Why do larval helminths avoid the gut of intermediate hosts?

In complex life cycles, larval helminths typically migrate from the gut to exploit the tissues of their intermediate hosts. Yet the definitive host's gut is overwhelmingly the most favoured site for adult helminths to release eggs. Vertebrate nematodes with one-host cycles commonly migrate to a site in the host away from the gut before returning to the gut for reproduction; those with complex cycles occupy sites exclusively in the intermediate host's tissues or body spaces, and may or may not show tissue migration before (typically) returning to the gut in the definitive host. We develop models to explain the patterns of exploitation of different host sites, and in particular why larval helminths avoid the intermediate host's gut, and adult helminths favour it. Our models include the survival costs of migration between sites, and maximise fitness (=expected lifetime number of eggs produced by a given helminth propagule) in seeking the optimal strategy (host gut versus host tissue exploitation) under different growth, mortality, transmission and reproductive rates in the gut and tissues (i.e. sites away from the gut). We consider the relative merits of the gut and tissues, and conclude that (i) growth rates are likely to be higher in the tissues, (ii) mortality rates possibly higher in the gut (despite the immunological inertness of the gut lumen), and (iii) that there are very high benefits to egg release in the gut. The models show that these growth and mortality relativities would account for the common life history pattern of avoidance of the intermediate host's gut because the tissues offer a higher growth rate/mortality rate ratio (discounted by the costs of migration), and make a number of testable predictions. Though nematode larvae in paratenic hosts usually migrate to the tissues, unlike larvae in intermediates, they sometimes remain in the gut, which is predicted since in paratenics mortality rate and migration costs alone determine the site to be exploited.     

Evolution of trophic transmission in parasites: why add intermediate hosts?

Although multihost complex life cycles (CLCs) are common in several distantly related groups of parasites, their evolution remains poorly understood. In this article, we argue that under particular circumstances, adding a second host to a single-host life cycle is likely to enhance transmission (i.e., reaching the target host). For instance, in several situations, the propagules of a parasite exploiting a predator species will achieve a higher host-finding success by encysting in a prey of the target predator than by other dispersal modes. In such a case, selection should favor the transition from a single- to a two-host life cycle that includes the prey species as an intermediate host. We use an optimality model to explore this idea, and we discuss it in relation to dispersal strategies known among free-living species, especially animal dispersal. The model found that selection favored a complex life cycle only if intermediate hosts were more abundant than definitive hosts. The selective value of a complex life cycle increased with predation rates by definitive hosts on intermediate hosts. In exploring trade-offs between transmission strategies, we found that more costly trade-offs made it more difficult to evolve a CLC while less costly trade-offs between traits could favor a mixed strategy.     

Physiological costs of growing fast: does accelerated growth reduce pay-off in adult fitness?

Accumulating evidence suggests that, in contrast to earlier assumptions, juvenile growth rates are optimised by means of natural and sexual selection rather than maximised to be as fast as possible. Owing to the generally accepted advantage of growing fast to adulthood, such adaptive variation strongly implies the existence of costs attached to rapid growth. By using four populations of protandrous copper butterflies with known differences in intrinsic growth rates within and across populations, we investigate a potential trade-off between rapid growth and the proportionate weight loss at metamorphosis. While controlling for effects of pupal time and mass, we demonstrate that (1) protandrous males, exhibiting higher growth rates, suffer a higher weight loss than females throughout, that (2) population differences in weight loss generally follow known differences in growth rates, and that (3) males have by 6 higher metabolic rates than females during pupal development. These results support the notion that a higher weight loss during the development to adulthood may comprise a physiological cost of rapid development, with the pay-off of accelerated growth being reduced by a disproportionally smaller adult sizeCo-ordinating editor: Leimer     

When does the variance of replicator fitness decrease?

For the general replicator dynamics with regular relative advantage functions (having non-singular Jacobian and symmetrized Jacobian), it will be shown that the variance of marginal fitness of the replicators strictly decreases along each trajectory of the replicator dynamics near an interior rest point if and only if this rest point is a regular evolutionarily stable state.     

How does competition affect the transmission of information?

The use of social information is a prerequisite to the evolution of culture. In humans, social learning allows individuals to aggregate adaptive information and increase the complexity of technology at a level unparalleled in the animal kingdom. However, the potential to use social information is related to the availability of this type of information. Although most cultural evolution experiments assume that social learners are free to use social information, there are many examples of information withholding, particularly in ethnographic studies. In this experiment, we used a computer-based cultural game in which players were faced with a complex task and had the possibility to trade a specific part of their knowledge within their groups. The dynamics of information transmission were studied when competition was within- or exclusively between-groups. Our results show that between-group competition improved the transmission of information, increasing the amount and the quality of information. Further, informational access costs did not prevent social learners from performing better than individual learners, even when between-group competition was absent. Interestingly, between-group competition did not entirely eliminate access costs and did not improve the performance of players as compared with within-group competition. These results suggest that the field of cultural evolution would benefit from a better understanding of the factors that underlie the production and the sharing of information.     

Is metamorphosis a critical interval in the early life of marine fishes?

Synopsis Small volumes of food in ‘stomach’, large proportions of empty ‘stomachs’ and small maximum sizes of prey during metamorphosis of cod,Gadus morhua, all point to an energy crisis during this developmental interval. The small size of the developing stomach of the 10–14 mm long cod may be partly responsible for the above mentioned difficulties during metamorphosis. A hypothesis based on Hjort's ‘critical stage theory’ is proposed, adding another critical interval at metamorphosis. Based on the presented data on cod and reports on other species in the literature, the new critical interval is discussed for larvae of marine fish species in general.     

Drugs and parasites: global experiments in life history evolution?

Recent work has shown that the evolution of Drosophila melanogaster resistance to attack by the parasitoid Asobara tabida is constrained by a trade-off with larval competitive ability. However, there are two very important questions that need to be answered. First, is this a general cost, or is it parasitoid specific? Second, does a selected increase in immune response against one parasitoid species result in a correlated change in resistance to other parasitoid species? The answers to both questions will influence the coevolutionary dynamics of these species, and also may have a previously unconsidered, yet important, influence on community structure.     

Do early life history characteristics determine species ranges and dispersal of coral reef fishes in the Indo‐Pacific?

The Indo‐Pacific consists of extensive continuous coastlines and many island groups of varying sizes and isolation. The species ranges of coral reef fishes vary enormously from Indo‐Pacific wide to highly endemic. There is also great variation in the early life history characteristics of coral reef fishes ( e.g . pelagic larval durations, spawning strategies and swimming abilities). We use individual‐based models (IBMs) to simulate the dispersal of coral reef fishes in the Indo‐Pacific. The development of dispersal strategies is explored based on ecological and geographical constraints. Simulations are presented for climatic and anthropogenically‐induced events.     

How does infection with parthenogenesis-inducing Wolbachia reduce the fitness of Trichogramma?

We analyzed the survival rate of the immature stages of Trichogramma species and lines that differed in their mode of reproduction. Specifically, we compared the mortality of arrhenotokous (W(-)), irrevertable thelytokous (W(-)), and Wolbachia-associated thelytokous (W(+)) forms. The embryonic mortality of the W(+) strains was significantly higher than that of the W(-) lines. The embryonic mortality was negligible for the arrhenotokous Trichogramma evanescens and the thelytokous Trichogramma cacoeciae, which is not infected with Wolbachia. Only 30% of the eggs of the Wolbachia-infected strains developed to adults, while the emergence rate of the Wolbachia-free strains was more than 78%, irrespective of the origin of the strains. More than 78% of the overall mortality in W(+) strains happened during the early stages of development. About 35% of embryos of W(+) strains remain in the mitotic stage even 48 h after oviposition. Most embryos of W(-) strains had already developed to cellular blastoderm after 6 h, regardless of strains. The mortality of immature stages in W(+) strains was mainly caused by the failure of the mitotic divisions.     

Is cardiorespiratory fitness related to quality of life in survivors of breast cancer?

The purpose of this study was to investigate whether indices of cardiorespiratory fitness are related to quality of life (QOL) in women survivors of breast cancer. Using the European Organization for Research and Treatment of Cancer QLQ-30 questionnaire, we assessed the QOL of 16 participants (age, 50 +/- 9 years; body mass, 66.6 +/- 9.6 kg). All participants performed incremental cycle ergometer exercise to determine several indices of cardiorespiratory fitness (e.g., peak oxygen uptake [.V(O2)peak, in L.min(-1), ml.kg(-1).min(-1)]), peak power output (PPO, in W), PPO/ body mass (W.kg(-1), peak heart rate (HRpeak, b.min(-1), peak ventilation (VEpeak), and .V(O2) and heart rate (HR) at the ventilatory (VT) and respiratory compensation (RCT) thresholds. Relationships between QOL and variables were assessed using Spearman rank-difference correlation tests. A significant inverse relationship (p < 0.05) was found for QOL scores and values for age (years) and body mass (kg) ( = -0.53), %HRpeak@VT ( = -0.59) and %VEpeak@VT ( = -0.61). A significant positive relationship (p < 0.05) was found for QOL and PPO/body mass ( = 0.59) and HRpeak ( = 0.78), .V(O2)@RCT (ml.kg(-1.min(-1) ( = 0.51), power output (PO, expressed as either W or W.kg(-1) at RCT, and HR at RCT ( = 0.54). No other significant relationship was found between QOL and variables obtained from the tests. In conclusion, these findings highlight possible relationships between cardiorespiratory fitness and well-being in survivors of breast cancer. From a practical point of view, our data emphasize the need for this population to engage in programmed cardiorespiratory exercise training, mainly designed to improve VT and RCT. The improvement of both submaximal indices can have a beneficial effect on QOL.     

To eject or to abandon? Life history traits of hosts and parasites interact to influence the fitness payoffs of alternative anti‐parasite strategies

Hosts either tolerate avian brood parasitism or reject it by ejecting parasitic eggs, as seen in most rejecter hosts of common cuckoos, Cuculus canorus, or by abandoning parasitized clutches, as seen in most rejecter hosts of brown‐headed cowbirds, Molothrus ater. What explains consistent variation between alternative rejection behaviours of hosts within the same species and across species when exposed to different types of parasites? Life history theory predicts that when parasites decrease the fitness of host offspring, but not the future reproductive success of host adults, optimal clutch size should decrease. Consistent with this prediction, evolutionarily old cowbird hosts, but not cuckoo hosts, have lower clutch sizes than related rarely‐ or newly parasitized species. We constructed a mathematical model to calculate the fitness payoffs of egg ejector vs. nest abandoner hosts to determine if various aspects of host life history traits and brood parasites’ virulence on adult and young host fitness differentially influence the payoffs of alternative host defences. These calculations showed that in general egg ejection was a superior anti‐parasite strategy to nest abandonment. Yet, increasing parasitism rates and increasing fitness values of hosts’ eggs in both currently parasitized and future replacement nests led to switch points in fitness payoffs in favour of nest abandonment. Nonetheless, nest abandonment became selectively more favourable only at lower clutch sizes and only when hosts faced parasitism by a cowbird‐ rather than a cuckoo‐type brood parasite. We suggest that, in addition to evolutionary lag and gape‐size limitation, our estimated fitness differences based on life history trait variation provide new insights for the consistent differences observed in the anti‐parasite rejection strategies between many cuckoo‐ and cowbird‐hosts.     

Evolution of complex life cycles in trophically transmitted helminths. II. How do life‐history stages adapt to their hosts?

We review how trophically transmitted helminths adapt to the special problems associated with successive hosts in complex cycles. In intermediate hosts, larvae typically show growth arrest at larval maturity (GALM). Theoretical models indicate that optimization of size at GALM requires larval mortality rate to increase with time between infection and GALM: low larval growth or paratenicity (no growth) arises from unfavourable growth and mortality rates in the intermediate host and low transmission rates to the definitive host. Reverse conditions favour high GALM size or continuous growth. Some support is found for these predictions. Intermediate host manipulation involves predation suppression (which decreases host vulnerability before the larva can establish in its next host) and predation enhancement (which increases host vulnerability after the larva can establish in its next host). Switches between suppression and enhancement suggest adaptive manipulation. Manipulation conflicts can occur between larvae of different ages/species a host individual. Larvae must usually develop to GALM before becoming infective to the next host, possibly due to trade‐offs, e.g. between growth/survival in the present host and infection ability for the next host. In definitive hosts, if mortality rate is constant, optimal growth before switching to reproduction is set by the growth/morality rate ratio. Rarely, no growth occurs in definitive hosts, predicted (with empirical support) when larval size on infection exceeds growth/mortality rate. Tissue migration patterns and residence sites may be explained by variations in growth/mortality rates between host gut and soma, migration costs and benefits of releasing eggs in the gut.     

Managing early ejaculation: what does the future hold?

Recent attention in the field of male sexual dysfunction has focused on erectile dysfunction. However, premature ejaculation (PE) is an extremely common condition that warrants clinical study and exploration of pharmacologic treatments. Until recently, PE was thought to be a behavioral problem for which the best remedy was a learned control technique. However, some drugs currently on the market, including sildenafil and the selective serotonin reuptake inhibitors, appear to have efficacy in the treatment of PE. More research is needed before FDA approval of such agents for this indication, but more and better options for men with PE are anticipated as attention to ejaculatory disorders grows and, hopefully, the associated stigma decreases.     

Possible origin of life between mica sheets: does life imitate mica?

The mica hypothesis for the origin of life proposes that life originated between the sheets of muscovite mica. This paper elaborates on two ways that life resembles what might have originated between mica sheets. First, enzymes: The configurations and dynamics of enzymes, with their substrates, cofactors, and sometimes transition metal ions, often resemble mica sheets, with their open-and-shut motions, acting on small molecules between them, sometimes assisted by transition metal ions. Second, organisms: Mica world had the potential to be a community or ecosystem of prebiotic organisms in a way unlike other models for the origin of life.