The Role of the Escape Swim Motor Network in the Organization of Behavioral Hierarchy and Arousal in Pleurobranchaea

The escape swimming pattern generator of the notaspid opisthobranchPleurobranchaea drives a high threshold, override behavior.The pattern generator is integrated with neural networks ofother behaviors so as to coordinate unitary behavioral expressionand to promote general behavioral arousal. These functions areseparately produced by different swim network elements. Oneset of swim premotor neurons, the A1/A10 ensemble, A3 and I_VS,generate the swim pattern and, through corollary activity, suppresspotentially conflicting feeding behavior by exerting broad inhibitionat major feeding network interneurons. A second set of swimneurons, the serotonergic As1–4 neurons, provides intrinsicneuromodulatory excitation to the swim pattern generator thatsustains the escape swim episode through multiple cycles. TheAs1–4 also provide neuromodulatory excitation to importantmodulatory, serotonergic cells in the feeding motor networkand locomotor network, and may have a general regulatory rolein the distributed serotonergic arousal network of the mollusk.The As1–4 appear to be also necessary to both avoidanceand orienting turning, and are therefore likely to be critical,multi-functional components upon which much of the organizationof the animal's behavior rests.     

Plasticity of Escape Responses: Prior Predator Experience Enhances Escape Performance in a Coral Reef Fish

Teleost and amphibian prey undertake fast-start escape responses during a predatory attack in an attempt to avoid being captured. Although previously viewed as a reflex reaction controlled by the autonomic nervous system, the escape responses of individuals when repeatedly startled are highly variable in their characteristics, suggesting some behavioural mediation of the response. Previous studies have shown that fishes are able to learn from past experiences, but few studies have assessed how past experience with predators affect the fast-start response. Here we determined whether prior experience with the smell or sight of a predator (the Dottyback, Pseudochromis fuscus) affected the escape response of juveniles of the Spiny Chromis (Acanthochromis polyacanthus). Results show that individuals exposed to any of the predator cues prior to being startled exhibited a stronger escape response (i.e., reduced latency, increased escape distance, mean response speed, maximum response speed and maximum acceleration) when compared with controls. This study demonstrates the plasticity of escape responses and highlights the potential for naïve reef fish to take into account both visual and olfactory threat cues simultaneously to optimise the amplitude of their kinematic responses to perceived risk.     

Escape from X inactivation

Although the process of X inactivation in mammalian cells silences the majority of genes on the inactivated X chromosome, some genes escape this chromosome-wide silencing. Genes that escape X inactivation present a unique opportunity to study the process of silencing and the mechanisms that protect some genes from being turned off. In this review, we will discuss evolutionary aspects of escape from X inactivation, in relation to the divergence of the sex chromosomes. Molecular characteristics, expression, and epigenetic modifications of genes that escape will be presented, including their developmental regulation and the implications of chromatin domains along the X chromosome in modeling the escape process.     

Localization of the site of effect of a wasp's venom in the cockroach escape circuitry

The parasitic wasp Ampulex compressa stings a cockroach Periplaneta americana in the neck, toward the head ganglia (the brain and subesophageal ganglion). In the present study, our aim was to identify the head ganglion that is the target of the venom and the mechanisms by which the venom blocks the thoracic portion of the escape neuronal circuitry. Because the escape responses elicited by a wind stimulus in brainless and sham-operated animals were similar, we propose that the venom effect is on the subesophageal ganglion. Apparently, the subesophageal ganglion modulates the thoracic portion of the escape circuit. Recordings of thoracic interneuron responses to the input from the abdominal giant interneurons showed that the thoracic interneurons receive synaptic drive from these interneurons in control and in stung animals. Unlike normal cockroaches, which use both fast and slow motoneurons for producing rapid escape movements, stung animals activate only the slow motoneuron. However, we show that in stung animals, the fast motoneuron still can be recruited with bath application of pilocarpine, a muscarinic agonist. These results indicate that the descending control from the subesophageal ganglion is presumably exerted on the premotor thoracic interneurons to motoneurons connection of the thoracic escape circuitry. Accepted: 19 December 1998     

Effect of Risk on Aspects of Escape Behavior by a Lizard, Holbrookia propinqua, in Relation to Optimal Escape Theory

Prey must balance gains from activities such as foraging and social behavior with predation risk. Optimal escape theory has been successful in predicting escape behavior of prey under a range of risk and cost factors. The optimal approach distance, the distance from the predator at which prey should begin to flee, occurs when risk equals cost. Optimal escape theory predicts that for a fixed cost, the approach distance increases as risk increases. It makes no predictions about approach distance for prey in refuges that provide only partial protection or about escape variables other than approach distance, such as the likelihood of stopping before entering refuge and escape speed. By experimentally simulating a predator approaching keeled earless lizards, Holbrookia propinqua, the predictions of optimal escape theory for two risk factors, predator approach speed and directness of approach were tested. In addition, predictions that the likelihood of fleeing into refuge without stopping and the speed of escape runs increase with risk, in this case predator approach speed, and that lizards in incompletely protective refuges permit closer approach than lizards not in refuges were also tested. Approach distance increased with predator approach speed and directness of approach, confirming predictions of optimal escape theory. Lizards were more likely to enter refuge and ran faster when approached rapidly, verifying that predation risk affects escape decisions by the lizards for escape variables not included in optimal escape theory. They allowed closer approach when in incompletely protective refuges than when in the open, confirming the prediction that risk affects escape decisions while in refuge. Optimal escape theory has been highly successful, but testing it has led to relative neglect of important aspects of escape other than approach distance.     

Structured Observations Reveal Slow HIV-1 CTL Escape

The existence of viral variants that escape from the selection pressures imposed by cytotoxic T-lymphocytes (CTLs) in HIV-1 infection is well documented, but it is unclear when they arise, with reported measures of the time to escape in individuals ranging from days to years. A study of participants enrolled in the SPARTAC (Short Pulse Anti-Retroviral Therapy at HIV Seroconversion) clinical trial allowed direct observation of the evolution of CTL escape variants in 125 adults with primary HIV-1 infection observed for up to three years. Patient HLA-type, longitudinal CD8+ T-cell responses measured by IFN-γ ELISpot and longitudinal HIV-1 gag, pol, and nef sequence data were used to study the timing and prevalence of CTL escape in the participants whilst untreated. Results showed that sequence variation within CTL epitopes at the first time point (within six months of the estimated date of seroconversion) was consistent with most mutations being transmitted in the infecting viral strain rather than with escape arising within the first few weeks of infection. Escape arose throughout the first three years of infection, but slowly and steadily. Approximately one third of patients did not drive any new escape in an HLA-restricted epitope in just under two years. Patients driving several escape mutations during these two years were rare and the median and modal numbers of new escape events in each patient were one and zero respectively. Survival analysis of time to escape found that possession of a protective HLA type significantly reduced time to first escape in a patient (p = 0.01), and epitopes escaped faster in the face of a measurable CD8+ ELISpot response (p = 0.001). However, even in an HLA matched host who mounted a measurable, specific, CD8+ response the average time before the targeted epitope evolved an escape mutation was longer than two years.     

Escape from X Inactivation Varies in Mouse Tissues

X chromosome inactivation (XCI) silences most genes on one X chromosome in female mammals, but some genes escape XCI. To identify escape genes in vivo and to explore molecular mechanisms that regulate this process we analyzed the allele-specific expression and chromatin structure of X-linked genes in mouse tissues and cells with skewed XCI and distinguishable alleles based on single nucleotide polymorphisms. Using a binomial model to assess allelic expression, we demonstrate a continuum between complete silencing and expression from the inactive X (Xi). The validity of the RNA-seq approach was verified using RT-PCR with species-specific primers or Sanger sequencing. Both common escape genes and genes with significant differences in XCI status between tissues were identified. Such genes may be candidates for tissue-specific sex differences. Overall, few genes (3–7%) escape XCI in any of the mouse tissues examined, suggesting stringent silencing and escape controls. In contrast, an in vitro system represented by the embryonic-kidney-derived Patski cell line showed a higher density of escape genes (21%), representing both kidney-specific escape genes and cell-line specific escape genes. Allele-specific RNA polymerase II occupancy and DNase I hypersensitivity at the promoter of genes on the Xi correlated well with levels of escape, consistent with an open chromatin structure at escape genes. Allele-specific CTCF binding on the Xi clustered at escape genes and was denser in brain compared to the Patski cell line, possibly contributing to a more compartmentalized structure of the Xi and fewer escape genes in brain compared to the cell line where larger domains of escape were observed.     

Characterization of tactile-sensitive interneurons in the abdominal ganglia of the cockroach,Periplaneta americana

Tactile stimulation of an insect's abdomen evokes various behaviors including grooming and vigorous escape responses. We tested a sample of 37 tactile-sensitive abdominal interneurons for various morphological and physiological characteristics, including their ability to excite thoracic interneurons that are known to integrate wind information conducted by giant interneurons in the classical escape response. The results suggest that abdominal tactile-sensitive interneurons are heterogeneous both in anatomical and physiological properties. In general, these cells are very small interganglionic interneurons that respond to tactile stimulation at more than one abdominal segment. However, the larger population contained virtually all types of cells. Some projected anteriorly, others posteriorly, and still others projected in both directions. For most cells, the soma was on the side opposite to their axons, but in 24% of the cells it was on the same side. Patterns of dendritic arbors also varied among cells. However, tactile sensitivity was in general consistent with the morphological bias noted in dendritic branch patterns. We were able to document the existence of tactile abdominal interneurons that connect directly to thoracic interneurons involved in escape (TI_As). However, instances of demonstrated connectivity were rare. One cell that did show connectivity (AI652) was characterized in detail, and its properties were appropriate for conducting tactile signals in a directional escape system. The dendritic arbors were biased to the side that was ipsilateral to the cell's soma and axon. As a result, this cell's abdominal inputs and thoracic outputs are on the same side. This pattern is appropriate for generating the sensory fields recorded previously in TI_As. Its axon was located in the ventral median tract, which should bring it close to the integrating region of the TI_As. © 1998 John Wiley & Sons, Inc. J Neurobiol 34: 227–241, 1998     

Kinetics of l-Alanine Escape from Xylem Vessels

Labeled ((3)H or (14)C) l-alanine was perfused through the xylem vessels of isolated tomato internodes (Lycopersicon esculentum cv. Moneymaker) at various concentrations (10(-6) molar to 10(-2) molar). At each concentration the escape of l-alanine from the xylem vessels was apparently a first order process, which is in agreement with Horwitz' (1958, Plant Physiology 33:81-93) model for irreversible escape from the xylem vessels. The escape constant (K) decreased at higher concentrations of l-alanine, which implies that Horwitz' model is inappropriate to describe the kinetics of l-alanine escape, and that the escape at least partly is a saturable process. To obtain data that relate the concentration of l-alanine in the xylem vessels and the escape rate of the amino acid, average escape rates per internode were measured and the corresponding concentrations were calculated from the integrated form of the Michaelis-Menten equation.AS THE CONCENTRATION DEPENDENCE OF THE ESCAPE RATE WAS BIPHASIC, THREE POSSIBLE MECHANISMS WERE CONSIDERED, ESCAPE BEING CAUSED BY: (a) saturable amino acid uptake of cells around the xylem vessels and diffusion into the free space; (b) saturable uptake of the cells around the xylem vessels, but at higher amino acid concentrations in the xylem vessels the number of cells, that participate in the uptake, increases; (c) two, simultaneously operating, saturable uptake systems in the cells around the xylem vessels.     

Some Possible Cases of Escape Mimicry in Neotropical Butterflies

The possibility that escape or evasive mimicry evolved in butterflies and other prey insects in a similar fashion to classical Batesian and Müllerian mimicry has long been advanced in the literature. However, there is a general disagreement among lepidopterists and evolutionary biologists on whether or not escape mimicry exists, as well as in which mimicry rings this form of mimicry has evolved. Here, we review some purported cases of escape mimicry in Neotropical butterflies and suggest new mimicry rings involving several species of Archaeoprepona, Prepona, and Doxocopa (the “bright blue bands” ring) and species of Colobura and Hypna (the “creamy bands” ring) where the palatability of butterflies, their ability to escape predator attacks, geographic distribution, relative abundance, and co-occurrence in the same habitats strongly suggest that escape mimicry is involved. In addition, we also indicate other butterfly taxa whose similarities of coloration patterns could be due to escape mimicry and would constitute important case studies for future investigation.     

Escape behaviors in insects

Escape behaviors are, by necessity, fast and robust, making them excellent systems with which to study the neural basis of behavior. This is especially true in insects, which have comparatively tractable nervous systems and members who are amenable to manipulation with genetic tools. Recent technical developments in high-speed video reveal that, despite their short duration, insect escape behaviors are more complex than previously appreciated. For example, before initiating an escape jump, a fly performs sophisticated posture and stimulus-dependent preparatory leg movements that enable it to jump away from a looming threat. This newfound flexibility raises the question of how the nervous system generates a behavior that is both rapid and flexible. Recordings from the cricket nervous system suggest that synchrony between the activity of specific interneuron pairs may provide a rapid cue for the cricket to detect the direction of an approaching predator and thus which direction it should run. Technical advances make possible wireless recording from neurons while locusts escape from a looming threat, enabling, for the first time, a direct correlation between the activity of multiple neurons and the time-course of an insect escape behavior.     

Strategic Escape Direction: Orientation,Turning, and Escape Trajectories of Zebra‐Tailed Lizards (Callisaurus draconoides)

A prey's body orientation relative to a predator's approach path may affect risk of fleeing straight ahead. Consequently, prey often turn before fleeing. Relationships among orientation, turn, and escape angles and between these angles and predation risk have not been studied in terrestrial vertebrates and have rarely been studied in the field. Escape angles are expected to lead away from predators and be highly variable to avoid being predictable by predators. Using approach speed as a risk factor, we studied these issues in the zebra‐tailed lizard, Callisaurus draconoides. Lizards fled away from human simulated predators, but most did not flee straight away. Escape angles were variable, as expected under the unpredictability hypothesis, and had modes at nearly straight away (i.e., 0°) and nearly perpendicular to the predator's approach path (90°). The straight away mode suggests maximal distancing from the predator; the other mode suggests maintaining ability to monitor the predator or possibly an influence of habitat features such as obstacles and refuges that differ among directions. Turn angles were larger when orientation was more toward the predator, and escape angles were closer to straight away when turn angles were larger. Turning serves to reach a favorable fleeing direction. When orientation angle was more toward the predator, escape angle was unaffected, suggesting that turn angle compensates completely for increased risk of orientation toward the predator. When approached more rapidly, lizards fled more nearly straight away, as expected under greater predation risk. Turn angles were unrelated to approach speed.     

Timing of Immune Escape Linked to Success or Failure of Vaccination

Successful vaccination against HIV should limit viral replication sufficiently to prevent the emergence of viral immune escape mutations. Broadly directed immunity is likely to be required to limit opportunities for immune escape variants to flourish. We studied the emergence of an SIV Gag cytotoxic T cell immune escape variant in pigtail macaques expressing the Mane-A*10 MHC I allele using a quantitative RT-PCR to measure viral loads of escape and wild type variants. Animals receiving whole Gag expressing vaccines completely controlled an SIV_mac251 challenge, had broader CTL responses and exhibited minimal CTL escape. In contrast, animals vaccinated with only a single CTL epitope and challenged with the same SIV_mac251 stock had high levels of viral replication and rapid CTL escape. Unvaccinated naïve animals exhibited a slower emergence of immune escape variants. Thus narrowly directed vaccination against a single epitope resulted in rapid immune escape and viral levels equivalent to that of naïve unvaccinated animals. These results emphasize the importance of inducing broadly directed HIV-specific immunity that effectively quashes early viral replication and limits the generation of immune escape variants. This has important implications for the selection of HIV vaccines for expanded human trials.     

Escape manoeuvres of schooling Clupea harengus

The escape behaviour of schooling herring startled by an artificial sound stimulus was observed by means of high speed video filming. Response latencies showed two distinct peaks, at 30 ms and c . 100 ms. Escape responses belonging to the two latency groups showed different turning rates during the first stage of the response, and showed different escape trajectories. We suggest that long latency escapes may be responses to startled neighbours or simply weak responses to the sound stimulus. In addition, the different contraction rates during the C-bend formation seen in the two latency groups may imply differences in the neuronal commands. The escape responses of herring were directed away from the stimulus more often than towards it (88% of the total). These away responses were more common in long latency responses, suggesting that the latter enable herring to be more accurate in discerning the direction of the threat. Startled fish contracting their body towards the stimulus (performing a towards response) appear to correct their escape course, since their escape trajectory distribution is non-uniformty distributed around 360° and directed away from the stimulus. We hypothesize that when herring are schooling, the ability of each fish to correct its trajectory following turns towards the stimulus is enhanced.     

Escape flights of yellowhammers and greenfinches: more than just physics

Wintering birds increase their fat reserves throughout the day, and impaired escape performance is often considered to be an important cost of fat reserves. Since lifting a larger mass requires more energy, if birds escape at maximum power output, an increase in mass will impair the escape flight. In this study we did not find support for mass-dependent escape performance for yellowhammers, Emberiza citrinella, and greenfinches, Carduelis chloris, with natural daily mass increases of 7-8%. This suggests either that the birds were not performing at maximum output at dawn, when light, or that maximum power output was higher at dusk, when heavy. Either way, the birds seemed to be able to put more effort into their escape flight when heavier. In both species, when alarmed, birds took off significantly faster and at a steeper angle than when not alarmed. Yellowhammers escaped at a higher speed and angle than greenfinches, and reacted faster to the predator model. This suggests that predator escape is more than just Newtonian physics, and may be influenced by behavioural, as well as morphological, adjustments. Different species may have evolved different responses to predation risk. Our results seem to be in disagreement with recent ideas about mass-dependent predation risk. However, to build up reserves, birds have to increase exposure time, which increases predation risk. This cost may be more important than impaired escape performance when relatively small, daily, changes in body mass are considered. Copyright 2000 The Association for the Study of Animal Behaviour.     

Escape latency is size independent in grey mullet

Using grey mullet Mugil cephalus as a model species, the hypothesis that escape latency increases with body size was tested. Minimum escape latency was c . 10 ms (mean ± s . d . 18·1 ± 13·7 ms; range 8–72 ms) and was independent of body size.     

Nuclear Mutations in Saccharomyces Cerevisiae That Affect the Escape of DNA from Mitochondria to the Nucleus

We have inserted a yeast nuclear DNA fragment bearing the TRP1 gene and its associated origin of DNA replication, ARS1, into the functional mitochondrial chromosome of a strain carrying a chromosomal trp1 deletion. TRP1 was not phenotypically expressed within the organelle. However, this Trp(-) strain readily gave rise to respiratory competent Trp(+) clones that contained the TRP1/ARS1 fragment, associated with portions of mitochondrial DNA (mtDNA), replicating in their nuclei. Thus the Trp(+) clones arose as a result of DNA escaping from mitochondria and migrating to the nucleus. We have isolated 21 nuclear mutants in which the rate of mtDNA escape is increased by screening for increased rates of papillation to Trp(+). All 21 mutations were recessive and fell into six complementation groups, termed YME1-YME6. In addition to increasing the rate of mtDNA escape, yme1 mutations also caused a heat-sensitive respiratory deficient phenotype at 37° and a cold-sensitive growth defect on complete glucose medium at 14°. While the other yme mutations had no detectable growth phenotypes, synergistic interactions were observed in two double mutant combinations: a yme1, yme2 double mutant failed to respire at 30° and a yme4, yme6 double mutant failed to respire at all temperatures tested. None of the respiratory defects were caused by loss of functional mtDNA. These findings suggest that yme1, yme2, yme4 and yme6 mutations alter mitochondrial functions and thereby lead to an increased rate of DNA escape from the organelle.     

Rainfall Cues and Flash-Flood Escape in Desert Stream Insects

Desert stream insects may use rainfall cues to anticipate and escape flash floods, but this has been studied in few taxa. We used controlled, replicated experiments to quantify the use of rainfall cues for flood escape in seven common desert stream insects. The hemipterans Curicta pronotata and Aquarius remigis responded consistently to rainfall cues by crawling vertically away from the water, in such a way that they may escape flash floods in nature. The coleopteran Gyrinus plicifer showed no response to rainfall cues. The hemipteran Ambrysus woodburyi did not exit the water but sought refuge under submerged rocks. Three taxa (Ranatra quadridentata, Corydalus texanus, and Rhantus atricolor) gave ambiguous results, although the latter apparently responded to environmental cues other than rainfall. We conclude that rainfall cues are a sufficient mechanism for flood escape in some taxa, but other desert stream insects may employ different strategies (behavioral, life history, or morphological) to survive floods.     

The Route of HIV Escape from Immune Response Targeting Multiple Sites Is Determined by the Cost-Benefit Tradeoff of Escape Mutations

Cytotoxic T lymphocytes (CTL) are a major factor in the control of HIV replication. CTL arise in acute infection, causing escape mutations to spread rapidly through the population of infected cells. As a result, the virus develops partial resistance to the immune response. The factors controlling the order of mutating epitope sites are currently unknown and would provide a valuable tool for predicting conserved epitopes. In this work, we adapt a well-established mathematical model of HIV evolution under dynamical selection pressure from multiple CTL clones to include partial impairment of CTL recognition, , as well as cost to viral replication, . The process of escape is described in terms of the cost-benefit tradeoff of escape mutations and predicts a trajectory in the cost-benefit plane connecting sequentially escaped sites, which moves from high recognition loss/low fitness cost to low recognition loss/high fitness cost and has a larger slope for early escapes than for late escapes. The slope of the trajectory offers an interpretation of positive correlation between fitness costs and HLA binding impairment to HLA-A molecules and a protective subset of HLA-B molecules that was observed for clinically relevant escape mutations in the Pol gene. We estimate the value of from published experimental studies to be in the range (0.01–0.86) and show that the assumption of complete recognition loss () leads to an overestimate of mutation cost. Our analysis offers a consistent interpretation of the commonly observed pattern of escape, in which several escape mutations are observed transiently in an epitope. This non-nested pattern is a combined effect of temporal changes in selection pressure and partial recognition loss. We conclude that partial recognition loss is as important as fitness loss for predicting the order of escapes and, ultimately, for predicting conserved epitopes that can be targeted by vaccines.     

Escape response in the damselfish Chromis cyanea (Pisces: Pomacentridae): A quantitative study

Models of different sizes, shapes, and colours evoked, oriented escape responses in schools of Chromis cyanea in their natural coral-reef habitat. Divers filmed these responses; from the films, the distance of the school from the model at the instant of response (reaction distance) was calculated. For a given model, reaction distance was independent of the speed of the model's approach. Darker coloured models were more effective in causing escape (yielded greater reaction distance) than lighter coloured models, suggesting contrast as an important feature of the stimulus. Larger models were more effective than smaller ones. Habituation to repeated stimuli was absent for inter-trial intervals from 0·5 to 4 min. Natural behaviour of Chromis suggests that cues such as sound, vibration, and details of shape, colours, and motion also play a role in triggering escape.