbriatus (Chelidae). Skin appendages also help provide camouflage and disruptive effects on the head. The increase in surface area produced by the appendages may be important in aquatic respiratory gas exchange in some species within the Kinosternidae.     

The size and conjugation of change of parameters of growth and energetics of fish caused by oscillation of temperature

Small deviations of temperature from stationary optimum values optimize the growth and energetics of fish. The positive influence on the growth and energetics of fish at fluctuations of temperature is more expressed when they occur as a result of spontaneous swimming in the temperature gradient field. Changes in growth rates, respiration intensity, and consumption of energy per unit of weight that occur under the influence of oscillation of temperature are closely correlated with each other.     

Method for Assigning Categorical Rank in Female <Emphasis Type="Italic">Pan troglodytes schweinfurthii</Emphasis> via the Frequency of Approaches

Establishing the order of a dominance hierarchy among female chimpanzees (Pan troglodytes) is complicated by the fact that they often forage solitarily, and aggressive interactions between them occur infrequently. Authors of previous studies have typically ranked females via the direction of submissive pant-grunts and the outcome of agonistic interactions. Given that higher rank correlates with higher reproductive success in female chimpanzees, assessing rank is important but may be limited by sparsely populated dominance matrices. I tested the hypothesis that rank predicts the direction of female approaches. There is a significant relationship among Gombe females between the frequency with which a female was approached and her dominance rank. Dominant females approached other females less often than they were approached. Though approached frequencies failed to meet the criteria for formal rank indicators, they may be useful as real indicators of subordination. Because approach interactions occur far more frequently than pant-grunts, they may be useful in assigning categorical rank when traditional dominance metrics are limited.     

Relapse processes after the extinction of instrumental learning: renewal, resurgence, and reacquisition

It is widely recognized that extinction (the procedure in which a Pavlovian conditioned stimulus or an instrumental action is repeatedly presented without its reinforcer) weakens behavior without erasing the original learning. Most of the experiments that support this claim have focused on several "relapse" effects that occur after Pavlovian extinction, which collectively suggest that the original learning is saved through extinction. However, although such effects do occur after instrumental extinction, they have not been explored there in as much detail. This article reviews recent research in our laboratory that has investigated three relapse effects that occur after the extinction of instrumental (operant) learning. In renewal, responding returns after extinction when the behavior is tested in a different context; in resurgence, responding recovers when a second response that has been reinforced during extinction of the first is itself put on extinction; and in rapid reacquisition, extinguished responding returns rapidly when the response is reinforced again. The results provide new insights into extinction and relapse, and are consistent with principles that have been developed to explain extinction and relapse as they occur after Pavlovian conditioning. Extinction of instrumental learning, like Pavlovian learning, involves new learning that is relatively dependent on the context for expression.     

Spontaneous Point Mutations That Occur More Often When Advantageous than When Neutral

Recent reports have called into question the widespread belief "that mutations arise continuously and without any consideration for their utility" (in the words of J. Cairns) and have suggested that some mutations (which Cairns called "directed" mutations) may occur as specific responses to environmental challenges, i.e., they may occur more often when advantageous than when neutral. In this paper it is shown that point mutations in the trp operon reverted to trp+ more frequently under conditions of prolonged tryptophan deprivation when the reversions were advantageous, than in the presence of tryptophan when the reversions were neutral. The overall mutation rate, as determined from the rates of mutation to valine resistance and to constitutive expression of the lac operon, did not increase during tryptophan starvation. The trp reversion rate did not increase when the cells were starved for cysteine for a similar period, indicating that the increased reversion rate was specific to conditions where the reversions were advantageous. Two artifactual explanations for the observations, delayed growth of some preexisting revertants and cryptic growth by some cells at the expense of dying cells within aged colonies, were tested and rejected as unlikely. The trp+ reversions that occurred while trp- colonies aged in the absence of tryptophan were shown to be time-dependent rather than replication-dependent, and it is suggested that they occur by mechanisms different from those that have been studied in growing cells. A heuristic model for the molecular basis of such mutations is proposed and evidence consistent with that model is discussed. It is suggested that the results in this and previous studies can be explained on the basis of underlying random mechanisms that act during prolonged periods of physiological stress, and that "directed" mutations are not necessarily the basis of those observations.     

Why do men marry and why do they stray?

Humans are quite unusual compared to other great apes in that reproduction typically takes place within long-term, iteroparous pairings--social arrangements that have been culturally reified as the institution of marriage. With respect to male behaviour, explanations of marriage fall into two major schools of thought. One holds that marriage facilitates a sexual division of labour and paternal investment, both important to the rearing of offspring that are born helpless and remain dependent for remarkably long periods (provisioning model). And the other suggests that the main benefits which men receive from entering into marriage derive from monopolizing access to women's fertility (mating effort model). In this paper, we explore extramarital sexual relationships and the conditions under which they occur as a means of testing predictions derived from these two models. Using data on men's extramarital sexual relationships among Tsimane forager-horticulturists in lowland Bolivia, we tested whether infidelity was more common when men had less of an opportunity to invest in their children or when they risked losing less fertility. We found that Tsimane men appear to be biasing the timing of their affairs to when they are younger and have fewer children, supporting the provisioning model.     

Pseudoterminalisation,terminalisation, and non-chiasmate modes of terminal association

Terminal associations occur commonly between meiotic homologues of the two smallest (S10, S11) chromosomes in the northern race of Cryptobothrus chrysophorus when they are either heterozygous or homozygous for distal supernumerary heterochromatic segments. A detailed examination of the origin and behaviour of these associations provides convincing evidence that they are non-chiasmate in character and so cannot be explained by either pseudoterminalisation or terminalisation. The same is true of the terminal associations involved in the persistent pseudomultiples that develop between non-homologues of Heteropternis obscurella when one or both of these carry distal heterochromatic segments. In both situations the C-bands involved in such terminal associations are entire and are never interrupted by non-banded material. In Cryptobothrus, similar associations can also develop between centromere regions when these are heterozygous or homozygous for proximal supernumerary heterochromatic segments.     

Kin-biased dispersal behaviour in the mango shield scale, Milviscutulus mangiferae

When fitness decreases with increasing density in a habitat, dispersal behaviour is expected to evolve. To avoid competition between kin, dispersal behaviour based on kin recognition should be more likely to occur when the individuals in a habitat are closely related. I tested this prediction with first-instar larvae (crawlers) of the mango shield scale, Milviscutulus mangiferae. The body size of adult females, a measure of fecundity, was larger when only one female was present on a leaf than when two were present. When I placed two crawlers on a leaf, they emigrated more frequently when they were siblings than when they were not related. I discuss the implication of the results for kin recognition in thelytokous parthenogenetic animals. Copyright 2000 The Association for the Study of Animal Behaviour.     

Character displacement in polyphenic tadpoles

Biologists have long known that closely related species are often phenotypically different where they occur together, but are indistinguishable where they occur alone. The causes of such character displacement are controversial, however. We used polyphenic spadefoot toad tadpoles (Spea bombifrons and S. multiplicata) to test the hypothesis that character displacement evolves to minimize competition for food. We also sought to evaluate the role of phenotypic plasticity in the mediation of competitive interactions between these species. Depending on their diet, individuals of both species develop into either a small-headed omnivore morph, which feeds mostly on detritus, or a large-headed carnivore morph, which specializes on shrimp. Laboratory experiments and surveys of natural ponds revealed that the two species were more dissimilar in their tendency to produce carnivores when they occurred together than when they occurred alone. This divergence in carnivore production was expressed as both character displacement (where S. multiplicata's propensity to produce carnivores was lower in sympatry than in allopatry) and as phenotypic plasticity (where S. multiplicata facultatively enhanced carnivore production in S. bombifrons, and S. bombifrons facultatively suppressed carnivore production in S. multiplicata). In separate experiments, we established that S. bombifrons (the species for which carnivore production was enhanced) was the superior competitor for shrimp. Conversely, S. multiplicata (the species for which carnivore production was suppressed and omnivore production enhanced) was the superior competitor for detritus. These results therefore demonstrate that selection to minimize competition for food can cause character displacement. They also suggest that both character displacement and phenotypic plasticity may mediate competitive interactions between species.     

Molecular mechanisms of production of symmetrical and asymmetrical chromosome exchanges

The formation of a heteroduplex is probably the first step leading to chromosome exchanges. Heteroduplexes occur by complementary association of two single DNA strands from different chromosomes. Therefore, repetitive DNA is the most common region involved in heteroduplex formation. DNA repeats are defined as polarized when they run the same, and antipolarized when they run opposite from centromere to telomere in two different chromosomes or chromosome arms. Paracentric inversions may easily account for the origin of antipolarized repeats. Palindromes are a special type of reversed repeats which always run the same from centromere to telomere independently of the existence or not of chromosomal rearrangements. Heteroduplexes leading to symmetrical exchanges can only occur by association of DNA strands with polarized repeats. On the other hand, antipolarized repeats are essential for the occurrence of asymmetrical rearrangements. Accordingly, the frequency of induced symmetrical and asymmetrical exchanges in a cell population may partially depend on the frequency of polarized and antipolarized repeats in the genome.