Repeated exposure affects susceptibility and responses of Atlantic salmon (Salmo salar) towards the ectoparasitic salmon lice (Lepeophtheirus salmonis).

Atlantic salmon (Salmo salar) is repeatedly exposed to and infected with ectoparasitic salmon lice (Lepeophtheirus salmonis) both in farms and in nature. However, this is not reflected in laboratory experiments where fish typically are infected only once. To investigate if a previous lice infection affects host response to subsequent infections, fish received 4 different experimental treatments; including 2 groups of fish that had previously been infected either with adult or infective salmon lice larvae (copepodids). Thereafter, fish in all treatment groups were infected with either a double or a single dose of copepodids originating from the same cohort. Fish were sampled when lice had developed into the chalimus, the pre-adult and the adult stage, respectively. Both the specific growth rate and cortisol levels (i.e. a proxy for stress) of the fish differed between treatments. Lice success (i.e. ability to infect and survive on the host) was higher in naïve than in previously infected fish (pre-adult stage). The expression of immune and wound healing transcripts in the skin also differed between treatments, and most noticeable was a higher upregulation early in the infection in the group previously infected with copepodids. However, later in the infection, the least upregulation was observed in this group, suggesting that previous exposure to salmon lice affects the response of Atlantic salmon towards subsequent lice infections.

The effect of the COVID-19 pandemic on the gender gap in research productivity within academia.

Using measures of research productivity to assess academic performance puts women at a disadvantage because gender roles and unconscious biases, operating both at home and in academia, can affect research productivity. The impact of the COVID-19 pandemic on research productivity has been the subject of a number of studies, including studies based on surveys and studies based on numbers of articles submitted to and/or published in journals. Here, we combine the results of 55 studies that compared the impact of the pandemic on the research productivity of men and women; 17 of the studies were based on surveys, 38 were based on article counts, and the total number of effect sizes was 130. We find that the gender gap in research productivity increased during the COVID-19 pandemic, with the largest changes occurring in the social sciences and medicine, and the changes in the biological sciences and TEMCP (technology, engineering, mathematics, chemistry and physics) being much smaller.

Adaptive host responses to infection can resemble parasitic manipulation.

Using a dynamic optimisation model for juvenile fish in stochastic food environments, we investigate optimal hormonal regulation, energy allocation and foraging behaviour of a growing host infected by a parasite that only incurs an energetic cost. We find it optimal for the infected host to have higher levels of orexin, growth and thyroid hormones, resulting in higher activity levels, increased foraging and faster growth. This growth strategy thus displays several of the fingerprints often associated with parasite manipulation: higher levels of metabolic hormones, faster growth, higher allocation to reserves (i.e. parasite-induced gigantism), higher risk-taking and eventually higher predation rate. However, there is no route for manipulation in our model, so these changes reflect adaptive host compensatory responses. Interestingly, several of these changes also increase the fitness of the parasite. Our results call for caution when interpreting observations of gigantism or risky host behaviours as parasite manipulation without further testing.

Extra-pair paternity explains cooperation in a bird species.

In many social animals, females mate with multiple males, but the adaptive value of female extra-pair mating is not fully understood. Here, we tested whether male pied flycatchers (Ficedula hypoleuca) engaging in extra-pair copulations with neighboring females were more likely to assist their neighbors in antipredator defense. We found that extra-pair sires joined predator-mobbing more often, approached predators more closely, and attacked predators more aggressively than males without extra-pair offspring in the neighboring nest. Extra-pair mating may incentivize males to assist in nest defense because of the benefits that this cooperative behavior has on their total offspring production. For females, this mating strategy may help recruit more males to join in antipredator defense, offering better protection and ultimately improving reproductive success. Our results suggest a simple mechanism by which extra-pair mating can improve reproductive success in breeding birds. In summary, males siring extra-pair offspring in neighboring nests assist neighbors in antipredator defense more often than males without extra-pair offspring.

Life history shifts in an exploited African fish following invasion by a castrating parasite.

Evolutionary theory predicts that infection by a parasite that reduces future host survival or fecundity should select for increased investment in current reproduction. In this study, we use the cestode Ligula intestinalis and its intermediate fish host Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania), as a model system. Using data about infection of E. sardella fish hosts by L. intestinalis collected for a period of 10 years, we explored whether parasite infection affects the fecundity of the fish host E. sardella, and whether host reproductive investment has increased at the expense of somatic growth. We found that L. intestinalis had a strong negative effect on the fecundity of its intermediate fish host. For the noninfected fish, we observed an increase in relative gonadal weight at maturity over the study period, while size at maturity decreased. These findings suggest that the life history of E. sardella has been shifting toward earlier reproduction. Further studies are warranted to assess whether these changes reflect plastic or evolutionary responses. We also discuss the interaction between parasite and fishery-mediated selection as a possible explanation for the decline of E. sardella stock in the lake.

Invest more and die faster: The life history of a parasite on intensive farms.

Organisms are expected to respond to alterations in their survival by evolutionary changes in their life history traits. As agriculture and aquaculture have become increasingly intensive in the past decades, there has been growing interest in their evolutionary effects on the life histories of agri- and aquacultural pests, parasites, and pathogens. In this study, we used salmon lice (Lepeophtheirus salmonis) to explore how modern farming might have affected life history evolution in parasites. We infected salmon hosts with lice from either farmed or unfarmed locations, and monitored life history traits of those parasites in laboratory conditions. Our results show that compared to salmon lice from areas unaffected by salmon farming, those from farmed areas produced more eggs in their first clutch, and less eggs later on; they achieved higher infestation intensities in early adulthood, but suffered higher adult mortality. These results suggest that salmon lice on farms may have been selected for increased investment in early reproduction, at the expense of later fecundity and survival. This call for further empirical studies of the extent to which farming practices may alter the virulence of agricultural parasites.

When to Reproduce? A New Answer to an Old Question.

We present a life-history model based on the assumptions that juvenile survival follows a negative exponential function and that fecundity gain increases linearly with time to maturity. This model predicts that the optimal fitness is achieved when survival at maturity is 0.368 (e(-1)). Survival at the time of maturity is therefore an invariant. We tested this prediction by using published data from infection experiments with mammalian nematodes, where both the initial number of juveniles colonizing a habitat (host) and the numbers surviving at the time of maturation were known. We found that the mean survival at maturity, both across and within species, was remarkably close to our predicted mean. As a control, we also looked at studies where the parasite species was adapted to a host species other than the one used in the reported experiment. In these experiments the mean survival at maturity differed from what our model predicted. Maturation at a fixed survival probability therefore appears as an adaptive trait evolved in a predictable environment, in this case, a host species. Our result further suggests that measures designed to increase juvenile parasite mortality, such as drugs or vaccines, will select for faster developmental rates.

How to deal with PCR contamination in molecular microbial ecology.

Microbial ecology studies often use broad-range PCR primers to obtain community profiles. Contaminant microbial DNA present in PCR reagents may therefore be amplified together with template DNA, resulting in unrepeatable data which may be difficult to interpret, especially when template DNA is present at low levels. One possible decontamination method consists in pre-treating PCR mixes with restriction enzymes before heat-inactivating those enzymes prior to the start of the PCR. However, this method has given contrasting results, including a reduction in PCR sensitivity. In this study, we tested the efficiency of two different enzymes (DNase 1 and Sau3AI) as well as the effect of dithiothreitol (DTT), a strong reducing agent, in the decontamination procedure. Our results indicate that enzymatic treatment does reduce contamination levels. However, DNase 1 caused substantial reductions in the bacterial richness found in communities, which we interpret as a result of its incomplete inactivation by heat treatment. DTT did help maintain bacterial richness in mixes treated with DNase 1. No such issues arose when using Sau3AI, which therefore seems a more appropriate enzyme. In our study, four operational taxonomic units (OTU) decreased in frequency and relative abundance after treatment with Sau3AI and hence are likely to represent contaminant bacterial DNA. We found higher within-sample similarity in community structure after treatment with Sau3AI, probably better reflecting the initial bacterial communities. We argue that the presence of contaminant bacterial DNA may have consequences in the interpretation of ecological data, especially when using low levels of template DNA from highly diverse communities. We advocate the use of such decontaminating approaches as a standard procedure in microbial ecology.

Intensive Farming: Evolutionary Implications for Parasites and Pathogens.

An increasing number of scientists have recently raised concerns about the threat posed by human intervention on the evolution of parasites and disease agents. New parasites (including pathogens) keep emerging and parasites which previously were considered to be 'under control' are re-emerging, sometimes in highly virulent forms. This re-emergence may be parasite evolution, driven by human activity, including ecological changes related to modern agricultural practices. Intensive farming creates conditions for parasite growth and transmission drastically different from what parasites experience in wild host populations and may therefore alter selection on various traits, such as life-history traits and virulence. Although recent epidemic outbreaks highlight the risks associated with intensive farming practices, most work has focused on reducing the short-term economic losses imposed by parasites, such as application of chemotherapy. Most of the research on parasite evolution has been conducted using laboratory model systems, often unrelated to economically important systems. Here, we review the possible evolutionary consequences of intensive farming by relating current knowledge of the evolution of parasite life-history and virulence with specific conditions experienced by parasites on farms. We show that intensive farming practices are likely to select for fast-growing, early-transmitted, and hence probably more virulent parasites. As an illustration, we consider the case of the fish farming industry, a branch of intensive farming which has dramatically expanded recently and present evidence that supports the idea that intensive farming conditions increase parasite virulence. We suggest that more studies should focus on the impact of intensive farming on parasite evolution in order to build currently lacking, but necessary bridges between academia and decision-makers.

Aromatic plants in nests of the blue tit Cyanistes caeruleus protect chicks from bacteria.

Several bird species add fresh fragments of plants which are rich in volatile secondary compounds to their nests. It has been suggested, although never tested, that birds use fresh plants to limit the growth of nest microorganisms. On Corsica, blue tits (Cyanistes caeruleus) incorporate fresh fragments of aromatic plants into their nests. These plants do not reduce infestation by nest ectoparasites, but have been shown to improve growth and condition of chicks at fledging. To understand the mechanisms underlying such benefits, we experimentally tested the effects of these plants on the bacteria living on blue tits. Aromatic plants significantly affected the structure of bacterial communities, in particular reducing bacterial richness on nestlings. In addition, in this population where there is a strong association between bacterial density and infestation by blood-sucking Protocalliphora blow fly larvae, these plants reduced bacterial density on the most infested chicks. Aromatic plants had no significant effect on the bacteria living on adult blue tits. This study provides the first evidence that fresh plants brought to the nests by adult birds limit bacterial richness and density on their chicks.

Local individual preferences for nest materials in a passerine bird.

BACKGROUND: Variation in the behavioural repertoire of animals is acquired by learning in a range of animal species. In nest-building birds, the assemblage of nest materials in an appropriate structure is often typical of a bird genus or species. Yet plasticity in the selection of nest materials may be beneficial because the nature and abundance of nest materials vary across habitats. Such plasticity can be learned, either individually or socially. In Corsican populations of blue tits Cyanistes caeruleus, females regularly add in their nests fragments of several species of aromatic plants during the whole breeding period. The selected plants represent a small fraction of the species present in the environment and have positive effects on nestlings. METHODOLOGY/PRINCIPAL FINDINGS: We investigated spatiotemporal variations of this behaviour to test whether the aromatic plant species composition in nests depends on 1) plant availability in territories, 2) female experience or 3) female identity. Our results indicate that territory plays a very marginal role in the aromatic plant species composition of nests. Female experience is not related to a change in nest plant composition. Actually, this composition clearly depends on female identity, i.e. results from individual preferences which, furthermore, are repeatable both within and across years. A puzzling fact is the strong difference in plant species composition of nests across distinct study plots. CONCLUSIONS/SIGNIFICANCE: This study demonstrates that plant species composition of nests results from individual preferences that are homogeneous within study plots. We propose several hypotheses to interpret this pattern of spatial variation before discussing them in the light of preliminary results. As a conclusion, we cannot exclude the possibility of social transmission of individual preferences for aromatic plants. This is an exciting perspective for further work in birds, where nest construction behaviour has classically been considered as a stereotypic behaviour.