Condition-dependent ecdysis and immunocompetence in the amphipod crustacean, Gammarus pulex

Effects of temperature, fungal infection and weight on intermoult duration and survival of starving earwig larvae

Moulting is a cornerstone of arthropods development. It can be determined by numerous factors such as body mass, temperature, and immunity. However, the effects of these factors can be dependent on each other, so that it is often difficult to predict whether and how they shape moulting, and whether their effects are additive or interactive. In this study, we addressed these questions by testing the effects of body mass, ambient temperature, fungal infection and their interaction on intermoult duration and survival in starved juveniles of the European earwig Forficula auricularia. We recorded the date of moult and death of a total of 207 earwig juveniles that were weighed, exposed to different doses of the entomopathogenic fungus Metarizium brunneum and then maintained at either 20 °C or 24 °C. Our results first reveal that juveniles moulted earlier when they were heavy compared to light on the day of exposure, as well as earlier when maintained at 24 °C compared to 20 °C. By contrast, pathogen exposure did not affect the moulting date. We also found that nymphs died faster when they were light compared to heavy on the day of exposure, when they were exposed to high (10⁶ and 10⁷ spores/ml) compared to low (10⁴, 10⁵ and 0 spores/ml) pathogen concentrations, and when they were maintained at 24 °C compared to 20 °C. We detected no sign of interaction between temperature, fungal infection and body mass on both moulting and survival. Overall, these findings shed light on the limited importance of infection on moulting in starved juveniles, and reveal that weight, temperature, and infection have additive effects on their survival. More generally, this study emphasizes that the three tested factors do not necessarily interact to shape key physiological processes in an insect.

MMP-14 regulates innate immune responses to Eriocheir sinensis via tissue degradation

Matrix metalloproteinases (MMPs) are a cluster of enzymes that degrade the extracellular matrix (ECM) and some intracellular proteins; as such, they play an important role in tissue regeneration, infant growth, animal reproduction, and immunity. Most research into MMPs focuses mainly on their effects on the mammalian immune system. However, it is not clear how MMPs affect immune processes in crustaceans. Here, we cloned the open reading frame (ORF) of Eriocheir sinensis (Chinese mitten crab) MMP-14 (EsMMP-14) to explore the role of MMPs in crustacean innate immune responses. RT-PCR results showed that stimulation of crab with LPS and poly I:C upregulated expression of EsMMP-14 markedly. Besides, following the stimulation of 20-Hydroxyecdysone, the expression level of EsMMP-14 increased robustly, suggesting that EsMMP-14 involved in the molt process of E. sinensis. Hematoxylin and eosin staining of hepatopancreas and intestine revealed that knocking down EsMMP-14 maintained morphology following infection by Bacillus thuringiensis. Moreover, downregulated expression of EsMMP-14 increased the survival rate of infected E. sinensis. These results show that EsMMP-14 plays a role in innate immune responses of E. sinensis and fills a gap in our knowledge about the function of MMPs in crustaceans.

Activation of the immune defence of the freshwater snail Lymnaea stagnalis by different immune elicitors

Understanding the outcomes of host-parasite interactions in nature is in high demand as parasites and pathogens are important for several ecological and evolutionary processes. Ecological immunology (ecoimmunology) has a key role in reaching this goal because immune defence is the main physiological barrier against infections. To date, ecoimmunological studies largely lean on measuring constitutive immune defences (components of defence that are always active). However, understanding the role of inducible components of immune function is important as the immune system is largely an inducible defence. Measuring such defences can be complicated as different parasites may activate different immune cascades, and expression of different immune traits may not be independent. We examined the suitability of different immune activation techniques for the freshwater snail Lymnaea stagnalis. By experimentally challenging snails with different immune elicitors [injection with snail saline (i.e. wounding), lyophilized Escherichia coli cells, lyophilized Micrococcus lysodeikticus cells, healthy snail gonad, and trematode-infected snail gonad; maintenance in microorganism-enriched water] and measuring phenoloxidase-like and antibacterial activity of their haemolymph, we found increased immune activity against some immune elicitors, but also decreased activity. Our findings suggest potentially complicated relationships among immune traits, and propose suitable techniques for ecological studies in this study system.

The role of moulting in parasite defence

Parasitic infections consist of a succession of steps during which hosts and parasites interact in specific manners. At each step, hosts can use diverse defence mechanisms to counteract the parasite's attempts to invade and exploit them. Of these steps, the penetration of parasites into the host is a key step for a successful infection and the epithelium is the first line of host defence. The shedding of this protective layer (moulting) is a crucial feature in the life cycle of several invertebrate and vertebrate taxa, and is generally considered to make hosts vulnerable to parasites and predators. Here, we used the crustacean Daphnia magna to test whether moulting influences the likelihood of infection by the castrating bacterium Pasteuria ramosa. This parasite is known to attach to the host cuticula before penetrating into its body. We found that the likelihood of successful parasite infection is greatly reduced if the host moults within 12 h after parasite exposure. Thus, moulting is beneficial for the host being exposed to this parasite. We further show that exposure to the parasite does not induce hosts to moult earlier. We discuss the implications of our findings for host and parasite evolution and epidemiology.

Immune defence under extreme ambient temperature

Owing to global climate change, the extreme weather conditions are predicted to become more frequent, which is suggested to have an even greater impact on ecological interactions than the gradual increase in average temperatures. Here, we examined whether exposure to high ambient temperature affects immune function of the great pond snail (Lymnaea stagnalis). We quantified the levels of several immune traits from snails maintained in a non-stressful temperature (15°C) and in an extreme temperature (30°C) that occurs in small ponds during hot summers. We found that snails exposed to high temperature had weaker immune defence, which potentially predisposes them to infections. However, while phenoloxidase and antibacterial activity of snail haemolymph were reduced at high temperature, haemocyte concentration was not affected. This suggests that the effect of high temperature on snail susceptibility to infections may vary across different pathogens because different components of invertebrate immune defence have different roles in resistance.