Immune response is energetically costly in white cabbage butterfly pupae

Parasite-driven coevolution has led hosts to develop a complicated and potentially costly defence machinery, consisting mainly of the immune system. Despite the evidence for the trade-offs between immune function and life-history traits, it is still obscure how the costs of using and maintaining the immune function are paid. We tested whether immune challenge is energetically costly for white cabbage butterfly (Pieris brassicae L.) diapausing pupa. Individuals challenged with nylon implant raised their standard metabolic rate nearly 8% compared to the controls. Hence, costs of activation of immune system in insect pupa can be expressed in energetic currency.     

Expression of immunity and general condition in the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae), in relation to origin and gender

In immunoecological studies, experimental effects usually explain a relatively small proportion of total variation observed in immune parameters, while the large amount of variation remains unexplained. It is crucial to be aware of such natural variation of immune parameters, which may overshadow the effects of the experiment. We examined factors responsible for variation in cellular immunity (estimated as hemocyte concentration) and general condition (estimated as fresh weight) in the Colorado potato beetle, Leptinotarsa decemlineata, originating from two neighboring potato fields, in a common garden experiment. Progeny of beetles collected from the “New” field, where potato was cultured for the first year, had significantly higher hemocyte concentration and fresh weight compared to individuals originating from the “Old” field, where potato had been cultured for several years. Furthermore, hemocyte concentration varied with respect to gender only in beetles originating from the New field, where males had a higher hemocyte concentration than females. No such sex differences were found in beetles originating from the Old field, suggesting that immune traits and general condition of insects originating from geographically close locations/populations may express different sources of variation. Therefore, generalization of immunity–life‐history trade‐offs based on one population/location should be treated with caution.     

Formation of melanin-based wing patterns is influenced by condition and immune challenge in Pieris brassicae

According to life‐history theory, trade‐offs emerge because organisms possess a limited amount of resources that they have to allocate between different bodily functions. Here, we tested whether there is a trade‐off between melanin‐based immune response and dark melanized wing patterning in the large white butterfly, Pieris brassicae L. (Lepidoptera: Pieridae), by activating the immune system of pupae and measuring the wing pigmentation of freshly emerged adults. In contrast to expectations, we did not find any negative associations between immune challenge and wing patterning. Furthermore, implanted and punctured male pupae tended to have larger and darker forewing tips as adults compared to controls. Both in females and males, different wing spots were affected by condition‐reflecting traits (e.g., pupal mass, brood), which suggest that formation of wing patterns may be a condition‐dependent process and/or heritable.     

Longer life span is associated with elevated immune activity in a seasonally polyphenic butterfly

Seasonal polyphenism constitutes a specific type of phenotypic plasticity in which short‐lived organisms produce different phenotypes in different times of the year. Seasonal generations of such species frequently differ in their overall lifespan and in the values of traits closely related to fitness. Seasonal polyphenisms provide thus excellent, albeit underused model systems for studying trade‐offs between life‐history traits. Here, we compare immunological parameters between the two generations of the European map butterfly (Araschnia levana), a well‐known example of a seasonally polyphenic species. To reveal possible costs of immune defence, we also examine the concurrent differences in several life‐history traits. Both in laboratory experiments and in the field, last instar larvae heading towards the diapause (overwintering) had higher levels of both phenoloxidase (PO) activity and lytic activity than directly developing individuals. These results suggest that individuals from the diapausing generation with much longer juvenile (pupal) period invest more in their immune system than those from the short‐living directly developing generation. The revealed negative correlation between pupal mass and PO activity may be one of the reasons why, in this species, the diapausing generation has a smaller body size than the directly developing generation. Immunological parameters may thus well mediate trade‐offs between body size‐related traits.     

Ants medicate to fight disease

Parasites are ubiquitous, and the ability to defend against these is of paramount importance. One way to fight diseases is self‐medication, which occurs when an organism consumes biologically active compounds to clear, inhibit, or alleviate disease symptoms. Here, we show for the first time that ants selectively consume harmful substances (reactive oxygen species, ROS) upon exposure to a fungal pathogen, yet avoid these in the absence of infection. This increased intake of ROS, while harmful to healthy ants, leads to higher survival of exposed ants. The fact that ingestion of this substance carries a fitness cost in the absence of pathogens rules out compensatory diet choice as the mechanism, and provides evidence that social insects medicate themselves against fungal infection, using a substance that carries a fitness cost to uninfected individuals.     

Transfer of Immunity from Mother to Offspring Is Mediated via Egg-Yolk Protein Vitellogenin

Insect immune systems can recognize specific pathogens and prime offspring immunity. High specificity of immune priming can be achieved when insect females transfer immune elicitors into developing oocytes. The molecular mechanism behind this transfer has been a mystery. Here, we establish that the egg-yolk protein vitellogenin is the carrier of immune elicitors. Using the honey bee, Apis mellifera, model system, we demonstrate with microscopy and western blotting that vitellogenin binds to bacteria, both Paenibacillus larvae – the gram-positive bacterium causing American foulbrood disease – and to Escherichia coli that represents gram-negative bacteria. Next, we verify that vitellogenin binds to pathogen-associated molecular patterns; lipopolysaccharide, peptidoglycan and zymosan, using surface plasmon resonance. We document that vitellogenin is required for transport of cell-wall pieces of E. coli into eggs by imaging tissue sections. These experiments identify vitellogenin, which is distributed widely in oviparous species, as the carrier of immune-priming signals. This work reveals a molecular explanation for trans-generational immunity in insects and a previously undescribed role for vitellogenin.     

Complexity and variability of gut commensal microbiota in polyphagous lepidopteran larvae

Background

The gut of most insects harbours nonpathogenic microorganisms. Recent work suggests that gut microbiota not only provide nutrients, but also involve in the development and maintenance of the host immune system. However, the complexity, dynamics and types of interactions between the insect hosts and their gut microbiota are far from being well understood.

Methods/Principal Findings

To determine the composition of the gut microbiota of two lepidopteran pests, Spodoptera littoralis and Helicoverpa armigera, we applied cultivation-independent techniques based on 16S rRNA gene sequencing and microarray. The two insect species were very similar regarding high abundant bacterial families. Different bacteria colonize different niches within the gut. A core community, consisting of Enterococci, Lactobacilli, Clostridia, etc. was revealed in the insect larvae. These bacteria are constantly present in the digestion tract at relatively high frequency despite that developmental stage and diet had a great impact on shaping the bacterial communities. Some low-abundant species might become dominant upon loading external disturbances; the core community, however, did not change significantly. Clearly the insect gut selects for particular bacterial phylotypes.

Conclusions

Because of their importance as agricultural pests, phytophagous Lepidopterans are widely used as experimental models in ecological and physiological studies. Our results demonstrated that a core microbial community exists in the insect gut, which may contribute to the host physiology. Host physiology and food, nevertheless, significantly influence some fringe bacterial species in the gut. The gut microbiota might also serve as a reservoir of microorganisms for ever-changing environments. Understanding these interactions might pave the way for developing novel pest control strategies.